In conventional X-ray microtomography (μCT), the three-dimensional (3D) distribution of the attenuation coefficient of X-rays is measured and reconstructed in a 3D volume. As spatial resolution increases, the refraction of X-rays becomes a significant phenomenon in the imaging process. Although this so-called phase contrast was initially a cumbersome feature in lab-based μCT, special phase retrieval algorithms were developed to exploit these effects. Clear advantages in terms of visualization and analysis can be seen when phase retrieval algorithms are applied, including an increased signal-to-noise ratio. In this work, this is demonstrated both on simulated and measured data.

Improved signal-to-noise ratio in laboratory-based phase contrast tomography.

With increased resolution in x-ray computed tomography, refraction adds increasingly to the attenuation signal. Though potentially beneficial, the artifacts caused by refraction often need to be removed from the image. In this paper, we propose a postprocessing method, based on deconvolution, that is able to remove these artifacts after conventional reconstruction. This method poses two advantages over existing projection-based (preprocessing) phase-retrieval or phase-removal algorithms. First, evaluation of the parameters can be done very quickly, improving the overall speed of the method. Second, postprocessing methods can be applied when projection data is not available, which occurs in several commercial systems with closed software or when projection data has been deleted. It is shown that the proposed method performs comparably to state-of-the-art methods in terms of image quality.

Postprocessing method for reducing phase effects in reconstructed microcomputed-tomography data.

Digital mammography has limitations in sensitivity, in particular for patients with a dense breast. Phase contrast techniques phase contrast mammography, PCM might increase the tissue contrast for breast imaging. Propagation based PCM with a dedicated 0.1-mm-focal spot size mammography unit was investigated in past years, showing higher image quality in magnification PCM than in absorption based DM. In this work the authors investigated, using breast phantoms, the dependence of image quality on increasing mean glandular dose with a 0.007-mm-focal spot size W-anode microfocus X-ray tube. They compared PCM imaging magnification Mi¾?i¾?i¾?2 to absorption based contact imaging Mi¾?i¾?i¾?1 and then to phase retrieval for phase imaging, at low 40 kV as well as high 80 kV beam energy. Phase imaging shows higher image contrast for glandular masses and microcalcifications with MGD similar to one-view mammography. The phase contrast power spectrum assumes higher values than for absorption imaging. Possibility of dose reduction was suggested by the adoption of phase retrieval PCM.

Image Quality and Radiation Dose in Propagation Based Phase Contrast Mammography with a Microfocus X-ray Tube: A Phantom Study

Liquid crystal display (LCD) of mammograms provides soft-copy results that differ in conventional and phase contrast mammography (PCM) PCM potentially offers the highest quality of sharpness and graininess, an edge emphasis effect on the object, and the highest image resolution However, when the image is displayed on an LCD, the resolution depends on the pixel pitch and the PCM image data must be diminished We investigated the observed effect on spatial resolution and contrast when conventional or phase contrast mammograms are viewed on an LCD Using the tissue-equivalent phantom (Model 1011A), a conventional mammogram and a magnification radiography image were obtained with a PCM system This phantom contains simulated fibers, microcalcifications, and masses The PCM image was reduced 1/175 to render it consistent with life size mammography using the nearest neighbor, bilinear, and bicubic interpolation methods The images were displayed on a five million (5M)-pixel LCD with 100 % magnification Ten mammography technicians observed the reduction images displayed on LCDs and reported their results In the detectability of the microcalcifications, there was no significant difference between conventional mammograms and reduced PCM images Regarding fibers and masses, detectability using reduced images was higher than those of conventional images The detectability using images reduced by the nearest-neighbor method was lower than those of images reduced by two other interpolation methods Bilinear interpolation was affected by the smoothing effect, while CNR was increased In addition, since the noise of PCM image was reduced by an air gap effect, high detectability of key image features was found Soft-copy display of phase-contrast mammograms is feasible with LCDs, while detectability of fibers and masses was best with bilinear interpolation and use of an air gap

Phantom-based comparison of conventional versus phase-contrast mammography for LCD soft-copy diagnosis

With use of the phase shift of X-rays that occurs when they pass through an object, phase-contrast imaging (herein referred to as “phase imaging”) can produce images different from those of conventional contact imaging (herein referred to as “conventional imaging”). For this reason, assessment of the image quality based on noise-equivalent quanta (NEQ) and detective quantum efficiency (DQE) which does not include object-based information may not be appropriate for comparison of image quality between phase and conventional images. As an alternative method, we conceived a new image-quality assessment method with images that contain information about an object. First, we constructed images with an object and without an object under the same imaging parameters; then, we obtained two-dimensional power spectra by Fourier transform of those images. Second, we calculated the radial direction distribution function with the power spectra, and the distribution of signal intensity, which we defined as a signal intensity distribution function (SIDF). In this way, differences in image quality were evaluated relatively based on the SIDF of the imaged object. In our study, we first confirmed that phase-imaging evaluation was not appropriate by comparing NEQ and DQE of conventional, magnification, and phase imaging. Further, comparing the image quality of projected plant seeds by employing conventional, magnification, and phase imaging, we found that the phase-imaging method provided a higher image quality regarding edge sharpness than did conventional and magnification imaging. Therefore, based on these results, our image assessment method is considered useful for evaluation of images which include object-based information.

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Image-quality assessment method for digital phase-contrast imaging based on two-dimensional power spectral analysis

X-ray beams irradiated from an x-ray tube with a point source or a sufficiently small focal spot produce edge-enhanced images in the boundary of an object due to the effect of phase shift of x-rays. This technique is called phase contrast imaging. A digital phase contrast imaging system based on a photostimulable phosphor plate (imaging plate) designed for mammography has been developed for clinical use recently and now commercially available. In this study, the digital phase contrast images of an acrylic plate and plant seeds were acquired without any increase of incident dose to detector when compared to conventional contact digital imaging. Improvement of image edge sharpness was evaluated in terms of spatial edge response and spectral analysis of the images. In addition, the improvement of the sharpness of the image was also evaluated in clinical mammograms. Our results indicated that higher image sharpness in the boundary of the object was observed. The power spectrum of the digital phase contrast image was found to be higher than that of the digital contact image at wide spatial frequency region. In conclusion, the commercially available phase contrast imaging system can provide breast images with details that are not available in conventional mammograms. The digital phase contrast imaging would be useful to detect diseases, especially microcalcifications, in mammograms without any increase of exposure dose.

Junko Sugiyama

Papers

Preliminary Evaluation of a Phase Contrast Imaging with Digital Mammography