Issues in quantification of cardiac PET studies.
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Algorithms↗
Biomedical subjects
Hugo W A M de Jong
Publications and source records attributed to Hugo W A M de Jong.
Acquisition settings for PET of 124I administered simultaneously with therapeutic amounts of 131I.
UNLABELLED: Radiation dosimetry of thyroid cancer therapy with 131I can be performed by coadministration of 124I followed by longitudinal PET scans over several days. The photons emitted by 131I may affect PET image quality. The aim of this study was to assess the influence of large amounts of 131I on PET image quality and accuracy with various acquisition settings. METHODS: Noise equivalent count (NEC) rates of 124I only were measured with a standard clinical PET scanner. Apart from the standard 350- to 650-keV energy window, 425- to 650-keV and 460- to 562-keV windows were used and data were acquired both with (2-dimensional) and without (3-dimensional [3D]) septa. A phantom containing 6 hot spheres, filled with a combination of 131I and 124I and with a sphere-to-background ratio of 18:1, was scanned repeatedly with energy window settings as indicated and emission and transmission scan durations of 7 and 3 min, respectively. NEC rates were calculated and compared with those measured with the phantom filled with only 124I. Sphere-to-background ratios in the reconstructed images were determined. One patient with known metastatic thyroid cancer was scanned using energy window settings and scan times as indicated 3 and 6 d after administration of 5.5 GBq of 131I and 75 MBq of 124I. RESULTS: The highest 124I-only NEC rates were obtained using a 425- to 650-keV energy window in 3D mode. In the presence of (131)I, the settings giving the highest NEC rate and contrast were 425-650 keV and 460-562 keV in 3D mode, with the clinical scans giving the highest quality images with the same settings. CONCLUSION: Acquisition in 3D mode with a 425- to 650-keV or 460- to 562-keV window leads to the highest image quality and contrast when imaging 124I in the presence of large amounts of 131I using a standard clinical PET scanner.
Aged↗
Monte Carlo-based down-scatter correction of SPECT attenuation maps.
Combined acquisition of transmission and emission data in single-photon emission computed tomography (SPECT) can be used for correction of non-uniform photon attenuation. However, down-scatter from a higher energy isotope (e.g. 99mTc) contaminates lower energy transmission data (e.g. 153Gd, 100 keV), resulting in underestimation of reconstructed attenuation coefficients. Window-based corrections are often not very accurate and increase noise in attenuation maps. We have developed a new correction scheme. It uses accurate scatter modelling to avoid noise amplification and does not require additional energy windows. The correction works as follows: Initially, an approximate attenuation map is reconstructed using down-scatter contaminated transmission data (step 1). An emission map is reconstructed based on the contaminated attenuation map (step 2). Based on this approximate 99mTc reconstruction and attenuation map, down-scatter in the 153Gd window is simulated using accelerated Monte Carlo simulation (step 3). This down-scatter estimate is used during reconstruction of a corrected attenuation map (step 4). Based on the corrected attenuation map, an improved 99mTc image is reconstructed (step 5). Steps 3-5 are repeated to incrementally improve the down-scatter estimate. The Monte Carlo simulator provides accurate down-scatter estimation with significantly less noise than down-scatter estimates acquired in an additional window. Errors in the reconstructed attenuation coefficients are reduced from ca. 40% to less than 5%. Furthermore, artefacts in 99mTc emission reconstructions are almost completely removed. These results are better than for window-based correction, both in simulation experiments and in physical phantom experiments. Monte Carlo down-scatter simulation in concert with statistical reconstruction provides accurate down-scatter correction of attenuation maps.
Algorithms↗
Monte Carlo simulations of pinhole imaging accelerated by kernel-based forced detection.
Pinhole collimation can provide both higher sensitivity and resolution than parallel hole collimation when used to image small objects. When objects are placed close to the pinhole, small pinhole diameters combined with high-magnification pinhole geometries yield ultra high resolution images. With Monte Carlo (MC) calculations it is possible to simulate accurately a wide range of features of pinhole imaging. The aim of the present work is to accelerate MC simulations of pinhole SPECT projections. To achieve speed-up, forced detection (FD), a commonly used acceleration technique, is replaced by a kernel-based forced detection (KFD) step. In KFD, instead of tracing individual photons from the source or last scatter position to the detector, a position dependent kernel (point spread function (PSF)) is projected on the detector. The PSFs for channel and knife edge pinhole apertures model the penetration effects through the aperture material. For simulations, the PSFs are pre-calculated and stored in tables. The speed-up and accuracy achieved by using KFD were validated by means of digital phantoms. MC simulations with FD and with KFD converge to almost identical images. However, KFD converges to an equal image noise level one to four orders of magnitude faster than FD, depending on the number of photons simulated. A simulator accelerated by KFD could serve as a practical tool to improve iterative image reconstruction.
Monte Carlo Method↗
Efficient fully 3-D iterative SPECT reconstruction with Monte Carlo-based scatter compensation.
Quantitative accuracy of single photon emission computed tomography (SPECT) images is highly dependent on the photon scatter model used for image reconstruction. Monte Carlo simulation (MCS) is the most general method for detailed modeling of scatter, but to date, fully three-dimensional (3-D) MCS-based statistical SPECT reconstruction approaches have not been realized, due to prohibitively long computation times and excessive computer memory requirements. MCS-based reconstruction has previously been restricted to two-dimensional approaches that are vastly inferior to fully 3-D reconstruction. Instead of MCS, scatter calculations based on simplified but less accurate models are sometimes incorporated in fully 3-D SPECT reconstruction algorithms. We developed a computationally efficient fully 3-D MCS-based reconstruction architecture by combining the following methods: 1) a dual matrix ordered subset (DM-OS) reconstruction algorithm to accelerate the reconstruction and avoid massive transition matrix precalculation and storage; 2) a stochastic photon transport calculation in MCS is combined with an analytic detector modeling step to reduce noise in the Monte Carlo (MC)-based reprojection after only a small number of photon histories have been tracked; and 3) the number of photon histories simulated is reduced by an order of magnitude in early iterations, or photon histories calculated in an early iteration are reused. For a 64 x 64 x 64 image array, the reconstruction time required for ten DM-OS iterations is approximately 30 min on a dual processor (AMD 1.4 GHz) PC, in which case the stochastic nature of MCS modeling is found to have a negligible effect on noise in reconstructions. Since MCS can calculate photon transport for any clinically used photon energy and patient attenuation distribution, the proposed methodology is expected to be useful for obtaining highly accurate quantitative SPECT images within clinically acceptable computation times.
Algorithms↗
Efficient simulation of SPECT down-scatter including photon interactions with crystal and lead.
A major image degrading factor in simultaneous Dual Isotope (DI) SPECT or simultaneous Emission-Transmission (ECT-TCT) imaging, is the detection of photons emitted by the higher energy isotope in the energy window used for imaging the lower energy isotope. In Tc-99m/Tl-201 DI-SPECT typically tens of percents of the total detected down-scatter is caused by lead x rays. In Tc-99m/Gd-153 ECT-TCT, a comparable fraction of the down-scatter originates from Tc-99m photons which only partly deposit their energy in the detector crystal (i.e., due to crystal interactions). Efficient simulation methods which model down-scatter can be used to optimize DI-SPECT or ECT-TCT imaging acquisition or reconstruction protocols. In this paper we adapt a previously proposed efficient down-scatter simulation method, to include the interactions of photons with the detector crystal and collimator lead. To this end, point spread function tables including crystal and lead interactions are precalculated. Subsequently, photons are traced through the patient body until their last scatter position, and the precalculated responses are used to project the photons onto the detector plane, while photon attenuation in the patient is taken into account. The approach is evaluated by comparing simulated Tc-99m down-scatter projections with measured projections. Incorporation of photon interaction with crystal and lead leads to significantly improved accuracy of the shape of down-scatter responses, while differences in total counts between simulated and measured projections typically decrease from tens of percents to a couple of percents. Calculating 60 down-scatter projections of an extended distribution on a 64 x 64 x 64 grid takes about three minutes on a PC with two 1.2 GHz processors. We conclude that accurate and efficient simulation of down-scatter is now possible including the major effects of the nonuniform mass density of the patient as well as photon interactions with the crystal and collimator lead.
Biophysical Phenomena↗