Medical Physics Seminar – Monday, September 26, 2011
CT protocols: image quality and dose *Important variations among different CT scanner methods of optimization and catastrophes to avoid
Frank Ranallo, Ph.D., DABR
Associate Professor(CHS), Medical Physics and Radiology, UW-School of Medicine & Public Health, Madison, WI USA
The sophistication and diagnostic power of CT scanners has increased greatly over the last decade. Unfortunately the general wisdom in understanding how to get the most out of these CT scanners has often not adequately followed these advancements. Ideally we should select CT scan and reconstruction techniques to (1) optimize image quality at a given dose, and then (2) properly decide the lowest dose that will consistently give us diagnostic image quality. We require diagnostic images - not "pretty" images.
Much of the present focus on reducing dose in CT scanners is simply centered on reducing the manual mA or effective mAs (mA x rotation time / pitch), or performing an equivalent operation with the automatic exposure control that will result in a decrease of mA or effective mAs. However, little attention has been given to optimizing the image quality AT A GIVEN DOSE. This last step is extremely important to both insuring properly diagnostic image and also allowing the lowest possible doses. There are many misconceptions in the CT medical imaging community, such as: One needs to increase the helical pitch to reduce the dose. This is very misleading; increasing the pitch should NEVER be used as a dose reduction strategy. Even the ACR, in its "Clinical Image Quality Guide,†recommends the use of a pitch "between 1.0 and 1.5" for most scanning, and actually requires it for pediatric scanning. This guide says “In general, a pitch of less than one (“overscanningâ€) is not recommended in pediatric patients.†A pitch less than one normally results in better image quality and can be used without any increase in the patient dose – contrary to most popular belief.
The operator’s interface for CT scanners with different manufacturers can be dramatically different, causing further confusion in the proper selection of techniques. Some scanners talk about mA, rotation time, and pitch, while others never mention mA, but talk about effective mAs, rotation time, and pitch. And one manufacturer actually talks about mAs when it is really referring to effective mAs. The automatic exposure control systems of different manufacturers operate very differently on several levels and this can lead to catastrophic effects if the system being used is not properly understood.
Another important consideration is the variation of the kV technique, with consideration of patient size and whether imaging with or without iodine contrast. At the UW we have pediatric protocols for 9 different patient sizes and adult protocols for 3 different sizes. The selected kV for most studies will vary over the full range of the CT scanner settings from 80 to 140 kV depending on patient size. This is required to provide properly diagnostic images at the lowest doses for all patient sizes.