Image quality of microcalcifications in digital breast tomosynthesis: effects of projection-view distributions

Abstract

Purpose: To analyze the effects of projection-view (PV) distribution on the contrast and spatial blurring of microcalcifications on the tomosynthesized slices (X-Y plane) and along the depth (Z) direction for the same radiation dose in digital breast tomosynthesis (DBT).

Methods: A GE GEN2 prototype DBT system was used for acquisition of DBT scans. The system acquires PV images from 21 angles in 3° increments over a ±30° range. From these acquired PV images, the authors selected six subsets of PV images to simulate DBT of different angular ranges and angular increments. The number of PV images in each subset was fixed at 11 to simulate a constant total dose. These different PV distributions were subjectively divided into three categories: uniform group, nonuniform central group, and nonuniform extreme group with different angular ranges and angular increments. The simultaneous algebraic reconstruction technique (SART) was applied to each subset to reconstruct the DBT slices. A selective diffusion regularization method was employed to suppress noise. The image quality of microcalcifications in the reconstructed DBTs with different PV distributions was compared using the DBT scans of an American College of Radiology phantom and three human subjects. The contrast-to-noise ratio (CNR) and the full width at half maximum (FWHM) of the line profiles of microcalcifications within their in-focus DBT slices (parallel to detector plane) and the FWHMs of the interplane artifact spread function (ASF) in the Z-direction (perpendicular to detector plane) were used as image quality measures.

Results: The results indicate that DBT acquired with a large angular range or, for an equal angular range,with a large fraction of PVs at large angles yielded superior ASF with smaller FWHM in the Z-direction. PV distributions with a narrow angular range or a large fraction of PVs at small angles had stronger interplane artifacts. In the X-Y focal planes, the effect of PV distributions on spatial blurring depended on the directions. In the X-direction (perpendicular to the chestwall), the normalized line profiles of the calcifications reconstructed with the different PV distributions were similar in terms of FWHM; the differences in the FWHMs between the different PV distributions were less than half a pixel. In the Y-direction (x-ray source motion), the normalized line profiles of the calcifications reconstructed with PVs acquired with a narrow angular range or a large fraction of PVs at small angles had smaller FWHMs and thus less blurring of the line profiles. In addition, PV distributions with a narrow angular range or a large fraction of PVs at small angles yielded slightly higher CNR than those with a wide angular range for small, subtle microcalcifications; however, PV distributions had no obvious effect on CNR for relatively large microcalcifications.

Conclusions: PV distributions affect the image quality of DBT. The relative importance of the impact depends on the characteristics of the signal and the direction (perpendicular or parallel) relative to the direction of x-ray source motion. For a given number of PVs, the angular range and the distribution of the PVs affect the degree of in-plane and interplane blurring in opposite ways. The design of the scan parameters of tomosynthesis systems would require proper consideration of the characteristics of the signals of interest and the potential trade-off of the image quality of different types of signals.

Figure 1

Geometry of the GE prototype GEN2 digital breast tomosynthesis system used in this study. In our DBT reconstruction, the coordinate system is oriented such that the X-direction was perpendicular to the x-ray source motion direction, the Y-direction was parallel to the x-ray source motion direction, the X-Y plane was parallel to the detector plane, and the Z-direction was perpendicular to the detector plane.

Figure 2

Six different subsets of PV images and their distributions: (column 1) uniform group, (column 2) non-uniform central group, and (column 3) non-uniform extreme group.

Figure 3

Regions of interest from a DBT slice of the ACR phantom reconstructed from 21 PVs by SART with selective diffusion regularization showing (a) the first, (b) the third, and (c) the fourth speck groups with nominal speck size of 0.54, 0.32, and 0.24 mm, respectively, selected for analysis in this study.

Figure 4

Comparison of the CNR using different PV distributions. The CNR of signals presented in Fig. 3 for three clusters of simulated microcalcifications selected from the ACR phantom images reconstructed by SART with selective diffusion regularization are compared. All values of signal 1 (nominal size 0.54 mm) and signal 2 (nominal size 0.32 mm) were obtained by averaging six repeated measurements. All values of signal 3 were obtained by averaging five simulated microcalcifications in the speck group of nominal size 0.24 mm and six repeated measurements. The error bars indicate one standard deviation of the measurements. Six subsets and the full set of projection views are compared. Data points of the same signal are connected by lines to facilitate reading the graph, not to indicate functional relationships.

Figure 5

Comparison of image blurring in three directions for the different PV distributions. The FWHMs of the line profiles in the (a) X-direction and (b) Y-direction on the X-Y focal planes and the FWHMs of the ASF in the (c) Z-direction for three clusters of simulated calcifications selected from the ACR phantom images reconstructed by SART with selective diffusion regularization are compared. The side length of one pixel is 0.1 mm. All values of signal 1 and signal 2 were obtained by averaging six repeated measurements. All values of signal 3 were obtained by averaging five simulated microcalcifications in the speck group of nominal size 0.24 mm and six repeated measurements. The error bars indicate one standard deviation of the measurements. Six subsets and the full set of projection views are compared.

Figure 6

Regions of interest from the in-focus DBT slices intersecting a microcalcification in three human subjects selected for analysis in this study. The DBT slices were reconstructed from 21 PVs by SART with selective diffusion regularization.

Figure 7

Comparison of the CNR using different PV distributions. The CNR of relatively large microcalcifications selected from DBT scans of three human subjects reconstructed by SART with selective diffusion regularization are compared.

Figure 8

Comparison of image blurring in three directions for the different PV distributions. The FWHMs of the line profiles in the (a) X-direction and (b) Y-direction on the X-Y focal planes and the FWHMs of the ASF in the (c) Z-direction for three relatively large microcalcifications selected from DBT of three human subjects reconstructed by SART with selective diffusion regularization are compared. The pixel size is 0.1 mm × 0.1 mm.