Breast tomosynthesis reconstruction with a multi-beam x-ray source

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1 Breast tomosynthesis reconstruction with a multi-beam x-ray source Ying Chen *a,b, Weihua Zhou a, Guang Yang c, Xin Qian c, Jianping Lu c,d, and Otto Zhou a Dept. of Electrical and Computer Engineering, Southern Illinois University, Carbondale, IL 62901; b Biomedical Engineering Graduate Program, Southern Illinois University, Carbondale, IL 62901; c Dept. of Physics and Astronomy, and Curriculum in Applied Sciences and Engineering, The University of North Carolina, Chapel Hill, NC 27599; d Lineberger Comprehensive Cancer Center, The University of North Carolina, Chapel Hill, NC c,d ABSTRACT As a new three-dimensional breast imaging technique, breast tomosynthesis allows the reconstruction of an arbitrary set of planes in the breast from a limited-angle series of x-ray projection images. The breast tomosynthesis technique has been demonstrated as promising to improve early breast cancer detection. This paper represents a preliminary phantom study and computer simulation results of different breast tomosynthesis reconstruction algorithms with a novel carbon nanotube based multi-beam x-ray source. Five representative tomosynthesis reconstruction algorithms, including back projection (BP), filtered back projection (FBP), matrix inversion tomosynthesis (MITS), maximum likelihood expectation maximization (MLEM), and simultaneous algebraic reconstruction technique (SART) were investigated. Tomosynthesis projection images of a phantom were acquired with the stationary multi-beam x-ray tomosynthesis system. Reconstruction results from different algorithms were studied. A computer simulation study was further done to investigate the sharpness of reconstructed in-plane structures and to see how effective each algorithm is at removing outof-plane blur with parallel-imaging geometries. Datasets with 9 and 25 projection images of a defined 3D spherical object were simulated with a total view angle of 50 degrees. Results showed that the multi-beam x-ray system is capable to generate 3D tomosynthesis images with faster speed compared with current commercial prototype systems. With simulated parallel-imaging geometry, MITS and FBP showed edge enhancement in-plane performance. BP, FBP and MLEM performed better at out-of-plane structure removal with larger number of projection images. Keywords: mammography, tomosynthesis, back projection (BP), filtered back projection (FBP), matrix inversion tomosynthesis (MITS), maximum likelihood (ML), simultaneous algebraic reconstruction technique (SART), carbon nanotube 1. INTRODUCTION Digital breast tomosynthesis refers to a novel X-ray imaging technique that provides three-dimensional (3D) information of the object with low dose to subjects [1]. Compared with standard mammography techniques, tomosynthesis imaging methods improve conspicuity of structures by removing the visual clutter associated with overlying anatomy [1-10]. Several breast tomosynthesis reconstruction algorithms have been proposed by different research groups, including [6] [7,8] Niklason and colleagues publication in 1997, Wu et al. s maximum likelihood iterative algorithm (MLEM), filtered back projection (FBP) algorithms [4,5,11-14], tuned-aperture computed tomography (TACT) reconstruction methods developed by Webber and investigated by Suryanarayanan et al [15-17], algebraic reconstruction techniques (ART) [9], etc. From 2004 through 2007, Dobbins and Chen reported the application of the matrix inversion tomosynthesis (MITS) technique in breast tomosynthesis [18,19] and proposed a Gaussian Frequency Blending (GFB) algorithm [4] to combine the MITS and FBP for better reconstruction. *adayingchen@gmail.com; phone ; fax

2 A few breast tomosynthesis prototype imaging systems have been designed by several research groups and commercial vendors including GE, Hologic and Siemens. Most of current tomosynthesis prototype systems applied the partial isocentric imaging geometries where the x-ray tube moves along an arc above the detector. [7,9,20,21] The x-ray tube s movement may introduce blur [5,22] to tomosynthesis images and cause patients discomfort. In this paper, we presented our preliminary tomosynthesis reconstruction study with a novel multi-beam x-ray source developed by Zhou et al. recently [23-25]. The breast tomosynthesis system was built up with fixed multi-beam fieldemission x-ray (MBFEX) sources based on unique properties of carbon nanotube electron emitters [24]. Parallel imaging configuration was applied to multi-beam x-ray system design where the path of the x-ray tube lies in a plane that is parallel to the detector plane [1,3]. The total scan time for a typical 25 views is only about 11.2 seconds [25]. Compared with commercial prototype systems, it increases the tomosynthesis imaging speed with simplified system design. With the fast-speed imaging and fixed x-ray sources, it may potentially reduce patients discomfort and the motion blur associated with x-ray tube s movement of typical prototype systems. 2.1 Preliminary phantom study 2. METHODS [23-25] Phantom experiments with the multi-beam x-ray system developed by Zhou et al. were investigated. Figure 1 shows a diagram of the parallel-imaging geometry of the system. The fixed multi-beam field-emission x-ray (MBFEX) sources were aligned up in a straight line along the x axis that is parallel to the detector plane. In this paper, the source to image distance (SID) was 687 mm. θ represents the view angle. For preliminary phantom experiment studies, a tomosynthesis dataset of nine projection images of a sponge phantom with embedded three objects simulating masses was acquired with stationary x-ray sources located at measurable positions. A technique of Mo/Mo spectrum/filtration was used with an exposure level of 3 mas for each projection image. The detector area was 19.5 x 24.4 cm with a pixel pitch of 127 μm. A 2x2 binning mode was used in this preliminary study. y Detector X-ray sources θ Z SID y x z Fig.1. Parallel-imaging geometry of the carbon nanotube based x-ray device Five representative algorithms including back projection (BP) [3], filtered back projection (FBP) [5], matrix inversion tomosynthesis (MITS) [18,19], maximum likelihood expectation maximization (MLEM) [7] and simultaneous algebraic reconstruction technique (SART) [9] were studied to generate reconstruction images. Previous studies [5,8,9] has demonstrated that, among those five reconstruction algorithms, BP is a common mathematical method to align up in-

3 focus subjects. MITS and FBP are fast-speed reconstruction methods with deblurring algorithms to remove out-of-plan blur. MLEM and SART are iterative methods to reconstruct three-dimensional information of the object. In our study, with a computer of 2GHz CPU and UNIX operating system, it takes less than 3 minutes for BP, FBP and MITS to generate all reconstruction images of the entire subject. For time-consuming iterative reconstruction methods, an accelerated MLEM has been investigated with improved sparse matrix data structure recently [26]. With a computer of 1.86 GHz CPU and 3GB limited memory size, it takes about 2 minutes to generate three MLEM reconstruction planes for each run. The computation time of SART is comparable with our accelerated MLEM reconstruction. 2.2 Preliminary computer simulation study Preliminary computer simulation study was also done to further investigate the reconstruction results and to see how above reconstruction algorithms differ from each other. A spherical object (4 mm in radius and located in a defined reconstruction plane z=20 mm away from the detector) was computer simulated as the target. Tomosynthesis datasets of nine and twenty-five projection images of the object were simulated by ray-tracing [3] method. Imaging parameters of the multi-beam x-ray device shown in figure 1 were used for geometries of the simulation. The view angle was set to be θ=50 0 for simulated evenly-spaced x-ray point sources. The pixel pitch of 254 μm (due to 2x2 binning) was used in simulation. The above five representative algorithms were then implemented with simulated tomosynthesis datasets to reconstruct images. A reconstruction plane spacing of 1 mm was used. Normalized in-plane and out-of-plane pixel intensities in the spatial domain were analyzed for evaluation. 3.1 Phantom study results 3. RESULTS Figure 2 shows a region of interest (ROI) of the low dose middle (0 0 ) projection image of the sponge phantom with the multi-beam x-ray source. Figure 3 shows reconstructed ROIs from five algorithms including back projection (BP) [3], filtered back projection (FBP) [5], matrix inversion tomosynthesis (MITS) [18,19], maximum likelihood expectation maximization (MLEM) [7] and simultaneous algebraic reconstruction technique (SART) [9] respectively. Fig.2. ROI of low dose middle (0 0 ) projection image of a phantom. (a) BP (b) FBP (c) MITS (d) MLEM (e) SART Fig.3. Breast tomosynthesis reconstructed ROI images of the phantom, at Z=80 mm away from the detector: (a) BP, (b) FBP), (c) MITS, (d) MLEM, (e) SART. In figure 2, one can barely identify the three embedded objects (simulating masses) on original middle projection ROI image. The margins and shapes of those objects are not visible. In figure 3, with five different tomosynthesis reconstructions, the visibility of three objects is better than that in figure 1. Margins and shapes are clearer. The

4 investigated five algorithms are capable to provide reconstruction of the phantom with three-dimensional location, shape and edge information. In this paper, we only included our preliminary phantom study results with a tomosynthesis dataset of 9 projection images. The phantom used for our preliminary study was dense. This also brought difficulty to exposure level selection. Recently, the multi-beam x-ray source tomosynthesis imaging system has been improved and is able to generate as many as 25 projection images. Further study will be done soon to improve the calibration and measurement accuracy of imaging parameters and to reduce reconstruction artifacts. 3.2 Computer simulation study results Tomosynthesis datasets of nine and twenty-five projection images were simulated and reconstructed by above described five representative reconstruction algorithms. Figure 4 shows the in-plane reconstruction performance with simulated tomosynthesis dateset of 25 projection images and view angle of Figure 4(a) through (e) are results from back projection (BP), filtered back projection (FBP), matrix inversion tomosynthesis (MITS), maximum likelihood expectation maximization (MLEM) and simultaneous algebraic reconstruction technique (SART) correspondingly. For each reconstruction algorithm, both three-dimensional mesh plot and two-dimensional line profile of normalized pixel intensities on the defined reconstruction planes passing through the center of simulated spherical object (z=20 mm away from the detector) were illustrated. Along x and y axe (pixel locations), a 300-pixel region surrounding the center of simulated spherical object was shown for better view. (a) BP (b) FBP

5 (c) MITS (d) MLEM (e) SART Fig.4. Comparison of in-plane line profiles on reconstruction planes at Z=20 mm away from the detector: (a) BP, (b) FBP), (c) MITS, (d) MLEM, (e) SART. As shown in figure 4, all five representative algorithms are able to reconstruct three-dimensional information of the simulated object. For in-plane performance, BP, MLEM and SART reconstruction algorithms showed sharp responses on the three-dimensional mesh plots and two-dimensional line profiles (fig.4a, 4d, 4e). MITS and FBP algorithms showed edge enhancement phenomena (fig.4b, 4c). This edge enhancement also exists for the partial isocentric tomosynthesis imaging configuration [2], which is common for current breast tomosynthesis commercial prototype systems. Figure 5 shows the comparison of out-of-plane line profiles of BP, FBP and MLEM on reconstruction planes at z=35 mm away from the detector. Figure 5(a), (b) and (c) show results with 9 projection images. Figure 5(d), (e) and (f) show results with 25 projection images. X axis represents the pixel location on reconstructed plane and a 300-pixel region of interest was shown for clarity. Y axis represents the pixel intensity on reconstructed image. For each reconstruction algorithm, the pixel intensities were normalized based on the in-plane (z=20 mm) reconstruction response accordingly.

6 9 proj (a) BP (b) FBP (c) MLEM 25 proj (d) BP (e) FBP (f) MLEM Fg.5. Comparison of out-of-plane line profiles on reconstruction planes at Z=35 mm away from the detector: (a) BP with 9 projection; (b) FBP with 9 projection; (c) MLEM with 9 projections; (d) BP with 25 projections; (e) FBP with 25 projections; (f) MLEM with 25 projections. As shown in figure 5, with a limited number of projection images, the out-of-plane performance shows noise and artifacts (fig.5a,5b,5c). With a larger number of projection images of twenty-five, BP, FBP and MLEM performed better at out-of-plane blur removal by showing relatively smooth profile. Edge enhancement performance existed for FBP (fig 5b, 5c). 4. CONCLUSION The stationary multi-beam x-ray tomosynthesis device developed by Zhou et al. has great potentials to challenge current breast tomosynthesis system designs. Our study has demonstrated that this x-ray tomosynthesis system is capable to generate three-dimensional information of the subject with representative reconstruction algorithms. Initial simulation study suggested that a bigger number of projection image will be beneficial to improve reconstruction image quality. Currently, an improved design has been implemented and demonstrated successful to generate 25 projection images. We are also working on improving calibration and measurement accuracy. Future phantom study will be done soon to further investigate reconstruction and imaging geometry optimization with this novel breast tomosynthesis device. ACKNOWLEDGMENTS We thank James T. Dobbins III, Ph.D., at Duke University Medical Center for helpful discussion.

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