Fulvia Arfelli

Investigation of the imaging quality of synchrotron-based phase-contrast mammographic tomography

We report the results of a systematic study of phase-contrast x-ray computed tomography in the propagation-based and analyser-based modes using specially designed phantoms and excised breast tissue samples. The study is aimed at the quantitative evaluation and subsequent optimization, with respect to detection of small tumours in breast tissue, of the effects of phase contrast and phase retrieval on key imaging parameters, such as spatial resolution, contrast-to-noise ratio, x-ray dose and a recently proposed ‘intrinsic quality’ characteristic which combines the image noise with the spatial resolution. We demonstrate that some of the methods evaluated in this work lead to substantial (more than 20-fold) improvement in the contrast-to-noise and intrinsic quality of the reconstructed tomographic images compared with conventional techniques, with the measured characteristics being in good agreement with the corresponding theoretical estimations. This improvement also corresponds to an approximately 400-fold reduction in the x-ray dose, compared with conventional absorption-based tomography, without a loss in the imaging quality. The results of this study confirm and quantify the significant potential benefits achievable in three-dimensional mammography using x-ray phase-contrast imaging and phase-retrieval techniques.

Diffraction-enhanced imaging: improved contrast and lower dose x-ray imaging

Conventional x-ray imaging relies almost entirely on differences in the absorption of x-rays between tissues to produce contrast. While these differences are substantial between bone and soft tissue, they are very small between different soft tissue types resulting in poor visualization of soft tissues. Diffraction enhanced imaging (DEI) is currently in development by several groups as a new imaging modality that exploits information contained within the x- ray scattering distribution at low angles. We have used the SYRMEP beam line at the Elettra Synchrotron facility in Trieste, Italy to image a variety of tissue specimens, together with several phantoms. Mono-energetic photons in the range 17 keV to 25 keV were used with an analyzer crystal which diffracted the x-rays onto a detector. We have obtained some spectacular images which display remarkable contrast and resolution. The images can be processed to separate the pure absorption and pure refraction effects in a quantitative manner. These images demonstrate that DEI provides tissue morphology information not accessible with conventional radiographic imaging. The contrast caused primarily by refraction as the x-ray passes from one tissue type to another in the specimen is evident. Since x-ray refraction is much less energy dependent than absorption there is considerable potential for extremely low dose imaging. We believe that the potential of this technique is considerable and we present dat to illustrate the quality of the images.

On the possibility of utilizing scattering-based contrast agents in combination with diffraction-enhanced imaging

Preliminary experiments have been carried out in order to evaluate the potential of the Diffraction Enhanced Imaging (DEI) technique in combination with contrast agents not based on X-ray absorption properties, but that provide strong scattering signals. The contrast agents tested in this study are microbubble echo-enhancing agents, usually used in ultrasound examinations, which are completely invisible with conventional X-ray absorption techniques. A DEI set-up has been implemented at the Medical Beamline at the synchrotron radiation facility ELETTRA (Trieste, Italy). The analyzer crystal is a single flat silicon crystal utilized in the [111] reflection. By shifting the analyzer crystal to different positions of the rocking curve it is possible to detect the scattered photons; in particular, if the sample consists of a large number of particles with size smaller than the pixel size of the detector, an overall effect can be visualized. Phantoms containing ultrasound contrast agents have been built and imaged at different angular positions of the analyzer crystal at 17 keV and 25 keV. For all the phantoms a much stronger contrast has been measured in comparison to the contrast evaluated from the images produced with normal absorption methods.

Diffraction-enhanced imaging utilizing different crystal reflections at Elettra and NSLS

Diffraction Enhanced Imaging (DEI) is a powerful X-ray imaging technique that allows the visualization of structures having different refraction and/or absorption properties with respect to the background. In DEI, the sample is irradiated with a monochromatic and highly collimated X-ray beam, and the outgoing beam is analyzed by means of a perfect crystal. A comparison was drawn among DEI images of a standard (ACR) and a custom phantom using different harmonic diffraction orders. Images were obtained at two different synchrotron beamlines, the SYRMEP beamline at Elettra and the X15A beamline at the NSLS (Brookhaven, NY), utilizing a double-crystal Si monochromator and a single-crystal Si analyzer, operated in the symmetric, non-dispersive Bragg configuration. The harmonic order was separated by placing a refractive prism between the two crystals of the monochromator. The use of the and the reflections resulted in a 5-fold improvement in the analyzer angular sensitivity, consequently enhancing the extinction and refraction contrasts with respect to the reflection. The detail visibility was improved by 1-2 orders of magnitude. By means of the refractive prism technique, even higher harmonics might be used, thus promising even better image quality.

Improvements in the field of radiological imaging at the SYRMEP beamline

One major goal of modern radiology is the improvement of image quality and subsequently the development of sophisticated radiographic methods which are capable of detecting low contrast and small size details in organic samples in particular in mammography where the requirements on contrast resolution and spatial resolution are extremely high. Significant improvements in image quality have been achieved by the SYRMEP (SYnchrotron Radiation for MEdical Physics) collaboration which has designed and built a beamline devoted to medical physics at the synchrotron radiation facility ELETTRA in Trieste (Italy). The detection system developed for digital mammography consists of a silicon pixel detector with a pixel size of 200 X 300 micrometers 2 used in the `edge on configuration in order to achieve a high conversion efficiency. The detector is equipped with a low noise VLSI amplifier chain; at present. Recently, a multilayer detector prototype has been implemented, consisting of a stack of three single silicon strip layers. This set-up provides a larger sensitive area and subsequently a reduction of the exposure time. Digital images of mammographic phantoms and of in vitro full breast tissue samples show a higher contrast resolution and lower absorbed dose when compared to conventional mammographic images. Besides, further promising studies have been initiated developing novel imaging methods based on the phase effects evidenced by the high degree of coherence of the SR source. At the SYRMEP beamline several experiments have been carried out in order to exploit the potentials of two different techniques, Phase Contrast and Diffraction Enhanced Imaging, respectively. Images showing better detail visibility and enhanced contrast were produced with dose lower or comparable to the conventional one.

Contrast enhanced refraction imaging

An attempt has been made, for the first time, to extend the capabilities of diffraction enhanced imaging (DEI) using low concentrations of a contrast agent. A phantom has been constructed to accommodate a systematic series of diluted bromine deoxyuridase (BrDU) samples in liquid form. This was imaged using a conventional DEI arrangement and at a range of energies traversing the Br K-edge. The images were analyzed to provide a quantitative measure of contrast as a function of X-ray energy and (BrDU) concentration. The results indicate that the particular experimental arrangement was not optimized to exploit the potential of this contrast enhancement and several suggestions are discussed to improve this further.