Medical, industrial and security x-ray systems should give images that demonstrate material contrast for accurate identification. Acquiring images simultaneously from coherently scattered x-rays plus primary is an efficient way to do so. In our projection imaging system for small biological samples, the same detector is used for both primary and scatter, necessitating attenuation of the post-object primary in order that both primary and scatter lie within the detector’s dynamic range. Consideration of the cross sections shows that the peak scatter-to-primary fluence ratio is nearly independent of photon energy. At lower energies, coherent scatter produces larger diffraction rings, which give better spatial separation of primary and scatter, but there is more attenuation and multibeam information disentanglement is more difficult. Previous development work for our system used a 110kV incident beam, with a 1.5-mm-thick post-object attenuator disk of 90% W/10% Cu alloy for each of the 15 pencil beams. In this work we investigate alternatives which are less attenuating. By reducing the kV and using K-edge filters the beam average energy is lowered to achieve greater primary image contrast. Preliminary primary beam results are shown.
Coherent x-ray scatter is material specific, and imaging systems utilizing information from coherently scattered x rays are promising for security and medical applications requiring material identification with high sensitivity. A persistent challenge for practical implementation of these systems has been slow image acquisition. Our approach to reducing acquisition time is to develop a multibeam projection imaging system rather than a volumetric (CT or otherwise) imaging system. Previously we implemented a synchrotron-based system with five coplanar pencil beams and continuous motion of the object. Now we present a tabletop x-ray scatter imaging system built using a rotating-anode x-ray tube and a scintillating, energy-integrating flat-panel detector. A conventional source is more challenging to use than a synchrotron beam due to polychromaticity, low intensity, beam divergence, and x-ray tube thermal considerations. Simulations were performed to determine the system layout that optimized the intensity and angular resolution of scatter signals. The tube is inclined 6.1° to reduce apparent focal spot size. The primary collimation allows for an array of up to three rows by five columns of pencil beams, 3mm diameter and 2 cm apart at the object midplane 35 cm from the source, to irradiate the object simultaneously. There is no scatter collimation and the multiplexed scatter signals are disentangled using a maximum-likelihood expectation maximization algorithm. Motorized translation stages scan the object through the beams. The system can image objects up to 10 × 10 × 10 cm3 and 1 kg. Post-object primary beam attenuators allow for the same detector to measure transmitted and scattered x rays simultaneously. Initial images acquired with the system are presented. Using 15 beams, a 6000-pixel scatter image of a 6 cm × 10 cm region was acquired in 4.6 min.
We report on the adaption of new particle physics gas proportional microdetectors filled with a xenon/methane gas mixture at high pressure as image receptors for medical x-ray imaging. Currently in radiology, all detectors used clinically merely integrate the energy deposited by the beam. The next generation of detectors for digital radiography and computed tomography (CT) will obtain extra information by counting individual photons and measuring their energy. This will enable (i) implementation of single exposure dual-energy radiography, (ii) for CT, reconstruction of images free of spectral artefacts, and (iii) for the same quantum efficiency, reduced image noise compared with images obtained by energy integration. The new gas microdetectors can measure individual photon energies at radiological fluence rates and have high spatial resolution. Prototype systems in our laboratory have demonstrated a limiting spatial resolution of 7.0 1p mm-' for a 30 kV x-ray spectrum and 11.9 1p mm-' for 50 kV. The energy resolution at 4 atm was 10% at 17.7 keV and 8% at 59.6 keV. The counting rate ability for unfiltered 30 kV x rays was < 2 x 106 mm-2 s-' at 3 atm of Xe:methane corresponding to an exposure rate < 25 mR s-1. Future systems in which the readout anodes are parallel to the photon direction will be capable of even higher rates.
All medical x-ray imaging today is done using the transmitted photons, i.e., those x-ray quanta which do not suffer any interaction within the patient. An alternative is to use the more plentiful scattered photons. Backscatter is almost entirely Compton (incoherent) scatter, which is principally sensitive to the number of electrons per unit volume. Forward scatter is dominated by coherent scatter, which is the basis of x-ray diffraction. Its cross section varies with angle and photon energy in a material-specific manner, even for amorphous materials. The dependence on Z and chemical structure allows it to be very useful in distinguishing tissues within the patient. Many workers have demonstrated utilization of both types of scatter in the lab, but it has been difficult to compare the performance of these systems with conventional transmission imaging. Therefore, we devised a semi-analytic model of scatter imaging. Our calculations predict that for some imaging tasks the contrast and signal-to-noise ratio achieved by collecting a portion of the scatter (in an annular cone) will be superior to that achieved by conventional transmission imaging, for the same number of photons incident on the patient. Our analysis is reliant on the limited published data for coherent scattering for biological materials.
Coherent x-ray scatter varies with angle and photon energy in a manner dependent on the chemical composition of the
scattering material, even for amorphous materials. Therefore, images generated from scattered photons can have much
higher contrast than conventional projection radiographs. We are developing a scatter projection imaging prototype at the
BioMedical Imaging and Therapy (BMIT) facility of the Canadian Light Source (CLS) synchrotron in Saskatoon, Canada.
The best images are obtained using step-and-shoot scanning with a single pencil beam and area detector to capture
sequentially the scatter pattern for each primary beam location on the sample. Primary x-ray transmission is recorded
simultaneously using photodiodes. The technological challenge is to acquire the scatter data in a reasonable time. Using
multiple pencil beams producing partially-overlapping scatter patterns reduces acquisition time but increases complexity
due to the need for a disentangling algorithm to extract the data. Continuous sample motion, rather than step-and-shoot,
also reduces acquisition time at the expense of introducing motion blur. With a five-beam (33.2 keV, 3.5 mm2 beam area)
continuous sample motion configuration, a rectangular array of 12 x 100 pixels with 1 mm sampling width has been
acquired in 0.4 minutes (3000 pixels per minute). The acquisition speed is 38 times the speed for single beam step-and-shoot.
A system model has been developed to calculate detected scatter patterns given the material composition of the
object to be imaged. Our prototype development, image acquisition of a plastic phantom and modelling are described.
Traditional projection x-ray imaging utilizes only the information from the primary photons. Low-angle coherent scatter
images can be made simultaneous to the primary images and provide additional information. To speed up acquisition
time for coherent scatter projection imaging, we developed disentangling algorithms for the overlapping scatter patterns
generated by multi pencil-beam geometries. A system at the Canadian Light Source synchrotron was configured which
utilizes a custom collimator designed to convert a 33.17 keV monoenergetic fan beam from a Laue monochromator into
multiple pencil beams by using 3 mm thick tungsten alloy stoppers. The pencil beams then travel through the sample and
are absorbed by a tungsten bar. A digital flat panel detector records the superimposed scatter patterns from the beams.
The sample is scanned through the beams using an automated step-and-shoot setup. The pixel value of the coherent
scatter image is generated by integrating the radial profile (scatter intensity versus scattering angle) over an angular
range. An MLEM-based iterative method and a least-squares method were developed to disentangle the scatter patterns.
Although past work has primarily been applied to medicine, other applications include non-destructive testing and
security.
The coherent scattering form factor defines a material's small angle x-ray scattering properties. These scattering properties can provide useful medical diagnostic information if properly utilized. Measurement of the coherent scattering form factor is difficult, requiring expensive equipment and long measurement times. We show that it is possible to measure the coherent scattering form factor using standard clinical equipment through a matrix equation. The matrix elements are constructed from knowledge of the input x-ray spectrum. In the ideal case, the form factor can be extracted from this equation by inverting the matrix. For typical x-ray spectra and form factors, however, the matrix tends to be poorly conditioned, leading to large errors upon inversion. We have developed a sub-matrix method that constructs a series of smaller, well-conditioned matrices that can be accurately inverted to give the required form factor. We show through numerical simulations that the sub-matrix method can accurately measure the form factor of common tissue materials. Root mean square deviations of 0.0502 and 0.0804 were calculated for the form factors of water and fat with 90 kV spectra and 0.2 mm of tungsten filtration. Over the measurement range, the form factors vary between approximately 0.5 and 2.5. We show that the optimal spectral shape when using the sub-matrix method is one that is strongly peaked at high energies and that using an improperly chosen spectrum can result in a significant loss of accuracy. We also demonstrate that the sub-matrix method is not readily applicable for measurements of strongly ordered materials.
Conventional medical x-ray imaging, based on the transmission of primary photons, works well to distinguish between hard and soft tissues. Up to 90% of the photons that reach the image receptor, however, are coherently or incoherently scattered, and so there is growing interest in utilizing scattered x rays for diagnosis. The semi-analytical model developed by our group predicts better contrast and signal-to-noise ratio for scatter imaging than for primary for some diagnostic examinations such as distinguishing white versus gray brain matter, and for mammography. Low-angle scattered photons can only be distinguished from primary on the basis of direction and consequently a well-collimated x-ray system is required. A hexagonal array of seven 1.5 mm diameter pinholes is designed and tested to record the diffraction pattern of plastic and water phantoms. These materials are amorphous solids and result in rotationally-symmetric diffraction patterns which are characteristic of the materials. The intensities of the diffraction patterns are numerically integrated over concentric rings and the scatter images are made by assigning the ring sums as the pixel values. For these measurements the tube is operated with technique factors ranging from 70 kV 2500 mAs to 120 kV 500 mAs. The scatter patterns are recorded on a storage phosphor image plate. Test images are made of 1 cm thick targets in air and in a water tank. The ultimate goal is to make scatter images of different kinds of tissues for better diagnostic information.
Although scattered radiation is generally regarded as a nuisance in radiological imaging, many innovative imaging concepts that use the scattered field have been demonstrated. A systematic approach, however, for analyzing the medical applications of x-ray scatter imaging has been lacking. We have therefore formulated a simple semi-analytic model that consists of imaging a target object against a background material of the same dimensions when both are situated within a water phantom. The target and background objects have small cross-sectional areas (1.0 mm2) to allow the omission of self-attenuation and multiple scatter within the objects. The incident energy fluence is kept constant so that similar doses are delivered by the various photon beams. For imaging white brain matter versus gray brain matter in a 15 cm thick water phantom, the maximum signal-to-noise ratio, over all photon energies, for images obtained with the forward scatter between 2 degrees - 12 degrees exceeds that of primary images for all object thicknesses less than or equal to 40 mm. The penalty in dose as a result of spectral blur is generally moderate. For example, using an 80 kV beam for the previous imaging task would require approximately a 24% dose increase relative to using a monoenergetic beam. A high-precision experimental apparatus has been assembled to validate our predictions.
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