News Column

"System and Method for Phase-Contrast X-Ray Imaging" in Patent Application Approval Process

September 4, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- A patent application by the inventors Stutman, Dan (Cockeysville, MD); Finkenthal, Michael (Columbia, MD), filed on February 6, 2014, was made available online on August 21, 2014, according to news reporting originating from Washington, D.C., by VerticalNews correspondents.

This patent application is assigned to The Johns Hopkins University.

The following quote was obtained by the news editors from the background information supplied by the inventors: "X-ray differential phase-contrast (DPC) imaging relies on the refraction of the X-rays passing through an object. Since for hard X-rays the refraction angles are in the .mu.-radian range, the basic technique used for DPC imaging is to angularly filter with .mu.-radian resolution the transmitted X-ray beam, thus converting the angular beam deviations from refraction into intensity changes on a conventional detector. The angular filtering is done using X-ray optics such as crystals or gratings.

"A fundamental advantage of DPC imaging is that it is sensitive to density gradients in the measured object rather than to its bulk X-ray absorption. In medical imaging for instance, refraction has a contrast enhancing effect at tissue boundaries, which enables the detection of soft tissues which are otherwise invisible in conventional X-ray imaging. The ultra-small angle scattering occurring in micro-structured soft tissue such as cartilage, tendon, ligament or muscle has also a volume contrast enhancing effect. Another benefit of DPC for medical imaging is that it can improve contrast and resolution at similar or lower dose than in conventional X-ray imaging. This is possible because DPC uses X-rays that are not absorbed by the body and because the soft tissue refraction coefficients decrease with X-ray energy much slower than the absorption ones. In particular, by using for DPC a spectrum with mean energy in the 50-80 keV range approximately, the soft tissue dose is minimized while refraction strongly dominates over absorption.

"X-ray phase-contrast is also of interest for imaging and non-destructive characterization in material sciences, in particular as concerns low-Z materials. The structure and defects of materials ranging from polymers, to fiber composites, to wood, and to engineered bio-materials can be probed on the micrometer scale using X-ray phase-contrast. Some of the techniques used for X-ray phase-contrast can also be applied with neutrons. Recently X-ray phase-contrast has gained attention in fusion energy research, where the capability of refraction based imaging to measure the density gradients in an object can be used for the diagnostic of high density plasmas in inertial confinement fusion (ICF) and other high energy density physics (HEDP) experiments.

"Until recently, research on X-ray DPC imaging has been done mostly at synchrotrons, using crystal optics; the high intensity of the synchrotron compensates for the low efficiency (less than a hundredth of a %) of the crystal optics. Although there are efforts to develop table-top synchrotrons, or to use narrow K.sub..alpha. lines from conventional tubes, the crystal method has not yet entered the domain of practical applications. It is thus of interest to develop more efficient DPC methods and optics, that can work with conventional medical or industrial X-ray tubes.

"A DPC method that can work with conventional X-ray sources is the Talbot-Lau shearing interferometry, in which micro-periodic optics such as gratings are used to angularly filter the refracted X-rays with .mu.-radian resolution. The Talbot interferometer includes first a 'beam-splitter' (typically a .pi.-shift phase grating), which divides (or 'shears') through the Talbot effect the incoming beam into few .mu.-radian wide beamlets. The Talbot effect consists in a 'replication' of the grating pattern by the wave intensity, at periodic distances along the beam, called Talbot distances, d.sub.T=k/.eta..sup.2g.sup.2/(2.lamda.), with .lamda. the X-ray wavelength, g the grating period, k=1, 2, . . . the order of the pattern, and .eta.=1 for a .pi./2 phase shifting grating or for an absorption grating, and .eta.=2 for a it phase grating. The beamsplitter thus creates at the 'Talbot distance' a micro-periodic fringe pattern, which changes shape (shifts) with respect to the unperturbed pattern when a refractive object is introduced in the beam. The differential phase-contrast imaging consists thus in measuring the changes in the fringe pattern induced by the object, with respect to the pattern without the object. To achieve .mu.-radian angular sensitivity at hard X-ray wavelengths, the period g must be in the .mu.m range, resulting in a Talbot distance of a few tens of cm.

"The fringe pattern can in principle be directly measured using a microscopic pixel detector. This is however quite inefficient. For most practical applications, the fringe pattern changes are converted into intensity changes on a macroscopic pixel detector by introducing an 'analyzer' absorption grating placed behind the beam-splitter and having the period of the Talbot pattern. Lastly, for such an interferometer to function with an extended spot X-ray tube, a 'source' absorption grating is placed in front of the source, thus dividing it into an array of quasi-coherent line sources.

"The gratings are made by micro-lithography in thin Si wafers or photoresist. The absorption gratings are difficult to fabricate; they are typically made by filling with gold the gaps in regular transmission gratings. The 'grating shearing method' described above has demonstrated performance similar to the crystal method at energies below a few tens of keV.

"This method is however less useful at energies above a few tens of keV. The reason is that it is difficult to fabricate micron-period absorption gratings with the thickness required to block higher energy X-rays. This is illustrated in FIG. 1A with a plot of the Au thickness needed for 95% absorption, as a function of the photon energy. As seen, several hundred .mu.m depth gratings would be needed in the range of interest for clinical DPC imaging. Depending on the grating period, the present technological limit is however around 50-100 .mu.m. This limits the contrast of the grating shearing method for high energy X-rays, as illustrated in FIG. 1B by the fringe contrast computed for an interferometer having 100 .mu.m thick, 4 .mu.m period Au analyzer grating (throughout this specification we used for X-ray phase-contrast and optics calculations the XWFP wave propagation code and the XOP optics package).

"Accordingly, it is desirable to develop improved types of optics to enable efficient DPC imaging at X-ray energies above a few tens of keV."

In addition to the background information obtained for this patent application, VerticalNews journalists also obtained the inventors' summary information for this patent application: "In accordance with implementations of the present disclosure, a differential phase contrast X-ray imaging system is disclosed. The imaging system can include an X-ray illumination system; a beam splitter grating arranged in a radiation path of the X-ray illumination system and operable to receive an incident X-ray beam and provide an interference pattern of X-rays; and a detector system arranged in a radiation path to detect X-rays after passing through the beam splitter grating and in a Talbot-Lau interferometer configuration with the beam splitter grating. The detector system can include a X-ray detector and an analyzer grating, wherein the analyzer grating is operable to intercept and block at least a portion of the interference pattern of X-rays before reaching the X-ray detector, wherein the beam splitter grating and the analyzer grating are arranged at a shallow angle relative to incident X-rays.

"In implementations, the analyzer grating has a longitudinal dimension, a lateral dimension that is orthogonal to the longitudinal dimension and a transverse dimension that is orthogonal to the longitudinal dimension and the lateral dimension, the analyzer grating comprising a pattern of optically dense regions each having a longest dimension along the longitudinal dimension and being spaced substantially parallel to each other in the lateral dimension such that there are optically rare regions between adjacent optically dense regions, wherein each optically dense region has a depth in the transverse dimension that is smaller than a length in the longitudinal dimension, wherein the analyzer grating is arranged with the longitudinal dimension at the shallow angle, wherein the shallow angle is less than 30 degrees.

"In implementations, the beam splitter grating is a transmission grating.

"In implementations, the analyzer gratings include more than one grating tiled on top of another grating.

"In implementations, the analyzer grating has a longitudinal dimension, a lateral dimension that is orthogonal to the longitudinal dimension and a transverse dimension that is orthogonal to the longitudinal dimension and the lateral dimension, the analyzer grating comprising a pattern of optically dense regions each having a longest dimension along the lateral dimension and being spaced in a divergent geometry from each other such that there are optically rare regions between adjacent optically dense regions, wherein each optically dense region has a depth in the transverse dimension that is smaller than a length in the longitudinal dimension and the lateral dimension, wherein the analyzer grating is arranged with the lateral dimension at the shallow angle, wherein the shallow angle is less than 30 degrees.

"In implementations, the X-ray illumination system can include an X-ray source; and a source grating arranged in a radiation path between the X-ray source and the beam splitter grating, wherein the source grating provides a plurality of substantially coherent X-ray beams.

"In implementations, the imaging system can include a vibration resistant mount operable to provide a support to the source grating, the beam-splitter grating, the analyzer grating.

"In implementations, the imaging system can include a rotation stage operable to rotate the X-ray illumination system, the beam splitter grating, the detection system about an object.

"In implementations, the beam splitter grating and the analyzer grating have grating patterns determined according Talbot-Lau conditions.

"In implementations, a field of view of the detector system is sized to image a human extremity.

"In implementations, the detection system is operable to capture a single image for each angle at which the rotation stage is rotated.

"In accordance with implementations of the present disclosure, a differential phase contrast X-ray imaging method is disclosed. The method can include providing an incident X-ray beam using an X-ray illumination system; receiving the incident X-Ray beam at a beam splitter grating that is arranged in a radiation path of the X-ray illumination system and providing an interference pattern of X-rays; and detecting, using a detecting system arranged in a radiation path, X-rays after passing through the beam splitter grating that is in a Talbot-Lau interferometer configuration with the beam splitter grating, wherein the detector system comprises a X-ray detector and an analyzer grating, wherein the analyzer grating is operable to intercept and block at least a portion of the interference pattern of X-rays before reaching the X-ray detector, wherein the beam splitter grating and the analyzer grating are arranged at a shallow angle relative to incident X-rays.

"In implementations, the analyzer grating has a longitudinal dimension, a lateral dimension that is orthogonal to the longitudinal dimension and a transverse dimension that is orthogonal to the longitudinal dimension and the lateral dimension, the analyzer grating comprising a pattern of optically dense regions each having a longest dimension along the longitudinal dimension and being spaced substantially parallel to each other in the lateral dimension such that there are optically rare regions between adjacent optically dense regions, wherein each optically dense region has a depth in the transverse dimension that is smaller than a length in the longitudinal dimension, wherein the analyzer grating is arranged with the longitudinal dimension at the shallow angle, wherein the shallow angle is less than 30 degrees.

"In implementations, the analyzer grating comprises more than one grating tiled and stacked on top of another grating.

"In implementations, the analyzer grating has a longitudinal dimension, a lateral dimension that is orthogonal to the longitudinal dimension and a transverse dimension that is orthogonal to the longitudinal dimension and the lateral dimension, the analyzer grating comprising a pattern of optically dense regions each having a longest dimension along the lateral dimension and being spaced in a divergent geometry from each other such that there are optically rare regions between adjacent optically dense regions, wherein each optically dense region has a depth in the transverse dimension that is smaller than a length in the longitudinal dimension and the lateral dimension, wherein the analyzer grating is arranged with the lateral dimension at the shallow angle, wherein the shallow angle is less than 30 degrees.

"In implementations, the method can include receiving the incident X-ray beam at a source grating that arranged in a radiation path between a X-ray source and the beam splitter grating; and proving a plurality of substantially coherent X-ray beams to the beam splitter grating.

"In implementations, the method can include rotating a rotation stage that is operable to support the X-ray illumination system, the beam splitter grating, the detection system about an object.

"In implementations, the method can include determining grating patterns for the beam splitter grating and the analyzer grating according to Talbot-Lau conditions.

"In implementations, a field of view of the detector system is sized to image a human extremity.

"In implementations, the method can include capturing a single image for each angle at which the rotation stage is rotated.

"It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

"Additional features, implementations, and embodiments consistent with the disclosure will be set forth in part in the description which follows, or may be learned by practice of the disclosure. The metes and bounds of the invention will be defined by means of the elements and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

"The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description, serve to explain the principles of the disclosure. In the figures:

"FIG. 1A shows gold thickness needed for 95% absorption, as a function of X-rays energy. Also shown in FIG. 1B the fringe contrast for a conventional grating interferometer having 100 .mu.m thick, 4 .mu.m period Au analyzer, and the contrast of a GAI interferometer having Au gratins of the same thickness, but inclined at 12.degree.. In the conventional interferometer at energies of clinical interest the analyzer becomes transparent to X-rays, drastically reducing the interferometer contrast, while in the GAI case a much higher contrast of 30% is obtained. The spectrum of a W anode X-ray tube, after transmission through 2 mm Al and 150 mm soft tissue is also plotted in FIG. 1B.

"FIG. 2 shows an optical layout of a glancing angle interferometer consistent with embodiments of the disclosure.

"FIG. 3A shows the horizontal FOV (field of view) vignetting in conventional interferometer; FIG. 3B shows a 'tiled' GAI grating layout for wide FOV at high energy consistent with embodiments of the disclosure. FIG. 3C shows the experimental GAI interferometer contrast at 80 kVp through 200 mm water. FIG. 3D shows the experimental GAI interferometer contrast with grating rotated and moved 10 cm off the beam axis, validating the 'tiled' grating design in FIG. 3B.

"FIGS. 4A-4C shows an example design for a clinical scanner for large extremity joints consistent with embodiments of the disclosure, where FIG. 4A shows a large FOV analyzer grating using three tiled wafers, FIG. 4B shows a side view of the scanner, and FIG. 4C shows a top view of the scanner.

"FIG. 5 shows a method for joining tiled GAI gratings consistent with embodiments of the disclosure.

"FIG. 6 shows an example top view of a GAI-TS (TS=tomosynthesis) scanner that can be used for the clinical evaluation of internal organs consistent with implementations of the present disclosure.

"FIG. 7A shows an interlaced phase-scan method consistent with embodiments of the disclosure; FIGS. 7B and 7C shows DPC-CT and attenuation-CT images of fresh pig soft tissue phantom obtained at 65 kVp/45 keV mean energy with clinically compatible does, respectively.

"FIGS. 8A and 8B show a full-view and a ROI (ROI=Region-of-Interest) DPC-CT images of soft tissue phantom at 65 kVp, respectively.

"FIG. 9A shows a full-coverage/full scan images of tissue phantom, FIGS. 9B and 9C show DPC and attenuation images, respectively, with limited-coverage/limited-angle scan, and FIG. 9D shows a full-coverage/full-scan and limited-coverage/limited-scan DPC images of bone/soft tissue phantom.

"FIGS. 10A and 10B shows incident and transmitted spectra through 150 mm tissue, for 80 and 90 kVp W anode tube filtered with Al/Cu and with Al/W, respectively."

URL and more information on this patent application, see: Stutman, Dan; Finkenthal, Michael. System and Method for Phase-Contrast X-Ray Imaging. Filed February 6, 2014 and posted August 21, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=3329&p=67&f=G&l=50&d=PG01&S1=20140814.PD.&OS=PD/20140814&RS=PD/20140814

Keywords for this news article include: The Johns Hopkins University.

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