Room: Exhibit Hall | Forum 9
Purpose: We are proposing an alternative, simple method of computing water equivalent thickness and evaluating the need for a correction application due to the presence of the pelvic bone when calculating SSDE, for patients with CT of the PELVIS.
Methods: Eleven patients with CT of the PELVIS were analyzed according to TG204 for computing the Deff and TG220 for Dw based on the mid-scanning range image, in most cases transversing the area with the largest concentration of bone. We introduce the use of the relative electron density Ï?e=1.3 to scale the lateral distance through the bone and Ï?e=1.0 though the rest of the tissue and, the definition of the radiological distance. The corrected-DeffTG204=Dwcorr that equals and replaces Dw=DwTG220 in our approach is computed by:Dw=Dwcorr=âˆš(APâˆ™LATrad ). The SSDE was computed with both Dw and Dwcorr and compared as well.
Results: There is a strong correlation noted between Deff (uncorrected as per TG204) and both Dw and Dwcorr and differences between the two in the range of (-1.61%, -6.35%) for the patients in the study. The scale-corrected Dwcorr is always larger than the DwTG220), due to a) the 1.3 increase of the distance into the bone and b) because the â€œwashoutâ€? of the bone in the mean HU value for the entire slice makes the rest of the tissue within the slice dominant on the computation of the Dw via the TG220 method.
Conclusion: SSDE calculations for the CT of the PELVIS does not benefit greatly by the use of Dw unless a modification of the formulation is performed in order to account better for the presence of the high density pelvic bone. An alternative and simpler corrective method that is based on radiological distance scaling has been shown. Details of the method and comparisons with TG220 will be shown further.