Many of the dose-reduction strategies described previously result in a trade-off of image quality. For example, if tube current or voltage is reduced, the radiation dose is reduced, but so is image quality. CT manufacturers have tried to come up with new technologies that can reduce dose without significantly compromising imaging quality. One of these techniques is dose modulation.
Dose modulation is a technique by which the CT scanner modifies the tube current in response to the patient's attenuation, to maintain the same image quality for the least possible tube current. Dose modulation can be performed in the z-axis where tube current changes along the length of the patient, in the xy-plane (angular modification), or can be a combination of the 2 (xyz–dose modulation). Recent studies demonstrated that dose modulation is capable of providing a reduction in radiation dose without significant image compromise, including for neuroradiology CT protocols, where up to 60% dose reduction was achieved for noncontrast CT of the brain in adult and pediatric patients; for CT studies of the cervical spine; and for CTA studies.
Implementation of dose modulation requires a team effort between radiologists, technicians, and physicists, who have to select a preset noise index (NI) that describes the level of noise acceptable to the radiologist for a given CT examination. The CT scanner then automatically selects, within a preset range, the tube current (milliampere) required to maintain the level of noise under the noise index, taking into consideration the patient's attenuation. Identification of optimal signal intensity-to-noise ratio for each type of CT protocol requires fine-tuning to lower the milliamperes as much as possible while preserving image quality. When we transitioned to dose modulation, we initially set the NI at a low value (as recommended by the manufacturer) and progressively increased it until the image quality was deemed insufficient. This decision was a consensus decision by the 9 faculty members of our neuroradiology section.
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