Material‐specific analysis using coherent‐scatter imaging

Abstract

Coherent-scatter computed tomography (CSCT) is a novel imaging method we are developing to produce cross-sectional images based on the low-angle (<10 degrees) scatter properties of tissue. At diagnostic energies, this scatter is primarily coherent with properties dependent upon the molecular structure of the scatterer. This facilitates the production of material-specific maps of each component in a conglomerate. Our particular goal is to obtain quantitative maps of bone-mineral content. A diagnostic x-ray source and image intensifier are used to acquire scatter patterns under first-generation CT geometry. An accurate measurement of the scatter patterns is necessary to correctly identify and quantify tissue composition. This requires corrections for exposure fluctuations, temporal lag in the intensifier, and self-attenuation within the specimen. The effect of lag is corrected using an approximate convolution method. Self-attenuation causes a cupping artifact in the CSCT images and is corrected using measurements of the transmitted primary beam. An accurate correction is required for reliable density measurements from material-specific images. The correction is shown to introduce negligible noise to the images and a theoretical expression for CSCT image SNR is confirmed by experiment. With these corrections, the scatter intensity is proportional to the number of scattering centers interrogated and quantitative measurements of each material (in g/cm3) are obtained. Results are demonstrated using both a series of poly(methyl methacrylate) (PMMA) sheets of increasing thickness (2-12 mm) and a series of 5 acrylic rods containing varying amounts of hydroxyapatite (0-0.400 g/cm3), simulating the physiological range of bone-mineral density (BMD) found in trabecular bone. The excellent agreement between known and measured BMD demonstrates the viability of CSCT as a tool for densitometry.

An X-ray scatter system for material identification in cluttered objects: A Monte Carlo simulation study

“…where FFðqÞ is the form factor, which is equal to the Fourier transform of the electron charge distribution in the scatterer [6]; q is the momentum transfer, which is related to the scatter angle h and the X-ray energy E through q ¼ ðE=hcÞ sinðh=2Þ; and h and c are Planck's constant and the speed of light in vacuum, respectively. GEANT4 and PENELOPE use the form factors for coherent scatter based on individual atoms (e.g., absent interference effects) from EPDL97 [36] (EPDL Evaluated Photon Data Library, 1997 version), which are the non-relativistic values calculated by Hubbell [37].…”

“…Its differential cross section per unit solid angle has shape described by a function known as the form factor. The form factor is proportional to the Fourier Transform of the electron charge distribution of the sample [6]. Therefore, whereas the X-ray attenuation measurement reflects only the attenuation coefficient of the sample, the X-ray coherent scatter pattern reflects the spatial configuration of the electrons in the sample.…”

Bone‐composition imaging using coherent‐scatter computed tomography: Assessing bone health beyond bone mineral density

“…This scanner has been presented in detail previously. 37,38 As described by Westmore et al, 37 the output of a diagnostic x-ray source ͑tungsten anode, 70 kVp, 0.6 mm focal spot͒ is filtered by 0.30 g / cm 2 gadolinium to reduce the spectral width ͓from 27% root-mean-square ͑rms͒ width to 14% rms width͔, thereby improving the angular resolution of the measured coherent-scatter patterns. Filtration reduces the x-ray intensity to 17% and produces a beam with a mean energy of 43 keV.…”

Small-angle scatter tomography with a photon-counting detector array

“…This method is termed as coherent scatter computerized tomography (CSCT) (Harding 1987, Batchelar and Cunningham 2002, Batchelar et al 2006). More importantly, CSCT has a larger collection solid angle with a collection efficiency much higher than that of the direct tomography setup.…”