In digital breast tomosynthesis (DBT), the reconstruction is calculated from x-ray projection images acquired over a small range of angles. One step in the reconstruction process is to identify the pixels that fall outside the shadow of the breast, to segment the breast from the background (air). In each projection, rays are back-projected from these pixels to the focal spot. All voxels along these rays are identified as air. By combining these results over all projections, a breast outline can be determined for the reconstruction. This paper quantifies the accuracy of this breast segmentation strategy in DBT. In this study, a physical phantom modeling a breast under compression was analyzed with a prototype next-generation tomosynthesis (NGT) system described in previous work. Multiple wires were wrapped around the phantom. Since the wires are thin and high contrast, their exact location can be determined from the reconstruction. Breast parenchyma was portrayed outside the outline defined by the wires. Specifically, the size of the phantom was overestimated along the posteroanterior (PA) direction; i.e., perpendicular to the plane of conventional source motion. To analyze how the acquisition geometry affects the accuracy of the breast outline segmentation, a computational phantom was also simulated. The simulation identified two ways to improve the segmentation accuracy; either by increasing the angular range of source motion laterally or by increasing the range in the PA direction. The latter approach is a unique feature of the NGT design; the advantage of this approach was validated with our prototype system.
Our next-generation tomosynthesis (NGT) system prototype introduces additional geometric movements to conventional Digital Breast Tomosynthesis (DBT) acquisition geometries, to provide isotropic super-resolution. These movements include x-ray source movement in the posteroanterior (PA) direction and detector movement in the z-direction (perpendicular to the breast support). The desired benefits of the NGT system are only achievable with precise geometric calibration. In our previous work, a geometric phantom with 24 point-like ball bearings (BB’s) at four different magnifications was designed and a geometric calibration method that minimizes the difference between the projected locations and the calculated locations of BB’s was tested. This study investigates a new calibration method using the same phantom, utilizing projected 2D equations of virtual line segments created by any two BB’s for more precise reconstruction of the various acquisition modes of the NGT system. The geometric parameters were solved with two approaches: (1) solving each projection individually and (2) solving all projections simultaneously. Furthermore, two algorithms to compensate for any possible inaccuracy in BB locations within the phantom, presumably by less than desired manufacturing precision, were developed and compared: (1) manually identifying and removing poorly positioned BB’s and (2) performing an iteration to re-calculate the BB locations. Magnification digital breast tomosynthesis was also performed to test the calibration method further. Tomographic image reconstructions successfully demonstrated isotropic super-resolution and magnified super-resolution.
A next generation tomosynthesis (NGT) system has been proposed to obtain higher spatial resolution than traditional digital breast tomosynthesis (DBT) by achieving consistent sub-pixel resolution. Resolution and linear acquisition artifacts can be further improved by creating multi-axis, x-ray tube acquisition paths. This requires synchronization of the x-ray generator, x-ray detector, and motion controller for an x-ray tube motion path composed of arbitrarily spaced x-ray projection points. We have implemented a state machine run on an Arduino microcontroller that synchronizes the system processes through hardware interrupts. The desired x-ray projection points are converted into two-dimensional motion segments that are compiled to the motion controller’s memory. The state machine then signals the x-ray tube to move from one acquisition point to another, exposing x-rays at each point, until every acquisition is made. The effectiveness of this design was tested based on speed of procedure and image quality metrics. The results show that the average procedure time, over 15 test runs for three different paths, took under 20 seconds, which is far superior to previous acquisition methods on the NGT system. In conclusion, this study shows that a state machine implementation is viable for fast and accurate acquisitioning in NGT systems.
Computed super-resolution (SR) is a method of reconstructing images with pixels that are smaller than the detector element size; superior spatial resolution is achieved through the elimination of aliasing and alteration of the sampling function imposed by the reconstructed pixel aperture. By comparison, magnification mammography is a method of projection imaging that uses geometric magnification to increase spatial resolution. This study explores the development and application of magnification digital breast tomosynthesis (MDBT). Four different acquisition geometries are compared in terms of various image metrics. High-contrast spatial resolution was measured in various axes using a lead star pattern. A modified Defrise phantom was used to determine the low-frequency spatial resolution. An anthropomorphic phantom was used to simulate clinical imaging. Each experiment was conducted at three different magnifications: contact (1.04x), MAG1 (1.3x), and MAG2 (1.6x). All images were taken on our next generation tomosynthesis system, an in-house solution designed to optimize SR. It is demonstrated that both computed SR and MDBT (MAG1 and MAG2) provide improved spatial resolution over non-SR contact imaging. To achieve the highest resolution, SR and MDBT should be combined. However, MDBT is adversely affected by patient motion at higher magnifications. In addition, MDBT requires more radiation dose and delays diagnosis, since MDBT would be conducted upon recall. By comparison, SR can be conducted with the original screening data. In conclusion, this study demonstrates that computed SR and MDBT are both viable methods of imaging the breast.
A method for geometric calibration of a next-generation tomosynthesis (NGT) system is proposed and tested. The NGT system incorporates additional geometric movements between projections over conventional DBT. These movements require precise geometric calibration to support magnification DBT and isotropic SR. A phantom was created to project small tungsten-carbide ball bearings (BB’s) onto the detector at four different magnifications. Using a bandpass filter and template matching, a MATLAB program was written to identify the centroid locations of each BB projection on the images. An optimization algorithm calculated an effective location for the source and detector that mathematically projected the BB’s onto the same locations on the detector as found on the projection images. The average distance between the BB projections on the image and the mathematically computed projections was 0.11 mm. The effective locations for the source and detector were encoded in the DICOM file for each projection; these were then used by the reconstruction algorithm. Tomographic image reconstructions were performed for three acquisition modes of the NGT system; these successfully demonstrated isotropic SR, magnified SR, and oblique reconstruction.
Multiplanar reconstruction (MPR) in digital breast tomosynthesis (DBT) allows tomographic images to be portrayed in
various orientations. We have conducted research to determine the resolution of tomosynthesis MPR. We built a
phantom that houses a star test pattern to measure resolution. This phantom provides three rotational degrees of freedom.
The design consists of two hemispheres with longitudinal and latitudinal grooves that reference angular increments.
When joined together, the hemispheres form a dome that sits inside a cylindrical encasement. The cylindrical
encasement contains reference notches to match the longitudinal and latitudinal grooves that guide the phantom’s
rotations. With this design, any orientation of the star-pattern can be analyzed. Images of the star-pattern were acquired
using a DBT mammography system at the Hospital of the University of Pennsylvania. Images taken were reconstructed
and analyzed by two different methods. First, the maximum visible frequency (in line pairs per millimeter) of the star
test pattern was measured. Then, the contrast was calculated at a fixed spatial frequency. These analyses confirm that
resolution decreases with tilt relative to the breast support. They also confirm that resolution in tomosynthesis MPR is
dependent on object orientation. Current results verify that the existence of super-resolution depends on the orientation
of the frequency; the direction parallel to x-ray tube motion shows super-resolution. In conclusion, this study
demonstrates that the direction of the spatial frequency relative to the motion of the x-ray tube is a determinant of
resolution in MPR for DBT.