Fast reconstruction of digital tomosynthesis using on-board images

Digital tomosynthesis (DTS) is a method to reconstruct pseudo three-dimensional (3D) volume images from two-dimensional x-ray projections acquired over limited scan angles. Compared with cone-beam computed tomography, which is frequently used for 3D image guided radiation therapy, DTS requires less imaging time and dose. Successful implementation of DTS for fast target localization requires the reconstruction process to be accomplished within tight clinical time constraints (usually within 2 min). To achieve this goal, substantial improvement of reconstruction efficiency is necessary. In this study, a reconstruction process based upon the algorithm proposed by Feldkamp, Davis, and Kress was implemented on graphics hardware for the purpose of acceleration. The performance of the novel reconstruction implementation was tested for phantom and real patient cases. The efficiency of DTS reconstruction was improved by a factor of 13 on average, without compromising image quality. With acceleration of the reconstruction algorithm, the whole DTS generation process including data preprocessing, reconstruction, and DICOM conversion is accomplished within 1.5 min, which ultimately meets clinical requirement for on-line target localization.     

Accelerating reconstruction of reference digital tomosynthesis using graphics hardware

The successful implementation of digital tomosynthesis (DTS) for on-board image guided radiation therapy (IGRT) requires fast DTS image reconstruction. Both target and reference DTS image sets are required to support an image registration application for IGRT. Target images are usually DTS image sets reconstructed from on-board projections, which can be accomplished quickly using the conventional filtered backprojection algorithm. Reference images are DTS image sets reconstructed from digitally reconstructed radiographs (DRRs) previously generated from conventional planning CT data. Generating a set of DRRs from planning CT is relatively slow using the conventional ray-casting algorithm. In order to facilitate DTS reconstruction within a clinically acceptable period of time, we implemented a high performance DRR reconstruction algorithm on a graphics processing unit of commercial PC graphics hardware. The performance of this new algorithm was evaluated and compared with that which is achieved using the conventional software-based ray-casting algorithm. DTS images were reconstructed from DRRs previously generated by both hardware and software algorithms. On average, the DRR reconstruction efficiency using the hardware method is improved by a factor of 67 over the software method. The image quality of the DRRs was comparable to those generated using the software-based ray-casting algorithm. Accelerated DRR reconstruction significantly reduces the overall time required to produce a set of reference DTS images from planning CT and makes this technique clinically practical for target localization for radiation therapy.     

Evaluation of three types of reference image data for external beam radiotherapy target localization using digital tomosynthesis (DTS)

Digital tomosynthesis (DTS) is a fast, low-dose three-dimensional (3D) imaging approach which yields slice images with excellent in-plane resolution, though low plane-to-plane resolution. A stack of DTS slices can be reconstructed from a single limited-angle scan, with typical scan angles ranging from 10 degrees to 40 degrees and acquisition times of less than 10 s. The resulting DTS slices show soft tissue contrast approaching that of full cone-beam CT. External beam radiotherapy target localization using DTS requires the registration of on-board DTS images with corresponding reference image data. This study evaluates three types of reference volume: original reference CT, exact reference DTS (RDTS), and a more computationally efficient approximate reference DTS (RDTSapprox), as well as three different DTS scan angles (22 degrees, 44 degrees, and 65 degrees) for the DTS target localization task. Three-dimensional mutual information (MI) shared between reference and onboard DTS volumes was computed in a region surrounding the spine of a chest phantom, as translations spanning +/-5 mm and rotations spanning +/-5 degrees were simulated along each dimension in the reference volumes. The locations of the MI maxima were used as surrogates for registration accuracy, and the width of the MI peaks were used to characterize the registration robustness. The results show that conventional treatment planning CT volumes are inadequate reference volumes for direct registration with on-board DTS data. The efficient RDTSapprox method also appears insufficient for MI-based registration without further refinement of the technique, though it may be suitable for manual registration performed by a human observer. The exact RDTS volumes, on the other hand, delivered a 3D DTS localization accuracy of 0.5 mm and 0.50 along each axis, using only a single 44 degrees coronal on-board DTS scan of the chest phantom.     

A multiple projection method for digital tomosynthesis.

A new method of optimized efficiency for the retrospective reconstruction of tomograms is presented. The method has been developed for use with isocentric fluoroscopic units and is capable of performing digital tomosynthesis of anatomical planes of user selected orientation and distance from the isocenter. Optimization of efficiency has been achieved by segmenting the reconstruction process into discrete transformations that are specific to groups of pixels, rather than performing pixel by pixel operations. These involve a number of projections of the acquired image matrices as well as parallel translations and summing. Application of this method has resulted in a significant reduction of computing time. The proposed algorithm has been experimentally tested on a radiotherapy simulator unit with the use of a phantom and the obtained results are reported and discussed.     

A new digital tomosynthesis method with less artifacts for angiography

A new nonlinear reconstruction method for tomosynthesis is described. This method is suited for "dilute" objects, i.e., objects in which most of the voxels have negligibly small absorption. Images of blood vessels filled with contrast material approximate this condition if the background is subtracted. The technique has been tested experimentally using a wire phantom and a prepared human heart. The results show significantly less artifacts than the well-known back projection. It is possible to get diagnostic image quality with a few projections. The reconstruction algorithm can be realized with dedicated real-time hardware.     

A novel background field removal method for MRI using projection onto dipole fields (PDF)

For optimal image quality in susceptibility-weighted imaging and accurate quantification of susceptibility, it is necessary to isolate the local field generated by local magnetic sources (such as iron) from the background field that arises from imperfect shimming and variations in magnetic susceptibility of surrounding tissues (including air). Previous background removal techniques have limited effectiveness depending on the accuracy of model assumptions or information input. In this article, we report an observation that the magnetic field for a dipole outside a given region of interest (ROI) is approximately orthogonal to the magnetic field of a dipole inside the ROI. Accordingly, we propose a nonparametric background field removal technique based on projection onto dipole fields (PDF). In this PDF technique, the background field inside an ROI is decomposed into a field originating from dipoles outside the ROI using the projection theorem in Hilbert space. This novel PDF background removal technique was validated on a numerical simulation and a phantom experiment and was applied in human brain imaging, demonstrating substantial improvement in background field removal compared with the commonly used high-pass filtering method.     

全数字化乳腺摄影与乳腺三维断层摄影对乳腺钙化的对比研究

目的:研究全数字化乳腺摄影（FFDM）及数字乳腺断层摄影（DBT）对于钙化征象及其在乳腺疾病诊断效能的对比。 方法:收集福建医科大学附属第二医院2013年6月~2017年4月期间就诊的97例经FFDM及DBT检查发现有钙化征象,并具有病理结果的患者,根据乳腺影像报告和数据系统进行阅片诊断,同时计算出在FFDM及COMBO（DBT结合FFDM）两种摄片模式下的钙化分数。 结果:通过秩和检验分析两种方法下钙化分数存在显著性差异（P<0.05）。通过ROC曲线分析两种方法在乳腺疾病诊断效能发现,两者曲线下面积均大于0.5,具有诊断价值,FFDM与COMBO敏感度（95.7%）一致;COMBO特异性（92%）较FFDM特异性（76%）高。 结论:COMBO相对于FFDM能更好地观察钙化征象,提高诊断效能。     

Megavoltage cone beam digital tomosynthesis (MV-CBDT) for image-guided radiotherapy: a clinical investigational system

Cone beam digital tomosynthesis (CBDT) is a new imaging technique proposed recently as a rapid approach for creating tomographic images of a patient in the radiotherapy treatment room. The purpose of this work is to investigate the feasibility of performing megavoltage (MV) CBDT clinically. A clinical investigational MV-CBDT system was installed on an existing LINAC. After the installation, the treatment machine can be operated in two distinct modes: (1) normal clinical treatment mode; (2) CBDT mode, in which tomographic images of the patient can be obtained using MV-CBDT. Various calibration and phantom measurements were performed on the system, followed by a patient study. Our phantom measurements have shown that: (1) for the same imaging dose, MV-CBDT has the same signal-difference-to-noise ratio as megavoltage cone beam computed tomography (MV-CBCT); (2) MV-CBDT has a better spatial resolution than MV-CBCT in the planes of reconstruction but a worse spatial resolution in the direction perpendicular to the planes of reconstruction. MV-CBDT patient images were also obtained and compared to that of MV-CBCT. We have demonstrated that it is clinically feasible to perform MV-CBDT in the treatment room for image-guided radiotherapy.     

Automated detection of microcalcification clusters for digital breast tomosynthesis using projection data only: a preliminary study

Digital breast tomosynthesis (DBT) is a promising modality for breast imaging in which an anisotropic volume image of the breast is obtained. We present an algorithm for computerized detection of microcalcification clusters (MCCs) for DBT. This algorithm operates on the projection views only. Therefore it does not depend on reconstruction, and is computationally efficient. The algorithm was developed using a database of 30 image sets with microcalcifications, and a control group of 30 image sets without visible findings. The patient data were acquired on the first DBT prototype at Massachusetts General Hospital. Algorithm sensitivity was estimated to be 0.86 at 1.3 false positive clusters, which is below that of current MCC detection algorithms for full-field digital mammography. Because of the small number of patient cases, algorithm parameters were not optimized and one linear classifier was used. An actual limitation of our approach may be that the signal-to-noise ratio in the projection images is too low for microcalcification detection. Furthermore, the database consisted of predominantly small MCC. This may be related to the image quality obtained with this first prototype.     

A novel method of radiochromic film dosimetry using a color scanner

A procedure that allows the improved extraction of the dose information based on the multicolor scanning of the radiochromic film is presented. The basic principle is the determination of the dose values from each color channel of the digital film image in RGB format by applying a nonlinear calibration function. The best estimate of the dose is then a weighted mean of the dose values derived from each color channel. The weighting factors are determined in such a way that the noise in the two-dimensional dose profile is at the minimum. The calculation of the weighting factors is presented; they are chosen to be proportional to the signal-to-noise ratio, Si/v(i)2, in all three color channels, i=red, green, or blue. The data reduction can be fully computerized, including the "cleaning" of the digital image from dust and scratches. It is highly reproducible, which is important for quality assurance, and easy to use. Our novel evaluation procedure combines the good response in the low dose range of the red color with the extended dose range of the blue and green color channels (response up to 10,000 Gy), making use of one single, steady evaluation function. Therefore, a smooth evaluation is possible in a wide dose range. For the interpretation of measurements with the radiochromic films the spatial inhomogeneity of the film's response to ionizing radiation is very important. Investigations on both film types, HS and MD55-2, as well as on the new EBT film have been carried out.     

A simple fixed-point approach to invert a deformation field

Inversion of deformation fields is applied frequently to map images, dose, and contours between the reference frame and the study frame. A prevailing approach that takes the negative of the forward deformation as the inverse deformation is oversimplified and can cause large errors for large deformations or deformations that are composites of several deformations. Other approaches, including Newton's method and scatter data interpolation, either require the first derivative or are very inefficient. Here we propose an iterative approach that is easy to implement, converges quickly to the inverse when it does, and works for a majority of cases in practice. Our approach is rooted in fixed-point theory. We build a sequence to approximate the inverse deformation through iterative evaluation of the forward deformation. A sufficient but not necessary convergence condition (Lipschitz condition) and its proof are also given. Though this condition guarantees the convergence, it may not be met for an arbitrary deformation field. One should always check whether the inverse exists for the given forward deformation field by calculating its Jacobian. If nonpositive values of the Jacobian occur only for few voxels, this method will usually converge to a pseudoinverse. In case the iteration fails to converge, one should switch to other means of finding the inverse. We tested the proposed method on simulated 2D data and real 3D computed tomography data of a lung patient and compared our method with two implementations in the Insight Segmentation and Registration Toolkit (ITK). Typically less than ten iterations are needed for our method to get an inverse deformation field with clinically relevant accuracy. Based on the test results, our method is about ten times faster and yet ten times more accurate than ITK's iterative method for the same number of iterations. Simulations and real data tests demonstrated the efficacy and the accuracy of the proposed algorithm.     

A novel multicolor immunofluorescence method using heat treatment

We describe a novel method for immunofluorescent detection of multiple antigens in a single paraffin-embedded tissue section. We hypothesized that if fluorescent dyes are resistant to heat treatment, then thermal inactivation of immunoglobulins during antigen detection procedures might make it possible to use multicolor immunofluorescence detection even if the primary antibodies are from the same species. We found that several fluorescent dyes, including fluorescein isothiocyanate (FITC), Cy3 and Cy5, were resistant to heating at 90 degrees Celsius for 15 min, whereas the antigenicities of the primary antibodies were lost completely. This novel method, which uses heat treatment between staining steps, has great advantages for multicolor immunofluorescence because unlabeled primary antibodies from the same species can be used. Therefore, by using this method not only 3 unlabeled mouse monoclonal antibodies but also 3 unlabeled rabbit antisera can be used as primary antibodies for multicolor immunofluorescence.     

A method for detecting myocardial abnormality by using a current-ratio map calculated from an exercise-induced magnetocardiogram

A method for making a current-ratio map to determine the ischaemic area of angina pectoris (AP) patients has been developed. This method uses a current-arrow map calculated using a QRS wave from 64-channel magnetocardiogram (MCG) signals. The current-ratio map can be calculated from the ratio of an exercise-induced current vector to an at-rest current vector. The MCG signals of eight patients with angina pectoris (AP) (six patients with effort AP and two patients with variant AP) and four healthy volunteers were measured before and after a two-step exercise test. The current-ratio maps of the six patients with effort AP showed three distinct patterns: a left-circumflex-artery (LCX) pattern; a right-coronary-artery (RCA) pattern; and a left-anterior-descending (LAD) pattern. The maximum current ratios of these three patterns differed from those of normal patterns. The patterns of two patients with variant AP were similar to normal patterns. Furthermore, a comparison of the current-ratio map before and after percutaneous-transluminal-coronary-angioplasty (PTCA) treatment indicated that the cardiac ischaemia was reduced in all patients. An appropriate criterion to diagnose abnormality in a patient with an ischaemic myocardial area seems to be a maximum current ratio exceeding 0.4 to 0.5. Based on these preliminary results, it is believed that the location of an ischaemic area (the coronary artery part) can be estimated by using the ischaemic current-ratio map pattern.