–The beam width on a 64-slice CT scanner is ~ 40 mm. –Collimation defines the total beam width in multidetector CT (MDCT) systems. –Collimation defines the section thickness on a single-slice scanner. –Collimators are located at the x-ray tube as well as at the x-ray detectors. –Bow tie filters also reduce scatter and patient dose. –Bow tie filters are made of a low Z material such as Teflon to minimize beam hardening differences. TABLE 5.1 Representative Values of X-ray Tube Power and Minimum Scan Times in CT Scanning –Bow tie filters attenuate little in the center, but attenuation increases with increasing distance from the central ray. –A bow tie filter is used to minimize the dynamic range of exposures at the detector.
–Heavy x-ray beam filtering reduces x-ray beam hardening effects. –The heavy filtration used with CT scanners typically produces a beam with an aluminum half-value layer (HVL) of up to 10 mm Al. –The typical filtration on a CT x-ray tube is ~ 6 mm Al. – Copper or aluminum filters are used to filter the x-ray beam. –The x-ray tube anode–cathode axis is positioned perpendicular to the imaging plane to reduce the heel effect. –CT x-ray tubes are very expensive, with the price of some tubes exceeding $200,000. –The Straton tube is relatively light and has very high heat dissipation rate that is > 60 kW. –Recent innovations in x-ray tube design include a rotating envelope vacuum vessel ( Straton tube). –Anode heat dissipation rates are ~ 10 kW. –X-ray tube anode heat capacities are high, and can exceed 4 MJ. –Heat loading on CT x-ray tubes is generally high, requiring high anode heat capacities. –Small x-ray tube focal spots are about half the size of the large focal spot and can tolerate no more than ~ 25 kW. –CT x-ray tubes have a large focal spot with a size of ~ 1 mm, which can tolerate a power loading of 100 kW. – Table 5.1 shows how x-ray tube power capabilities have increased since the early 1970s. –A tube voltage of 120 kV and tube current of 830 mA corresponds to 100 kW. –Power loading on CT x-ray tubes can be as high as ~ 100 kW. –Tube current of 800 mA and rotation time of 0.3 s corresponds to 240 mAs. – Table 5.1 shows how x-ray tube rotation times have been reduced since the introduction of CT scanners into clinical practice in the early 1970s. –Time for a 360-degree rotation of the x-ray tube currently ranges between 0.3 and 2 seconds. –Tube currents increase when the path length increases, as in a lateral abdominal projection compared to an anteroposterior (AP) projection. –Tube currents are frequently modulated as the x-ray tube rotates around the patient. –Modern CT scanners make use of slip ring technology in which high voltage is supplied to the tube through contact rings in the gantry. –High-frequency power supplies are used in computed tomography (CT), capable of providing stable tube currents and voltages.
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