Radionuclide Imaging MII 3073 RADIONUCLIDE IMAGING SYSTEM

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Theodore Beasley
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1 Radionuclide Imaging MII 3073 RADIONUCLIDE IMAGING SYSTEM

2 Preamplifiers and amplifiers The current from PMT must be further amplified before it can be processed and counted (the number of electrons yielded is still small). Typically, this amplification is divided into: 1. Preamplifier: located close to PMT, increases the number of charges sufficiently to allow the current to be transmitted to amplifier 2. Amplifier: main amplifier that increases the current of electrical pulses as much as a thousand-fold.

3 Pulse-height analyzer The amplitude of each electrical pulse from amplifiers is measured in the electrical circuits of pulse-height analyzer (PHA). Generates a plot of the number of pulses against their energy, called pulse-height spectrum. The pulse-height analyzer is often used to select Z pulses that correspond to a range of the acceptable energies, known as energy window. Eg: A window setting of 20% for 140keV 99m Tc means a range of kev energy will be accepted and counted.

5 Computers in nuclear medicine Used for acquisition, storage and processing of data. For each Z pulse that is accepted by the pulse-height analyzer, one count is added to the storage location that corresponds to its x,y location determined by the positioning circuit. The data storage can be visualized as a matrix. Each position within the matrix corresponds to a pixel within the image and is assigned a unique address composed of the row and column of its location. A 64 x 64 matrix has 4096 pixels; a 128 x 128 has pixels. The greater the number of pixels, the smaller the size of a pixel for a given FOV, and the better the resolution of the image.

7 Rectilinear Scanner Rectilinear scanner is one of the early scintillation imaging devices. Can be divided into two: 1. Single detector assembly: these automatic scanners, called rectilinear scanners, make a map of the location of radioactive materials in a patient s body. 2. Multiple detector assemblies: consists of two opposing detector heads, each head consisting of multiple NaI crystals and multiple associated photomultiplier tubes, each crystal with a focused collimator. Rectilinear: Refers to images with no geometric distortion in which the scales in the horizontal and vertical directions are identical.

8 In a rectilinear scanner, detector head is moved over the organ of interest in a straight line with the help of an electric motor. After travel at a specific distance, the detector head either steps up or down on a small distance and then continues the straight line motion in the opposite direction. The detector head assembly consists of a collimator, NaI(Tl) crystal and PMT with preamplifier. The information gathered by the detector head is continuously relayed to a photographic system using 14 x 17 inch x-ray film. This system is moved in a light-tight box in synchronization with the detector head. Dark areas on the film represent high areas of radioactivity in the organ, and vice versa.

11 Scintillation Scanner/Gamma Camera In gamma camera, the whole organ or a large part of the body is imaged simultaneously. This feature makes the study of rapid dynamic processes possible. A scintillation camera can physically be divided into two parts: 1. Detector head contains the collimator and NaI(Tl) crystal with PMT and associated electronics and is mounted on a stand where it can be easily moved up or down or rotated in any desired position with hand-held controls or computerdriven automatic. 2. Console houses the power supplies and operational controls including the display system.

12 The outputs from the PMTs are converted into three signals, two of which (X and Y) give the spatial location of the scintillation while the third (Z) represents the energy deposited in the crystal by the gamma ray. To improve their quality these signals then pass through correction circuits. The Z signal goes to a PHA, which tests whether the energy of the gamma ray is within the window. The ability of a gamma camera to accurately image sources of activity depends on several factors such as: 1. Intrinsic resolution of the camera (the ability of the camera to distinguish between two closely spaced interactions in the crystal). 2. The gamma-ray energy. 3. The choice of collimator. 4. The distance from the collimator face to the source.

14 The following steps are taken to obtain an image with a scintillation camera: 1. Select the study to be performed. 2. Select the radiopharmaceutical used and its dose. 3. Select the PHA parameters (peak energy and % window). 4. Select an appropriate collimator (with respect to energy and spatial resolution). 5. Select the mode: accumulation of a certain number of counts or exposure for fixed amount of time. 6. Select the appropriate intensity of the display system (cathode ray tube). 7. Position the patient under the camera. 8. Start and finish of the exposure. 9. Development of the film. If more than one view is to be displayed on the same film, then the development of the film takes place at the end of the study.

Quality control of Gamma Camera. By Dr/ Ibrahim Elsayed Saad 242 NMT

Quality control of Gamma Camera By Dr/ Ibrahim Elsayed Saad 242 NMT WHAT IS QUALITY? The quality of a practice is to fulfill the expectations and demands from: Patient Clinicain Your self Quality assurance