While tube current is varied cyclically in relation to the starting tube current value in angular tube current modulation, longitudinal tube current modulation aims to produce uniform noise levels across various regions. For this, the operator uses the following manufacturer-specific methods: the reference noise index (GE Healthcare Technologies, Waukesha, Wis), reference image acquisition (Philips Medical Systems, Best, the Netherlands), reference tube current-time product value (Siemens Medical Solutions, Forchheim, Germany), or reference standard deviation or image quality level (Toshiba Medical Systems, Tokyo, Japan). This gives the operator the required level of image quality for input to the algorithm (McCollough, Bruesewitz, and Kofler 2006.)c.
Angular-longitudinal tube current modulation-this involves the simultaneous combination of angular and longitudinal (x-, y-, and z-axis) tube current modulation. The tube current is varied during both gantry rotation and along the z-axis of the patient (i. e., from the anteroposterior direction to the lateral direction, and from the shoulders to the abdomen). As before, the operator uses the manufacturer-specific methods to indicate the desired level of image quality. Since the x-ray dose is adjusted according to the patient-specific attenuation in all three planes, this method is the most comprehensive approach to dose reduction (McCollough, Bruesewitz, and Kofler 2006)d.
Automatic exposure control (AEC)-while the operator determines the image quality requirements, the CT system determines the right tube current-time product. However, it may not be that easy to determine the image quality requirements for the various patient age groups and CT examination types. Depending on the diagnostic task, the operator can make a choice between low noise and a low dose to get the required image quality. Based on this, the CT system will make adjustments to the tube current during the gantry rotation, during movement along the z-axis, or during movement in all three dimensions (McCollough, Bruesewitz, and Kofler 2006).
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