Inverse filtering approach to measure directional in-plane modulation transfer function using a sphere phantom for a digital tomosynthesis system

We propose a method to measure the directional in-plane modulation transfer function (MTF) of a digital tomosynthesis system using a sphere phantom. To assess the spatial resolution of an in-plane image of the tomosynthesis system, projection data of a sphere phantom were generated within a limited data acquisition range of 40°, and reconstructed by the FDK algorithm. To measure the in-plane MTF, we divided the Fourier transform of the reconstructed sphere phantom by that of the ideal sphere phantom, and then performed plane integral along the fz-direction. When dividing, small values in the denominator can introduce estimation errors, and these errors were reduced by the proposed method. To evaluate the performance of the proposed method, the in-plane MTF estimated by simulation and experimental data was compared to the ideal in-plane MTF generated by computer simulations using a point object. For quantitative evaluation, we measured frequency values at half-maximum and full-maximum of the directional in-plane MTF along the three different directions (i.e., f_0° -, f_30° -, and f_60° -directions) and compared them with those of the ideal in-plane MTF. Although the sphere phantom has been regarded as an inappropriate object due to the anisotropic characteristics of tomosynthesis image, our results show that the proposed method has a reliable estimation performance, demonstrating the sphere phantom is still suitable for measuring the directional in-plane MTF for a digital tomosynthesis system.

In-line phase-contrast breast tomosynthesis: a phantom feasibility study at a synchrotron radiation facility

The major objective is to adopt, apply and test developed in-house algorithms for volumetric breast reconstructions from projection images, obtained in in-line phase-contrast mode. Four angular sets, each consisting of 17 projection images obtained from four physical phantoms, were acquired at beamline ID17, European Synchroton Radiation Facility, Grenoble, France. The tomosynthesis arc was  ±32°. The physical phantoms differed in complexity of texture and introduced features of interest. Three of the used phantoms were in-house developed, and made of epoxy resin, polymethyl-methacrylate and paraffin wax, while the fourth phantom was the CIRS BR3D. The projection images had a pixel size of 47 µm  ×  47 µm. Tomosynthesis images were reconstructed with standard shift-and-add (SAA) and filtered backprojection (FBP) algorithms. It was found that the edge enhancement observed in planar x-ray images is preserved in tomosynthesis images from both phantoms with homogeneous and highly heterogeneous backgrounds. In case of BR3D, it was found that features not visible in the planar case were well outlined in the tomosynthesis slices. In addition, the edge enhancement index calculated for features of interest was found to be much higher in tomosynthesis images reconstructed with FBP than in planar images and tomosynthesis images reconstructed with SAA. The comparison between images reconstructed by the two reconstruction algorithms shows an advantage for the FBP method in terms of better edge enhancement. Phase-contrast breast tomosynthesis realized in in-line mode benefits the detection of suspicious areas in mammography images by adding the edge enhancement effect to the reconstructed slices.

Breast tomosynthesis using the multiple projection algorithm adapted for stationary detectors

OBJECTIVE

The aim of this study is to investigate the validity of using the Multiple Projection Algorithm (MPA) for Breast Tomosynthesis (BT) using real projection images acquired with phantoms at a clinical setting.

METHODS

The CIRS-BR3D phantom with ranging thicknesses between 3 cm and 6 cm was used for all image quality evaluations. Five sets of measurements were acquired, each comprised of a 2D mammographic image followed by a set of 25 projections within an arc length of 50°. A reconstruction algorithm based on the MPA was adapted for partial isocentric rotation using a stationary detector. For reference purposes, a Back Projection (BP) algorithm was also developed for this geometry. The performance of the algorithms was evaluated, in combination with pre-filtering of the projections, in comparative studies that involved also a comparison between tomosynthesis slices and 2D mammograms.

RESULTS

Evaluation of tomosynthesis slices reconstructed with BP and MPA showed close performance for the two algorithms with no considerable differences in feature detection, size and appearance of the background tissue with the MPA running faster the overall process. Pre-filtering of the projections, led to better BT images compared to non-filtering. Increased thickness resulted in limited detection of the features of interest, especially the smaller sized ones. In these cases, the filtered BT slices allowed improved visualization due to removed superimposed tissue compared to the 2D images. The different breast-like slab arrangements in phantoms of the same thickness demonstrated a slight influence on the quality of reconstructed features.

CONCLUSIONS

The MPA which had been applied previously to reconstruct tomograms from projections acquired at synchrotron facilities, is a time efficient algorithm, and is fully compliant with and can be successfully used in BT clinical systems. Compared to 2D mammography, BT shows advantage in visualizing features of small size and for increased phantom thickness or features within a dense background with superimposed structures.

A review of breast tomosynthesis. Part II. Image reconstruction, processing and analysis, and advanced applications

Many important post-acquisition aspects of breast tomosynthesis imaging can impact its clinical performance. Chief among them is the reconstruction algorithm that generates the representation of the three-dimensional breast volume from the acquired projections. But even after reconstruction, additional processes, such as artifact reduction algorithms, computer aided detection and diagnosis, among others, can also impact the performance of breast tomosynthesis in the clinical realm. In this two part paper, a review of breast tomosynthesis research is performed, with an emphasis on its medical physics aspects. In the companion paper, the first part of this review, the research performed relevant to the image acquisition process is examined. This second part will review the research on the post-acquisition aspects, including reconstruction, image processing, and analysis, as well as the advanced applications being investigated for breast tomosynthesis.

Comparison of visual grading and free-response ROC analyses for assessment of image-processing algorithms in digital mammography

OBJECTIVE

To compare two methods for assessment of image-processing algorithms in digital mammography: free-response receiver operating characteristic (FROC) for the specific task of microcalcification detection and visual grading analysis (VGA).

METHODS

The FROC study was conducted prior to the VGA study reported here. 200 raw data files of low breast density (Breast Imaging-Reporting and Data System I-II) mammograms (Novation DR, Siemens, Germany)-100 of which abnormal-were processed by four image-processing algorithms: Raffaello (IMS, Bologna, Italy), Sigmoid (Sectra, Linköping, Sweden), and OpView v. 2 and v. 1 (Siemens, Erlangen, Germany). Four radiologists assessed the mammograms for the detection of microcalcifications. 8 months after the FROC study, a subset (200) of the 800 images was reinterpreted by the same radiologists, using the VGA methodology in a side-by-side approach. The VGA grading was based on noise, saturation, contrast, sharpness and confidence with the image in terms of normal structures. Ordinal logistic regression was applied; OpView v. 1 was the reference processing algorithm.

RESULTS

In the FROC study all algorithms performed better than OpView v. 1. From the current VGA study and for confidence with the image, Sigmoid and Raffaello were significantly worse (p<0.001) than OpView v. 1; OpView v. 2 was significantly better (p=0.01). For the image quality criteria, results were mixed; Raffaello and Sigmoid for example were better than OpView v. 1 for sharpness and contrast (although not always significantly).

CONCLUSION

VGA and FROC discordant results should be attributed to the different clinical task addressed.

ADVANCES IN KNOWLEDGE

The method to use for image-processing assessment depends on the clinical task tested.

Comparison of algorithms for out-of-plane artifacts removal in digital tomosynthesis reconstructions

Digital tomosynthesis is a method of limited angle reconstruction of tomographic images produced at variable heights, on the basis of a set of angular projections taken in an arc around human anatomy. Reconstructed tomograms from unprocessed original projection images, however, are invariably affected by tomographic noise such as blurred images of objects lying outside the plane of interest and superimposed on the focused image of the fulcrum plane. The present work investigates the performance of two approaches for generation of tomograms with a reduced noise: a generalised post-processing method, based on constructing a noise mask from all planes in the reconstructed volume, and its subsequent subtraction from the in-focus plane and a filtered Multiple Projection Algorithm. The comparison between the two algorithms shows that the first method provides reconstructions with very good quality in case of high contrast features, especially for those embedded into a heterogeneous background.

Monte Carlo simulation of breast imaging using synchrotron radiation

PURPOSE

Synchrotron radiation (SR), being the brightest artificial source of x-rays with a very promising geometry, has raised the scientific expectations that it could be used for breast imaging with optimized results. The "in situ" evaluation of this technique is difficult to perform, mostly due to the limited available SR facilities worldwide. In this study, a simulation model for SR breast imaging was developed, based on Monte Carlo simulation techniques, and validated using data acquired in the SYRMEP beamline of the Elettra facility in Trieste, Italy. Furthermore, primary results concerning the performance of SR were derived.

METHODS

The developed model includes the exact setup of the SR beamline, considering that the x-ray source is located at almost 23 m from the slit, while the photon energy was considered to originate from a very narrow Gaussian spectrum. Breast phantoms, made of Perspex and filled with air cavities, were irradiated with energies in the range of 16-28 keV. The model included a Gd(2)O(2)S detector with the same characteristics as the one available in the SYRMEP beamline. Following the development and validation of the model, experiments were performed in order to evaluate the contrast resolution of SR. A phantom made of adipose tissue and filled with inhomogeneities of several compositions and sizes was designed and utilized to simulate the irradiation under conventional mammography and SR conditions.

RESULTS

The validation results of the model showed an excellent agreement with the experimental data, with the correlation for contrast being 0.996. Significant differences only appeared at the edges of the phantom, where phase effects occur. The initial evaluation experiments revealed that SR shows very good performance in terms of the image quality indices utilized, namely subject contrast and contrast to noise ratio. The response of subject contrast to energy is monotonic; however, this does not stand for contrast to noise ratio, since there is a range of optimal performance for SR (18-21 keV). In comparison to conventional mammography, SR shows improved subject contrast for energies lower than the mean energy of each spectrum.

CONCLUSIONS

The comparison of the results of the two models, conventional and SR, proved that SR exhibits better performance in the majority of cases. The proposed simulation model offers the possibility to perform exhaustive search to evaluate the performance of SR in clinical applications such as breast imaging.

Breast tomosynthesis

Digital mammography has been well-evaluated for the diagnosis of breast cancer. The scientific data show that mammography alone, especially in dense breast parenchyma, has its weaknesses. These weaknesses are due to the low contrast of tumors in comparison with the surrounding parenchyma and the overlying structures that mask tumors. The initial results from tomosynthesis studies show a tendency for better imaging and higher accuracy and lower recall rates with tomosynthesis. We present in this article a literature review of the development of breast tomosynthesis and follow it with case examples.