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Relationship between Noise, Dose, and Pitch in Cardiac Multi–Detector Row CT¹

¹ From the CT Clinical Innovation Center, Department of Radiology, Mayo Clinic College of Medicine, 200 First Street SW, Rochester, MN 55905. Recipient of a Certificate of Merit award for an education exhibit at the 2005 RSNA Annual Meeting. Received April 12, 2006; revision requested May 30 and received July 6; accepted July 10. A.N.P., C.H.M., J.Z., and J.G.F. receive research grants from Siemens Medical Solutions, Malvern, Pa; C.H.M. also receives a research grant from GE Healthcare, Waukesha, Wis; J.G.F. has an educational license from GE Healthcare and participates in a CME course sponsored by E-Z-Em, Lake Success, NY; M.R.B. has no financial relationships to disclose. Address correspondence to A.N.P. (e-mail: primak.andrew{at}mayo.edu).

In spiral computed tomography (CT), dose is always inversely proportional to pitch. However, the relationship between noise and pitch (and hence noise and dose) depends on the scanner type (single vs multi–detector row) and reconstruction mode (cardiac vs noncardiac). In single detector row spiral CT, noise is independent of pitch. Conversely, in noncardiac multi–detector row CT, noise depends on pitch because the spiral interpolation algorithm makes use of redundant data from different detector rows to decrease noise for pitch values less than 1 (and increase noise for pitch values > 1). However, in cardiac spiral CT, redundant data cannot be used because such data averaging would degrade the temporal resolution. Therefore, the behavior of noise versus pitch returns to the single detector row paradigm, with noise being independent of pitch. Consequently, since faster rotation times require lower pitch values in cardiac multi–detector row CT, dose is increased without a commensurate decrease in noise. Thus, the use of faster rotation times will improve temporal resolution, not alter noise, and increase dose. For a particular application, the higher dose resulting from faster rotation speeds should be justified by the clinical benefits of the improved temporal resolution.

© RSNA, 2006

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