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Instruments for Radiation Detection and Measurement Lab # 4 • 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 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 registred 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. 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 • 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 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 highresolution, all-purpose, and 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 • NaI(Tl) crystals used as the detector • 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 g rays and hence the sensitivity of the detector X, Y Positioning Circuit • When a g ray interacts in the crystal, its exact location is determined by the X, Y positioning circuit • Many PM tubes are mounted on the NaI(Tl) crystal in scintillation cameras • After g-ray interaction in the crystal, a maximum amount of light will be received by the PM tube nearest to the point of interaction Pulse Height Analyzer • circuit that sums up the output of all PM tubes to produce a pulse known as the Z pulse that represents the energy of a g ray • The SCA analyzes the amplitude of the Z pulses and selects only those of desired energy by the use of appropriate peak energy and percent window settings • In many scintillation cameras, the energy selection is made automatically by pushbutton type isotope selectors designated for different radionuclides such as 131I, 99mTc Pulse Height Analyzer • 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 – Single Photon Emission Computed Tomography – Positron Emission Tomography Tomographic Imagers • mathematical algorithms, to reconstruct the images at distinct focal planes (slices).
