The dose of radiation delivered during a cardiac CT examination can be decreased by implementing features such as tube current modulation, cardiac phase–specific scanning, and increased pitch.
For instance, all scanners are designed so that the x-ray tube current can be modulated, or varied, while the gantry spins around the patient. The tube current thus can be decreased when the beam crosses areas of the body with less tissue or lower tissue density, thereby lowering the total overall dose of radiation needed (9). In general CT examinations, dose reductions on the order of 20%–40% have been achieved with these approaches (10–12).
A dose-reduction approach specific to cardiac CT is to decrease the x-ray tube current during phases of the cardiac cycle that are expected to contain fairly large amounts of motion. Images acquired during these phases are expected to be of lower quality and of less value for interpretation; thus, the radiologist may choose that they not be captured for use in image reconstruction. Dose reductions of up to 50% have been achieved with this feature (13). This option may or may not be successful at individual facilities; many radiologists appear to demand images from all cardiac cycles to review regardless of their temporal resolution quality.
A third means of reducing the effective dose is to use a higher pitch value. This method is particularly used for selective patients with higher-than-normal heart rates, as with the new dual x-ray tube CT scanner discussed earlier (14). The overall dose of radiation delivered can also be substantially decreased with this method.
Combining two or more of these dose reduction options during a cardiac CT study may result in even greater decreases in the dose of radiation delivered.
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