The feasibility of a regional CTDI_vol 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 vol (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 tu...

Radiation Dose to the Breast by 64-slice CT

“…The organ is affected not only by the scanner radiation output level, usually characterized by CTDI, but also by various other factors, such as patient size, study protocol, scanner model, and x-ray energy spectrum. A number of studies have been conducted with the aim of developing a robust yet simple method with acceptable accuracy for organ dose estimation (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). These studies employed either experimental methods using physical phantoms or numerical methods using validated Monte Carlo programs, although the majority of them were based on the latter because of the flexibility of simulation.…”

Practical dose point‐based methods to characterize dose distribution in a stationary elliptical body phantom for a cone‐beam C‐arm CT system

“…7. Comparison of the performances of the proposed methods as a function of the number of dose points used (4,5,6,7,14, and 23). The differences of CTDI w (1/3, 2/3) and CTDI w (1/2, 1/2) were 8.7% (±2.8%) and 6.8% (±1.0%), respectively.…”

“…9,10 The mean dose and the dose distribution over the central phantom plane (i.e., z = 0, dose maximum image) are valuable in that they enable cross-CT scanner or cross-acquisition protocol comparisons of radiation dose levels. Previous studies [12][13][14] have explored estimation approaches for systemindependent organ doses utilizing the CTDI-based mean dose such as weighted CTDI (CTDI w ). Although the mean dose is valuable, no analytic expression is available for the mean dose and the dose distribution in the PMMA phantom.…”

“…The planar dose distributions were then estimated using subsets of the point dose measurements using two proposed methods: (1) the proximity-based weighting method (method 1) and (2) the dose point surface fitting method (method 2). Twenty-eight different dose point distributions with six different point number cases (4,5,6,7,14, and 23 dose points) were evaluated to determine the optimal number of dose points and their placement in the phantom. The performances of the methods were determined by comparing their results with those of the validated MC simulations.…”

Size-specific dose estimates: Localizer or transverse abdominal computed tomography images?

“…Recently, the American Association of Physicists in Medicine (AAPM) report 204 [11] , has proposed a new method, "size specific dose estimate" (SSDE) to represent more accurate estimations of patient doses. SSDE takes into account patient size in order to enable users to optimize CTDIvol based on patient's physical dimensions [12][13][14][15][16][17] . Look-up tables of this report provide conversion factors that can be applied on CTDIvol to calculate SSDE for appropriate phantom sizes (16 and 32 cm).…”