Estimating organ doses from tube current modulated CT examinations using a generalized linear model

Abstract

Purpose: Currently, available Computed Tomography dose metrics are mostly based on fixed tube current Monte Carlo (MC) simulations and/or physical measurements such as the size specific dose estimate (SSDE). In addition to not being able to account for Tube Current Modulation (TCM), these dose metrics do not represent actual patient dose. The purpose of this study was to generate and evaluate a dose estimation model based on the Generalized Linear Model (GLM), which extends the ability to estimate organ dose from tube current modulated examinations by incorporating regional descriptors of patient size, scanner output, and other scan-specific variables as needed.

Methods: The collection of a total of 332 patient CT scans at four different institutions was approved by each institution's IRB and used to generate and test organ dose estimation models. The patient population consisted of pediatric and adult patients and included thoracic and abdomen/pelvis scans. The scans were performed on three different CT scanner systems. Manual segmentation of organs, depending on the examined anatomy, was performed on each patient's image series. In addition to the collected images, detailed TCM data were collected for all patients scanned on Siemens CT scanners, while for all GE and Toshiba patients, data representing z-axis-only TCM, extracted from the DICOM header of the images, were used for TCM simulations. A validated MC dosimetry package was used to perform detailed simulation of CT examinations on all 332 patient models to estimate dose to each segmented organ (lungs, breasts, liver, spleen, and kidneys), denoted as reference organ dose values. Approximately 60% of the data were used to train a dose estimation model, while the remaining 40% was used to evaluate performance. Two different methodologies were explored using GLM to generate a dose estimation model: (a) using the conventional exponential relationship between normalized organ dose and size with regional water equivalent diameter (WED) and regional CTDI_vol as variables and (b) using the same exponential relationship with the addition of categorical variables such as scanner model and organ to provide a more complete estimate of factors that may affect organ dose. Finally, estimates from generated models were compared to those obtained from SSDE and ImPACT.

Results: The Generalized Linear Model yielded organ dose estimates that were significantly closer to the MC reference organ dose values than were organ doses estimated via SSDE or ImPACT. Moreover, the GLM estimates were better than those of SSDE or ImPACT irrespective of whether or not categorical variables were used in the model. While the improvement associated with a categorical variable was substantial in estimating breast dose, the improvement was minor for other organs.

Conclusions: The GLM approach extends the current CT dose estimation methods by allowing the use of additional variables to more accurately estimate organ dose from TCM scans. Thus, this approach may be able to overcome the limitations of current CT dose metrics to provide more accurate estimates of patient dose, in particular, dose to organs with considerable variability across the population.

Keywords: CT; Monte Carlo simulations; generalized linear model; organ dose estimation; tube current modulation.

Figure 1

A visual illustration of the range of patient sizes used in this study.

Figure 2

Graphic illustration of pooled reference organ dose values across all three CT scanner systems normalized by global CTDI _vol versus global WED and the corresponding fit of the data.

Figure 3

Graphic illustration of pooled reference organ dose values across all three CT scanner systems normalized by regional CTDI _vol versus regional WED and the corresponding fit of the data.

Figure 4

Reference organ dose values versus regional WED showing actual simulated organ doses for individual CT scanner manufacturers.

Figure 5

Reference organ dose values normalized by regional CTDI _vol versus regional WED. As compared to Fig. 4, once organ dose is normalized by regional CTDI _vol the dose variability among CT scanner manufacturers decreases.

Figure 6

Organ‐specific fits for Siemens data shown along with reference organ dose values normalized by regional CTDI _vol versus regional WED. R² values for each organ‐specific fit is listed in Table 3 under Siemens for regional CTDI _vol as normalization factor and regional WED as the size metric. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 7

Organ‐specific fits for GE dataset shown along with reference organ dose values normalized by regional CTDI _vol versus regional WED. R² values for each organ‐specific fit is listed in Table 3 under GE for regional CTDI _vol as normalization factor and regional WED as the size metric. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8

Organ‐specific fits for Toshiba dataset shown along with reference organ dose values normalized by regional CTDI _vol versus regional WED. R² values for each organ‐specific fit is listed in Table 3 under Toshiba for regional CTDI _vol as normalization factor and regional WED as the size metric. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 9

Organ‐specific fits for pooled dataset (Siemens + GE + Toshiba) dataset shown along with reference organ dose values normalized by regional CTDI _vol versus regional WED. R² values for each organ‐specific fit is listed in Table 3 under Pooled for regional CTDI _vol as normalization factor and regional WED as the size metric. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 10

Organ dose estimation model constructed using the GLM with the categorical variable “Breasts”.

Figure 11

Mean percent difference, including error bars, between the reference method (Monte Carlo simulations) and each dose estimation model for each organ. The plus signs represent P<0.05, indicating statistically significant difference between the estimates calculated using the model and the reference method, Monte Carlo simulations.

Figure 12

Typical z‐axis‐only TCM functions of thorax (left) and abdomen/pelvis exams (right) shown for Siemens, GE, and Toshiba for three patients with similar regional WED.

Figure 13

(a) Simulated absolute organ doses for pediatric and adult patients scanned on Toshiba scanner at different sites versus WED. (b) Simulated organ doses normalized by CTDI _vol,Regional and shown as a function of WED.