The purpose of this study is to evaluate the performance of an antiscatter grid and its potential benefit on image quality for a full-field digital mammography ͑FFDM͒ detector geometry at energies typical for temporal subtraction contrast-enhanced ͑CE͒ breast imaging. The signal intensities from primary, scatter, and glare were quantified in images acquired with an a-Si/ CsI͑Tl͒ FFDM detector using a Rh target and a 0.27 mm Cu filter at tube voltages ranging from 35 to 49 kV. Measurements were obtained at the center of the irradiation region of 20-80 mm thick breastequivalent phantoms. The phantoms were imaged with and without an antiscatter grid. Based on these data, the performance of the antiscatter grid was determined by calculating the primary and scatter transmission factors ͑T P and T S ͒ and Bucky factors ͑B f ͒. In addition, glare-to-primary ratios ͑GPRs͒ and scatter-to-primary ratios ͑SPRs͒ were quantified. The effect of the antiscatter grid on the signal-difference-to-noise ratio ͑SDNR͒ was also assessed. It was found that T P increases with kV but does not depend on the phantom thickness; T P values between 0.81 and 0.84 were measured. T S increases with kV and phantom thickness; T S values between 0.13 and 0.21 were measured. B f decreases with kV and increases with phantom thickness; B f ranges from 1.4 to 2.1. GPR is nearly constant, varying from 0.10 to 0.11. SPR without an antiscatter grid ͑SPR − ͒ ranges from 0.35 to 1.34. SPR − decreases by approximately 9% from 35 to 49 kV for a given phantom thickness and is 3.5 times larger for an 80 mm thick breast-equivalent phantom than for a 20 mm thick breastequivalent phantom. SPR with an antiscatter grid ͑SPR + ͒ ranges from 0.06 to 0.31. SPR + increases by approximately 23% from 35 to 49 kV for a given phantom thickness; SPR + is four times larger for an 80 mm breast-equivalent phantom than for a 20 mm breast-equivalent phantom. When imaging a 25 mm PMMA plate at the same mean glandular dose with and without an antiscatter grid, the SDNR is 4% greater with a grid than without. For an 75mm PMMA plate, the SDNR is 20% greater with a grid. In conclusion, at the higher x-ray energy range used for CE-DM and CE-DBT, an antiscatter grid significantly reduces SPR and improves SDNR. These effects are most pronounced for thick breasts.
Feature tracking was developed to efficiently compute motion measurements from volumetric ultrasound images. Prior studies have demonstrated the motion magnitude accuracy and computation speed of feature tracking. However, the previous feature tracking implementations were limited by performance of their calculations in rectilinear coordinates. Also, the previous feature tracking approaches did not fully explore the three dimensional (3- D) nature of volumetric image analysis or utilize the 3-D directional information from the tracking calculations. This study presents an improved feature tracking method which achieves further computation speed gains by performing all calculations in the native spherical coordinates of the 3-D ultrasound image. The novel method utilizes a statistical analysis of tracked directions of motion to achieve better rejection of false tracking matches. Results from in vitro tracking of a speckle target show that the new feature tracking method is significantly faster than correlation search and can accurately determine target motion magnitude and 3-D direction.
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