Search

International audience; Edge illumination is an X-ray phase-contrast imaging technique that can be efficiently applied to both synchrotron radiation and laboratory sources. Its implementation with these two types of setups is here described, and a recently developed method to perform quantitative retrieval of the object attenuation and refraction properties is presented. We report results obtained at two synchrotron radiation facilities and with one of the setups installed in our laboratories at University College London, which show that very high angular sensitivities can be obtained with this technique. The effect of different experimental parameters on the achievable sensitivity is also analyzed. The obtained results will be a useful guide for the design and optimization of future experimental layouts

A silicon strip detector for the SYRMEP (SYncrotron Radiation for MEdical Physics) experiment has been designed and realised. The main features of this detector are AC-coupling through integrated coupling capacitors, DC biasing by means of a gated punch-through structure, bulk contact on the junction side through a forward-biased p/sup +/ implant and integrated fan-in on active silicon. Results of laboratory tests of the detector parameters, allowing a thorough evaluation of the technological solutions employed, are presented. The functionality of the detectors and the charge collection linearity have been tested with different /spl gamma/ sources, using a hybrid, low-noise front-end electronics.

A synchrotron radiation beamline devoted to medical imaging has been built by the SYRMEP collaboration at Elettra in Trieste, Italy, and is now in operation. The SYRMEP imaging system is based on the combination of a monochromatic, laminar X-ray beam, with a laminar, high-efficiency pixel silicon detector. To image soft tissue, including small, low-contrast details, as in mammography, beam energies should be chosen in the range from 15 keV to 30 keV and the detector should be capable of single-photon counting. The SYRMEP beamline originates from a bending magnet light port and consists basically of a Be window, a slit system and a Si(1,1,1) crystal monochromator. The experimental area is equipped with a radiation flux monitor, a sample movement stage, and a positioning system holding the silicon detector. Images are obtained by keeping the detector stationary with respect to the beam and by scanning the sample through the beam itself. The current detector is a linear array of 200/spl times/300 /spl mu/m/sup 2/ pixels, each coupled to its electronic counting chain on a custom VLSI read-out chip. We present a summary of beam studies, and digital images of standard RMI 160 and RMI 180 Ackermann Mammographic phantoms. Results show the high contrast resolution, the good spatial resolution and the large dynamic range which constitute the defining characteristics of the SYRMEP imaging system.

We present images of a mammographic test object obtained using a linear array silicon pixel detector capable of single-photon counting. The detector pixel size was 200 x 300 microns2 and images were acquired by scanning the test object between the laminar detector and the x-ray source with a scanning step of 100 microns. A molybdenum anode tube was used with two different filtrations: 2 mm aluminium and 25 microns molybdenum. Conventional film-screen images were also obtained in order to compare spatial and contrast resolution. In our digital images it is possible to recognize low-contrast details having dimensions smaller than or equal to the dimensions of details visible by means of a clinical mammographic unit. The detection of microcalcifications smaller than 150 microns was possible only when using the Mo filtration. However a copper wire of 50 microns diameter was detectable when embedded in a simulated tissue. We discuss in detail the mean glandular doses (MGDs) delivered during the image acquisition. The MGDs necessary to obtain good-quality images are always smaller than at a conventional mammographic unit. Since MGDs depend on the x-ray spectrum, the dose reduction becomes larger when the applied spectrum is harder than in film-screen acquisition (Al filtration and 35 kVp).

The SYRMEP project (SYnchrotron Radiation for MEdical Physics) intends to improve mammography performances using two non-traditional means: synchrotron radiation and silicon detectors. A silicon crystal (the monochromator) allows to choose the optimal beam energy that leads to the maximum signal to noise ratio, while the detectors are arranged to form a matrix of pixels and coupled to low noise high-gain VLSI integrated circuits making the single photon counting technique possible. We present the data acquisition system implemented to read out the VLSI circuit with particular emphasis on two dedicated CAMAC modules that generate the control signals and store the chip digital output on memories that can be accessed via the CAMAC bus. It has been useful in testing the chip performances and can be considered a prototype of the final SYRMEP data acquisition system.

A silicon strip detector for the SYRMEP (SYnchrotron Radiation for MEdical Physics) experiment has been designed and realised. The main features of this detector are AC-coupling through integrated coupling capacitors, DC bias of the strips by means of a gated punch-through structure, bulk contact on the junction side through a forward-biased p + implant, thinned entrance window for the incoming radiation (in an “edge-on” geometry) and integrated fan-in on active silicon. Results of laboratory tests of the detector parameters, allowing a thorough evaluation of the technological solutions employed, are presented.

The SYRMEP collaboration (Synchrotron Radiation for Medical Physics) is developing a novel imaging device to be used with a laminar monochromatic synchrotron radiation X-ray beam for applications in digital mammography. Its main components are silicon microstrip detectors and a dedicated VLSI readout chip based on the single photon counting concept. The innovative geometry-several overlapped detectors with strips parallel to the beam direction-determines a matrix of pixels, each of them having an area of 200/spl times/300 /spl mu/m/sup 2/, which can detect up to 10/sup 6/ photons/(mm/sup 2/ s) with 90% efficiency at 25 keV. Front-end and readout electronics are expandable in a modular fashion and can be coupled to successive receptor versions with increasing number of channels, while a VME based DAQ system offers full testing facilities. In our view this system is providing specific and original contributions to digital mammography but it could also be useful in other modern radiology fields.

We derive a Fourier formulation of coded-aperture x-ray phase-contrast imaging, based on the wave theory of optics in the Fresnel approximation. We use this model to develop a flexible, efficient, and general simulation algorithm that can be easily adapted to other implementations of x-ray phase contrast imaging. Likewise, the algorithm enables a simple extension to 2D aperture designs, different acquisition schemes, etc. Problems related to numerical implementation of the algorithm are analyzed in detail, and simple rules are derived that enable us to avoid or at least mitigate them. Finally, comparisons with experimental data and data obtained with a different simulation algorithm are presented to validate the model and demonstrate its advantages in practical implementations. This also enabled us to demonstrate an increase in computational speed of more than one order of magnitude over a previous algorithm. (C) 2013 Optical Society of America

The use of microbubbles as a contrast agent for x-ray phase contrast imaging could both transform x-ray imaging into a “functional” modality and enable much needed monitoring of targeted drug delivery. To realize these benefits, it is essential to be able to quantify bubble concentration in a given tissue volume. We developed and validated a model that enables this to be achieved not only for phase-retrieved images obtained by processing multiple frames but also on “single-shot” images, a likely necessity in in-vivo implementations. Our experimental validation was based on analyzer-based imaging, but extension to other phase-based modalities is straightforward.

We have developed a totally incoherent, non-interferometric x-ray phase contrast imaging (XPCI) method. This is based on the edge illumination (EI) concept developed at the ELETTRA synchrotron in Italy in the late `90s. The method was subsequently adapted to the divergent beam generated by a conventional source, by replicating it for every detector line through suitable masks. The method was modelled both with the simplified ray-tracing and with the more rigorous wave-optics approach, and in both cases excellent agreement with the experimental results was found. The wave-optics model enabled assessing the methods' coherence requirements, showing that they are at least an order of magnitude more relaxed than in other methods, without this having negative consequences on the phase sensitivity. Our masks have large pitches (up to 50 times larger than in grating interferometry, for example), which allows for manufacturing through standard lithography, scalability, cost-effectiveness and easiness to align. When applied to a polychromatic and divergent beam generated by a conventional source, the method enables the detection of strong phase effects also with uncollimated, unapertured sources with focal spots of up to 100 μm, compatible with the state-of-the-art in mammography. When used at synchrotrons, it enables a contrast increase of orders of magnitude over other methods. Robust phase retrieval was proven for both coherent and incoherent sources, and additional advantages are compatibility with high x-ray energies and easy implementation of phase sensitivity in two directions simultaneously. This paper briefly summarizes these achievements and reviews some of the key results.

In the last few years the interest for application of silicon radiation detectors to the detection of X-rays in the 15–30 keV region has been growing. One of the main reasons of such an interest is the development of devices for medical and biological applications, in particular for the diagnostic radiology of soft tissues. For these applications, to ensure a high detection efficiency is of crucial importance. This leads to the use of “on purpose” geometrical arrangements and technological solutions for the detector, as well as the use of specifically designed read-out electronics. In this paper we present results and images obtained with silicon microstrip detectors in a “pixel-like” geometry, with the X-ray beam impinging parallel to the strips. An efficiency close to 80% has been obtained for photons of energies around 20 keV. The ongoing developments, concerning the realisation of a dedicated silicon detector for digital radiology at the SYRMEP (SYnchrotron Radiation for MEdical Physics) beam line of the ELETTRA synchrotron radiation facility, are described.

The SYRMEP collaboration is developing a digital mammography project using a synchrotron radiation monochromatic X-ray beam and a silicon pixel detector. The combination of a monochromatic laminar beam with a high efficiency laminar detector, capable of single-photon counting, allows one to minimize the radiation dose delivered to the sample, while maximizing contrast resolution and dynamic range. The SYRMEP detector is a silicon microstrip device used in an innovative configuration in which radiation impinges on the side rather than on the surface of the chip and is therefore totally absorbed within the detector active volume. The high contrast resolution and spatial resolution (

The SYRMEP (SYnchrotron Radiation for MEdical Physics) collaboration is now developing a microstrip silicon detector with high granularity and high efficiency for research in digital mammography. The project is planning to use a monochromatic X-ray beam, from a bending magnet, of ELETTRA, the synchrotron facility installed in Trieste. The detector is a matrix of pixels, formed by the slacking of different layers of strip silicon pads, with pixel dimensions determined by the pitch and the thickness of the pads themselves. The final detector, an AC-coupled FOXFET-type high resistivity silicon pad, will cover the whole beam region of 4 x 150 mm(2). Different pixel sizes have been tested (from 0.5 x 0.5 mm(2) to 0.3 x 0.2 mm(2)). The dead volume in the front of the detector has been reduced in order to reach 80% efficiency in the photon collection. The stacking of the layers has been obtained by means of screen printing deposits of non-conductive glue whose height has been chosen in order to minimize the inter-layer dead region and the cross talk of superimposed pixels, We present here the results of our studies in terms of imaging and the status of the final detector.

We discuss the problem of signal diffusion among neighbouring pixels in x-ray phase contrast imaging (XPCi) specifically for coded-aperture (CA) XPCi, but many of the discussed observations are directly transferable to other XPCi modalities. CA XPCi exploits the principle of pixel edge illumination by means of two CA masks. The first mask, placed in contact with the detector, creates insensitive regions between adjacent pixels; the second one, placed immediately before the sample, creates individual beams impinging on the boundaries between sensitive and insensitive regions on the detector, as created by the detector mask. In this way, edge illumination is achieved for all pixels of an area detector illuminated by a divergent and polychromatic beam generated by a conventional source. As the detector mask redefines the resolution properties of the detector, sample dithering can be used to effectively increase the system spatial resolution, without having to apply any post-processing procedure (e.g., deconvolution). This however creates artifacts in the form of secondary fringes (which have nothing to do with phase-related secondary fringes) if there is signal diffusion between adjacent pixels. In non-dithered images, signal diffusion between adjacent pixels causes a reduction in image contrast. This effect is investigated both theoretically and experimentally, and its direct implications on image quality are discussed. The interplay with the sample positioning with respect to the detector pixel matrix, which also has an effect on the obtained image contrast, is also discussed.

This work analyzes the performance of the coded-aperture based x-ray phase contrast imaging approach, showing that it can be used at high x-ray energies with acceptable exposure times. Due to limitations in the used source, we show images acquired at tube voltages of up to 100 kVp, however, no intrinsic reason indicates that the method could not be extended to even higher energies. In particular, we show quantitative agreement between the contrast extracted from the experimental x-ray images and the theoretical one, determined by the behavior of the material’s refractive index as a function of energy. This proves that all energies in the used spectrum contribute to the image formation, and also that there are no additional factors affecting image contrast as the x-ray energy is increased. We also discuss the method flexibility by displaying and analyzing the first set of images obtained while varying the relative displacement between coded-aperture sets, which leads to image variations to some extent simi...

There is currently much interest in developing X-ray Phase Contrast Imaging (XPCI) systems which employ laboratory sources in order to deploy the technique in real world applications. The challenge faced by nearly all XPCI techniques is that of efficiently utilising the x-ray flux emitted by an x-ray tube which is polychromatic and possesses only partial spatial coherence. Techniques have, however, been developed which overcome these limitations. Such a technique, known as coded aperture XPCI, has been under development in our laboratories in recent years for application principally in medical imaging and security screening. In this paper we derive limitations imposed upon source polychromaticity and spatial extent by the coded aperture system. We also show that although other grating XPCI techniques employ a different physical principle, they satisfy design constraints similar to those of the coded aperture XPCI.

X-ray phase contrast imaging is a very promising technique which may lead to significant advancements in medical imaging. One of the impediments to the clinical implementation of the technique is the general requirement to have an x-ray source of high coherence. The radiation physics group at UCL is currently developing an x-ray phase contrast imaging technique which works with laboratory x-ray sources. Validation of the system requires extensive modelling of relatively large samples of tissue. To aid this, we have undertaken a study of when geometrical optics may be employed to model the system in order to avoid the need to perform a computationally expensive wave optics calculation. In this paper, we derive the relationship between the geometrical and wave optics model for our system imaging an infinite cylinder. From this model we are able to draw conclusions regarding the general applicability of the geometrical optics approximation.

We present an x-ray phase contrast imaging method based on coded apertures sensitive to phase effects in two directions simultaneously. To date, this is the only non-free-space propagation approach with this capability. Whereas the use of free-space propagation methods is limited to synchrotron radiation or microfocal x-ray sources, which impose severe limits in terms of practical applicability, coded-aperture based methods have been shown to provide synchrotronlike phase contrast enhancements with conventional x-ray sources. A two-directional sensitive method working with conventional sources could create a breakthrough in medical imaging, where two-directional sensitivity is often a mandatory requirement.

A novel prototype step and shoot x-ray system, the intelligent imaging system (I-Imas), has been developed in the UCL Radiation Physics laboratories. The system uses collimators to split the beam into two: the first, "scout" beam, strongly attenuated, identifies regions of interest in the image. This information is then fed back to the system so that the intensity of the second beam is modulated to optimize the dose distribution--i.e., "interesting" regions of the sample receive a higher dose, whereas other regions receive a lower one. Such systems may be capable of improved diagnostic capability at the same overall patient dose levels as typical when using large area digital plates. This acquisition mode means that large overall images are obtained by aligning many smaller images. This paper investigates the effects that this acquisition modality has on the overall spatial resolution of the system. We review different modulation transfer function (MTF) evaluation techniques and those shown to be optimal are used in the investigation of two considerations key to such a system: (i) whether there is a minimum size sensor whose MTF can accurately be determined using these techniques and (ii) whether the MTF of the large overall image differs significantly from those of the many constituent images. As the use of step and shoot systems is becoming more and more widespread, both are important considerations. We found that, for a fixed pixel pitch, the MTF is determined marginally less accurately the smaller the sensor area, with the perceived resolution varying by up to 0.1 lp/mm. It was also found that use of such a step and shoot technique does cause a very small overall degradation in resolution. The resolution of overall images was calculated to be 0.1 lp/mm lower than that of the individual images acquired.

A novel technique based on coded apertures which makes x-ray phase contrast imaging (XPCi) feasible with conventional sources has been recently developed at University College London. This technique is expected to enable for the first time real-world applications of XPCi in fields as diverse as medical, biological and scientific imaging, industrial inspections and security scans.This paper focuses on XPCi applications in homeland security. A simulation of coded-aperture XPCi, which was experimentally validated in a previous work, has been used to produce images of potential threat objects, and to quantitatively assess detail visibility improvements in terms of contrast and signal-to-noise ratio over conventional techniques based on absorption.Free-space propagation phase contrast images taken with synchrotron radiation (SR) are also presented. Although not directly related to the coded-aperture technique used in the simulation, these can be considered indicative of the image quality that would be made available through coded-aperture XPCi with conventional sources: previous studies have demonstrated that this technique provides image enhancements compatible with those obtained with free-space propagation phase contrast with SR. Substantial enhancements in image quality and improvement in detail detectability in cluttered environments are demonstrated in this way. The results presented in this study and the software used to generate them will provide the basis for the design and realization of a dedicated imaging prototype.

The detection limit of invasive carcinoma by standard prone scintimammography appears to be >=1 cm diameter. Since it is desirable to detect lesions at very earliest stages of growth (0.8 cm or less), the development of prototype scintimammographic systems with improved imaging performances is a primary goal. Here, the authors propose a dedicated high resolution breast imaging scanner for SPECT around the Vertical Axis of Rotation (VAOR) with the breast in prone position. The rotating detector module consists of a compact position sensitive photomultiplier tube (PS-PMTHamamatsu R7600-00-C8 coupled to a CsI(Tl) scintillating array. The compactness is the peculiarity of this detector module to allow the lodging in the breast interspace to work close to the chest wall and to minimize the tumor to collimator front distance. A preliminary study was performed by a breast phantom and 5" small FOV gamma camera with an actual rotation radius of 6.5 cm. Images were reconstructured using a filtered back projection implementation. Tumor SNR from planar images shown very low values: on the contrary reconstructed images strongly enhanced the contrast of a tumor 1 cm sized. A further enhancement was obtained by the compact detector module that was able to achieve higher SNR values for tumor less 1 cm sized).

A 3-layer edge-on silicon microstrip detector for the SYRMEP/FRONTRAD project has been designed and realised. The image matrix is made by 764 pixels with dimensions 300 (thickness of the single detectors) × 200 (strip pitch) μm2. The system has a sensitive area of 50 × 1 mm2, an inter-layer distance of ≈ 100 μm and an efficiency of ≈ 80% for 20 keV photons. The image is acquired by scanning the object across the beam cross-section and the overall statistics on the single pixel is obtained by summing up the information of corresponding pixels in the three layers, thereby reducing the effect of possible noisy or not functioning pixels. Experimental results obtained at the SYRMEP/FRONTRAD beam line of the ELETTRA synchrotron radiation machine with this innovative detector are presented.

The SYRMEP (SYnchrotron Radiation for MEdical Physics) imaging system is based on the combination of a monochromatic laminar X-ray beam, with a laminar high-efficiency pixel silicon detector. The use of synchrotron radiation X-rays has the advantage of minimising the radiation dose delivered to the sample because it is possible to choose the best energy for the considered radiological examination. The high efficiency of the detector (about 90% in 18-28 keV energy range) also lowers this dose figure. Since the detector is capable of single-photon counting, the information contained in the beam is fully exploited, leading to high contrast resolution. The present phase of the SYRMEP project involves a systematic study of in vitro breast tissue samples. The same samples are then imaged by means of a conventional mammographic Mo anode tube used in the common diagnostic practice and recorded on film/screen system. The comparison of the two imaging modalities shows the advantages, with special regard to contrast resolution, of the SYRMEP imaging system compared with conventional film-screen devices.

A linear-array, silicon pixel detector, capable of counting single photons, was applied to mammography by using a synchrotron radiation beam. Images were obtained of both a mammographic phantom and a breast-tissue sample. The phantom image was acquired with a mean glandular dose of 0.32 mGy. This detector combined with a synchrotron radiation beam allows acquisition of high-contrast, low-dose images of soft tissues.

Recently the application of small imagers in nuclear medicine is growing, particularly in scintimammography. In this paper we propose the use of the Hamamatsu R7600-C8 Position Sensitive Photomultiplier Tube (PSPMT) for detection image optimization in scintimammography through the evaluation of image performances of detector pixellation and its interaction with a collimator lattice . To this aim, a number of CsI(Tl) scintillating arrays with different pixel size and thickness were coupled to the same PSPMT. Considering the very high intrinsic spatial resolution, a look up table was realized to correct accurately the gain non uniformities. The results show an overall energy resolution FWHM, from flood field irradiation source, in close agreement with the individual crystal response and an uniformity response within ± 10%. Finally an analysis of tumor signal to noise ratio (SNR) as a function of detection pixellation was performed, utilizing a breast phantom. Tumor SNR values are highest for 5 mm thick CsI(Tl) arrays but shown slight differences with pixel size. The best tumor SNR value for 6 cm breast thick was obtained with 1.5 × 1.5 mm2 crystal pixel size. The collimator used was a low energy all purpose, parallel hole collimator with 22 mm length, 1.5 mm hexagonal hole, 0.2 mm Pb septa, currently utilized in scintimammography.