Asymmetric fan transmission CT on SPECT systems

Abstract

For proper attenuation correction of SPECT images, a set of 3D attenuation maps specific to the imaging slices is needed. Among the many different approaches for deriving the attenuation maps, fan beam transmission CT (FBTCT), performed on the same SPECT system as emission imaging, has many promising and clinically practical features. The major problem of FBTCT is that the current SPECT systems do not have a large enough field of view (FOV) to cover the typical cross-sectional size of patients. To address this problem, we have developed a novel asymmetric fan (AsF) sampling scheme to extend the FOV to practical sizes for clinical TCT imaging on existing SPECT systems. This AsF scheme samples only half of the intended FOV in each projection; the other half would be sampled in an opposing projection after detector rotation. We have implemented the AsF sampling on a three-head SPECT system through a specially designed source-collimator assembly. We have modified the conventional convolution backprojection algorithm to facilitate simple and fast image reconstruction. The feasibility of the approach is confirmed by the quality of the derived TCT images of various phantoms and human subjects. The AsF sampling scheme could also have applications in other general transmission CT systems.

An Overview of Attenuation and Scatter Correction of Planar and SPECT Data for Dosimetry Studies

“…Asymmetric fan-beam collimators can be used to provide attenuation maps by imaging line sources, or moving point sources, located at their focus. 20,21 As illustrated in Figure 3C, such systems will partially truncate the cross-section of the patient unless data is acquired for a 360- One problem with use of this design is that the fan-beam collimator is also used for emission imaging. This difficulty can be overcome, by using photons from a medium-energy scanning-point source to create an asymmetric fan-beam transmission projection through a parallel-hole collimator by penetrating the septa of the collimator.…”

The design and performance of a simultaneous transmission and emission tomography system

“…One approach used a 30" slant hole collimator with a scanning line source orthogonal to the axis of rotation (scanning along the axis of rotation) [40]. Another approach used an asymmetric fan-beam geometry with the line source oriented parallel to the axis of rotation and fixed at an off-center focal line [41], [42]. However, both the slant hole collimator and this geometry cover only a portion of the field of view for a particular angular view.…”

An efficient reconstruction method for nonuniform attenuation compensation in nonparallel beam geometries based on Novikov's explicit inversion formula

“…With the Fourier coefficients, we can obtain the re-phased projection data (6) in which the angular positions at each radial distance are aligned. These sampled points of the re-phased projection data , derived from the measurements , are illustrated in Fig.…”

“…The flat detector of 60 cm was equidistantly sampled by 256 bins (with a size of 0.23 cm). Different from their purpose of increasing the field-of-view (FOV) with data truncation [6], we expanded the detector size to avoid the truncation (the size is slightly larger than the currently available size of 54 cm of Siemens E-CAM system for the purpose of testing the algorithm). The simulated noise-free sinogram and its reconstructed image, as well as their horizontal profiles, are shown in Fig.…”

“…Recently an explicit inversion formula for the AtRT has been derived by Novikov for parallel-beam (PB) collimation geometry using wave propagation and generalized distribution theories [2], and later a simpler derivation of an equivalent formula was given by Natterer [3]. In many clinical applications, however, nonparallel beam (NPB) collimation geometries may be desired, such as fan-beam (FB), asymmetrical fan-beam (AsF), or spatially varying focal-length fan-beam (VFF) geometries [4]- [6]. For FB geometry, an explicit inversion formula was discovered by Bukhgeim and Kazantsev via solving the boundary condition problem of a transport equation [7], [8].…”