Frank N. Ranallo PhD, Presenter: Grant, General Electric Company

Timothy Peter Szczykutowicz PhD, Abstract Co-Author: Equipment support, General Electric Company Research Grant, Siemens AG

The proper selection of the pitch and rotation time not only affects the total scan time but also the image quality and dose in ways that are commonly misunderstood. A proper understanding of the effect of changing the pitch and rotation time in can provide improve image quality and/or reduced patient dose.

Presently in CT scanning there is an essential need to provide correct guidance in the proper selection of pitch and rotation time for optimal CT imaging. There exists a widespread misconception concerning the role of pitch in patient dose with modern multi-slice scanners, particularly with the use of mA modulation techniques. We investigated the relationship of pitch and rotation time to image quality, dose, and scan duration, with CT scanners from different manufacturers in a way that clarifies this misconception.This source of this misconception may concern the original role of pitch with single slice CT scanners.

As expected we found that the dose is generally independent of the selected effective mAs (mA*time/ pitch) with manual mA technique settings; the image noise is often, but not always independan of the selected effective mAs. The dose and image noise are often independent of the selected pitch and /or rotation time with automatic mA modulation techniques. However we did find that on certain scanners the use of a pitch just above 0.5 provided images of equal image noise at a lower dose compared to the use of a pitch just below 1.0.

The misconception that the use of a lower pitch over-irradiates patients by wasting dose is clearly false. The use of a lower pitch provides images of equal or better image quality at the same patient dose, whether using manual mA or automatic mA modulation techniques. By decreasing the pitch and the rotation times by equal amounts, both helical and patient motion artifacts can be reduced without affecting the exam time. The use of lower helical pitch also allows better scanning of larger patients by allowing a greater scan effective mAs, if the exam time can be extended. The one caution with the use of low pitch is not related to patient dose, but to the length of the scan time if the rotation time is not set short enough.

Ranallo, F, Szczykutowicz, T, The Correct Selection of Pitch and Rotation Time for Optimal CT Scanning – “The Big Misconception”; The Effects of Pitch on Image Quality and Patient Dose in Both Manual mA and AEC - mA Modulation Scanning Modes.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14014812.html