Instruments for Radiation Detection

and Measurement

Lab # 3

• In nuclear medicine it is important to ascertain the

– Presence– Type– Intensity– Energy of radiations emitted by radionuclides

• Two commonly used devices

– Gas-filled detectors – Scintillation detectors

Gas-Filled Detectors

• The operation of a gas-filled detector is based on the ionization of gas molecules by radiations, followed by collection of the ion pairs as current with the application of a voltage between two electrodes.

• The measured current is primarily proportional to the applied voltage and the amount of radiations.

ionization of gas moleculesby radiations

collection of the ion pairs as current withthe application of a voltage between two electrodes

The measured current isprimarily proportional to the applied voltage and the amount of radiations.

Gas-Filled Detectors

Gas-Filled Detectors

Gas-Filled Detectors

• The two most commonly used gas-filled detectors are

– Ionization chambers• Cutie-Pie counters used for measuring high

intensity radiation sources, such as output from x-ray machines

• Dose calibrators measures the activity of radiopharmaceuticals

– Geiger-Müller (GM) counters.

Dose Calibrators

• one of the most essential instruments for measuring the activity of radionuclides– Cylindrically shaped – Sealed chamber with a central well – Filled with argon and traces of halogen at high

pressure

Geiger-Müller (GM) Counters

• One of the most sensitive detectors.• Used for the measurement of exposure delivered by a

radiation source and called survey meters.• Primarily used for area survey for contamination with

low-level activity.• It is usually battery operated.

Scintillation Detecting Instruments

• g-ray detecting equipment• Most commonly used:

– well counters– Thyroid probes– g or scintillation

• All these instruments are g-ray detecting devices • Consist of:• Collimator (excluding well counter) • Sodium iodide detector• Photomultiplier tube• Preamplifier• Pulse height analyzer• Display or Storage

• Scintillation detectors consist of scintilator emitting flashes of light after absorbing gamma or x radiation.

• The light photons produced are then converted to an electrical pulse by means of a photomultiplier tube.

• The pulse is amplified by a linear amplifier, sorted by a pulse-height analyzer and then registered as a count.

• Different solid or liquid scintillators are used for different types of radiation.

• In nuclear medicine, sodium iodide solid crystals with a trace of thallium NaI(Tl) are used for gamma and x ray detection.

g rays from a source interact in the sodium iodide detector and light photons are emitted.

The light photons will strike the photocathode of a photomultiplier(PM) tube and a pulse is generated at the end of the PM tube.

The pulse is first amplified by a preamplifier and then by a linear amplifier

Scintillation Camera

• also known as a gamma camera• consists of :• Collimator• Detector• X, Y positioning circuit• PM tubes • Preamplifiers• Linear amplifiers• PHA • Display or storage

Collimator

• In all nuclear medicine equipment for imaging a collimator is attached to the face of a sodium iodide detector to limit the field of view so that all radiations from outside the field of view are prevented from reaching the detector.

• Made of lead and have a number of holes of different shapes and sizes.

Collimator

• Classification of collimators used in scintillation cameras depends primarily on

1. The type of focusing

2. The thickness of the holes

• Depending on the type of focusing

1. parallel hole

2. Pinholet

3. Converging

4. Diverging type

Pinhole collimators are used in imaging small

organs such as thyroid glands

Converging collimators are employed when

the target organ is smaller than the size of the detector

divergingcollimators are used in imaging

organs such as lungs that are larger than the

size of the detector

Parallel hole collimators are most commonly used in

nuclear medicine procedures.

• Parallel hole collimators are classified as – High-resolution– All-purpose– High-sensitivity types.

• The size and number of holes the same for all these collimator

• The only change is in the thickness.• High sensitivity collimators are made with

smaller thickness than all-purpose collimators• High-resolution collimators are made thickest of

all.

Detector

• Sodium iodide crystal doped with a very small amount of thallium [NaI(Tl)] is most commonly used.– The choice of NaI(Tl) crystals for g-ray detection is

primarily due to their reasonable density (3.67 g/cm3) and high atomic number of iodine (Z = 53)

– That result in efficient production of light photons• Rectangular in shape• Have the dimension between 33 X 43 cm and 37 X 59

cm with thickness varying between 0.64 cm and 1.9 cm• The most common thickness is 0.95 cm• The 0.64-cm thick detectors are usually used in portable

cameras for nuclear cardiac studies

Detector

• Increasing the thickness of a crystal increases the probability of complete absorption of ɣ rays and hence the sensitivity of the detector

thicknessabsorption

sensitivity

Photomultiplier Tube

• A PM tube consists of

1. Light-sensitive photocathode at one end

2. A series (usually 10) of metallic electrodes called dynodes in the middle

3. Anode at the other end– All enclosed in a vacuum glass tube.

• Fixed on to the NaI(Tl) crystal• The number of PM tubes in the thyroid probe and the

well counter is one whereas in scintillation cameras it varies from19 to 94 which are attached on the back face of the NaI(Tl) crystal

Photomultiplier Tube

• When a lightphoton from the NaI(Tl) crystal strikes the photocathode photoelectrons are emitted and accelerated toward the immediate dynode

• The accelerated electrons strike the dynode and more secondary electrons are emitted, which are further accelerated

• The process of multiplication of secondary electrons continues until the last dynode is reached, where a pulse of 105 to 108 electrons is produced

• The pulse is then attracted to the anode and finally delivered to the preamplifier

Preamplifier

• The pulse from the PM tube is small in amplitude and must be amplified before further processing.

Linear Amplifier

• The output pulse from the preamplifier is further amplified and properly shaped by a linear amplifier.

• The amplified pulse is then delivered to a pulse height analyzer for analysis as to its voltage.

Pulse Height Analyzer

• Gamma rays of different energies can arise from a source, either – from the same radionuclide – or from different radionuclides– or due to scattering of grays in the source

• The pulses coming out of the amplifier may be different in amplitude due to differing g-ray energies

• The pulse height analyzer (PHA) is a device that selects for counting only those pulses falling within preselected voltage amplitude intervals and rejects all others

Pulse Height Analyzer

• A pulse height analyzer normally selects only one range of pulses and is called a single-channel analyzer (SCA).

• A multichannel analyzer (MCA) is a device that can simultaneously sort out pulses of different energies into a number of channels.

• In many scintillation cameras, the energy selection is made automatically by pushbutton type isotope selectors designated for different radionuclides such as 131I, 99mTc

• In some scintillation cameras, two or three SCAs are used to select simultaneously two or three g rays of different energies

Display and Storage

• most cameras employ digital computers in acquiring, storing, and processing of image data

Tomographic Imagers

• limitation of the scintillation cameras is that they depict images of three-dimensional activity distributions in two-dimensional displays

• One way to solve this problem is to obtain images at different angles around the patient such as anterior, posterior, lateral, and oblique projections

• Success of the technique is limited because of the complexity of structures surrounding the organ of interest

1. Single Photon Emission Computed Tomography2. Positron Emission Tomography

Tomographic Imagers

• Single Photon Emission Computed Tomography– uses g-emitting radionuclides such as 99mTc, 123I,

67Ga, 111In• Positron Emission Tomography• uses beta+ emitting radionuclides such as 11C, 13N, 15O,

18F, 68Ga, 82Rb

Tomographic Imagers

• mathematical algorithms, to reconstruct the images at distinct focal planes (slices).