Algorithms for density and composition-discrimination imaging for fourth-generation CT systems

Abstract

This paper shows that if the off-beam idle detectors in the detection ring of a fourth-generation x-ray computed tomography (CT) system are used to measure the scattered radiation, it is numerically feasible to reconstruct electron-density images to supplement the conventional attenuation-coefficient images of transmitted radiation. It is also shown that by combining these two images, composition changes can be detected with the aid of an effective-atomic-number indicator. The required image-reconstruction algorithms are developed and tested against Monte Carlo simulated measurements, for a variety of phantom configurations. In spite of the relatively poor statistical quality of scattering measurements, it is demonstrated that electron-density images of reasonable quality can be obtained. In addition, it is shown that composition discrimination is possible for materials of effective atomic number greater than five, in the photon energy range of a typical medical x-ray CT system operating at 102 kVp. The obtained supplementary electron-density and composition images can be useful in radiotherapy planning and for studying tumour histology, as well as in industrial and security applications where identification of materials based on density and composition is important.

A nonrotating multiparameter 3-D X-ray imaging system-Part I: modeling and reconstruction

“…0018-9499/04$20.00 © 2004 IEEE Since then, perhaps two major undertakings are worth noting. The first is the work of [8] which utilized radiation scattering that can be readily recorded in 4th generation CT systems to produce electron-density and atomic-number images. The second is the recent work of [9] which demonstrated a one-side scatter tomography system with a fan beam that utilized a source (88 keV) along with six-element array of planar HPGe detectors.…”

“…The response of a transmission detector corresponding to the beam can be expressed as One can put the problem as (8) where describes how contributes to . Since the number of available transmission measurements is only , while there are unknown values, the systems of equations produced by (6) results in an underdetermined problem.…”

“…The scattering measurements were used to compensate for the underdetermination of the transmission measurements, by providing an estimate of at each voxel from one of the corresponding scattering detectors. For example, a bottom scattering measurement gives (9) Adding an estimate of the photoelectric coefficient, to the value of from (9) should lead to a value of the total coefficient, , equivalent to that obtained from (8). If there is a mismatch between the two values, the process is repeated iteratively until acceptable matching occurs.…”

On the intricacy of imaging with incoherently-scattered radiation

“…However, some workers, Prettyman et al [13] and Busono and Hussein [14], directly determined the f attenuation factor using independent CT data. The factor g can then be estimated from the f values adjusted by the inverse energy dependence of the Compton scattering cross-section.…”

“…(2). The usual practice is to assume equivalent single incident and scattered energies [14,18], but effort should be devoted to developing more accurate methods.…”