Dedicated cone-beam breast CT: Data acquisition strategies based on projection angle-dependent normalized glandular dose coefficients

Background: Dedicated cone-beam breast computed tomography (CBBCT) using short-scan acquisition is being actively investigated to potentially reduce the radiation dose to the breast. This would require determining the optimal x-ray source trajectory for such short-scan acquisition. Purpose: To quantify the projection angle-dependent normalized glandular dose coefficient ((Formula presented.)) in CBBCT, referred to as angular (Formula presented.), so that the x-ray ray source trajectory that minimizes the radiation dose to the breast for short-scan acquisition can be determined. Materials and Methods: A cohort of 75 CBBCT clinical datasets was segmented and used to generate three breast models – (I) patient-specific breast with heterogeneous fibroglandular tissue distribution and real breast shape, (II) patient-specific breast shape with homogeneous tissue distribution and matched fibroglandular weight fraction, and (III) homogeneous semi-ellipsoidal breast with patient-specific breast dimensions and matched fibroglandular weight fraction, which corresponds to the breast model used in current radiation dosimetry protocols. For each clinical dataset, the angular (Formula presented.) was obtained at 10 discrete angles, spaced 36° apart, for full-scan, circular, x-ray source trajectory from Monte Carlo simulations. Model III is used for validating the Monte Carlo simulation results. Models II and III are used to determine if breast shape contributes to the observed trends in angular (Formula presented.). A geometry-based theory in conjunction with center-of-mass ((Formula presented.)) based distribution analysis is used to explain the projection angle-dependent variation in angular (Formula presented.). Results: The theoretical model predicted that the angular (Formula presented.) will follow a sinusoidal pattern and the amplitude of the sinusoid increases when the center-of-mass of fibroglandular tissue ((Formula presented.)) is farther from the center-of-mass of the breast ((Formula presented.)). It also predicted that the angular (Formula presented.) will be minimized at x-ray source positions complementary to the (Formula presented.). The (Formula presented.) was superior to the (Formula presented.) in 80% (60/75) of the breasts. From Monte Carlo simulations and for homogeneous breasts (models II and III), the deviation in breast shape from a semi-ellipsoid had minimal effect on angular (Formula presented.) and showed less than 4% variation. From Monte Carlo simulations and for model I, as predicted by our theory, the angular (Formula presented.) followed a sinusoidal pattern with maxima and minima at x-ray source positions superior and inferior to the breast, respectively. For model I, the projection angle-dependent variation in angular (Formula presented.) was 16.4%. Conclusion: The heterogeneous tissue distribution affected the angular (Formula presented.) more than the breast shape. For model I, the angular (Formula presented.) was lowest when the x-ray source was inferior to the breast. Hence, for short-scan CBBCT acquisition with (Formula presented.) aligned with axis-of-rotation, an x-ray source trajectory inferior to the breast is preferable and such an acquisition spanning 205° can potentially reduce the mean glandular dose by up to 52%.