BASIC PHYSICS&

RADIOPHARMACY&

INSTRUMENTATIONin

NUCLEAR MEDICINEby

Prof. Dr. Haluk B. Sayman

Basic Physics• Molecules are the smallest particles of a substance and formed

by at least two atoms. Atoms are the basic units of the matter and consists of a dense positively charged nucleus which is surrounded by a cloud of negatively charged electrons. Over 99.9% of an atom’s mass is concentrated in the nucleus, with protons and neutrons having roughly equal mass.

• Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of p (+) and n in a nucleus. Number of protons is atomic number (Z) and number of p+n is mass number (A).

• Electrons that are bound to atoms have stable energy levels (orbits) and can undergo transitions between them by absorbing or emitting photons.

Basic Physics• There are about 2,450 known isotopes of the

one hundred odd atomic no. elements in the Periodic Table. You can imagine the size of a table of isotopes relative to that of the Periodic Table! The unstable isotopes lie above or below the Nuclear Stability Curve (A>83). These unstable isotopes attempt to reach the stability curve by splitting into fragments, in a process called Fission, or by emitting particles and/or energy in the form of radiation. This latter process is called Radioactivity.

Basic Physics• Chemical elements have two atoms with identical number of

protons and neutrons in their nuclei. Atoms with equal numbers of proton but a different number of neutrons form isotopes.

• Stability of isotopes is affected by the ratio of protons to neutrons. If they are unstable they reach to stable state by radioactive decay.

• The most common forms of radioactive decay are:– Alpha decay: Emitting He nucleus (2p+2n)– Beta decay: Transformation of n into p or p into n.– Gamma decay: Electromagnetic radiation

• Other forms: internal conversion (nucleus is excited by an orbital electron and emits gamma ray that ejects another electron)

Basic Physics• Half life

– Physical half life– Biological half life– Effective half life

1/Teff=1/Tphy+1/Tbio

• Radioactive decay formulaeNt=N0e-λt

Nt: Number of atoms after t time elapsedN0: Number of atoms at the beginninge: Natural logarithm base (2,71828)λ: Disintegration constantt: time-: indicates loss

Basic Physics• Units of Radioactivity

– The SI or metric unit of radioactivity is named after Henri Becquerel, in honour of his discovery of radioactivity, and is called the becquerel with the symbol Bq. The becquerel is defined as the quantity of radioactive substance that gives rise to a decay rate of 1 decay per second.

– The traditional unit of radioactivity is named after Marie Curie and is called the curie, with the symbol Ci. The curie is defined as the amount of radioactive substance which gives rise to a decay rate of 3.7 x 1010 decays per second. (1mCi=37MBq)

Radiopharmacy• Production of Radiotracers

– Reactor products-Radionuclides (I-131)– Generators (Tc-99m, Rb-82)

– Cyclotron products (F-18, O-15, C-11)

Radiopharmacy• Cold kits

– Non radioactive compounds easy to store

Radiopharmacy• Quality Control

– Physicochemical Tests• Physical properties• pH• Radionuclidic purity-different radionuclides• Radiochemical purity-radionuclide leased from compound• Chemical purity• Stability• Radioactivity

– Biological Purity Tests• Sterility-represents the absence of bacteria or phungi• Pyrogenity-substances causing fever• Toxicity tests-endotoxins or exotoxins• Bioligical distribution

Instrumentation• The instruments used in Nuclear medicine attempt to

detect the radiation which results from a process called gamma decay or by a beta decay known as annihilation of a positron, which gives rise to the emission of two gamma rays.

• The radioactivity is generally administered to the patient in the form of a radiopharmaceutical - the term radiotracer is also used. This follows some physiological pathway to accumulate for a short period of time in some part of the body. A good example is 99mTc-tin colloid which following intravenous injection accumulates mainly in the patient's liver. The substance emits gamma-rays while it is in the patient's liver and we can produce an image of its distribution using a nuclear medicine imaging system. This image can tell us whether the function of the liver is normal or abnormal or if sections of it are damaged from some form of disease.

Instrumentation• Scintillation Detectors:

The radiation detector used in almost all conventional nuclear medicine equipments is the thallium activated sodium iodide crystal.The modern PET cameras has LSO, BGO or LSYSO type crystals.

• These crystals turn invisible gamma radiation into visible light, namely photons.One or more photomultiplier tubes (PMT) coupled to these crystals change the photons to the electrical currents.

Instrumentation• Photomultiplier Tube

Instrumentation

• The sketch shows the construction of a typical solid-crystal scintillation detector. One gamma ray is shown entering the crystal, where it interacts and produces electrons which, in turn, generate photons in the visible range. One such photon is shown moving form the crystal, through a light-coupling system (designed to pass the light with minimum losses) onto a photocathode, where it ejects electrons from the material. One of these photons accelerated toward a dynode maintained at a positive potential relative to the photocathode. The energy gained by the electron results in the ejection of several electrons upon impact on the dynode (only two are show for clarity). The cascade produced...dynode to dynode...illustrates the amplification achieved at the output level.

Instrumentation• Scintillations produced in the crystal are detected by a

large number of PM tubes which are arranged in a two-dimensional array.

• The output voltages generated by these PM tubes are fed to a position circuit which produces four output signals called ±X and ±Y. These position signals contain information about where the scintillations were produced within the crystal. In the most basic gamma camera design they are fed to a cathode ray oscilloscope

• Note that the position signals (Z) also contain information about the intensity of each scintillation. This intensity information can be derived from the position signals by feeding them to a summation circuit (marked ∑ in the figure) which adds up the four position signals to generate a voltage pulse which represents the intensity of a scintillation

Instrumentation• Collimators

– The collimator is a device which is attached to the front of the gamma camera head. It functions something like a lens used in a photographic camera but this analogy is not quite correct because it is rather difficult to focus gamma-rays. Nevertheless in its simplest form it is used to block out all gamma rays which are heading towards the crystal except those which are travelling at right angles to the plane of the crystal.

– Made of lead which absorbs gamma rays.

• Parallel hole collimator

• Pin hole collimator

• Diverging (magnify) or converging (minify) collimators

Instrumentation• Block Diagram of a gamma camera

Instrumentation• Types of Cameras:

– Scanning Camera– Planar Camera– SPECT (Single Photon Emission

Computerized Tomography)– PET (Positron Emission Tomography)– Hybrid Cameras (SPECT/CT or PET/CT)– Near future …... (PET/MRI)

Instrumentation

Instrumentation• PET Camera:

– If we administer a positron-emitting radiopharmaceutical to a patient an emitted positrons can annihilate with a nearby electron and two gamma-rays will be emitted in opposite directions. These gamma-rays can be detected using a ring of radiation detectors encircling the patient and tomographic images can be generated using a computer system.

Instrumentation

Radiopharmacy

Instrumentation• Image examples:

Instrumentation• Image

examples:

Instrumentation• Hard-Copy Devices

– Multiformat film Cameras– Video printers– Polaroid printers– Laser printers– PACS systems