1. Instruments for Radiation Detection and Measurement
Lab # 3

2. 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

3. 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.

4. Gas-Filled Detectors 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 molecules
by radiations

5. Gas-Filled Detectors

6. Gas-Filled Detectors

7. 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.

8. 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

9. 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.

10. 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

11. 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.

12. The light photons will strike the photocathode of a photomultiplier
(PM) tube and a pulse is generated at the end of the PM tube.
g rays from a source interact in the sodium iodide detector and light photons are emitted.
The pulse is first amplified by a preamplifier and then by a linear amplifier

13. 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

15. 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.

16. Collimator
Classification of collimators used in scintillation cameras depends primarily on
The type of focusing
The thickness of the holes
Depending on the type of focusing
parallel hole
Pinholet
Converging
Diverging type

17. Converging collimators are employed when
the target organ is smaller than the size of the detector
Pinhole collimators are used in imaging small
organs such as thyroid glands

18. Parallel hole collimators are most commonly used in
nuclear medicine procedures.
diverging
collimators are used in imaging organs such as lungs that are larger than the
size of the detector

19. 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.

20. 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

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

22. Photomultiplier Tube A PM tube consists of
Light-sensitive photocathode at one end
A series (usually 10) of metallic electrodes called dynodes in the middle
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

23. 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

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

25. 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.

26. 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

27. 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

28. Display and Storage
most cameras employ digital computers in acquiring, storing, and processing of image data

29. 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
Single Photon Emission Computed Tomography
Positron Emission Tomography

30. 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

31. Tomographic Imagers
mathematical algorithms, to reconstruct the images at distinct focal planes (slices).