CT protocol management: simplifying the process by using a master protocol concept

Abstract

This article explains a method for creating CT protocols for a wide range of patient body sizes and clinical indications, using detailed tube current information from a small set of commonly used protocols. Analytical expressions were created relating CT technical acquisition parameters which can be used to create new CT protocols on a given scanner or customize protocols from one scanner to another. Plots of mA as a function of patient size for specific anatomical regions were generated and used to identify the tube output needs for patients as a function of size for a single master protocol. Tube output data were obtained from the DICOM header of clinical images from our PACS and patient size was measured from CT localizer radiographs under IRB approval. This master protocol was then used to create 11 additional master protocols. The 12 master protocols were further combined to create 39 single and multiphase clinical protocols. Radiologist acceptance rate of exams scanned using the clinical protocols was monitored for 12,857 patients to analyze the effectiveness of the presented protocol management methods using a two-tailed Fisher's exact test. A single routine adult abdominal protocol was used as the master protocol to create 11 additional master abdominal protocols of varying dose and beam energy. Situations in which the maximum tube current would have been exceeded are presented, and the trade-offs between increasing the effective tube output via 1) decreasing pitch, 2) increasing the scan time, or 3) increasing the kV are discussed. Out of 12 master protocols customized across three different scanners, only one had a statistically significant acceptance rate that differed from the scanner it was customized from. The difference, however, was only 1% and was judged to be negligible. All other master protocols differed in acceptance rate insignificantly between scanners. The methodology described in this paper allows a small set of master protocols to be adapted among different clinical indications on a single scanner and among different CT scanners.

Figure 1

Depiction of an example of how noticing a protocol is not reaching the maximum tube current can lead to a change of parameters (higher pitch and/or faster rotation time) and, thus, a reduction in scan time can be made without compromising image quality.

Figure 2

Example box and whisker plot used to assess a protocol by analyzing what mA values the AEC system was using. The median is shown with a horizontal line, the extent of the 90th and 10th percentiles are shown with a thick line, the extent of all points outside the 90th and 10th percentiles but not considered outliers are shown with a thin line, and outliers are individually shown with a cross. The data were taken from a routine abdominal/pelvis scan. Each box and whisker represents the mA distribution from a single patient exam. The horizontal axis value was computed by taking the sum of the average of each patient's lateral and anterior/posterior width measured from CT localizer radiograph images.

Figure 3

Example plot used to obtain reference minimum and maximum mA values.

Figure 4

Flowchart detailing the proper use of mA data and (1), (2) in creating protocols on the same scanner at different CNR levels and customizing these protocols to a different scanner.

Figure 5

Box and whisker plot of the mA as a function of patient size for the (top) low dose B7 master protocol and (bottom) higher dose B1 master protocols from a LightSpeed VCT scanner for the medium patient size range. The median is shown with a horizontal line, the extent of the 90th and 10th percentiles are shown with a thick line, the extent of all points outside the 90th and 10th percentiles but not considered outliers are shown with a thin line, and outliers are individually shown with a cross. The minimum/maximum mA values for the top and bottom graphs were 15/200 and 50/640, respectively.

Figure 6

Box and whisker plot of the mA as a function of patient size for the routine abdominal pelvis protocol from three different CT scanners for the medium patient size range. The LightSpeed VCT, Optima 660, and Discovery HD 750 systems are shown from top to bottom, respectively. The median is shown with a horizontal line, the extent of the 90th and 10th percentiles are shown with a thick line, the extent of all points outside the 90th and 10th percentiles but not considered outliers are shown with a thin line, and outliers are individually shown with a cross. The minimum/maximum mA values for the top, bottom, and middle graphs were 30/400, 40/320 and 50/500, respectively.