Quasi-monochromatic X-ray Beam Research Articles

Monte Carlo simulations of synchrotron X-ray dose affecting root growth during in vivo tomographic imaging

Synchrotron X-ray computed tomography (XCT) has been increasingly applied to study the in vivo dynamics of root growth and rhizosphere processes. However, minimizing radiation-induced damage to root growth warrants further investigation. Our objective was to develop a robust approach for modeling and evaluating ways to reduce synchrotron X-ray dose effects on root growth during in vivo imaging. Wheat roots growing in soil were exposed to X-rays during XCT experiments resolved in space (3D) plus time (4D). The dose rate and cumulative absorbed dose in roots were modelled using the Monte Carlo code FLUKA for different experimental conditions of polychromatic and quasi-monochromatic X-ray beam configurations. The most impactful factors affecting damage to roots were incident X-ray energy spectrum, stored current in the accelerator machine, position of the root in the soil, and possibly the number of exposures during the 4D XCT experiments. Our results imply that radiation dose during in vivo imaging of plant roots can be diminished by using monochromatic radiation at the highest energy suitable for a given sample thickness and field of view, and by controlling the rotation axis of off-centered roots to increase attenuation of radiation by the soil matrix.

Development of a precise evaluation technique for the pre-sampled point spread function of X-ray imaging detectors with sub-micrometer resolution

This article presents a method for precisely measuring the charge spread inside a silicon sensor, before it is sampled by the pixels of an X-ray imaging detector. The shape of the charge spread or pre-sampled point spread function (PSF) of the silicon sensor was demonstrated with a sub-micrometer resolution. The measurement setup comprises quasi-monochromatic X-ray beams emitted by a synchrotron radiation source, 5-μm-diameter tungsten pinhole, sub-μm-resolution X-ray beam profiler, and photodiode array in automation mode for step-scanning measurements. We obtained a value of 16.5–17.1 μm (full width at half-maxima; sensor thickness: 650 μm; bias voltage: 750 V) based on preliminary experiments using 7.3 keV X-rays at BL19LXU of SPring-8. This value is consistent with the expected value obtained from the convolution of the beam profiles and pre-sampled PSFs corresponding to multiple photons for a dopant density of ∼1012cm−3 within a range of approximately 1 μm. We believe that our measurement method has sufficient potential for evaluating the pre-sampled PSF of a photodiode array with an accuracy of 1 μm or less. The single-photon capability of imaging detectors depends on the detector noise as well as the pre-sampled PSF. Our measurement method, with high spatial resolution, is beneficial in the sense that it optimizes the pixel design of X-ray image sensors using a sample piece with a limited number of pixels.

Spectroscopic imaging at compact inverse Compton X-ray sources.

While K-edge subtraction (KES) imaging is a commonly applied technique at synchrotron sources, the application of this imaging method in clinical imaging is limited although results have shown its superiority to conventional clinical subtraction imaging. Over the past decades, compact synchrotron X-ray sources, based on inverse Compton scattering, have been developed to fill the gap between conventional X-ray tubes and synchrotron facilities. These so called inverse Compton sources (ICSs) provide a tunable, quasi-monochromatic X-ray beam in a laboratory setting with reduced spatial and financial requirements. This allows for the transfer of imaging techniques that have been limited to synchrotrons until now, like KES imaging, into a laboratory environment. This review article presents the first studies that have successfully performed KES at ICSs. These have shown that KES provides improved image quality in comparison to conventional X-ray imaging. The results indicate that medical imaging could benefit from monochromatic imaging and KES techniques. Currently, the clinical application of KES is limited by the low K-edge energy of available iodine contrast agents. However, several ICSs are under development or already in commissioning which will provide monochromatic X-ray beams with higher X-ray energies and will enable KES using high-Z elements as contrast media. With these developments, KES at an ICS has the ability to become an important tool in pre-clinical research and potentially advancing existing clinical imaging techniques.

BoCXS: A compact multidisciplinary X-ray source

Many research and application areas demand photon sources capable of producing quasi-monochromatic X-ray beams in the multi-keV energy range with reasonably high fluxes and compact footprints. Besides industrial, research, commercial and cultural heritage applications, various biomedical applications could benefit from the features of this type of X-ray sources. The main target of our proposal is the production of X-ray beams with energy range tunability, monochromaticity, flux and pulse duration, compatible with those required by various pre-clinical imaging modalities and material characterization techniques, through inverse Compton scattering. This kind of source involves an infrastructure much more compact than a large Synchrotron facility and can be realized on a laboratory scale enabling an easier access to a local community of users.

Inverse Compton radiation: a novel x-ray source for K-edge subtraction angiography?

Coronary angiography is clinically used worldwide to diagnose diseases of coronary arteries. Despite its effectiveness, this technique is quite invasive and it is associated with significant risks due to the arterial catheterisation needed to inject the contrast agent. A valid alternative is using the K-edge subtraction (KES) method, which is based on the subtraction of two images acquired at energies bracketing the K-edge of the contrast element. The enhanced sensitivity of KES allows the intravenous injection of the contrast agent, thus reducing the risks of catheterisation. This technique can be effectively implemented by using intense and quasi-monochromatic x-ray beams. Synchrotron radiation has been proven to work well for this purpose, but its cost and size prevent a widespread clinical application. Inverse Compton sources are among the most promising innovative sources of intense and quasi-monochromatic x-rays. These sources are intrinsically more compact than those based on synchrotron radiation. In this work, the potential application of inverse Compton radiation to KES angiography is investigated. To this purpose, after a short review of the physics behind the inverse Compton process, an analytical framework is described. The proposed model is based on the application of the KES algorithm to calculate the SNR of details inside a suitable mathematical phantom. That allowed us to identify the characteristics of an inverse Compton source required for KES imaging. In particular, it was estimated that a photon fluence of 108 ph mm−2 is necessary to detect signals of clinical interest. Novel sources based on inverse Compton promise to achieve this requirement with an acquisition time of few hundreds of ms. This feature, together with compactness, broad two-dimensional radiation field, absence of harmonic contamination and the ability to deliver high photon fluxes also at high energies, makes this kind of sources promising for KES angiography and other diagnostic applications.

Development and characterization of a new soft x-ray diagnostic concept for tokamaks

Diagnosing soft x-ray (SXR) emission from tokamaks represents a unique source of information, since it allows the study of several plasma parameters, such as the electron and ion temperature, the investigation of the ionization equilibrium, particle and impurities transport and the study of MHD fluctuations and disruptions. A new SXR diagnostic system called EXODUS (Enhanced X-ray Optimized Detector for Use in multiple Scenarios) is under development with the aim to obtain energy resolved SXR emission profiles from the plasma with a high time resolution (<0.1 ms). The system is based on the Gas Electron Multiplier (GEM) technology coupled with the new data acquisition system especially designed for GEM called GEMINI, which gives the possibility to obtain information about the energy deposited in the detector by the incoming radiation using the so called Time-Over-Threshold technique on each detector channel. The information of the deposited energy allows the study of the SXR emission from the plasma resolved in space, energy and time. There are several advantages in the use of GEM based detector in the harsh environment of a tokamak. First of all, it offers very high rate capabilities (up to 1 MHz/mm2), giving the possibility to obtain sub-ms time resolution together with sub-mm spatial resolution. The high counting rate and the high spatial resolution allow the installation of the detector in a pinhole camera very close to the tokamak vacuum barrier, producing very high statistic measurements. Finally, if properly designed, GEM detectors are intrinsically insensitive to gamma and neutron irradiation, allowing obtaining an excellent signal to noise ratio. In this contribution we describe the design and the production of the GEM based detector with the EXODUS system, together with the characterization of the detector response under quasi-monochromatic x-ray beam obtained using fluorescence of different materials. EXODUS electronics and its architecture allows the measurement of the energy of each detected photon and its time of arrival, resulting in a strong improvement of the energy and time resolution, compared to the old electronics used so far.

X-ray source with quasi-monochromatic parallel beam

In inertial confined fusion experiments, an excellent-performance and high-efficiency X-ray source plays an important role in X-ray radiography schemes. Indeed, it can be used in a variety of X-ray experimental techniques. The mono-chromaticity, flux intensity, degree of collimation (the radiation can be transported long distances without loss), and spot size of the X-ray source affect the quality of imaging. Ray-tracing simulations, which are validated by experimental results, demonstrate that high-intensity collimated X-ray beams can be produced from an isotropic X-ray source. Therefore, a method of improving the performance of an X-ray source from a laser-produced plasma is presented. A spherically bent crystal is used to collimate mono-chromatic X-rays emitted from a laser-produced plasma. Here we design a spherically bent crystal spectrometer system for collimating the laser-produced X-rays. The system performance is experimentally tested at the Shenguang Ⅱ (SGⅡ) laser facility located in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. The beam divergence is measured by using a metal grid placed downstream from the crystal, the metal grid that possesses wires with 60 μm in diameter and 127 μm in period. An imaging plate (IP) is placed at various distances downstream from grid. The quality of the generated beam is monitored by measuring the dimensions of the grid image formed by the beam on IP. While the narrow range of wavelength is measured with a spherically bent crystal spectrometer. Experimental results show that the spherically bent crystal spectrometer system can produce quasi-monochromatic (10&lt;sup&gt;-3&lt;/sup&gt; &lt; △ &lt;i&gt;λ/λ&lt;/i&gt; &lt;10&lt;sup&gt;-2&lt;/sup&gt;) X-ray beams with a high degree of collimation (less than 2 mrad divergence), uniform spot size (~500 μm), and a relative tenability in the wide spectral range. The influences of various experimental parameters on the quality of beam collimation are evaluated in two ways. They can be investigated in test experiments by representing the beam divergence distribution as a function of Bragg angle. In another study of the effect of the aberrations, when the incident beam on the spherically bent crystal is not normal, the beam is less collimated in the tangential plane, and out of collimation in the sagittal plane. Following the ray-tracing method, we analyze the diffracted beam divergence produced by the astigmatic aberration. The qualitative conclusion is that the good agreement with the experimental results is obtained. By fully utilizing limited Bragg angle range, the spherically bent crystal spectrometer system can realize collimated diffracted X-ray beams with divergence of less than 1 mrad by using a laser-produced plasma X-ray source under the appropriately experimental parameters.

A novel approach to scatter-free imaging for the improvement of breast cancer Detection

Compton scattering is one of the main causes of image degradation in X-ray imaging. This is particularly noticeable in mammography where details of interest feature low contrast in comparison to the surrounding tissue. This work shows the feasibility of obtaining scatter-free images by using a quasi-monochromatic X-ray beam and a pixellated spectroscopic detector. This work presents characterisation of the imaging system and quantitative imaging data of a low contrast test object. An improvement in contrast by 8% was observed compared to images obtained including scattered radiation. Comparison with a conventional setup showed an increase in the image quality factor when scatter has been removed.

Recent developments on techniques for differential phase imaging at the medical beamline of ELETTRA

Over the last decade different phase contrast approaches have been exploited at the medical beamline SYRMEP of the synchrotron radiation facility Elettra in Trieste, Italy. In particular special focus has been drawn to analyzer based imaging and the associated imaging theory and processing. Analyzer based Imaging (ABI) and Diffraction Enhanced Imaging (DEI) techniques have been successfully applied in several biomedical applications. Recently it has been suggested to translate the acquired knowledge in this field towards a Thomson Backscattering Source (TBS), which is presently under development at the Frascati National Laboratories of INFN (Istituto Nazionale di Fisica Nucleare) in Rome, Italy. Such source is capable of producing intense and quasi-monochromatic hard X-ray beams. For the technical implementation of biomedical phase imaging at the TBS a grating interferometer for differential phase contrast imaging has been designed and successfully tested at SYRMEP beamline.

Evaluation of energy distribution of quasi-monochromatic x-ray beams for sources with extremely high instantaneous flux using a k-edge subtraction technique

The characterization of novel x-ray sources includes the measurement of the photon flux and the energy distribution of the beam produced. The aim of the BEATS2 experiment at INFN-LNF is the study of medical applications of an x-ray source based on Thomson relativistic backscattering. This source is expected to produce pulsed quasi-monochromatic x-ray beam with an instantaneous flux of 1020ph/s in pulses 10ps long and with an average energy of 20keV. A direct measurement of energy distribution of this beam with traditional detectors, such as HPGe, CZT or CdTe, is very difficult because of the extremely high photon flux. For this reason we have planned to use a technique based on beam filtration using k-edge absorbing foils in the energy range of interest (16–22keV). By measuring the photon flux of the photon beam filtered with appropriate thicknesses of k-edge absorbers, it is possible to retrieve the energy distribution of the incident x-ray beam. The energy resolution obtainable depends on the number and energy separation of k-edges of filter used.A preliminary test of the technique was made using an x-ray tube with a tungsten anode at 22kVp, filtered with 3.1mm of Al providing an x-ray spectrum with an energy distribution similar to that expected from a Thomson source. The comparison between the results obtained directly measuring this spectrum with an HPGe detector and using the k-edge subtraction technique showed good agreement.

Proton induced quasi-monochromatic x-ray beams for soft x-ray spectroscopy studies and selective x-ray fluorescence analysis

We present the analytical features and performance of an x-ray spectroscopy end station of moderate energy resolution operating with proton-induced quasi-monochromatic x-ray beams. The apparatus was designed, installed and operated at the 5.5 MV Tandem VdG Accelerator Laboratory of the Institute of Nuclear Physics, N.C.S.R. "Demokritos," Athens. The setup includes a two-level ultrahigh vacuum chamber that hosts in the lower level up to six primary targets in a rotatable holder; there, the irradiation of pure element materials-used as primary targets-with few-MeV high current (~μA) proton beams produces intense quasi-monochromatic x-ray beams of selectable energy. In the chamber's upper level, a six-position rotatable sample holder hosts the targets considered for x-ray spectroscopy studies. The proton-induced x-ray beam, after proper collimation, is guided to the sample position whereas various filters can be also inserted along the beam's path to eliminate the backscattered protons or/and to absorb selectively components of the x-ray beam. The apparatus incorporates an ultrathin window Si(Li) spectrometer (FWHM 136 eV at 5.89 keV) coupled with low-noise electronics capable of efficiently detecting photons down to carbon Kα. Exemplary soft x-ray spectroscopy studies and results of selective x-ray fluorescence analysis are presented.

Energy distribution measurement of narrow-band ultrashort x-ray beams via K-edge filters subtraction

The characterization of novel x-ray sources includes the measurement of the photon flux and the energy distribution of the produced beam. The aim of BEATS2 experiment at the SPARC-LAB facility of the INFN National Laboratories of Frascati (Rome, Italy) is to investigate possible medical applications of an x-ray source based on Thomson relativistic back-scattering. This source is expected to produce a pulsed quasi-monochromatic x-ray beam with an instantaneous flux of 1020 ph/s in pulses 10 ps long and with an average energy of about 20 keV. A direct measurement of energy distribution of this beam is very difficult with traditional detectors because of the extremely high photon flux. In this paper, we present a method for the evaluation of the energy distribution of quasi-monochromatic x-ray beams based on beam filtration with K-edge absorbing foils in the energy range of interest (16–22 keV). The technique was tested measuring the energy distribution of an x-ray beam having a spectrum similar to the expected one (SPARC-LAB Thomson source) by using a tungsten anode x-ray tube properly filtered and powered. The energy distribution obtained has been compared with the one measured with a HPGe detector showing very good agreement.

Evaluation of the absorbed dose to the breast using radiochromic film in a dedicated CT mammotomography system employing a quasi-monochromatic x-ray beam.

A dual modality SPECT-CT prototype system dedicated to uncompressed breast imaging (mammotomography) has been developed. The computed tomography subsystem incorporates an ultrathick K-edge filtration technique producing a quasi-monochromatic x-ray cone beam that optimizes the dose efficiency of the system for lesion imaging in an uncompressed breast. Here, the absorbed dose in various geometric phantoms and in an uncompressed and pendant cadaveric breast using a normal tomographic cone beam imaging protocol is characterized using both thermoluminescent dosimeter (TLD) measurements and ionization chamber-calibrated radiochromic film. Initially, two geometric phantoms and an anthropomorphic breast phantom are filled in turn with oil and water to simulate the dose to objects that mimic various breast shapes having effective density bounds of 100% fatty and glandular breast compositions, respectively. Ultimately, an excised human cadaver breast is tomographically scanned using the normal tomographic imaging protocol, and the dose to the breast tissue is evaluated and compared to the earlier phantom-based measurements. Measured trends in dose distribution across all breast geometric and anthropomorphic phantom volumes indicate lower doses in the medial breast and more proximal to the chest wall, with consequently higher doses near the lateral peripheries and nipple regions. Measured doses to the oil-filled phantoms are consistently lower across all volume shapes due to the reduced mass energy-absorption coefficient of oil relative to water. The mean measured dose to the breast cadaver, composed of adipose and glandular tissues, was measured to be 4.2 mGy compared to a mean whole-breast dose of 3.8 and 4.5 mGy for the oil- and water-filled anthropomorphic breast phantoms, respectively. Assuming rotational symmetry due to the tomographic acquisition exposures, these results characterize the 3D dose distributions in an uncompressed human breast tissue volume for this dedicated breast imaging device and illustrate advantages of using the novel ultrathick K-edge filtered beam to minimize the dose to the breast during fully-3D imaging.

Preliminary results of a Multi-Energy CT system for small animals

A new multi-energy CT for small animals is being developed at the Physics Department of the University of Bologna, Italy. The system makes use of a set of quasi-monochromatic X-ray beams, with energy tunable in a range from 26 to 72 keV. These beams are produced by Bragg diffraction on a Highly Oriented Pyrolytic Graphite crystal. With quasi-monochromatic sources it is possible to perform multi-energy investigation in a more effective way, as compared with conventional X-ray tubes. Triple-energy projection allows to combine a set of three quasi-monochromatic images of an object, in order to obtain a corresponding set of three single-tissue images, which are the mass-thickness map (product of density and thickness) of three reference materials. The triple-energy technique can be applied to the mass-thickness-map reconstruction of a contrast medium, because it is able to remove completely the signal due to other tissues (i.e. the structural background noise). Here, we present a description of the system and the methods for combining the images at the different energies. We also show some results of images acquired at three different energies and of the reconstructed information about the contrast medium (Iodine). We achieved that the measurement of the Iodine mass-thickness agrees very well with the known concentration of the injected Iodine solution.

A method to remove the projection error in triple-energy radiography with contrast medium

The density-map reconstruction of a radiological contrast medium is affected by noise arising from the background lack of homogeneity (the so-called “projection error”) if images of the medium are collected starting from quasi-monochromatic X-ray beams. This noise, especially for a dual-energy reconstruction algorithm, becomes more significant than the statistical fluctuations of the photon-transmitted flux, dramatically reducing the accuracy and the sensitivity of the reconstruction. In this work, we investigate the efficacy of the triple-energy technique, which is based on the simultaneous acquisition of three monochromatic images of the same target injected with contrast medium. A theoretical analysis allows to estimate the sensitivity and the accuracy of the reconstructed density map compared with the dual-energy one (i.e., the density map reconstructed acquiring only two monochromatic images). To validate the theory, a set of experimental measurements was performed: results show that triple energy drastically reduces the projection errors (from 10 to 60 times smaller than the dual-energy one), making it negligible with respect to the statistical noise.

Advantages of quasi-monochromatic X-ray sources in absorption mammography

Mammography is a challenging field of medical imaging. Early detection of breast cancer requires identifying small contrast details. The choice of the appropriate monochromatic energy enhances the visibility of such details. Thomson scattering source can provide tunable quasi-monochromatic X-ray beams. In this work, we investigate by Monte Carlo simulations the optimal monochromatic energy to image mammographic phantoms. In order to mimic a Thomson scattering source, we consider the effect on image quality of the presence of an energy spread and of the presence of higher-order harmonics.

Development of a Multi-Energy CT for Small Animals: Characterization of the Quasi-Monochromatic X-Ray Source

A new multi-energy CT for small animals is operative (now only in scanning mode) at the Physics Department of the University of Bologna. The system makes use of a set of quasi-monochromatic X-ray beams produced by means of an Highly-Oriented Pyrolytic Graphite Bragg monochromator. This source is able to provide beams with energy tunable in a range from 20 to 70 keV. Here we present a complete characterization of the source. A theoretical model of the source has been analyzed, according to the known Zachariasen's theory for diffracting crystals. The beams have also been characterized in resolution and intensity, over the accessible range, and we present here some measured spectra. The monochromator system demonstrated an energy spread of about 3 keV (Full Width at Half Maximum-FWHM) at a beam energy of 26 keV. At the same energy, the intensity of the output beam is about 6% of the primary beam.

Effect of x‐ray energy dispersion in digital subtraction imaging at the iodine ‐edge—A Monte Carlo study

The effect of the energy dispersion of a quasi-monochromatic x-ray beam on the performance of a dual-energy x-ray imaging system is studied by means of Monte Carlo simulations using MCNPX (Monte Carlo N-Particle eXtended) version 2.6.0. In particular, the case of subtraction imaging at the iodine K-edge, suitable for angiographic imaging application, is investigated. The average energies of the two beams bracketing the iodine K-edge are set to the values of 31.2 and 35.6 keV corresponding to the ones obtained with a compact source based on a conventional x-ray tube and a mosaic crystal monochromator. The energy dispersion of the two beams is varied between 0 and 10 keV of full width at half-maximum (FWHM). The signal and signal-to-noise ratio produced in the simulated images by iodine-filled cavities (simulating patient vessels) drilled in a PMMA phantom are studied as a function of the x-ray energy dispersion. The obtained results show that, for the considered energy separation of 4.4 keV, no dramatic deterioration of the image quality is observed with increasing x-ray energy dispersion up to a FWHM of about 2.35 keV. The case of different beam energies is also investigated by means of fast simulations of the phantom absorption.

Initial study of quasi-monochromatic X-ray beam performance for X-ray computed mammotomography

We evaluate the feasibility, benefits, and operating parameters of a quasimonochromatic beam for a newly developed x-ray cone beam computed mammotomography application. The value of a near monochromatic x-ray source for fully 3D dedicated mammotomography is the expected improved ability to separate tissues with very small differences in attenuation coefficients while maintaining dose levels at or below that of existing dual view mammography. In previous studies, simulations for a range of tungsten tube potentials, K-edge filter materials, filter thicknesses, and a 12 cm uncompressed breast, with a digital flat-panel CsI(Tl) detector model, indicated that thick, rare earth filter materials may provide optimized image quality. Figures of merit computed included: lesion contrast under different filtering conditions; ratio of measured lesion contrast with and without filtering; and exposure efficiency (SNR/sup 2//exposure). Initial experiments are performed with a custom built x-ray mammotomography system, cerium foil filters, and plastic breast and lesion tissue-equivalent slabs. Simulation results showed that tube potentials of 50-70 kVp with filters of Z=57-63 yielded quasimonochromatic x-ray spectra with improved FOMs. Initial experimental measurements corroborate simulation results in that, relative trends and rank order of contrast ratios and exposure efficiency were in agreement. These studies show that this approach can be implemented practically with simple hardware and yield improved exposure efficiency versus the unfiltered or minimally filtered case.

K-edge digital subtraction imaging based on a dichromatic and compact x-ray source

This work proposes a compact dichromatic imaging system for the application of the K-edge digital subtraction technique based on a conventional x-ray tube and a monochromator system. A quasi-monochromatic x-ray beam at the energy of iodine K-edge is produced by Bragg diffraction on a mosaic crystal. Two thin adjacent beams with energies that bracket the K-edge discontinuity are obtained from the diffracted beam by means of a proper collimation system. They are then detected using an array of Si detectors. A home-made phantom is used to study the image quality as a function of iodine concentration. Signal and signal-to-noise ratio analysis has also been performed. The results are compared with theoretical expectations.