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Radiation Sources in medicine diagnostic Radiology Computed Tomography IAEA International Atomic Energy Agency Day 7 – Lecture 1(4) Objective • To become familiar with the technology of Computed Tomography (CT) scanners. • To become familiar with specific radiation risks associated with this equipment. IAEA 2 Contents • Scanner technology, main specifications; • Technical and clinical developments in computed tomography; • Management of patient doses by optimizing scan protocols; • Equipment malfunction affecting radiation protection; • Quality control. IAEA 3 Computed Tomography Scanners CT scanners use one or more narrow, fan-shaped x-ray beams that rotate around the patient to produce a series of thin, transverse images of the patient. • The x-ray beam, attenuated by the patient, reaches a detector array consisting of several hundred separate detectors which record the intensity of the x-ray beam at each point. • Gas detectors (usually Xenon) convert x-ray photons into electrical signals. Solid state detectors perform an indirect conversion, using photo-diodes coupled to scintillator systems. IAEA 4 Computed Tomography (Cont.) • The CT image is mathematically reconstructed from the measured data. IAEA 5 Computed Tomography (cont) • CT operates with an x-ray tube potential in the range of perhaps 90-140 kV peak. • The geometry of the x-ray beam is primarily determined by the size of the x-ray tube focal spot (0.5 - 2 mm) and prepatient collimation. • Shaped attenuation filters and / or detector collimation are often used to minimize scattered radiation reaching the detectors and hence degrading the image. IAEA 6 Computed Tomography (cont) The introduction of slip rings in the gantry structure enabled the development of helical scanners and more recently multi-slice (multi detector-array) CT, where the x-ray tube rotates continuously while the patient couch moves through the gantry. The main advantages of helical (and multi-slice) CT are: • faster scanning which allows, for example, an examination in a single held breath; • the possibility to choose, after scanning, the position and spacing of reconstructed images. IAEA 7 Computed Tomography (cont) Multislice CT scanner IAEA 8 Computed Tomography (cont) • These technological developments have led to an increasing use of CT in routine patient management and an increasing variety of examinations. • Patient doses from CT examinations are relatively high (10 -100 mSv). Substantial dose variations for similar examinations can arise from different imaging protocols or between scanner types. • To minimize unnecessary radiation exposure, prior clinical justification of all CT examinations should be encouraged. Imaging protocols must be optimized to provide the required clinical information with the minimum dose to the patient. IAEA 9 Scan Parameters influencing Patient Dose Patient dose depends on the intrinsic qualities of the CT scanner (scanner geometry, beam geometry, collimation and filtration, etc). However, changes in a range of selectable operating parameters can also significantly affect patient radiation dose. i.e. • x-ray tube potential, tube current, exposure time. • slice thickness, number of slices (or helical rotations). • slice interval (incremental mode) or pitch factor (helical mode). • window width, matrix size and field of view. IAEA 10 Malfunctions affecting radiation protection • x-ray tube voltage / current inaccuracy and inconsistency; • x-ray tube output inconsistency; • discrepancies between measured exposure and image quality parameters and the manufacturer’s data: e.g. image noise, resolution, slice thickness, CT number values and uniformity, Computed Tomography Dose Index (CTDI). Radiation protection issues are generally affected more by poor understanding and management of the selected parameters than by poor equipment performance. IAEA 11