Optimizing the Acquisition Geometry for Digital Breast Tomosynthesis Using the Defrise Phantom Raymond J. Acciavatti, Alice Chang, Laura Woodbridge, and Andrew D. A. Maidment University of Pennsylvania, Department of Radiology, 3400 Spruce St., Philadelphia PA 19104 E-mail: Raymond.Acciavatti@uphs.upenn.edu, calic@seas.upenn.edu, wolau@seas.upenn.edu, Andrew.Maidment@uphs.upenn.edu ABSTRACT In cone beam computed tomography (CT), it is common practice to use the Defrise phantom for image quality assessment. The phantom consists of a stack of plastic plates with low frequency spacing. Because the x-ray beam may traverse multiple plates, the spacing between plates can appear blurry in the reconstruction, and hence modulation provides a measure of image quality. This study considers the potential merit of using the Defrise phantom in digital breast tomosynthesis (DBT), a modality with a smaller projection range than CT. To this end, a Defrise phantom was constructed and subsequently imaged with a commercial DBT system. It was demonstrated that modulation is dependent on position and orientation in the reconstruction. Modulation is preserved over a broad range of positions along the chest wall if the input frequency is oriented in the tube travel direction. By contrast, modulation is degraded with increasing distance from the chest wall if the input frequency is oriented in the posteroanterior (PA) direction. A theoretical framework was then developed to model these results. Reconstructions were calculated in an acquisition geometry designed to improve modulation. Unlike current geometries in which the x-ray tube motion is restricted to the plane of the chest wall, we consider a geometry with an additional component of tube motion along the PA direction. In simulations, it is shown that the newly proposed geometry improves modulation at positions distal to the chest wall. In conclusion, this study demonstrates that the Defrise phantom is a tool for optimizing DBT systems. Keywords: Defrise phantom, image quality, modulation, digital breast tomosynthesis (DBT), reconstruction, anisotropy, analytical modeling, optimization. 1. INTRODUCTION It is common practice to use the Defrise phantom for assessing image quality in cone beam computed tomography (CT). One application of the Defrise phantom is comparing the benefits of various reconstruction algorithms in CT. 1 In this study, we consider the potential merit of using the Defrise phantom in digital breast tomosynthesis (DBT), which acquires projection images over a smaller angular range than CT. We have experimentally analyzed a Defrise phantom using a commercial DBT system (Selenia Dimensions, Hologic Inc., Bedford, MA). The phantom was built using a laser cutter to create 10 plastic rectangular plates (15.0 cm × 5.0 cm × 0.29 cm) with a constant air gap (0.29 cm). The phantom was placed at various positions along the chest wall side of the DBT system, with the plastic-air spacings oriented along the tube travel direction. Images were acquired at 33 kVp and 59 mAs with a W/Al target-filter combination. With the phantom centered at the chest wall, Figure 1(a) shows a slice along the mid-depth of the object using the Selenia Dimensions reconstruction algorithm. The resultant signal profile [Figure 2(a)] can be used to quantify the image quality. In particular, the peak-to-peak amplitude (Pk-Pk) measures the relative ability to discern the plastic-air spacings. Figure 2(b) illustrates that image quality is preserved at various positions along the chest wall. The change in Pk-Pk varies between +38.6% and –5.8% relative to the midpoint of the chest wall. To determine whether image quality is preserved in other directions, the Defrise phantom was rotated by 90°, so that the plastic-air spacings were oriented along the posteroanterior (PA) direction (i.e., the direction perpendicular to the plane of x-ray tube motion). Because different filtering is applied in this direction in the reconstruction, Pk-Pk is plotted on a different scale. Comparing Figure 1(b) with Figure 2(b) shows that Pk-Pk is degraded with increasing PA position, indicating poor image quality. The edges of the phantom also show blurring artifacts along this direction [Figure 1(b)]. Medical Imaging 2014: Physics of Medical Imaging, edited by Bruce R. Whiting, Christoph Hoeschen, Despina Kontos, Proc. of SPIE Vol. 9033, 903315 · © 2014 SPIE · CCC code: 1605-7422/14/$18 · doi: 10.1117/12.2043988 Proc. of SPIE Vol. 9033 903315-1 Downloaded From: http://spiedigitallibrary.org/ on 07/15/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx