Evaluation of x‐ray scatter properties in a dedicated cone‐beam breast CT scanner

Abstract

The magnitude of scatter contamination on a first-generation prototype breast computed tomography (CT) scanner was evaluated using the scatter-to-primary ratio (SPR) metric. The SPR was measured and characterized over a wide range of parameters relevant to breast CT imaging, including x-ray beam energy, breast diameter, breast composition, isocenter-to-detector distance, collimated slot thickness, and grid ratio. The results demonstrated that in the absence of scatter reduction techniques, the SPR levels for the average breast (e.g., 14 cm diameter 50/50 composition cylindrical phantom) are quite high (approximately 0.5 at the center of the phantom for 80 kVp in true cone-beam CT geometry), and increases as the diameter of the phantom is increased (to approximately 1.0 at the center of a 18 cm diameter 50/50 phantom). The x-ray beam energy and the phantom compositions had only minimal impact on the measured SPR. When an ideal bowtie filter was used, the SPRs at the central axis of the 14 and 18 cm cylindrical phantoms were reduced while the SPRs at the edge of the phantoms were increased. Lastly, collimation in the vertical direction had a significant impact on the SPRs at the central axis of the phantoms. These high SPR levels might lead to cupping artifacts and increased noise in the reconstructed CT images, and this suggests that efficient scatter rejection and/or correction techniques may be required to improve the quality and accuracy of cone beam CT images.

Monte Carlo study of the effects of system geometry and antiscatter grids on cone‐beam CT scatter distributions

“…Other authors have shown that for some configurations, bowties are beneficial in terms of SPR reduction and image uniformity. 43,47,54,60 Bowties were omitted from the current study because they are not commonly used in all configurations studied here, to avoid further increasing the already large parameter space, and to maintain the focus of the current work on effects of system geometry in and of itself. Similarly, the simulations did not account for the effects of finite focal spot size.…”

“…There is therefore no simple, nominal choice of filter for each configuration that would permit a fair comparison across this range of geometry, object size, and kVp. Rather, the design of bowtie filters optimal to a particular configuration is a continuing area of work, with results indicating potential advantages in x-ray scatter for some configurations (e.g., breast CT 43 and Linac CBCT 47,60 ). For simplicity in an already broad space of parameters considered below, the simulations considered scenarios without a bowtie.…”

“…In musculoskeletal extremity imaging, Zbijewski et al 3 found ∼20% scatter-induced cupping and showed that antiscatter grids significantly reduce such artifact and improve contrast-to-noise ratio (CNR) without increase in imaging dose. In breast imaging, Kwan et al 43 measured ∼50% SPR at the detector for an average-size breast (14 cm diameter) and showed the benefit of a bowtie filter in reducing scatter. For mobile C-arm imaging, Schafer et al 44 showed that an antiscatter grid did not offer an improvement in CNR per unit square root dose.…”

Methodology for generating a 3D computerized breast phantom from empirical data

“…The gantry rotates over 360°in the horizontal plane around the breast and acquires 500 conebeam projection images in 16.6 s. Each breast is scanned separately and the mean fibroglandular radiation dose delivered to the breast was constrained to be the same as twoview mammography. [16][17][18][19][20][21][22] The low-dose acquisition of the breast data as well as the cone-beam geometry of the CT system results in image degradation due to scatter radiation and considerable quantum noise. Therefore correction of scatter and noise is a necessary part of this project because the nonuniformity of the background may cause the denser tissue ͑fibroglandular tissue͒ values to be lower than the less-dense ͑adipose tissue͒ values and vice versa.…”

“…There are several groups currently researching dedicated breast CT at the University of California at Davis, [16][17][18][19][20][21][22] Duke University, [23][24][25][26][27][28][29][30][31] University of Rochester, [32][33][34][35][36] University of Massachusetts Medical School, [37][38][39] and University of Texas M.D. Anderson Cancer Center.…”