Geoffrey Dr. Harding

X-Ray diffraction measurements of some plastic materials and body tissues

X-ray diffraction allows the investigation of the atomic or molecular structure of materials. The combination of diffractometry with computerized tomography enables spatially resolved imaging of the diffraction properties of extended objects as described in more detail in a companion article [Harding et al., Med. Phys. 14, 515 (1987)]. We present measured diffraction patterns of some plastics and several biological materials, which allow further optimization of our method and the selection of suitable application areas.

Energy-dispersive X-ray diffraction tomography

A novel tomographic imaging technique is described based on the energy analysis, at fixed angle, of coherent X-ray scatter excited in an object by polychromatic radiation. The authors term their technique energy-dispersive X-ray diffraction tomography (EXDT). It permits the X-ray diffraction properties of small voxels of an extended object to be measured in vivo. Tomographic information is obtained directly without the need to reconstruct from projections. EXDT images of a simple test object comprising water and various plastic materials are presented to illustrate the feasibility of the technique. Potential applications of EXDT in bone imaging are discussed.

Dual-energy Compton scatter tomography

A dual-energy Compton scatter imaging technique, analogous to that used in computed tomography and conventional radiography, has been explored. It is shown that this technique allows the photoelectric component of the attenuation factors for the primary and scattered radiations to be derived. Compton scatter images of the ear, corrected for photo-attenuation using dual energy, have been obtained with a novel Compton scatter scanner (Comscan) and are presented here. In addition, dual-energy Compton scatter imaging allows photo-effect slice images to be obtained which are a useful extension to the customary Compton images. The application of dual-energy Compton scatter to imaging bony structures and densitometry is illustrated.

Coherent scatter in radiographic imaging: a Monte Carlo simulation study

The significance of coherently scattered radiation in radiographic imaging is investigated using the Monte Carlo simulation technique. Recent data on the form factor of liquid water, which take into account intermolecular interference effects, have been used for the calculation of the coherent differential cross section. The spatial distribution of scattered radiation in the detection plane was calculated separately for coherent and incoherent single and multiple scattering. In the pencil beam geometry, it is found that coherent scattering leaving the object, is almost exclusively single scattering, is concentrated near but not exactly at the transmitted primary beam and dominates over multiple incoherent scattering in this region even for thick objects and polyenergetic radiation. Some consequences concerning the performance of grids, the choice of phantom materials and a new imaging method are given.

A new fluorescent X-ray source for photon scattering investigations

A description is given of the design and performance of a novel fluorescent X-ray source, comprising a demountable conical target enclosed in a conical anode X-ray tube. Measurements are presented of the spectral purity of the fluorescent emission and the variation in photon flux with the applied tube voltage. A Ta target tube (Kalpha 1=57.532 keV) powered by a standard 3.75 kW, 160 kV high voltage generator has a source brightness of 109 photons/s sr mm2. This is 500 times greater than that of the brightest available 241-Am radionuclide sources. Some potential applications of the fluorescent X-ray source are listed.

A K edge filter technique for optimization of the coherent-to-Compton scatter ratio method.

The ratio method involves forming the ratio of the elastic to inelastic x-ray scatter signals from a localized region of a scattering medium to determine its mean atomic number. An analysis is presented of two major error sources influencing the ratio method: firstly statistical (photon) noise and secondly multiple scattering and self-attenuation of the primary and scatter radiations in the medium. It is shown that a forward scattering geometry minimizes errors of both types for substances composed of elements with low and medium atomic number. However, owing to the small energy separation (approximately 100 eV) of coherent and Compton scatter for this geometry, they cannot be distinguished directly with semiconductor (e.g., Ge) detectors. A novel K edge filter technique is described which permits separation of the elastic and Compton signals in the forward-scatter geometry. The feasibility of this method is demonstrated by experimental results obtained with Ta fluorescence radiation provided by a fluorescent x-ray source filtered with an Er foil. The extension of this technique to the in vivo measurement of low momentum transfer inelastic scattering from biological tissues, possibly providing useful diagnostic information, is briefly discussed.

Energy Resolved X-Ray Diffraction CT

Low angle x-ray scattering at diagnostic energies in narrow beam geometry is due to coherent (Rayleigh) and incoherent (Compton) scattering. It has been found that single coherent scatter dominates below 10 deg. Interference effects with coherent scatter leads to diffraction patterns which differ from material to material. A technique, analogous to conventional CT, allows the reconstruction of the 2D distribution of the x-ray diffraction properties within an object slice, as demonstrated recently? Use of the bremsstrahlung spectrum of an x-ray tube permits short measuring times, but causes a significant energy broadening of the diffraction curves, thus deteriorating the maximum contrast obtainable by diffraction imaging. With energy resolved photon counting of the scattered x-ray quanta this broadening can be corrected, yielding an image contrast approaching that of a monochromatic x-ray source.