Instruments for Radiation Detection and Measurement Lab 3

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.

Gas-Filled Detectors collection of the ion pairs as current with the application of a voltage between two electrodes ionization of gas molecules by radiations The measured current is primarily proportional to the applied voltage and the amount of radiations.

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 Na. I(Tl) are used for gamma and x ray detection.

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

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

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

• 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 [Na. I(Tl)] is most commonly used. – The choice of Na. I(Tl) crystals for g-ray detection is primarily due to their reasonable density (3. 67 g/cm 3) 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 thickness absorption 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 Na. I(Tl) crystal • The number of PM tubes in the thyroid probe and the well counter is one whereas in scintillation cameras it varies from 19 to 94 which are attached on the back face of the Na. I(Tl) crystal

Photomultiplier Tube • When a lightphoton from the Na. I(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 131 I, 99 m. Tc • 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 twodimensional 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 Tomography 2. Positron Emission Tomography

Tomographic Imagers • Single Photon Emission Computed Tomography – uses g-emitting radionuclides such as 99 m. Tc, 123 I, 67 Ga, 111 In • Positron Emission Tomography • uses beta+ emitting radionuclides such as 11 C, 13 N, 15 O, 18 F, 68 Ga, 82 Rb

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