A three-dimensional ray-driven attenuation, scatter and geometric response correction technique for SPECT in inhomogeneous media

Abstract

The qualitative and quantitative accuracy of SPECT images is degraded by physical factors of attenuation, Compton scatter and spatially varying collimator geometric response. This paper presents a 3D ray-tracing technique for modelling attenuation, scatter and geometric response for SPECT imaging in an inhomogeneous attenuating medium. The model is incorporated into a three-dimensional projector-backprojector and used with the maximum-likelihood expectation-maximization algorithm for reconstruction of parallel-beam data. A transmission map is used to define the inhomogeneous attenuating and scattering object being imaged. The attenuation map defines the probability of photon attenuation between the source and the scattering site, the scattering angle at the scattering site and the probability of attenuation of the scattered photon between the scattering site and the detector. The probability of a photon being scattered through a given angle and being detected in the emission energy window is approximated using a Gaussian function. The parameters of this Gaussian function are determined using physical measurements of parallel-beam scatter line spread functions from a non-uniformly attenuating phantom. The 3D ray-tracing scatter projector-backprojector produces the scatter and primary components. Then, a 3D ray-tracing projector-backprojector is used to model the geometric response of the collimator. From Monte Carlo and physical phantom experiments, it is shown that the best results are obtained by simultaneously correcting attenuation, scatter and geometric response, compared with results obtained with only one or two of the three corrections. It is also shown that a 3D scatter model is more accurate than a 2D model. A transmission map is useful for obtaining measurements of attenuation and scatter in SPECT data, which can be used together with a model of the geometric response of the collimator to obtain corrected images with quantitative and diagnostically accurate information.

Technological Development and Advances in Single-Photon Emission Computed Tomography/Computed Tomography

“…X-ray transmission imaging in SPECT/CT also provides a patientspecific map of Compton coefficients that can be used for model-based scatter estimation [78][79][80][81] to compensate the radionuclide data for scatter. A final method of improving image quality models the geometrical configuration of the radionuclide collimator (ie including parallel-hole or pinhole collimators) that corrects the SPECT for the geometrical response of the collimator [82][83][84][85]. These compensation methods are becoming common for improving the spatial resolution, contrast, and signal-to-noise characteristics of SPECT imaging, including those obtained with SPECT/CT ( Figure 6).…”

“…These errors are inherent to radionuclide imaging but can be compensated using patient-specific information derived from CT in a SPECT/CT scanner. For example, radionuclide data can be compensated for errors from photon attenuation and scatter radiation [82,[94][95][96] using patient-specific images or "maps" of linear attenuation coefficients derived from CT. When correlated CT data are available, they also can be used to define the size and shape of target regions, and thereby compensate the image of the apparent distribution of activity for partial volume errors caused by the finite spatial resolution of the radionuclide imaging system [32].…”

“…When correlated CT data are available, they also can be used to define the size and shape of target regions, and thereby compensate the image of the apparent distribution of activity for partial volume errors caused by the finite spatial resolution of the radionuclide imaging system [32]. Therefore, while it often is noted in the scientific literature that SPECT is not quantitative [97][98][99][100] and suffers both accuracy and precision errors, it increasingly is being recognized that SPECT/ CT with appropriate reconstruction algorithms can compensate the radionuclide data for photon attenuation, partial volume errors, and scatter radiation [29,76,82,[94][95][96]101] in a way that significantly improves the accuracy of radionuclide quantitation in comparison to measurements obtained with SPECT alone.…”

Spatiotemporal scatter models for dynamic SPECT

“…Generally, it is computationally intensive to calculate spatial projection functions that contain scatter as determined by Monte Carlo simulation [8] or analytic ray-driven algorithms [9].…”

“…Projection bins were 6 mm × 6 mm. Uniform attenuation and scatter at 140 keV were simulated with use of a ray-driven projector and analytic line integrals [14], [15], [9]. Depth-dependent collimator response was not simulated.…”