design for pet ct facility
TRANSCRIPT
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Medical Physics
Graduate Project
NASIR IQBAL
MEDICAL PHYSICS
Govt. College of Science, Wahdat Road, Lahore.
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Medical Physics i
RADIATION SHIELDING AND DESIGN REQUIREMENT OF PET-CT FACILITY
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Medical Physics ii
Student Name: Nasir Iqbal
Class: BS (Hons) Physics
Roll No: 2277
Semester: 8
Title of the Project
RADIATION SHIELDING AND DESIGN REQUIREMENT OF PET-CT FACILITY
Signed by:
XProf. Dr. Ejaz Ahmed
Head of Physics Department GCS Lahore
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Experience
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Table of Contents
Sr. No. Title Page no.
1 Chapter-1 Introduction 1-9
1.1 Radiations 1
1.2 Types of Radiations 1
1.2.1 Ionizing Radiations 1
1.2.2 Non-Ionizing Radiations 4
1.3 Detection of Radiations 4
1.4 Radiation Exposures 6
1.5 Radionuclides 7
1.6 Production of Radionuclides 7
1.6.1 Cyclotrons Produced Radionuclides 8
1.6.2 Nuclear Reactor Produced Radionuclides 8
1.6.2.1 Nuclear Fission 8
1.6.2.2 Neutron Activation 8
2 Chapter-2 Nuclear Medicine 10-19
2.1 Introduction to Nuclear Medicine 10
2.2 Comprehensive Definition of Nuclear Medicine 11
2.2.1 Nuclear Medicine Diagnosis 12
2.2.2 Nuclear Medicine Therapy 13
2.3 Model of the Nuclear Medicine Department 14
2.3.1 Ideal Design of Nuclear Medicine Department 15
2.4 Radiation Protection in Nuclear Medicine Department 17
2.4.1 Patient as a Radioactive Source 17
2.4.2 Radiation Dose Limits 17
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2.4.3 The Transportation of the Radioactive Material 18
3 Chapter-3 PET-CT 20-26
3.1 Introduction to PET-CT 20
3.2 Positron Emission Tomography 20
3.2.1 Radionuclide Used in PET Imaging 22
3.3 Computed Tomography 23
3.4 Design of PET-CT Clinics 23
3.5 Radiation Safety and Controlling Radiation Exposures 24
3.5.1 Distance 24
3.5.2 Time 25
3.5.3 Radiation Contamination Control 25
3.5.4 Shielding 25
4 Chapter-4 Literature Review 27-29
5 Chapter-5 Method for Shielding Calculation 30-35
5.1 Gamma Radiations 30
5.2 Shielding Calculation 30
5.2.1 Patient Uptake Room 32
5.2.2 PET-CT Scanning Room 33
5.2.3 Hot Lab 34
6 Chapter-6 Shielding Calculations for SKMCH Site 36-47
6.1 Cancer Hospital, Lahore 37
6.1(a) Shielding of Patient Uptake Room of SKMCH 37
6.1(b) Shielding of PET-CT Imaging Room 38
6.1(c) Shielding of Hot Lab 39
6.2 Cancer Hospital, Karachi 41
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6.2(a) Shielding of Uptake Room 42
6.2(b) Shielding of Scanning Room 43
6.2(c) Shielding of Hot Lab 44
6.3 Comparison of Calculated and Existing Shielding of
SKMCH Karachi
45
6.4 Comparison of Calculated Shielding of SKMCH Lahore
and Karachi
46
7 Chapter-7 Results and Discussions 48-49
8 Chapter-8 Conclusions 50
9 Chapter-9 References 51-52
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List of Figures
Figure no. Description Page no.
1 Penetrability of Ionizing Radiation 2
2 Radiation Exposures to Public 6
3 Design of Nuclear Medicine Department 15
4 Radiation Dose Limits 17
5 Radioactive Labels 18
6 Scanning Process of PET imaging 19
7 Physics of PET 20
8 Decay by Positron Emission 21
9 Functioning of CT Imaging 22
10 Relation of Exposure with Distance 23
11 Tungsten Shielded Syringes 25
12 Site Plan of PET/CT at Cancer Hospital Lahore 36
13 Site Plan of PET/CT Suite Cancer Hospital Karachi 40
14 Existing and Calculated Shielding of Cancer Hospital
Karachi
44
15 Comparison between Calculated Shielding of Lahore and
Karachi
44
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Medical Physics 1
Chapter-1 Introduction
1.1 Radiation:
Every particle is composed of atoms. Some of the atoms are the lighter ones like hydrogen and
some of them are heavier atoms, such as uranium. Normally, the heavier atoms have more
unstable nuclei as compared to lighter atoms. This is because of the imbalance in the proportion
of the neutrons and protons in the nucleus. These unstable nuclei gain their stability after
emitting excess energy in the form of fast moving energetic particles. This energy is known as
Radiation and such unstable nuclei are called radionuclides. The emission of radiation by these
radioactive nuclides is referred to as radioactivity.
1.2 Types of Radiations:
Radiations are classified on the basis of intensity of energy, frequency and wavelength. The
higher the frequency, the lesser would be the wavelength of a particular wave. Following are
the two main types of radiations.
Ionizing Radiations
Non-Ionizing radiations
1.2.1 Ionizing Radiations:
So by this it is understood that the Radiations that has the ability to remove firmly bound
electrons from atoms which results in creating ions. These are referred to as Ionizing radiations.
People usually think about this type of radiation. Frequency, wavelength, penetrability and
LET (Linear Energy Transformation) varies with respect to the radiation. These radiations have
higher frequencies and very small wavelengths. These have higher penetrating ability that is
why they are very harmful to environment. The advantages of this type of radiation are in
generation of electricity in nuclear reactor, diagnosing and treatment of cancer in nuclear
medicine etc. Our main focus will be more on ionizing radiations (We are in this thesis
concentrate on ionizing radiation only). There are four major types of ionizing radiations:
Alpha radiations
Beta radiations
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Gamma Radiations
Neutron Radiation
Figure 1 Penetrability of Ionizing Radiation
http://www.australian-radiation-services.com.au/index.php?page=19
Alpha Radiations:
Alpha radiations are simply helium nuclei composed of two protons and two neutrons, which
are released from the nucleus of an atom when the neutron to proton ratio is very low. The unit
which is used to express the energy of radiation is the electron volt.
Alpha particles loose their energy in short distances due to their large ionization potential (high
LET). Alpha particles are relatively massive and slow, and usually can not pass through a
normal sheet of paper or the external layer of skin. As a result, these charged particles represent
a main hazard only when taken into the body, where the energy they release will be completely
absorbed by small volumes of tissue.
Beta Radiations ():
Beta radiation are high energy and fast speed electrons or positrons released by specific types
of radioactive nuclei (Fluorine-18). The production of beta particles is known as beta decay.
They are denoted by the Greek letter beta (). There are two forms of beta decay, Beta negative
decay and Beta Positive Decay +, which produces electron and positron respectively.
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Beta particles have an extremely small mass as compared to alpha particles. Thus for a
particular energy, beta particles have higher speed, low LET which enables them to possess
higher penetrability over alpha. Therefore beta particles have a small energy loss, which means
that their ability to penetrate in any material is much larger than alpha particles. The reaction
for the beta decay is as follows,
n p + e- + Ve- ( For electron emission or Beta negative decay )
p n + e+ + Ve ( For Positron emission or Beta Positive decay )
When shielding the beta radiation, we came across the electromagnetic radiation which is
simply the secondary X-rays commonly known as Bremsstrahlung, produced by the fast
moving electrons. Shielding for beta radiation should be made of those materials which have
small atomic number to decrease the amount of bremsstrahlung produced. The penetration of
beta particles depends on their energy. For example, beta particle having energy of 1 Mev will
move about 3.49 m in air. Following table gives the thicknesses of materials in inches to absorb
beta radiation.
Energy (Mev) Plastic (inches) Concrete (inches) Aluminium (inches)
0.5 0.11 0.06 0.06
1.0 0.2 0.09 0.09
2.0 0.31 0.19 0.19
3.0 0.42 0.31 0.31
Gamma Radiations:
Gamma radiations are the type of electromagnetic rays and are denoted by the Greek letter .
It is a high energy ionizing electromagnetic radiation having high frequency and a very short
wavelength which is measured in some nanometres (billionth of a meter). Particle interactions
such as the process of electron-positron annihilation and radioactive decay produce gamma
rays.
Gamma photons have energy of about 10,000 times as the photons in the visible range of the
electromagnetic spectrum. They have no mass and are not charged electrically. Gamma
photons move at the speed of light and can travel up to thousands of meters in air due to their
high energy. They can penetrate in different kind of materials including human tissue. Owing
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to this property these are used in medical imaging. Dense materials like lead, concrete are
commonly used as shielding to reduce the intensity of gamma photons.
Neutron Radiations:
Neutron is the form of radiation, similar to gamma radiation, which have a great ability to
penetrate in the materials. Neutrons are not electrically charged, therefore they are unaffected
by the electric fields of atoms of absorber materials. Neutron attenuation is achieved mostly
through elastic and inelastic scatter, which decrease the energy of the neutron until it is
absorbed in the material used for shielding. Elastic scatter is where the neutron collides with
the target nucleus and bounces off exactly like the collision of the two pool balls. The neutron
loses some of its energy and this energy is transferred to the target nucleus during the collision.
Light elements are best for slowing down neutrons by elastic scatter and so materials with high
hydrogen content, such as water, concrete, and plastic are used for this purpose.
Inelastic scatter is a type of scattering in which the incoming neutrons impart some of their
energy to the scattering material and excite the target nuclei. The excited target nuclei emit
gamma rays as it return to its ground state. Neutron capture is the process where neutrons are
captured by the target nuclei which then de-excite by emitting another particle or gamma ray.
Neutrons are most effectively shielded by materials containing low atomic number absorbers.
Neutrons are slowed to thermal energies by elastic collision and then they are captured by
nuclei of the shielding material. Materials commonly used to shield neutrons are concrete,
water, and polyethylene.
1.2.2 Non-Ionizing Radiations:
Radiation with sufficient energy that can make the atoms to move in a molecule or enough
energy that results in the vibration of atoms but do not have the tendency to remove the
electrons from their orbits, such radiations are known as Non-Ionizing radiations. Radio waves,
Sound waves and the visible light are the common examples of non-ionizing radiations.
1.3 Detection of Radiation:
Our senses cannot detect the presence of radiation. We can detect radiations indirectly by using
some scientific method or techniques. As light affects the Photo-Films, similarly radiation also
does affect. So, these films are used to detect and record radiation levels. There are also other
materials which emit light when exposed to radiation. Such materials are known as scintillators
and the detectors made of these materials are known as Scintillation Detectors. The intensity
of light emitted by the scintillator is proportional to the radiation intensity. Another type of
instrument is the Geiger-Mueller counter, which is the most commonly used instrument for
easy and quick detection of radiation. In this, electric current is measured which is produced
when radiation passes through an inert gas. [1]
Following are some of the instruments used to detect radiation.
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(a) Gas- Filled Counters:
The detectors in which we use some type of inert gas (usually air) to detect the radiations are
knows as Gas-Filled counters. When the radiation passes through the gas detector in the
presence of electric field, it ionizes the gas and as a result of that the electrons are produced.
By collecting and measuring these charged electrons, we can detect the presence of the nuclear
radiations. A detector can not only simply record the presence of the nuclear radiations but at
the same time it also tells us about the energy and the type of the radiation. There are three
main types of gas-filled counters. The construction is almost the same of these three counters.
In each type the instrument consists of a container to hold the gas and two electrodes across
which there is a potential difference. These detectors mainly differ in having different potential
differences, auxiliary circuits and the gaseous filling. The names of them are given below:-
Ionization Chamber
Proportional Counter
Geiger-Mueller Counter[2]
(b) Semi-Conductor Detectors:
Semi-conductor radiation detectors are widely used for the detection, imaging and
spectroscopy of different types of radiations such as gamma rays, x-rays etc. The basic
principle of semiconductor detectors is similar to that of gas filled counters. When radiations
interact with matter, the passage of ionizing particles creates electron-hole pairs which are then
collected by the electric eld. The advantage of the semiconductor detector is that the average
energy needed to create an electron-hole pair is approximately 10 times smaller than the energy
required for gas ionization.
The above table shows the comparison of energies (required to produce an electric signal)
between the inert gases and semiconductors. Hence the amount of ionization produced for a
particular energy is an order of magnitude greater which results in increased energy resolution.
So these are more sensitive as compared to gas-filled radiation detectors. But their disadvantage
is that it is not possible to build large scale detectors since process of crystal growth has its size
limitations. But by integrating many smaller detectors together large area position sensitive
detectors have been built. At single photon level it is possible to build detectors for high and
low radiation fields. [3]
He 41.2 eV Si 3.6 eV
Xe 22.3 eV Ge 2.84 eV
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1.4 Radiation Exposures:
As mentioned above that the effects of non-ionizing radiation can be neglected because they
are low energy radiations but exposure to ionizing radiation has the potential to be damaging
and in some cases even deadly. This brings the importance to an idea of radiation safety called
ALARA which means As Low As Reasonably Achievable. Because of the fact that results
of exposure at low levels are not known, the idea of ALARA expects that exposure rates should
be kept as low as it can be possible considering the social and economic factors. Law makers
have made specific limits in order to minimize the exposure of the radiation because of the
known and unknown dangerous effects of the radiations. These limits are made to control the
doses to occupationally exposed workers. These Workers have willingly accepted the risk of
exposure. Dose limits are also given for the people of the general public, who are
unintentionally and possibly unknowingly being exposed of radiation. These limits are made
compulsory at the federal and as well as at the state level.
As listed in the Nuclear Regulatory Commissions Title 10 Code of Federal Regulations Part
20 (10CFR 20), occupationally exposed workers are not to exceed 0.05 Sv (5 rem) per year
while members to the public are limited to 0.001 Sv (100 mrem) per year (NRC 1992). [4]
Fast development in technology using ionizing radiation leave challenges to make sure the
safety of the public, the patients and to the occupationally-exposed workers. There are several
methods by which the minimum radiation exposure can be maintained within the certain safety
limits, made by law makers. Usually the technique of installing highly attenuating material like
lead or concrete within the adjacent structures is used. Normally the computational method like
Monte Carlo codes (MCNPX) is used to calculate the shielding of a specific area which is
required to be in the safely limits of radiation exposure enforced by the federal and state
regulations. These codes are based on the attenuation coefficient of the material which is to be
used in shielding.
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Figure 2 Radiation Exposures to Public
http://www.themebuilders.com/ProtectYourFAMILYfromRadiation.html
1.5 Radio-nuclides:
Radionuclide is also known as radioactive nuclides, is basically an atom having unstable nuclei.
It is considered as the nucleus having excess energy which is available to be transferred either
to a newly created radiation particle within the nucleus or to an atomic electron. Many types of
cancer are treated by the help of these radiations emitting radio-nuclides. These are introduced
into a particular region either surgically or by ingestion or injection attached to a
pharmaceutical which is then taken to the specific cancerous tissue.
1.6 Production of Radio-nuclides:
Although there exist naturally occurring radionuclide but in nuclear medicine, we use
artificially produced radio-nuclides. Because in diagnosing and therapy of cancer, only those
nuclides can be used which have the half-lives of few minutes so that diagnosing and therapy
can be done. Radio-nuclides which are used in nuclear medicines are produced by radionuclide
generators, nuclear reactors or cyclotrons accelerators.
Following are the technique of producing radio-nuclides.
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1.6.1 Cyclotrons Produced Radio-nuclides:
Radio-nuclides produced in cyclotrons and other accelerators by the bombardment of high
energy charged particles on stable nuclei. Usually protons and alpha particles are used in the
production of radio-nuclides made for the nuclear medicine. Kinetic energy is provided to the
high energy charged particles to overcome the coulomb repulsion of the target nuclei. Gallium-
67 is the commonly used radionuclide produced by cyclotrons. Usually the radio-nuclides
produced by cyclotrons are poor in neutrons therefore they decay by positron emission or
electron capture. The cyclotrons used in the medical field are normally placed near the Positron
Emission Tomography (PET) facility because of the short half-lives of the radio-nuclides
produced by cyclotrons. As they decay in a very short time. F-18 (Fluorine-18) is an
exceptional radionuclide because of its ideal long half-life, which is 110 minutes.
1.6.2 Nuclear Reactors Produced Radio-nuclides:
Nuclear reactors are another important source of producing radio-nuclides used in nuclear
medicines. Neutrons being neutral in nature penetrate easily into the nucleus as compared to
the other charged particles without accelerated to high energies. Nuclear fission and neutron
activation are the two main processes by which radio-nuclides can be produced in a nuclear
reactor.
1.6.2.1 Nuclear Fission
The breaking of an atomic nucleus into two smaller nuclei is known as nuclear fission. Some
fission reactions impulsively occur without providing any energy and some of them require
energy to overcome the binding forces of the nucleus. This energy is often provided by the
absorption of the neutrons. Neutrons can only induce fission in some very heavy nuclei. But
high energy neutrons can induce fission in such several nuclei.
Special kind of nuclear reactors are used to produce medically useful radionuclides from the
fission products of stable target material. The samples which are to be irradiated are inserted
through the ports which exist between the fuel elements in the reactor core. The most
commonly used fission products in nuclear medicines are molybdenum-99 (Mo-99), Xenon-
131 (Xe-133) and Iodine-31 (I-31). These radionuclides can be separated chemically from other
fission products so that there should not remain any stable isotopes of radionuclides. Therefore
the concentration or the specific activity can be increased and volume of the injected dose can
be minimized.
1.6.2.2 Neutron Activation
Neutrons which are produced by the fission of uranium in the nuclear reactor can be bombarded
on a stable target to produce radionuclides. The process in which the stable target material
captures neutron to produce radioactive nuclei is known as Neutron Activation. The most
common thermalized neutron reaction in neutron activation produced radionuclides is that
when the stable nucleus captures a neutron, it suddenly emits gamma ray. Other thermal
neutron capture reaction is followed by the emission of an alpha particle or a proton. Most
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Medical Physics 9
neutron activation uses the (n, ) gamma emission method because thermalized neutrons can
only induce in some low atomic mass target nuclei. All radionuclides which are produced by
neutron activation decay by Beta-Negative particle emission.
The common examples of neutron activation produced radionuclides which are used in nuclear
medicine are listed below:
Phosphorous 32 production: 31P (n, ) 32P T1/2 = 14.3 days
Chromium 51 production: 50Cr (n, ) 51Cr T1/2 = 27.8 days [5]
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Chapter-2 Nuclear Medicine
2.1 Introduction of Nuclear Medicine:
Nuclear Medicine has a complex history because there has always been a close relation
between discoveries in atomic and nuclear physics, and their use in medicine. It starts from
scientific discoveries, like the discovery of x-rays in 1895, when Roentgen a German scientist
included an X-ray image of the skeletal structure of his wifes hand. Then the natural
radioactivity in 1896 and artificial radioactivity in 1934 were discovered. Pierre Curie
demonstrated the considerably damaging effects of radiations who was induced a radiation
burn on his hand. In 1936 Artificial radioactivity was first demonstrated clinically at The
University of California. A thyroid cancer patient's was treated successfully with radioactive
iodine in 1946. However clinical use of Nuclear Medicine did not begin until the early 1950s.
Nuclear medicine is also known as Radionuclide imaging or scintigraphy. It is a medical
imaging method which basically involves the following steps:
The introduction of a radioactive substance either by ingestion or by injecting directly
into the body.
The detection of the emitted gamma radiations by a detector which is placed outside
the body but close to the skin surface.
The detecting instrument which detects the emitted radiations is known as Gamma
Camera.
A nuclear medicine facility consists of at least a gamma camera, a qualified physician and a
nuclear medicine technologist. When more than one technical member is there, a technical
director is responsible for supervision of the technical staff. [6]
Nuclear medicine is a medical facility which uses safe practices to image the body. It is
matchless because it is an anatomy (Internal study) based technique which tells about functions
of organs and their structure. Nuclear medicine is used in the diagnosis, therapy and prevention
of many diseases like cancer etc. Nuclear medicine mechanism is one of the most safe and
sound diagnostic imaging techniques which are currently accessible.
Following are the specific scans which can be obtained by using nuclear medicine procedures:
Gastro-intestinal system
Endocrine system
Central nervous system
Genito-urinary system
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Medical Physics 11
Skeletal system
Pulmonary system
Diagnosis of infection
Tumor diagnosis and therapy
Nuclear medicine therapy
Myocardial perfusion
Nuclear cardiology
2.2 Comprehensive Definition of Nuclear Medicine:
Nuclear medicine is the branch of medical imaging sciences that uses applications of physics
such as radiations to provide information about the structure and functioning of a particular
organ of a person or for the treatment of a specific disease. The kidneys, cardiac muscles, liver,
thyroid and other organs can be easily diagnosed, and the functional disorder can be known.
Radiations are also used for the treatment of abnormal or the cancerous cells (Tumor). Nuclear
medicine techniques are usually painless medical tests that help the physicians to diagnose
easily medical conditions.
There are about more than 10,000 hospitals worldwide which uses radioisotopes in medicine.
90% of the radioisotopes are used for diagnosis. The most widely used radioisotope in
diagnosis is technetium-99 (Tc-99) which is a daughter nucleus of Molybdenum. It accounts
for about 80% of all nuclear medicine techniques worldwide.
In developed countries (26% of world population) the frequency of diagnostic nuclear
medicine is 1.9% per year, and the frequency of therapy with radioisotopes is about one tenth
of this. In the USA there are some 18 million nuclear medicine procedures per year among 305
million people, and in Europe about 10 million among 500 million people. In Australia there
are about 560,000 per year among 21 million people. The use of radiopharmaceuticals in
diagnosis is growing at over 10% per year.[7]
Nuclear medicine was developed by physicians in the 1950s. In the beginning, they used
iodine-131 (I-131) for diagnosis. But then they used it for therapy especially for the treatment
of thyroid cancer, when further they came to know about its energy and its decay scheme.
Nuclear medicine is the most consistent method for making diagnoses and answering
appropriate treatments for many diseases. The department of Nuclear medicine is classified
into two groups:
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Medical Physics 12
1. Nuclear medicine Diagnosis
2. Nuclear medicine Therapy.
2.2.1 Nuclear medicine Diagnosis
Diagnostic techniques in nuclear medicine use radioactive isotopes which emit radiations.
These radioisotopes are usually short-lived isotopes. There are very few radionuclides which
are used to diagnose human disease in diagnostic nuclear medicine department. A
pharmaceutical is attached to a particular radionuclide which works as a carrier to gather in a
particular part of the body which is to be scanned. This is known as radiopharmaceutical. It
emits radiation and the detector like gamma camera or a PET (Positron Emission Tomography)
scanner is used which detects the presence of radiation so in this way the particular organ is
observed. These special types of radiation sensitive detectors are used to transform the gamma
radiation of the tracers into images.
The gamma camera is one of the basic medical imaging tool which is used to view and analyze
the images of the human body or the delivery of medically ingested, inhaled, or injected
radionuclides emitting gamma rays in nuclear medicine department. Radiopharmaceutical is
given to a patient. This starts emitting radiations when it is accumulated in the specific area of
interest. This energy or radiation is detected by gamma camera. This device work together with
a computer to measure the amount of radioisotope absorbed by the body and to give special
pictures explaining details on both the structure and functioning of organs and tissues.
Positron Emission Tomography (PET) is a recently developed technique which is more precise
and accurate as compared to other imaging techniques. A positron-emitting radionuclide like
F-18 is administered to the patient by injection, and it is collected at the particular target tissue.
When it decays it emits a positron, which suddenly combines with a neighbouring electron
(Annihilation process) to emit two identical gamma rays in approximately opposite directions
at the same time. These radiations are detected by a PET scanner and give very precise and
accurate indication of their origin. PET technique is widely used in oncology studies, with
fluorine-18 as the radioisotope combined with a pharmaceutical to produce
radiopharmaceutical FDG (Fluoro Dseoxy Glucose). It has proven to be the best method of
detecting and diagnosing most cancers. It is also well used in brain and heart scanning.
PET/CT is also a latest technique which is used for diagnostic oncology studies. This is
basically the combined form of X-ray Computed Tomography (CT) and Positron Emission
Tomography (PET). PET scan gives the important body functions, such as blood flow, oxygen
use, and glucose metabolism to help doctors in assessing the functioning of organs and tissues.
CT scan uses special x-ray equipment attached to the gantry, to produce several images of the
inside of the body. These pictures can then be explained by a radiologist on a computer monitor
as printed images. Thus, it is a very powerful tool by which the functional and anatomical
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imaging of a specific organ can be obtained and a wide variety of diseases can be diagnosed.
It gives 30% better diagnosis as compared to the traditional gamma camera alone.
PET and PET/CT scans are performed to:
Detect the cancer.
Determine the dispersed cancer in the body.
Evaluate the effectiveness of a treatment, such as cancer therapy.
Determine if a cancer has returned after treatment.
Determine blood flow to the heart muscle.
Determine the effects of a heart attack on areas of the heart like cardiac muscles.
Identify areas of the heart muscle that would benefit from a procedure such as
angioplasty or angiography.
Assess brain abnormalities for example tumors, memory disorders and other central
nervous system disorders.
To map normal human brain and heart function.
2.2.2 Nuclear Medicine Therapy
The unique characteristics of radioisotopes are used for the treatment of the abnormal cells in
therapeutic nuclear medicine. The radioisotope emits small amounts of radiation which will act
on target cells. This irradiation can be for the purpose of a curative treatment (like thyroid
cancer), palliative treatment (for instance for bone pain) or to reduce an organs function (for
example an over-active thyroid). Because of the fact that the rapidly growing cells are highly
sensitive to radiation, some cancerous growths can be destroyed or controlled by irradiating
the specific area containing the growth.
External irradiation is one of the therapeutic techniques used for the treatment of cancerous
cells. This is also known as Tele-Therapy. As by its name suggests, It is carried out using a
gamma beam from a radioactive cobalt-60 source placed externally. In developed countries the
much versatile and modified linear accelerators are now being used as a high-energy x-ray
source. An external radiation procedure is known as the Gamma Knife Radio-surgery. In this
technique the gamma radiations are focused from 201 sources of cobalt-60 on a very precise
and accurate area of the brain having abnormal cells or commonly known as Tumor. There are
about over 30,000 patients which are treated annually.
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Medical Physics 14
Internal radionuclide therapy is similar to diagnostic nuclear medicine in which the patient is
administered by a radiopharmaceutical which goes to the targeted area. In this case, the
medicine is a combination of a pharmaceutical and a radioisotope which is usually a gamma or
beta emitter. The short-range radiotherapy is becoming the main means of treatment and it is
known as Brachy-Therapy. Iodine-131, being one of the most successful radio-therapy nuclide,
is commonly used for the treatment of thyroid cancer.
Another common example of radiation therapy is the treatment of the disease known as
leukaemia which involves a bone marrow transplant. In this case the defective bone marrow is
first killed off with a lethal dose of radiation before it is being transplanted with healthy bone
marrow from a donor.
To eliminate and control the dispersed cancers, a recently developed therapy is used which is
known as Targeted Alpha Therapy (TAT) or alpha radio-immuno-therapy. When the alpha
emitting radionuclide goes to the targeted cells, the short range highly energetic alpha
radiations suddenly transfers its energy which results in the eradication of the cancerous cells
in the particular tissue. Targeted Alpha Therapy using lead-212 can be used for the treatment
of pancreatic, ovarian and melanoma (skin) cancers.
Radionuclide therapy has increasingly become successful for the treatment of many diseases
with less harmful side-effects. The basic idea in any therapeutic procedure is to focus the
radiations to well-defined target volumes of the patient.
Some common types of radionuclide therapy are:
Treatment of over-active thyroid
Treatment of thyroid cancer
Palliative treatment of bone pain caused by metastatic cancers
Treatment of blood disorders
Chronic inflammatory rheumatism
Treatment of Non-Hodgkins lymphoma
2.3 Model of a Nuclear Medicine Department:
The designing of the nuclear medicine department should be done in that way which can reduce
the un-necessary exposure of the radiations to the general public and as well as the
occupationally exposed workers. While designing this facility, further modifications and
extension of this facility for the future should also be kept in mind. This can be done in the
following manner:
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Medical Physics 15
Because of the continuous emission of radiations, the transportation of the radioactive
material should be minimized. For the achievement of this, the dose preparation room
and the dose administration room (injection room) must be bordering to each other or
should be very close.
Appropriate shielding should be done in those areas storing radioactive material. To
avoid the radiation exposure, the radioactive material itself should be completely stored
in a well-shielded container.
Entrance should be limited so that the general public must not access the controlled
areas because the exposures in such areas are very high. There must be a separate toilet
and a waiting area for the radionuclide administered patients. Only the authorized
occupationally exposed workers should have access to such areas.
The storage of radioactive waste should be in those areas which cannot be accessible to
the general public.
Every precaution should be taken to make sure that the radiation doses received by the
people should be less than the dose limits given by the nuclear regulatory authority.
2.3.1 Ideal Design of Nuclear Medicine Department:
An ideal design of nuclear medicine department must include the features given below:
The imaging or the scanning rooms should be large enough, having a minimum area of
approximately 30 meter square. Because the imaging machines like gamma camera,
PET/CT etc requires large area.
The Hot Lab should be in particular order, having separate workbenches for keeping
the record and making certain radiopharmaceutical. Hot lab is basically the area used
for storing, preparing and dispensing the radio-pharmaceuticals (combination of
radionuclide and a particular pharmaceutical).
Sufficient shielding should be done to the Hot lab. Radiation protection officer should
calculate the amount of shielding required for the Hot lab depending upon the emission
of radiations, intensity and the usage of radioactive nuclides.
The waiting area of injected patients should be separated from the staff and from the
people of the general public. It should also display the radiation warning signs.
Radiation warning signs should also be displayed on Hot Lab or Radio-pharmacy as
well as on any area having radioactive material.
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Medical Physics 16
There should be shielded bins in every injection rooms so that the injection syringes
must be completely disposed and the emission of radiation from syringes can be
stopped.
Injection rooms and the Hot lab should be close enough to each other in order to
minimize the transportation of radioactive materials.
Radio-pharmacy and the storage area of radioactive waste should not be accessible to
the members of the general public.
The areas having radiation exposures should be sufficiently shielded.
Figure 3 Design of Nuclear Medicine Department
http://nuclearsafety.gc.ca/eng/lawsregs/guidancedocuments/published/html/gd52
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Medical Physics 17
2.4 Radiation Protection in Nuclear Medicine Department:
There are lots of useful benefits of radiations in nuclear medicine such as in diagnosing
particular disease and also for the treatment of various diseases known as therapy. But there
are also disadvantages and harmful effects of radiations if their exposure is increased from a
particular permissible values made by International Atomic Energy Agency. So the use of
radiation should be monitored in order to take benefits without having any harmful effects to
the environment or to the people.
Following measures should have to be taken in order to get maximum radiation protection in
Nuclear Medicine Department.
2.4.1 Patients as a Radioactive Source:
When the patient is administered by the radiopharmaceutical either by ingestion or by any
means, he/she should be treated as a radioactive source. For diagnostic purposes, the dose of
radioisotope administered to a particular person is not so high and generally the patient does
not need to be admitted in to the hospital. This dose is approximately a small fraction of the
annual allowed public dose and normally it is unnecessary to give radiation protection advice
to the patients family. However the precautions must be taken for the patient undergoing
radiotherapy and it depends on the amount of radiopharmaceutical administered to the patient,
the exposure in the area surrounding the patient and the daily direct interactions between patient
and the other people.
As it is mentioned in the International Radiation Safety Standards In order to restrict the
exposure of any members of the household of a patient who has undergone a therapeutic
procedure with sealed or unsealed radionuclides and members of the public, such a patient shall
not be discharged from hospital before the activity of radioactive substances in the body falls
below the level specified. [8]
2.4.2 Radiation Dose limits:
As previously mentioned, the dose limit made by International Atomic Energy Agency must
be obeyed. These limits should be enforced by the national regulatory authorities and also by
the state in order to be protected from the dangerous effects of the radiations. Remaining in the
radiation dose limits, the radiological risk can be minimized to such extent that it goes to
negligible level and it could not have a significant effect.
Following figure shows the dose limits specifically for each organ:
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Medical Physics 18
Figure 4 Radiation Dose Limits
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1556/v6/fig012.html
2.4.3 The Transportation of Radioactive materials:
Transporting the radioactive material carries great radiological risks. The transportation of the
radioactive materials inside and outside the hospital should be carried out under the precautions
and rules made by the International Atomic Energy Agency (IAEA). Inside the Nuclear
Medicine Department, it involves the transfer of radiopharmaceutical from hot lab to the
injection or the dose administration room and the transfer of radioactive waste from these
injection rooms to the waste storage area etc. To get maximum protection from these radiation
emitting materials, these transportation should be minimized as far as it could be possible and
the material should be placed in a perfectly shielded container. These containers are also known
as packages. Packages are made of a perfectly good and rigid material in order to be safe from
the externally occurring accidents like fire etc. These packages should be made from those
materials which have higher attenuating coefficients to stop the radiation leakage.
Label should be displayed on the surface of the package which gives the dose rate of the
radioactive carrying package. Following figure shows the label of the radioactive package:
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Medical Physics 19
Figure 5 Radioactive Labels
http://www.mcleancargo.com/tools/HAZMAT_Labels.htm
Different colored labels give different dose rate which are as under:
I- White D 0.005 mSv/h
II- Yellow 0.005
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Medical Physics 20
Chapter-3 PET/CT
3.1 Introduction to PET-CT:
PET/CT is a combination of two modern scanning techniques which gives both the functional
and anatomical view of the particular organ scanned. These techniques are Positron Emission
Tomography (PET), and Computed Tomography. This combined technique of imaging
recently gained great favour due to its multi-functional way of scanning and diagnosing
different diseases, especially in oncology studies (Cancer related studies).
PET/CT is basically highly sensitive and a new imaging technique although either functional
diagnosis by PET or anatomical diagnosis by CT can be obtained. This tool is generally used
to obtain both PET and CT images simultaneously. PET/CT combines the two images and
therefore functional and anatomical information is obtained.
3.2 Positron Emission Tomography:
Positron Emission Tomography is a nuclear diagnosing method which uses the ideal properties
of the radioisotope that decay by emitting the positron (similar to electron but opposite in
charge). The commonly used radionuclide is F-18 (Fluorine-18) which is combined with a
pharmaceutical to produce radiopharmaceutical known as FDG-18 (Fluoro-Deoxy-Glugose).
This radiopharmaceutical is administered the patient through injection. This pharmaceutical
goes to its area of interest like tissues etc. When this radionuclide decays by +, it emits high
energy positron which is then annihilates with nearby electron rapidly to produce two identical
high energy photons in approximately opposite directions. Because of their high energy, these
photons also have a great probability to escape from the body. Externally present detectors
detect these photons and determine the position of the radioactive decay occurred in the body.
This process occurs very rapidly because of the short life time of the positron in the electron
rich material such as tissue etc. PET scanner is made up of collection of detectors which
surrounds the body which is to be scanned. It generates electrical signals by converting these
photons into electrical pulses. PET scanner takes series of images over a time that is why it is
known as Tomography (slices of images). The scanning process is shown in the following
diagram:
Figure 6 Scanning Process of PET Imaging
http://legacyweb.triumf.ca/welcome/petscan.html
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Medical Physics 21
When the positron emitted by radiopharmaceutical is combined with electron it produces
Positronium, for a very short period of time. Positronium is an analogy to the hydrogen like
state, when the proton which makes up the nucleus in a hydrogen atom is replaced by positron.
This state of Positronium remains for about 10-10 seconds before the process called annihilation
takes place. When annihilation occurs, the mass of the positron and the mass of the electron is
transformed into electromagnetic energy by the Einsteins Mass-Energy Equation given as under:
E = mc2 Equation ---- 3.0
E = mpc2 + mec
2 Equation ---- 3.1
Where c is the speed of light, mp is mass of the positron and me is mass of the electron. Putting the values of these in the above equation:
mp = 9.1093826211031 Kg
me = 9.10938188231 1031 kg
c = 3108m/s
E = (9.109382611031) (3108)2 + (9.1093818831 1031) (3108)2
E = 1.022 Mev (Mega Electron Volt)
Hence the above calculated energy is the energy of the emitted photons. As already discussed
that the number of emitting photons in annihilation is 2, Therefore each photon has an energy
of 511 kev (Kilo Electron Volt).
The process is shown schematically in the following figure:
Figure 7 Physics of PET
http://dels-old.nas.edu/ilar_n/ilarjournal/42_3/4203_Positron.shtml
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The process of annihilation also occurs at higher order like 3 or more than these photons may
be produced but they are about 0.03 % of the total annihilations that is why they can be
neglected. The annihilation process in Positron emission tomography has many useful
properties which are very beneficial for imaging techniques. For example, the photons which
are emitted in this process are high energy photons which fall in the gamma rays region in the
electro-magnetic light spectrum. So these photons have enough energy and probability to
escape from the body and the detectors which are placed outside the body can easily detect
their presence. One more positive use of the property of annihilation is that it gives quite good
approximation of where the radioactive material was present in the body.
3.2.1 Radionuclides used in PET:
As already discussed above that those radioactive nuclei are feasible which decay by + or
technically known by Positron emission. Few other things should also be taken into
consideration before using a particular radioisotope like half-life and cost etc. Although there
are lots of other radioactive nuclei which can be used for diagnosis purposes in positron
emission tomography but the most widely used radionuclide is F-18 (Fluorine-18) due to its
extraordinary unique properties. F-18 FDG is a non-specific tracer for metabolic activity that
is taken up normally in the brain, heart, bone marrow, bowel, kidneys and activated muscles,
and concentrates in many metabolically active tumors.[10]
Following table discuss some of the important radioactive nuclei and their specific properties,
which can be used to identify and diagnose particular disease in PET technique.
Figure 8 Table taken from the book PET Physics, Instrumentation and scanners by
Michael E. Phelps
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3.3 Computed Tomography:
Computed tomography is one of the methods used to investigate the inner depths of the body
by taking series of images of the organ. This technique was made possible by the invention of
the Computer.
Computer tomography is a medical imaging technique which converts the 3 dimensional
human anatomy into 2 dimensional projection images. It produces the image by using series of
small x-ray detectors and a computer. The x-ray sources present in the gantry of the CT
machine are rotated around the body which is to be scanned and the consequent data is obtained
from different angles. This data is then processed in the computer which create image on the
screen. In conventional old x-ray machines, the images which were obtained had a
disadvantage that the shadows of the images were overlapping. But these computer based
techniques remove this major problem and hence we get clear images of inner body.
Following figure explains the working of computed tomography machine:
Figure 9 Functioning of CT Imaging
http://www.themesotheliomalibrary.com/ct-scan.html
3.4 Design of PET-CT Clinics:
Modern PET-CT facility in nuclear medicine department involve following equipments:
1. A scanner room consist of PET-CT machine having double gantry and a separate well
shielded control room.
2. Hot lab where the doses or in technical words radiopharmaceutical are prepared.
3. Ideally two or more than two uptake rooms where the patient is asked to rest over there
before having PET scan.
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These areas must be properly shielded in order to provide maximum protection against the high
energy electromagnetic waves to the authorized personnel and other staff working in the areas
closer to these rooms. These waves are photons having high energy of about 511 kev.
3.5 Radiation Safety and Controlling Exposure of Radiations:
To have a radiation safety against the harmful effects of the radiations, there are four main
techniques by which the effect and intensity of the radiations can be reduced. By altering these
below mentioned factors, the exposure of the radiation can be controlled.
1. Distance 2. Time 3. Contamination Control 4. Shielding
3.5.1 Distance:
Radiation exposure rate from its source is dependent upon the distance. Exposure and distance
are related with inverse square law. When the distance is doubled, the exposure rate from the
source becomes . Mathematically this can be written as,
X2 = X1 (d1/d2)2 Equation --
-- 3.2
Where,
X2 is the final exposure rate after altering the distance, at point 2.
X1 is the initial exposure rate from the distance at point 1.
d1 is the distance at point 1.
d2 is the distance at point 2.
Figure 10 Relation of Exposure with Distance
http://www.epa.gov/rpdweb00/understand/protection_basics.html
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Medical Physics 25
3.5.2 Time:
The amount of the radiation exposure also depends upon the time spent with the radioactive
source. If the time spent of person with any radioactive material is large enough so ones
exposure would also be very high. So the time factor should also be taken into consideration
when dealing with radiation protection. Different radioactive sources have different exposure
rates. For example, while diagnosing patients by simple x-ray source, the time spent for the
radiation worker should be minimized in order to have maximum radiation safety. This can be
done by not operating the x-ray machine when the staff is near to this radioactive source.
3.5.3 Radiation Contamination Control:
Radiation contamination is also commonly known as radiological contamination. According to
the International Atomic Energy Agency, it is defined as the radioactive substances on
surfaces, or within solids, liquids or gases (including the human body), where their presence is
unintended or undesirable, or the process giving rise to their presence in such places. This
terminology only explains about the radioactivity but not give the corresponding involved
radiation hazard. The methods of controlling the contamination are designed to avoid its spread
to other work surfaces and also to reduce the interaction of the radioactive material and the
personnel working in that area. [11]
3.5.4 Shielding:
To minimize the radiation exposure in diagnostic nuclear medicine and diagnostic radiology,
shielding is the common technique used to prevent and reduce the exposure to patients, staff
and the members of the general public. As already discussed that different radiation have
different exposure rate so the amount of shielding required would also be different which varies
with the radiations. This shielding depends upon the intensity, energy, number of the
radioactive source and the geometry of the radiations produced by the radioactive material. The
shielding material should have a good absorption coefficient or technically known as
attenuation coefficient in order to reduce the exposure of the radiations. The calculations of the
shielding thickness can be done by using traditional formulas or by computer based software
commonly known as Monte Carlo (e.g MCNPX code).
The radiation exposure rates in nuclear medicine facility can ranges over 100Rem/hour (1 Rem
is equal to 10 milli-sievert). Specific exposure rate constant () also known as gamma factor
is used to calculate the exposure rate from a radioactive nuclide at any distance. Its unit is
R.cm2/mCi.h . Mathematically its formula can be written as,
Exposure Rate = A/d2 Equation ---- 3.3
Where,
= specific exposure rate
A = Activity in milli-curie
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Medical Physics 26
d = it is the distance in centimeters from a point source of radioactivity
This specific exposure rate constant measure only the significant amount of radiation exposure
and ignore the photons below certain energy which do not have any adverse effects. For
example, 30 means that the specific exposure rate constant for photons having energy greater
than or equal to 30 kev (kilo electron volts.) [12]
For shielding, Materials like lead, tungsten and the lead equivalent glasses are used in nuclear
medicine facility to reduce the exposure from the small syringes and containers having
radioactive material. During dose preparation and dose administration, syringes are shielded
which reduce the exposure to the radiation worker. Person carrying radioactive material should
wear disposable gloves, lab coats, lead aprons, TLD ring and body film badges (Dosimeters).
Following figure is the tungsten shielded syringe containing radioactive material.
Figure 11 Tungsten Shielded Syringes
http://www.tungsten-alloy.com/radiotherapy-radiation.htm
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Chapter-4 Literature Review
In a wonderful study of Positron Emission Tomography and Computed Tomography
shielding conducted by AAPM Task Group 108 et al. explained the requirements and the
measures which should be taken for shielding a PET-CT facility. This technique of imaging
gives different challenges. The emitted photons by the process of annihilation have energy of
about 511 kev associated with the positron emitting radionuclide which is really a large amount
of energy as compared to the other diagnostic imaging procedures. In order to have maximum
radiation safety, appropriate shielding is needed on the adjacent walls, floors and also on the
ceiling of the room containing the radioactive source or PET-CT machine. When the
radiopharmaceutical is administered to the patient, he/she should be considered a radioactive
source and has to remain in the shielded area until the amount of permissible amount of activity
is achieved. In this study, they mentioned the methods and the calculations for approximating
shielding conditions of PET-CT facility. The information related to the radionuclide used in
PET studies (F-18) like half-life and its decay scheme etc is also given. The factors and some
examples are also given for estimating the shielding in a PET design. The shielded rooms are
PET scanning or diagnosing room, control room, uptake room, hot lab and independent toilets
for radiopharmaceutical administered patients. The concept of effective half-life of the
radioisotope is also mentioned in this study. They have also included the tables and
approximate graphs of the transmission factor of radiations for steel, concrete and lead at
energy of about 511 kev. [13]
Robert L. Metzger and Kenneth A. Van Riper et al. at Monte Carlo 2005 topical meeting in A
Monte Carlo Shielding Model for PET-CT Clinics demonstrated their view related with
dealing with the shielding of PET-CT suite. They have given the estimated calculation of
shielding a PET-CT department us with different methods. They have explained the complete
process of the scanning of patients. They have focused mainly on the two models of shielding
calculations which are Monte carlo model and Mercurad Model, and compared their
readings. After the successful shielding of a PET-CT clinic in a particular hospital, they have
checked the exposure rates with a Radcal Model 10X5-1800 ionization chamber which can
measure the low transmission rates of energetic radiations in a short interval of time and found
out the reliability of both the models. They have also mentioned design dose limits specifically
for the PET workers, as they are very much exposed to highly energetic photons as compared
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Medical Physics 28
to other imaging techniques. Different occupancy factors are also given for different areas in
this study. [14]
In another study carried out by the International Atomic Energy Agency (IAEA) in their
Radiation Protection in Nuclear Medicine Department safety report series no. 115, IAEA,
Vienna (1996) explained some of the main safety standards which should be taken into
consideration in order to meet the requirements given by this authority. The precautions which
need to be taken in the nuclear medicine department are Design consideration, dose limits, the
radioactive patients, the special problems related to some kind of accidents and transportation
of the radioactive material inside and outside the department. [15]
A study conducted by Jerrold T. Bushberg et al. in The Essential Physics of Medical Imaging
explained the techniques of medical imaging by using gamma camera and computed
tomography etc. They have also mentioned the functioning and basic operations of these
machines. Specific radionuclides and radiopharmaceutical, and their unique characteristics
which are used for the imaging techniques are also included. The production and the processing
of these radionuclides are also given. This study also explained the production of radionuclides
from different cyclotrons, nuclear reactors and radionuclide generators. Different types of
detectors and their functioning is also mentioned. These detectors include Gas filled Counters,
Scintillation detectors and Semi-conductor detectors. It also explained the radiation protection
against the radioactive sources and the effects of harmful ionizing radiations. [16]
Jon A. Anderson et al. at the department of radiology, The university of Texas South-western
medical centre at Dallas, demonstrated site planning and radiation safety in the PET facility in
this study. This study explained some of the basic problem which are associated with PET-CT
clinic and the implementation of the safety standards while handling radioactive materials. The
safety techniques which are mentioned in this study can be made practical in any other situation
after appropriate changing for different workloads, the number of radiopharmaceutical
administered to the patients, use of different radioisotope and other operating considerations.
[17]
In a study performed by Michael E. Phelps et al. in PET Physics, Instrumentation and
Scanners explained one of the recently developed PET-CT, a nuclear imaging technique. This
study mentioned the functioning, advantages, precision and accuracy, and the basic physics
behind the Positron emission tomography. The process of annihilation of positron and electron
resulting in highly energetic two photons, their range and their non-colinearity are also included
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Medical Physics 29
in this study. The detection and the material used for the detection of these highly energetic
photons are also discussed. [18]
James E. Turner et al. explains in the study Atoms, Radiations, and Radiation Protection
about the protection against the externally present radiations. He focused mainly on the factors
by which the radiation exposure depends which are distance, time and shielding. He
demonstrated the importance of the shielding factor especially for the shielding of high energy
gamma rays. Few other important concepts, like interaction of photons with matter, are also
mentioned which are very necessary to understand while dealing with radiations.[19]
In another study presented by Radiation and Nuclear Safety Authority, Helsinki Finland
describes the dose constraints and the limits made by international nuclear regulatory authority.
The instructions and precautions which should be given to the patient both orally and in writing
are also mentioned in this study. Handlings with the different situations when abnormal events
in the use of radiations take place are also discussed. [20]
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Chapter-5 Method for Shielding Calculations
As already discussed, in PET-CT clinics, highly energetic photons are emitted from positron
emitting radionuclides of energy about 511 keV. These are extremely penetrating radiations
therefore; heavy shielding is required in all aspects of radiation protection. Different clinics
require different shielding. There are different methods for calculating desired shielding. But
the method used for estimating the required shielding in this thesis is based on the formulations
mentioned in the report of AAPM task group 108. Following are the factors, on which the
amount of shielding mainly depends,
The type of radiation which is to be shielded.
The activity of the source used in the clinic.
The permissible dose rate.
When choosing the material for radiation shielding, the first consideration which should be
taken into account would be the personnel protection. An appropriate shielding will reduce the
large amount of energy of radiation in a small penetration distance without emitting any other
harmful radiations or secondary radiations like Bremsstrahlung. However there are also other
factors which have an impact on the choice of shielding materials. They are mentioned as
under,
Space for the shielding material.
Weight of the material.
Cost of the material.
The effectiveness for the shielding material is determined by the interaction between the
incident radiation and the atoms of the absorbing medium. This interaction is mainly depends
on the type of radiation, the energy of the radiation and the atomic number of the absorbing
material. As mentioned in the beginning, the alpha and the beta radiations are not energetic as
compared to gamma radiations and they do not have sufficient penetrating power, so they do
not contribute in shielding calculations. Gamma and the neutron radiations are highly
penetrating and require enough shielding. In this, only gamma rays will be discussed, as
neutron radiations are not used in hospitals for medical purposes. [21]
5.1 Gamma Radiations:
Unlike alpha and beta radiations, gamma rays do not lose its energy continuously when it
passes through a medium. This is the reason that gamma rays have much penetrating power as
compared to alphas and betas. When gamma rays passes through the material used for the
shielding of radiations, it is attenuated by an exponential factor. This means that gamma rays
cannot be stopped completely no matter how thick the shielding material is used. But it is
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Medical Physics 31
possible that the energy of radiation can be decreased to a permissible level. Mathematically,
the dose rate of gamma rays can be calculated is written below,
Dt = Do e-t Equation ---- 5.1
Where Dt is the dose rate when radiation passes through the shielding material of thickness t
and is the linear absorption coefficient. This coefficient of linear absorption is dependent
upon the type of material used for shielding and the energy of gamma rays. In S.I units, the
unit for linear absorption coefficient is m-1. This equation is only true for the narrow beam of
radiation coming from the source.
For the case of broad beam, the term Dose Build Up factor is used, which is gives below,
Dt = BDu e-t Equation ---- 5.2
Where, Du is known as dose rate from the unscattered gamma rays. The value for the build-up
factor B is dependent upon the energy of the incident radiation, shielding material and the
thickness of the shielding material used. The values for this factor normally looked up in the
tables and are not calculated, when shielding a nuclear facility. The concept of half value layer
(HVL) is normally used when quickly estimating the required shielding. This is defined as
The thickness of the shielding material required to reduce the intensity to half of its incident
value. The mathematical expression for using the half value layer is given below,
Ds = Do (HVL) n Equation ---- 5.3
Where Ds is the desired shielding dose rate, Do is the initial dose rate and n is the number of
half value layers.
5.2 Shielding Calculation:
The radiopharmaceuticals used in the technique of Positron Emission tomography have a very
short life and usually they are administered to a patient in a large quantity as compared to the
other diagnostic technique in nuclear medicine. When the radiopharmaceutical is administered
to the patient, the patient becomes the active source of radiation that is continuously emitting
radiations. After the dose administration, there come several phases which all need to be
shielded so that the dose limits should be reduced to the internationally recommended dose
constraints made by the international atomic energy agency. These phases are,
Uptake Phase
Imaging or Scanning Phase
Decay phase
These phases are specifically related to the patients. However, the rooms in which radionuclide
are made and stored also need to be shielded. Therefore the rooms which must be shielded are,
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Medical Physics 32
Patient Uptake Room
Hot Lab
Imaging Room
Cyclotron Room
The radioisotope used in PET technique is F-18 (Fluorine-18). It is normally used because of
its longer life time (109.8 min) and has average dose rate constant. If this radioisotope is taken
into account while shielding, then rest of the all radioisotopes will automatically be shielded.
Its radioactive can be determined by using the concept of the total dose. The total dose is given
by,
Dt = (Do/E) (1 - e- Et) Equation ---- 5.4
Dose reduction factor for a specific time t can be calculated as,
Rt = Dt/(Do t) Equation ---- 5.5
Putting the value of Dt in the above equation, we get,
Rt = (Do/E) (1 - e- Et) 1/(Do t) Equation ---- 5.5
Rt = (1 - e- Et)/( E t) Equation ---- 5.6
Now substituting the value of E in the above equation, the equation becomes,
Rt = [1.443 T1/2 (1 - e- Et)]/t Equation ---- 5.7
Where Dt is the total dose, E is decay constant, Do is the initial dose rate. T1/2 is the half-life
of radioisotope and Rt is the dose reduction factor.
5.2.1 Patient Uptake Room:
After the dose administration to the patient, the patient has to wait in this room for about 30 to
90 min in order to reduce the muscular and skeletal uptake of the dose. This shielding of this
area should be made so that the permissible level can be achieved. Following is the formulation
of estimating the shielding in uptake room.
The dose rate from the patient at a distance d is given by,
Dtu = 0.092 Ao tu Rtu /d2 Equation ---- 5.8
Where Ao is the average activity administered to the patient, tu is the uptake time and d is the
same distance between the source and the point of interest.
If Nw be the number of patients per week then the weekly dose Dw can be calculated as,
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Medical Physics 33
Dw = Nw 0.092 Ao tu Rtu /d2 Equation ---- (A)
Now the transmission factor can be calculated as,
B = P/ (Dw U T) Equation ---- 5.9
Putting the values of Dw in the above equation,
B = P / [(Nw 0.092 Ao tu Rtu /d2)(U T)]
B = P d2/ [(Nw 0.092 Ao tu Rtu U T] Equation--- (B)
Where P is the maximum permissible dose, U is the use factor and T is the occupancy factor.
Example:
Calculate the transmission factor and wall thickness for patient waiting room where
administered activity Ao is 500 Mbq, uptake time is 30 min, number of patients per week
are 50 and the dimensions of the room are 10 10 ft2. The wall thickness of room is 4.5
inches of concrete.
Solution:
Since F-18 has a dose rate constant = 0.143Sv.m2/Mbq.hr , d = 5ft = 1.52m + 0.3m + 0.11=
1.94m , A = 500 Mbq and P= 20 Sv/hr, T = 1, U = 1, Rtu = 0.91, tu = 30 min.
Putting these values in the following formulation,
B = P d2/ [(Nw 0.092 Ao tu Rtu U T]
B = 0.0717
Using the table, the required shielding thickness = 9 cm of concrete.
5.2.2 PET-CT Scanning Room:
As already discussed above that computed tomography uses X-rays which is mostly operated
on 100-250kev of X-rays photons but these are very less energetic as compared to the 511kev
photons generated by the process of annihilation. Therefore the CT is automatically shielded
when shielding these highly energetic photons used for PET scanning. The activity of the
radioisotope is reduced by the factor Fu because of the time delay required for the uptake stage
between the dose administration and the actual scanning. This factor is decreased
exponentially. One more point which should be taken into consideration is that in most cases,
the patient void 15% of the total activity administered. This is known as the patient reduction
factor. Remaining activity is 85%. The dose reduction factor is given as,
Fu = e -0.693 tu / T1/2 Equation ---- 5.10
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Medical Physics 34
Where, tu is the uptake time. Now the weekly dose for the scanner room could be formulated
as below,
Dw = [Nw 0.092 Ao tI RtI Fu]/ d2 Equation -- (C)
Hence the transmission factor could be,
B = P d2/ [(Nw 0.092 Ao ti Rti U T Fu] Equation -- (D)
Or,
B = P / [Dw UT]
Example:
Calculate the transmission factor and wall thickness for scanner room where
administered activity Ao in patient is 500MBq, uptake time is 30 min, number of patients
per week 50 and the dimensions of the room are 1010 ft2 and imaging time is 20 min.
The existing wall is 9 inches thick.
Solution:
Since F-18 has a dose rate constant = 0.143Sv.m2/Mbq.hr , d = 5ft = 1.52m + 0.3m + 0.23=
2.05m , A = 500 Mbq and P= 20 Sv/hr, T = 1, U = 1, Rtl = 0.94, ti = 20 min, Fu = 0.83.
B = P d2/ [(Nw 0.85 0.092 Ao ti Rti U T Fu]
B = 0.1663
By using the table given at the end, the shielding thickness = 6 cm of concrete.
5.2.3 Hot Lab:
This is the area where all the radioisotopes are stored. So the activity in this area is very high.
Therefore it is necessary to shield this room accurately. If At be the total radioactivity present
in the room, d be the distance between the source and the point where the shielding is needed
to be installed. be the dose rate constant associated with it. Normally the one foot of distance
is also added to the point of interest in all cases for calculating radiation shielding. Therefore,
the equivalent dose rate can be calculated as,
Ho = At / d2 Equation ---- (E)
Thus, the transmission factor can be calculated as,
BL = Po /Ho Equation ---- (F)
Where,
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Medical Physics 35
Bo is the transmission factor for the hot lab and Po is the permissible dose made by regulatory
authorities.
Example
Calculate the transmission factor and wall thickness for hot lab where maximum activity
is 50 104 Mbq and dimensions of the room are 1010 ft2.
Solution:
Since F-18 has a dose rate constant = 0.143Sv.m2/Mbq.hr, d = 5ft = 1.52m, A = 50104
Mbq and P= 10 Sv/hr.
Putting these values in the expression, we have,
Ho = At / d2 = (0.143)(50104)/ (1.52)2
= 3.095 104 Sv/hr.
Therefore the transmission factor can be calculated as,
BL = Po /Ho
= 10/ (3.095 104)
BL = 3.23 10- 4
Using the table for above transmission factor, Shielding thickness = 43mm of lead.
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Chapter-6 Shielding Calculation of SKMCH Site
The shielding calculation for PET/CT facility of Shaukat Khanum Memorial Cancer Hospital
is based on the requirements and parameters which are given by the hospital. The different
parameters used in the calculation of shielding are given in the table below. Furthermore, it
should be noted that the estimated radiation shielding in this report is carried out by using the
formulas given by the AAPM Task group report no.108 PET and PET-CT Shielding
Requirements.
Name of Parameters Symbol Value
Radioisotope used in PET F-18 Fluorine-18
Half-life of F-18 T1/2 110 min
Number of Patients per week Nw 30
Average Administered Activity Ao 370 MBq
Uptake Time Tu 90 min
Average Scan Time TI 20 min
Dose Rate Constant for F-18 0.143Sv.m2/Mbq.hr
Dose Reduction factor Rtu 0.76 for tu = 90 min
Exponential Reduction Factor for
Uptake Time
Fu 0.57 for tu = 90 min
Permissible Dose rate for Hot Lab PH 10 Sv/h
Permissible Dose for Uptake Room PU 20 Sv/h
Permissible dose for Imaging Room PI 20 Sv/h
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Medical Physics 37
6.1 Shaukat Khanum Cancer Hospital and Research Centre, Lahore
Figure 12 Site Plan for PET/CT Suite of SKMCH Lahore
6.1(a) Shielding of Patient Uptake Room:
As per the requirement of the nuclear regulatory authority of Pakistan, Shaukat Khanum
Hospital has three uptake rooms for single PET-CT scanner. The dimensions of these rooms
are mentioned in the figure given below. The shielding calculation for this room could be
estimated as given below.
Wall # 2 0.3m
0.3m
Wall # 1 Wall # 3
Wall # 4
1.17 m
1.67 m 1.52m
2.44 m
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Medical Physics 38
As already given the maximum permissible dose P= 20Sv, occupancy factor T=1, the use
factor U=1 from the parameters given by the hospital and d= 1.67m taken from the above
figure, the transmission factor and the required shielding for wall 1 and 3 are calculated. Results
are discussed in the following table. The shielding calculation is carried out by the formulation
provided by the AAPM task group.
Calculations for wall 1 and 3 of patient uptake room
Parameters Symbol Values Reference
Weekly Dose Dw 417.43 Sv Equation: A
Transmission Factor B 0.0479 Equation: B
Thickness for Shield X 24cm concrete
Now calculating for the wall 2 and 4, and the distance between the patient and the point of
interest is taken from the above figure, which is d= 1.17m
Calculations for wall 2 and 4 of patient uptake room
Parameters Symbol Values Reference
Weekly Dose Dw 850.44 Sv Equation: A
Transmission factor B 0.0235 Equation: B
Thickness for Shield X 29cm of concrete
6.1(b) Shielding of Imaging Room:
Following figure shows the dimensions of this room. By using the given parameters, the
shielding calculation is carried out.
Wall # 2 0.3m
0.3m
Wall # 1 Wall # 3
2.66 m
4.19 m 4.3m
7.3 m
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Wall # 4
By using the given parameters and the distance given from the figure above d = 4.19 m,
calculations of transmission factor and the shielding thickness of the material for the walls 1
and 3 are discussed in the following table.
Calculations for the walls 1 and 3 of Imaging Room
Parameters Symbol Values Reference
Weekly Dose Dw 10.61 Sv Equation: C
Transmission Factor B 1.88 Equation: D
Thickness for Shield X Already Enough Concrete
Now the distance for wall 2 and 4 between the source and the wall which need to be shielded
is given by the above figure, which is d = 2.66m. The transmission factor and the required
shielding are discussed in the following table.
Calculations for the walls 2 and 4 of Imaging Room
Parameters Symbol Values Refer