KEYWORDS: Image segmentation, Breast, Tissues, Cancer, Digital breast tomosynthesis, Computer simulations, Signal to noise ratio, Signal attenuation, Motion models, X-rays
Our lab has built a next-generation tomosynthesis (NGT) system utilizing scanning motions with more degrees of freedom than clinical digital breast tomosynthesis systems. We are working toward designing scanning motions that are customized around the locations of suspicious findings. The first step in this direction is to demonstrate that these findings can be detected with a single projection image, which can guide the remainder of the scan. This paper develops an automated method to identify findings that are prone to be masked. Perlin-noise phantoms and synthetic lesions were used to simulate masked cancers. NGT projections of phantoms were simulated using ray-tracing software. The risk of masking cancers was mapped using the ground-truth labels of phantoms. The phantom labels were used to denote regions of low and high risk of masking suspicious findings. A U-Net model was trained for multiclass segmentation of phantom images. Model performance was quantified with a receiver operating characteristic (ROC) curve using area under the curve (AUC). The ROC operating point was defined to be the point closest to the upper left corner of ROC space. The output predictions showed an accurate segmentation of tissue predominantly adipose (mean AUC of 0.93). The predictions also indicate regions of suspicious findings; for the highest risk class, mean AUC was 0.89, with a true positive rate of 0.80 and a true negative rate of 0.83 at the operating point. In summary, this paper demonstrates with virtual phantoms that a single projection can indeed be used to identify suspicious findings.
Objects created by 3D printers are increasingly used in various medical applications. Today, affordable 3D printers, using Fused Deposition Modeling are widely available. In this project, a commercially available 3D printer was used to replicate a conventional radiographic contrast detail phantom. Printing materials were selected by comparing their x-ray attenuation properties. Two replicas were printed using polylactic acid, with different filling patterns. The printed phantoms were imaged by a clinical mammography system, using automatic exposure control. Phantom images were visually and quantitively compared to images of the corresponding conventional contrast detail phantom. Visual scoring of the contrast detail elements was performed by a medical physics student. Contrast-to-noise ratio (CNR) was calculated for each phantom element. The diameter and thickness of the smallest visible phantom object were 0.44 mm and 0.09 mm, respectively, for both filling patterns. For the conventional phantom, the diameter and thickness of the smallest visible object were 0.31 mm and 0.09 mm. Visual inspection of printed phantoms revealed some linear artefacts. These artefacts were however not visible on mammographic projections. Quantitively, average CNR of printed phantom objects followed the same trend with an increase of average CNR with increasing disk height. However, there is a limitation of detail objects with disk diameters below 1.25 mm, caused by the available nozzle size. Based upon the encouraging results, future work will explore the use of different materials and smaller nozzle diameters.
KEYWORDS: Digital breast tomosynthesis, Image quality, Breast, Image restoration, Prototyping, 3D image reconstruction, Sensors, X-rays, X-ray sources, Super resolution
A next generation tomosynthesis (NGT) prototype has been developed to investigate alternative scanning geometries for digital breast tomosynthesis (DBT). Performance of NGT acquisition geometries is evaluated to validate previous phantom experiments. Two custom NGT acquisition geometries were compared to a conventional DBT geometry. Noise power spectra are used to describe features of specimen image reconstructions and compare acquisition geometries. NGT acquisition geometries improve high-frequency performance with superior isotropic super resolution, reduced out-of-plane blurring, and better overall reconstruction quality. NGT combines benefits of narrow- and wide-angle tomosynthesis in a single scan improving high-frequency spatial resolution and out-of-plane blurring, respectively.
The mathematical underpinnings of a novel reconstruction algorithm are presented that can facilitate 4D tomosynthesis for the purpose of guiding needle breast biopsies in real-time. Conventional tomosynthesis reconstruction algorithms produce motion artifacts when applied to a continuous tomosynthesis acquisition of a moving biopsy needle. The novel algorithm proposed in this work successfully overcomes this by using differences in slow-scan data to identify variational regions in the reconstructed volume, and adaptively reconstruct those regions to eliminate motion. The algorithm has been tested using simulated images, where reconstructed images of a moving needle had significantly better clarity than the conventional algorithm.
The impact of the angular range in conventional DBT is a trade-off in image quality; increasing angular range improves in-depth resolution and isotropic sampling across the detector, but compromises in-plane resolution. Our next generation tomosynthesis (NGT) system is capable of two-dimensional source trajectory and incorporates narrow- and wide-angle acquisition in orthogonal directions for a single tomosynthesis scan. In this work, performance of NGT geometries for high- and low-frequency objects across the detector was evaluated via computer simulations. We showed that NGT geometries preserve high in-plane resolution and present highly isotropic sampling, thus combine the benefits of narrow- and wide-angle tomosynthesis.
KEYWORDS: Breast, Digital breast tomosynthesis, 3D modeling, Skin, Fractal analysis, Tissues, Prototyping, Computer simulations, Signal attenuation, Principal component analysis
Virtual clinical trials (VCTs) have been used widely to evaluate digital breast tomosynthesis (DBT) systems. VCTs require realistic simulations of the breast anatomy (phantoms) to characterize lesions and to estimate risk of masking cancers. This study introduces the use of Perlin-based phantoms to optimize the acquisition geometry of a novel DBT prototype. These phantoms were developed using a GPU implementation of a novel library called Perlin-CuPy. The breast anatomy is simulated using 3D models under mammography cranio-caudal compression. In total, 240 phantoms were created using compressed breast thickness, chest-wall to nipple distance, and skin thickness that varied in a {[35, 75], [59, 130), [1.0, 2.0]} mm interval, respectively. DBT projections and reconstructions of the phantoms were simulated using two acquisition geometries of our DBT prototype. The performance of both acquisition geometries was compared using breast volume segmentations of the Perlin phantoms. Results show that breast volume estimates are improved with the introduction of posterior-anterior motion of the x-ray source in DBT acquisitions. The breast volume is overestimated in DBT, varying substantially with the acquisition geometry; segmentation errors are more evident for thicker and larger breasts. These results provide additional evidence and suggest that custom acquisition geometries can improve the performance and accuracy in DBT. Perlin phantoms help to identify limitations in acquisition geometries and to optimize the performance of the DBT prototypes.
KEYWORDS: Fourier transforms, Digital breast tomosynthesis, Super resolution, Breast, Anisotropy, Modulation transfer functions, Contrast transfer function, X-rays, X-ray sources, Image quality, Digital x-ray imaging
Our prototype next-generation tomosynthesis (NGT) system is a tool for investigating novel acquisition geometries in digital breast tomosynthesis (DBT). One such geometry is a non-isocentric acquisition in which the detector descends in the superior-to-inferior direction during the scan. The advantage of this geometry is examined through analysis of super-resolution (SR) with a high-frequency test pattern. In clinical DBT reconstructions, SR is only achieved if the test frequency is oriented parallel with the direction of source motion. In the non-isocentric geometry, SR can be achieved isotropically; that is, for all orientations of the test pattern.
KEYWORDS: Signal to noise ratio, Chest, Tissues, X-ray sources, Digital breast tomosynthesis, Visibility, Virtual reality, Prototyping, Physics, Clinical trials
It is standard for the x-ray source in conventional digital breast tomosynthesis (DBT) acquisitions to move strictly along the chest wall of the patient. A prototype, next-generation tomosynthesis (NGT) system has been developed that is capable of acquiring customized geometries with source motion parallel and perpendicular to the chest wall. One well-known consequence of acquiring projections with the x-ray source anterior to the chest wall is that a small volume of tissue adjacent to the chest wall is missed. Here we evaluate strategies in DBT to avoid missing tissue while improving overall image quality. Acquisition geometries tested in this study include the conventional (control), “T-shape,” and “bowtie” geometries. To evaluate the impact of moving the x-ray source away from the chest wall, the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured as a function of location within the reconstructed volume. Using simulations and physical experiments, the SNR and CNR were compared with conventional DBT. Simulations of two different phantoms were performed: a “tube” phantom and a “lattice” phantom. Experiments with uniform and textured phantoms were also conducted. While the image quality was slightly reduced immediately adjacent to the chest wall, there was no missed tissue and both the T-shape and Bowtie geometries exhibited SNR and CNR improvement over the vast majority of the reconstruction volume; the overall result being an improvement in image quality with both the T-shape and bowtie geometries.
KEYWORDS: Iterative methods, Interference (communication), Digital breast tomosynthesis, Calibration, Systems modeling, Denoising, X-rays, Transform theory, Data analysis
The majority of the denoising algorithms available in the literature are designed to treat signal-independent Gaussian noise. However, in digital breast tomosynthesis (DBT) systems, the noise model seldom presents signal-independence. In this scenario, variance-stabilizing transforms (VSTs) may be used to convert the signaldependent noise into approximately signal-independent noise, enabling the use of ‘off-the-shelf’ denoising techniques. The accurate stabilization of the noise variance requires a robust estimation of the system’s noise coefficients, usually obtained using calibration data. However, practical issues often arise when calibration data are required, impairing the clinical deployment of algorithms that rely on variance stabilization. In this work, we present a practical method to achieve variance stabilization by approaching it as an optimization task, with the stabilized noise variance dictating the cost function. An iterative method is used to implicitly optimize the coefficients used in the variance stabilization, leveraging a single set of raw DBT projections. The variance stabilization achieved using the proposed method is compared against the stabilization achieved using noise coefficients estimated from calibration data, considering two commercially available DBT systems and a prototype DBT system. The results showed that the average error for variance stabilization achieved using the proposed method is comparable to the error achieved through calibration data. Thus, the proposed method can be a viable alternative for achieving variance stabilization when calibration data are not easily accessible, facilitating the clinical deployment of algorithms that rely on variance stabilization.
A next generation tomosynthesis (NGT) prototype is under development to investigate alternative acquisition geometries for digital breast tomosynthesis (DBT). A positron emission tomography (PET) device will be integrated into the NGT prototype to facilitate DBT acquisition followed immediately by PET acquisition (PET-DBT). The aim of this study was to identify custom acquisition geometries that (1) improve dense/adipose tissue classification and (2) improve breast outline segmentation. Our lab’s virtual clinical trial framework (OpenVCT) was used to simulate various NGT acquisitions of anthropomorphic breast phantoms. Five custom acquisition geometries of the NGT prototype, with posteroanterior (PA) x-ray source motion ranging from 40-200 mm in 40 mm steps, were simulated for five phantoms. These acquisition geometries were compared against the simulation of a conventional DBT acquisition geometry. Signal in the reconstruction was compared against the ground truth on a voxel-by-voxel basis. The segmentation of breast from air is performed during reconstruction. Within the breast, we use a threshold-based classification of glandular tissue. The threshold was varied to produce a receiver operating characteristic (ROC) curve, representing the proportion of true fibroglandular classification as a function of the proportion of false fibroglandular classification at each threshold. The area under the ROC curve (AUC) was the figure-of-merit used to quantify adipose-glandular classification performance. Reconstructed breast volume estimation and sensitivity index (d’) were calculated for all image reconstructions. Volume overestimation is highest for conventional DBT and decreases with increasing PA source motion. AUC and d’ increase with increasing PA source motion. These results suggest that NGT can improve PET-DBT attenuation corrections over conventional DBT.
KEYWORDS: Digital breast tomosynthesis, X-rays, Visualization, Modulation transfer functions, X-ray detectors, Super resolution, Statistical analysis, Spatial resolution, Sensors
Tomosynthesis acquires projections over a limited angular range and thus samples an incomplete projection set of the object. For a given acquisition geometry, the extent of tomosynthesis sampling can be measured in the frequency domain based on the Fourier Slice Theorem (FST). In this paper we propose a term, “sampling comprehensiveness”, to describe how comprehensively an acquisition geometry samples the Fourier domain, and we propose two measurements to assess the sampling comprehensiveness: the volume of the null space and the nearest sampled plane. Four acquisition geometries, conventional (linear), T-shape, bowtie, and circular geometries, were compared on their comprehensiveness. The volume of the null space was estimated as the percentage of voxels subtended by zero slices in the sampled Fourier space. For each voxel in the frequency space, the nearest sampled plane and the distance to that plane were recorded. Among the four, the circular geometry was determined to be the most comprehensive based on the two measurements. We review tomosynthesis sampling with a finite number of projections and discuss how the sampling comprehensiveness should be interpreted. We further suggest that the decision on a system geometry should consider multiple factors including the sampling comprehensiveness, the task to be performed, the thickness of the imaged object, system specifications, and reconstruction algorithm.
Tomosynthesis has become a vital interventional tool for breast biopsy procedures. It is used to orient, advance and confirm the biopsy needle’s movement. However, at the end of a procedure, success is determined only after the biopsy sample shows the presence of the targeted lesion. Contrarily, failures, such as a target miss, are realized only after healthy tissue has been incorrectly excised. If real-time 4D tomosynthesis is made possible, it could not only guide and confirm the needle advancement but also anticipate any inadvertent target displacement and prevent healthy tissue damage. This study explores three classes of novel reconstruction algorithms that facilitate real-time 4D tomosynthesis guided biopsy procedures namely, Image-Processed algorithm, Segmented algorithm and Difference-Exploiting algorithm. A conventional tomosynthesis reconstruction algorithm applied to an incrementally moving needle shows a blurred needle tip - a consequence of superimposing and averaging the back-projections where the tip exists at different positions. The Image-Processed algorithm contrast-enhances all the back-projections before reconstruction thereby curbing the blurring and producing a more discernible needle tip. Pixel-based Segmented and Difference-Exploiting algorithms reconstruct individual pixels differently. The Segmented algorithm uses only the latest back-projection to reconstruct the pixels of the needle thereby capturing its most recent position. The Difference-Exploiting algorithm utilizes the superimposed differences of back-projections that helps in selectively identifying those elements, like the moving needle, that show a variation. Reconstructing these elements differently compared to other static elements of the breast allows capturing them in real-time. This work details the formulation of the three algorithms.
Our previous work showed that digital breast tomosynthesis (DBT) supports super-resolution (SR). Clinical systems are not yet designed to optimize SR; this can be demonstrated with a high-frequency line-resolution pattern. SR is achieved if frequencies are oriented laterally, but not if frequencies are oriented in the perpendicular direction; i.e., the posteroanterior (PA) direction. We are developing a next-generation tomosynthesis (NGT) prototype with new trajectories for the x-ray source. This system is being designed to optimize SR not just for screening, but also for diagnostic mammography; specifically, for magnification DBT (M-DBT). SR is not achieved clinically in magnification mammography, since the acquisition is 2D. The aim of this study is to investigate SR in M-DBT, and analyze how anisotropies differ from screening DBT (S-DBT). We have a theoretical model of a high-frequency sinusoidal test object. First, a conventional scanning motion (directed laterally) was simulated. In the PA direction, SR was not achieved in either S-DBT or M-DBT. Next, the scanning motion was angled relative to the lateral direction. This motion introduces submillimeter offsets in source positions in the PA direction. Theoretical modeling demonstrated that SR was achieved in M-DBT, but not in S-DBT, in the PA direction. This work shows that, with the use of magnification, anisotropies in SR are more sensitive to small offsets in the source motion, leading to insights into how to design M-DBT systems.
KEYWORDS: Digital breast tomosynthesis, Prototyping, Physics, Sensors, Super resolution, X-ray sources, Spatial resolution, Contrast transfer function, Ray tracing
Digital breast tomosynthesis (DBT) systems utilize an isocentric acquisition geometry which introduces imaging artifacts that are deleterious to image reconstructions. The next-generation tomosynthesis (NGT) prototype was designed to incorporate various x-ray source and detector motions for the purpose of investigating alternative acquisition geometries for DBT. Non-isocentric acquisition geometries, acquisitions that vary the image magnification between projection images, are capable of ameliorating aliasing and other artifacts that are intrinsic to conventional DBT. We used virtual clinical trials (VCTs) to develop custom acquisition geometries for the NGT prototype. A high-resolution (5μm voxel size) star pattern test object was simulated to compare the high-frequency performance of isocentric with non-isocentric image reconstructions. A tilted bar pattern test object was also simulated to compare multiplanar reconstructions (MPR) of isocentric and non-isocentric acquisition geometries. Two source- and detector-motion paths were simulated to obtain super-sampled image reconstructions of the test objects. An aliasing-sensitive metric was used to evaluate spatial resolution performance for two orthogonal frequency orientations. Pairwise comparisons were made for the two frequency orientations between the isocentric and non-isocentric acquisition geometries. Non-isocentric acquisition geometries show an improvement over isocentric acquisition geometries. The greatest improvement was 75.2% for frequencies aligned perpendicular to x-ray source motion, which is the direction of frequencies for which DBT is prone to aliasing. Both frequency orientations exhibit super resolution for non-isocentric geometries. MPR of the tilted bar pattern show zdependent degener
KEYWORDS: Super resolution, Anisotropy, Digital breast tomosynthesis, X-rays, Fourier transforms, Breast, Image resolution, Digital x-ray imaging, Image quality
Our previous work explored the use of super-resolution as a way to improve the visibility of calcifications in digital breast tomosynthesis. This paper demonstrates that there are anisotropies in super-resolution throughout the reconstruction, and investigates new motion paths for the x-ray tube to suppress these anisotropies. We used a theoretical model of a sinusoidal test object to demonstrate the existence of the anisotropies. In addition, high-frequency test objects were simulated with virtual clinical trial (VCT) software developed for breast imaging. The simulated objects include a lead bar pattern phantom as well as punctate calcifications in a breast-like background. In a conventional acquisition geometry in which the source motion is directed laterally, we found that super-resolution is not achievable if the frequency is oriented in the perpendicular direction (posteroanteriorly). Also, there are positions, corresponding to various slices above the breast support, at which super-resolution is inherently not achievable. The existence of these anisotropies was validated with VCT simulations. At locations predicted by theoretical modeling, the bar pattern phantom showed aliasing, and the spacing between individual calcifications was not properly resolved. To show that super-resolution can be optimized by re-designing the acquisition geometry, we applied our theoretical model to the analysis of new motion paths for the x-ray tube; specifically, motions with more degrees of freedom and with more rapid pulsing (submillimeter spacing) between source positions. These two strategies can be used in combination to suppress the anisotropies in super-resolution.
KEYWORDS: Digital breast tomosynthesis, Breast, Sensors, Computer simulations, Super resolution, Clinical trials, Imaging systems, Statistical analysis, Tomography, Analytical research
Digital breast tomosynthesis (DBT) reduces breast tissue overlap, which is a major limitation of digital mammography. However, DBT does not show significant improvement in calcification detection, because of the limited angle and small number of projections used to reconstruct the 3D breast volume. Virtual clinical trials (VCTs) were used to evaluate the benefits of computed super resolution (SR) and the optimal combination of the acquisition parameters to improve calcification detection in DBT. We simulated calcifications that were embedded into software breast phantoms. DBT projections of the breast phantoms with and without calcifications were synthesized. We simulated detector elements of 0.085 mm and reconstructed DBT images using 0.0425 mm and 0.085 mm voxels. Channelized Hotelling observers (CHOs) were trained and tested to simulate five virtual readers. Differences in area under the curve (AUC) between SR images and images synthesized with 0.085 mm voxels were calculated using the one-shot multiple-reader multiple-case receiver operator curve (MRMC ROC) methods. Our results show that the differences in AUC is approximately 0.10, 0.03 and 0.03 for DBT images simulated using calcifications sizes 0.001 mm3, 0.002 mm3, and 0.003 mm3, respectively. SR shows a substantial improvement for calcification detection in DBT. The impact of SR on calcification detection is more prominent for small calcifications.
KEYWORDS: Super resolution, Digital breast tomosynthesis, Anisotropy, Fourier transforms, Breast, X-rays, Image quality, Digital mammography, Digital x-ray imaging
Our previous work showed that digital breast tomosynthesis (DBT) systems are capable of super-resolution, or subpixel resolution relative to the detector. Using a bar pattern phantom, it is possible to demonstrate that there are anisotropies in super-resolution throughout the reconstruction. These anisotropies are lessened in acquisition geometries with narrow spacing between source positions. This paper demonstrates that by re-arranging the source positions in the scan, the anisotropies can be minimized even further. To this end, a theoretical model of the reconstruction of a high-frequency sinusoidal test object was developed from first principles. We modeled the effect of clustering additional source positions around each conventional source position in fine increments (submillimeter). This design can be implemented by rapidly pulsing the source during a continuous sweep of the x-ray tube. It is shown that it is not possible to eliminate the anisotropies in a conventional DBT system with uniformly-spaced source positions, even if the increments of spacing are narrower than those used clinically. However, super-resolution can be achieved everywhere if the source positions are re-arranged in clusters with submillimeter spacing. Our previous work investigated a different approach for optimizing super-resolution through the use of detector motion perpendicular to the breast support. The advantage of introducing rapid source pulsing is that detector motion is no longer required; this mitigates the need for a thick detector housing, which may be cumbersome for patient positioning.
The limited angle and limited number of projections in digital breast tomosynthesis (DBT) produce under-sampled datasets that may compromise calcification detection. Small breast lesions, such as microcalcifications, may not be discernible without sufficient sampling in the reconstructed DBT images. We propose a virtual clinical trial (VCT) method to evaluate the calcification detection in DBT using computer simulations of breast phantoms, images, and virtual readers. We used multiple-reader multiple-case (MRMC) receiver operating characteristic (ROC) analyses to evaluate the performance of channelized Hotelling observers (CHOs) in calcification detection. The angular motion path of the x-ray source was varied to simulate different DBT acquisition geometries. We simulated continuous and step-and-shoot x-ray source motion and three angular motion paths: ±7.5°, ±15°, and ±25°. The detection of calcifications is affected by the angular motion path, particularly for the ±25° angular range, combined with continuous tube motion, larger detector element sizes (0.14 mm) and larger reconstructed voxel sizes (0.10 mm). When an angular range of ±25° is compared to ±7.5°, the difference in the area under the curve (AUC) is -0.030 (d’ ratio=0.633) and -0.067 (d’ ratio=0.584), for one and two- voxel calcifications (0.1 mm³ and 0.2 mm³), respectively. There is no significant difference in calcification detection using images acquired with ±7.5° and ±15°. The results provide insight on the impact of angular range for calcification detection, an ongoing limitation of tomosynthesis.
It has been reported and discussed that electrical current can be produced when an insulating material interacts with ionizing radiation. We have found that high-resolution images can be obtained from insulating materials if this current is guided by an electric field to the pixels of a TFT array. The charge production efficiency of insulators is much smaller than that of photoconductor materials such as selenium, silicon, or other conventional semiconductors. Nevertheless, when the intensity of the ionizing radiation is sufficiently high, a charge sensitive TFT imaging array with only dielectric material can produce high MTF images with contrast resolution proportional to the intensity of the radiation. The function of the dielectric in this new detector may be similar to that of an ionization chamber. Without the semiconductor charge generating material, the dielectric imaging detector does not exhibit charge generation fatigue or charge generation saturation. Prototype detectors have been tested using diagnostic x-ray beams with energy ranging from 25 kVp to 150 kVp, and therapeutic 2.5MV, 6MV, 10MV, and 15MV photon beams (with and without an electron built-up layer), electron beams, broad area proton beams, and proton pencil beams in the energy range of 150 MeV. High spatial resolution images up to the Nyquist frequency have been demonstrated. The physics, structure, and the imaging properties as well as the potential application of this detector will be presented and discussed.
KEYWORDS: Modulation transfer functions, 3D acquisition, Digital breast tomosynthesis, Contrast transfer function, Prototyping, Image quality, 3D image processing, Spatial resolution, Image restoration
A next generation tomosynthesis (NGT) prototype was designed to investigate alternative scanning geometries for digital breast tomosynthesis (DBT). The NGT system uses a 2D plane as an address space for the x-ray source, and onedimensional linear detector motion to determine an acquisition geometry. This design provides myriad acquisition geometries for investigation. This system is also capable of magnification DBT. We performed image quality measurements to evaluate performance of the NGT system for both contact and magnification imaging in 2D and 3D. The modulation transfer function (MTF) was computed using the slanted-edge method to evaluate spatial resolution. The first zero of the MTF was observed to increase by a factor of the magnification. In-plane spatial resolution performance for 3D was measured using an in-house metric, and was found to be commensurate to the MTF. This metric uses a star pattern as an input object to produce the contrast transfer function (CTF). The 2D noise power spectra (NPS) were calculated to evaluate the degradation of image quality due to noise. 3D NPS were also calculated for various 3D image reconstructions. 3D renditions of the NPS show how the NGT can sample a broader range of frequencies in the Fourier domain than conventional DBT. The system’s lag was measured and found not to affect 3D image reconstructions significantly. A wax calcification phantom was constructed and imaged using the NGT system. The performance of this system has been evaluated and the results suggest that image quality is sufficient for clinical investigation.
Star pattern test objects are used to evaluate the high-contrast performance of imaging systems. These objects were used to investigate alternative scanning geometries for a prototype next-generation tomosynthesis (NGT) system. The NGT system has 2D planar source motion and linear detector motion, and is capable of myriad acquisition geometries. We designed a virtual star pattern with a voxel size of 5𝜇m, and used it to evaluate the spatial resolution performance of the NGT system for three different acquisition geometries. The Open Virtual Clinical Trials (OpenVCT) framework was used to simulate virtual star patterns for acquisition geometries of the NGT system. Simulated x-ray projections of the virtual phantom were used to create super-sampled 3D image reconstructions. Using the same acquisition geometries on the NGT system, a physical star pattern was imaged to create experimental 3D image reconstructions. The simulated and physical data were compared qualitatively by visual inspection, and quantitatively using an in-house metric. This metric computes the Fourier transform radially for one quadrant of the star pattern to discern the limit of spatial resolution (LSR) and the existence of aliasing. The results exhibit the same characteristics in terms of super-resolution and Moiré patterns (arising from aliasing) with visual inspection. The simulated LSR for the 12 conditions analyzed are all within 3% of the physical data. Aliasing was determined to be present in the same simulated image reconstructions as the experimental complements. Super-resolution is observed for two of the three NGT acquisition geometries in the experimental and simulated images.
In digital breast tomosynthesis (DBT), projection images are acquired as the x-ray tube rotates in the plane of the chest wall. We constructed a prototype next-generation tomosynthesis (NGT) system that has an additional component of tube motion in the perpendicular direction (i.e., posteroanterior motion). Our previous work demonstrated the advantages of the NGT system using the Defrise phantom. The reconstruction shows higher contrast and fewer blurring artifacts. To expand upon that work, this paper analyzes how image quality can be further improved by customizing the motion path of the x-ray tube based on the object being imaged. In simulations, phantoms are created with realistic 3D breast outlines based on an established model of the breast under compression. The phantoms are given an internal structure similar to a Defrise phantom. Two tissue types (fibroglandular and adipose) are arranged in a square-wave pattern. The reconstruction is analyzed as a binary classification task using thresholding to segment the two tissue types. At various thresholds, the classification of each voxel in the reconstruction is compared against the phantom, and receiver operating characteristic (ROC) curves are calculated. It is shown that the area under the ROC curve (AUC) is dependent on the x-ray tube trajectory. The trajectory that maximizes AUC differs between phantoms. In conclusion, this paper demonstrates that the acquisition geometry in DBT should be personalized to the object being imaged in order to optimize the image quality.
KEYWORDS: Breast, Image segmentation, Digital breast tomosynthesis, Digital x-ray imaging, X-ray imaging, X-rays, Sensors, Signal detection, Computer simulations, Yield improvement
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.
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.
KEYWORDS: Calibration, Sensors, Super resolution, Digital breast tomosynthesis, X-ray sources, Reconstruction algorithms, Detection and tracking algorithms, X-rays, Breast
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 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.
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.
KEYWORDS: Digital breast tomosynthesis, Stars, Super resolution, Spatial resolution, X-rays, Modulation, Visible radiation, Breast, X-ray sources, Domes
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.
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