Correlation of size-dependent conversion factor and body-mass-index using size-specific dose estimates formalism in CT examinations.

PURPOSE: The aim of this investigation is to establish the relationship between the size-dependent conversion factor (fsize ) and the body-mass-index (BMI) and to test whether BMI can be substituted for the conventionally used patients' anterior-posterior (AP) and lateral (LAT) dimensions for calculation of fsize . By calculating fsize on the basis of BMI instead of the AP and LAT measurements, size-specific dose estimates (SSDE) could be determined prior to image acquisition.

METHODS: Our institute utilizes a dose monitoring software to record radiation exposure during CT examinations. The datasets gathered during each examination contain information regarding the scan protocol, the volumetric computed tomography dose index (CTDIvol ), SSDE and BMI. fsize is traditionally calculated through measurement of AP and LAT dimensions. In this work, the dose monitoring system calculates AP and LAT diameters at the middle of scout views. For purpose of this investigation, we used data from 13544 patients who underwent CT examinations of the torso, head or knee (both) to compare fsize as calculated from the AP and LAT dimensions to fsize calculated as a function of BMI.

RESULTS: In the examinations of the torso, we observed an exponentially decreasing correlation between fsize and BMI. In the examinations of the head and knee (both), fsize reflected an almost independent behavior to BMI.

CONCLUSION: This study demonstrates that it is possible to estimate fsize by using the patients' BMI for the torso as well as the head and knee CT, thereby enabling calculation of the probable SSDE prior to image acquisition on the basis of the presumed CTDIvol provided by the scanner. By providing information on the expected patient dose prior to image acquisition, this method is advantageous over the traditional calculation of fsize via the AP and LAT dimensions.