A comparison of breast and lung doses from chest CT scans using organ-based tube current modulation (OBTCM) vs. Automatic tube current modulation (ATCM)

Abstract

Purpose: The purpose of this work was to estimate and compare breast and lung doses of chest CT scans using organ-based tube current modulation (OBTCM) to those from conventional, attenuation-based automatic tube current modulation (ATCM) across a range of patient sizes.

Methods: Thirty-four patients (17 females, 17 males) who underwent clinically indicated CT chest/abdomen/pelvis (CAP) examinations employing OBTCM were collected from two multi-detector row CT scanners. Patient size metric was assessed as water equivalent diameter (D_w ) taken at the center of the scan volume. Breast and lung tissues were segmented from patient image data to create voxelized models for use in a Monte Carlo transport code. The OBTCM schemes for the chest portion were extracted from the raw projection data. ATCM schemes were estimated using a recently developed method. Breast and lung doses for each TCM scenario were estimated for each patient model. CTDI_vol -normalized breast (nD_breast ) and lung (nD_lung ) doses were subsequently calculated. The differences between OBTCM and ATCM normalized organ dose estimates were tested using linear regression models that included CT scanner and D_w as covariates.

Results: Mean dose reduction from OBTCM in nD_breast was significant after adjusting for the scanner models and patient size (P = 0.047). When pooled with females and male patient, mean dose reduction from OBTCM in nD_lung was observed to be trending after adjusting for the scanner model and patient size (P = 0.085).

Conclusions: One specific manufacturer's OBTCM was analyzed. OBTCM was observed to significantly decrease normalized breast relative to a modeled version of that same manufacturer's ATCM scheme. However, significant dose savings were not observed in lung dose over all. Results from this study support the use of OBTCM chest protocols for females only.

Keywords: breast and lung dose; organ-based modulation; tube current modulation.

Fig. 1

a) Segmented image and b) voxelized patient model of a female patient who underwent clinically indicated CAP CT exam for use in MC simulations. The segmented image contains an outline of 120° fluence reduction zone.

Fig. 2

a) Extracted OBTCM (blue) and predicted ATCM (red) tube current schemes as a function of patient position overlaid atop of a patient topogram. Polar plots of b) OBTCM and c) ATCM as a function of tube gantry angle. For b) and c), the radial axes correspond to the tube current while the polar axes correspond to the gantry angle.

Fig. 3

Three‐dimensional rendering of a) OBTCM and b) predicted ATCM tube current schemes. Note that in b), for OBTCM, the tube current is reduced on the anterior surface along the entire length of the scan and not just over the breast region.

Fig. 4

Correlation of a) OBTCM CTDI_vol and b) ATCM CTDI_vol with respect to D_w.

Fig. 5

Plot of OBTCM CTDI_vol in relation to predicted ATCM CTDI_vol. The dashed line represents the correlation between OBTCM and ATCM CTDI_vol of which the coefficient of determination (R ²) was observed to be 0.58. The solid line represents the line of identity (CTDI_vol from OBTCM equal to CTDI_vol from ATCM).

Fig. 6

a) Coronal and b) axial dose distribution for patient #11 which had similar CTDI_vol values for the OBTCM (9.9 mGy) and estimated ATCM (11.4 mGy) exams. For this patient, there was a 9% breast dose saving but an 11% lung dose penalty. The green line in the coronal view represents the position where the axial slice was taken.