RSNA 2016

Abstract Archives of the RSNA, 2016


SSE21-06

Comparison of Size-Specific Dose Estimate Conversion Factors for Fixed Tube Current and Tube Current Modulated Computed Tomography

Monday, Nov. 28 3:50PM - 4:00PM Room: S403A



Anthony Hardy, BS, Los Angeles, CA (Presenter) Nothing to Disclose
Maryam Bostani, PhD, Los Angeles, CA (Abstract Co-Author) Research support, Siemens AG
Christopher H. Cagnon, PhD, Los Angeles, CA (Abstract Co-Author) Nothing to Disclose
Michael F. McNitt-Gray, PhD, Los Angeles, CA (Abstract Co-Author) Institutional research agreement, Siemens AG Research support, Siemens AG
PURPOSE

The purpose of this work is to compare the conversion factors used to calculate size-specific dose estimate (SSDE) as outlined in TG 204 for tube current modulation (TCM) with different modulation strengths to those of fixed tube current (FTC).

METHOD AND MATERIALS

A series of abdominal CT protocols were performed on three cylindrical polymethyl methacrylate (PMMA) phantoms with diameters of 32 cm, 16 cm, and 8 cm on a 64-slice MDCT scanner (Definition AS64, Siemens). The scans were performed using the same kV, collimation, rotation time, and pitch. Several scans were performed using fixed mAs (200) and TCM (CareDose4D, Quality Reference mAs = 200). To examine the effects of different scanner parameters, both Adult and Child options were used as well as three different modulation strengths for TCM: Very Weak, Average, and Very Strong. For each scan, a 0.6-cc ionization chamber was placed in the center hole of each phantom and the air kerma measurement was recorded for each condition. In addition, the scanner reported CTDIvol was recorded and used to normalize the air kerma values in order to produce conversion factors similar to those described in TG 204. Phantom size was described in terms of water equivalent diameter (WED). The conversion factors for all different strengths were plotted against WED. Percent differences were reported between conversion factors from all three strengths and those from FTC scans.  

RESULTS

For the child abdomen protocol, differences between FTC and TCM conversion factors were 7%, 7%, and 5% at 32 cm; 6%, 1%, and 4% at 16 cm; and 55%, 13%, and 72% at 8 cm for Average, Very Weak, and Very Strong modulation, respectively. For the adult abdomen protocol, differences between FTC and TCM conversion factors were 21%, 17%, and 23% at 32 cm; 26%, 4%, and 23% at 16 cm; and 46%, 13%, and 63% at 8 cm for Average, Very Weak, and Very Strong modulation, respectively.

CONCLUSION

FTC conversion factors were systematically larger than those of TCM at the three modulation strengths. TCM conversion factors possessed a similar exponential relationship relative size to the FTC factors with the 32 and 16 cm phantoms. At 8 cm, surprisingly, TCM factors deviated from this relationship in a reproducible manner. This deviation is currently being investigated.

CLINICAL RELEVANCE/APPLICATION

FTC conversion factors may not be applicable for TCM protocols depending patient size as they yield an inaccurate estimate of SSDE.