The feasibility of a regional CTDIvol to estimate organ dose from tube current modulated CT exams

Abstract

Purpose: In AAPM Task Group 204, the size-specific dose estimate (SSDE) was developed by providing size adjustment factors which are applied to the Computed Tomography (CT) standardized dose metric, CTDI(vol). However, that work focused on fixed tube current scans and did not specifically address tube current modulation (TCM) scans, which are currently the majority of clinical scans performed. The purpose of this study was to extend the SSDE concept to account for TCM by investigating the feasibility of using anatomic and organ specific regions of scanner output to improve accuracy of dose estimates.

Methods: Thirty-nine adult abdomen/pelvis and 32 chest scans from clinically indicated CT exams acquired on a multidetector CT using TCM were obtained with Institutional Review Board approval for generating voxelized models. Along with image data, raw projection data were obtained to extract TCM functions for use in Monte Carlo simulations. Patient size was calculated using the effective diameter described in TG 204. In addition, the scanner-reported CTDI(vo)l (CTDI(vol),global) was obtained for each patient, which is based on the average tube current across the entire scan. For the abdomen/pelvis scans, liver, spleen, and kidneys were manually segmented from the patient datasets; for the chest scans, lungs and for female models only, glandular breast tissue were segmented. For each patient organ doses were estimated using Monte Carlo Methods. To investigate the utility of regional measures of scanner output, regional and organ anatomic boundaries were identified from image data and used to calculate regional and organ-specific average tube current values. From these regional and organ-specific averages, CTDI(vol) values, referred to as regional and organ-specific CTDI(vol), were calculated for each patient. Using an approach similar to TG 204, all CTDI(vol) values were used to normalize simulated organ doses; and the ability of each normalized dose to correlate with patient size was investigated.

Results: For all five organs, the correlations with patient size increased when organ doses were normalized by regional and organ-specific CTDI(vol) values. For example, when estimating dose to the liver, CTDI(vol),global yielded a R(2) value of 0.26, which improved to 0.77 and 0.86, when using the regional and organ-specific CTDI(vol) for abdomen and liver, respectively. For breast dose, the global CTDI(vol) yielded a R(2) value of 0.08, which improved to 0.58 and 0.83, when using the regional and organ-specific CTDI(vol) for chest and breasts, respectively. The R(2) values also increased once the thoracic models were separated for the analysis into females and males, indicating differences between genders in this region not explained by a simple measure of effective diameter.

Conclusions: This work demonstrated the utility of regional and organ-specific CTDI(vol) as normalization factors when using TCM. It was demonstrated that CTDI(vol),global is not an effective normalization factor in TCM exams where attenuation (and therefore tube current) varies considerably throughout the scan, such as abdomen/pelvis and even thorax. These exams can be more accurately assessed for dose using regional CTDI(vol) descriptors that account for local variations in scanner output present when TCM is employed.

FIG. 1.

(a) An example of a chest exam's TCM profile, illustrating the variation of tube current along patient's z-axis and within the axial plane (the high frequency component). (b) An example of an abdomen/pelvis exam's TCM profile, illustrating three different attenuating regions: lungs, abdomen, and pelvis. CTDI_vol reported by the scanner is based on the global average tube current (dashed line).

FIG. 2.

An example of an abdomen/pelvis exam's TCM profile, illustrating three different attenuating regions: lungs, abdomen, and pelvis. The horizontal dashed line through all three regions represents the average tube current over the entire scan length (i.e., global average tube current). The horizontal line at each section represents the average tube current over that specific section.

FIG. 3.

An example of a chest exam's TCM profile, illustrating three different attenuating regions: shoulder, low-attenuation region (encompasses most of the lungs, excluding shoulders, scapula, and abdomen). The horizontal dashed line through all three regions represents the average tube current over the entire scan length. The horizontal line at each section represents the average tube current over that specific section.

FIG. 4.

Absolute Monte Carlo simulated kidney doses (mGy) versus effective diameter (a) and CTDI_vol values versus effective diameter (b)–(d). In this small dataset with a small range of different sizes, TCM mode appears to result in lower doses for smaller patients and higher doses for larger patients.

FIG. 5.

For kidneys, (b) normalized dose by regional and organ-specific CTDI_vol show a higher correlation with size compared to (a) global CTDI_vol. Similar improvements were seen for liver.

FIG. 6.

Absolute Monte Carlo simulated breast dose (mGy) versus effective diameter (a) and CTDI_vol values versus effective diameter (b)–(d). Similar to the abdominal organs and abdomen/pelvis CTDI_vol values, for this small number of patients TCM appears to result in lower doses for smaller and higher doses for larger patients.

FIG. 7.

For breasts, (b) normalized organ dose by regional and organ-specific CTDI_vol show a higher correlation with patient size than (a) global CTDI_vol.

FIG. 8.

Gender-specific analysis performed on the lung data. Absolute lung doses (a) and CTDI_vol (b) values are separated into females and males and illustrated versus effective diameter. A visible separation between females and males is demonstrated by the absolute lung doses; however, CTDI_vol values do not depict a similar conclusion.

FIG. 9.

Gender-specific analysis performed on the lung data. Lung doses are separated into females and males and normalized by global (a), regional (b), and lung-specific CTDI_vol (c). All three plots demonstrate an obvious segregation between females and males in the chest models. This gender separation is not seen in the abdominal organ liver (d).