Larry G. Byars

MRI-Assisted PET Motion Correction for Neurologic Studies in an Integrated MR-PET Scanner

Head motion is difficult to avoid in long PET studies, degrading the image quality and offsetting the benefit of using a high-resolution scanner. As a potential solution in an integrated MR-PET scanner, the simultaneously acquired MRI data can be used for motion tracking. In this work, a novel algorithm for data processing and rigid-body motion correction (MC) for the MRI-compatible BrainPET prototype scanner is described, and proof-of-principle phantom and human studies are presented. Methods: To account for motion, the PET prompt and random coincidences and sensitivity data for postnormalization were processed in the line-of-response (LOR) space according to the MRI-derived motion estimates. The processing time on the standard BrainPET workstation is approximately 16 s for each motion estimate. After rebinning in the sinogram space, the motion corrected data were summed, and the PET volume was reconstructed using the attenuation and scatter sinograms in the reference position. The accuracy of the MC algorithm was first tested using a Hoffman phantom. Next, human volunteer studies were performed, and motion estimates were obtained using 2 high-temporal-resolution MRI-based motion-tracking techniques. Results: After accounting for the misalignment between the 2 scanners, perfectly coregistered MRI and PET volumes were reproducibly obtained. The MRI output gates inserted into the PET list-mode allow the temporal correlation of the 2 datasets within 0.2 ms. The Hoffman phantom volume reconstructed by processing the PET data in the LOR space was similar to the one obtained by processing the data using the standard methods and applying the MC in the image space, demonstrating the quantitative accuracy of the procedure. In human volunteer studies, motion estimates were obtained from echo planar imaging and cloverleaf navigator sequences every 3 s and 20 ms, respectively. Motion-deblurred PET images, with excellent delineation of specific brain structures, were obtained using these 2 MRI-based estimates. Conclusion: An MRI-based MC algorithm was implemented for an integrated MR-PET scanner. High-temporal-resolution MRI-derived motion estimates (obtained while simultaneously acquiring anatomic or functional MRI data) can be used for PET MC. An MRI-based MC method has the potential to improve PET image quality, increasing its reliability, reproducibility, and quantitative accuracy, and to benefit many neurologic applications.

The SMART scanner: a combined PET/CT tomograph for clinical oncology

A combined PET/CT tomograph with the unique capability to acquire accurately aligned functional and anatomical images for any part of the human body has been designed and built. The PET/CT, or SMART scanner, was developed by combining a Siemens Somatom AR.SP spiral CT scanner with a partial ring rotating ECAT ART PET tomograph. All components are mounted on a common rotational support within a single gantry that has an axial depth of 110 cm. The PET and CT components can be operated either separately or in combined mode. In combined mode, the CT images are used to correct the PET data for scatter and attenuation. Fully quantitative whole-body images can be obtained for an axial extent of up to 100 cm in an imaging time of less than one hour. When operated in PET mode alone, transmission scans are acquired with two 15 mCi cesium sources. We report the first performance measurements from the scanner, and present some illustrative clinical studies.

SPMD cluster-based parallel 3D OSEM

This study empirically compares two approaches to parallel 3D OSEM that differ as to whether calculations are assigned to nodes by projection number or by transaxial plane number. For projection space decomposition (PSD), the forward projection is completely parallel, but backprojection requires a slow image synchronization. For image space decomposition (ISD), the communication associated with forward projection can be overlapped with calculation, and the communication associated with backprojection is more efficient. To compare these methods, an implementation of 3D OSEM for three PET scanners is developed that runs on an experimental, 9-node, 18-processor cluster computer. For selected benchmarks, both methods exhibit speedups in excess of 8 for 9 nodes, and comparable performance for the tested range of cluster sizes.

Evaluation of single photon transmission for the HRRT

A dedicated whole human brain positron emission tomograph (PET), the High Resolution Research Tomograph (ECAT HRRT) is utilized to evaluate attenuation correction using single photon based transmission scanning. The patented transmission procedure uses a 740 MBq Cs-137 point source, which is extended into the FOV and collimated to flood the opposing detectors only. An attenuation map is then calculated iteratively using a blank and transmission scan, scaled to 511 keV, and re-projected using inverse Fourier rebinning to estimate the 3D attenuation correction. We have evaluated the accuracy of the single-based transmission procedure and attenuation correction process. In particular, we compare variance weighted OSEM and a dedicated TR algorithm with regularization (MAP-TR) for the reconstruction of the /spl mu/-image. Contamination from emission is estimated from a mock scan without moving the source. Results of a measurement of the patient dose during HRRT transmission scans show a 4 times lower dose compared to patient transmission scans on the ECAT HR.

Lossless data compression for short duration 3D frames in positron emission tomography

H/sub 2/O/sup 15/ 3D bolus studies and other dynamic 3D protocols in positron emission tomography (PET) require frame durations of five seconds or less as the protocol begins, with required frame durations increasing as the study progresses. A goal in PET acquisition is sustained 3D frame duration of ten seconds, with shorter frame durations allowed for a limited time. Transfer of projection arrays to an acquisition hard disk is a major limit of frame duration. Data compression can be used to increase the effective disk throughput and therefore decrease the sustained frame duration. The authors discuss the use of lossless compression hardware in a modern PET 3D acquisition system. The hardware uses an implementation of Lempel-Ziv compression with an estimated sustained throughput of 20 megabytes per second and compression ratios of 3 to 10 for short duration 3D projections arrays. Compression use can decrease the minimum sustainable frame duration to less than 10 seconds for an ECAT EXACT HR.<<ETX>>

Optimizing rod window width in positron emission tomography

A technique that determines optimal rod window width for rotating rod transmission studies in positron emission tomography (PET) is discussed. Rod windowing reduces noise in rotating rod transmission studies. Lines-of-response (LOR) which intersect the rods generate primarily true coincidence events. LORs which pass far from the rods generate random and scatter events. Since the angular position of the rotating rods is known in real-time, LORs, which produce mostly noise are gated off.<<ETX>>

A rotating PET camera using BGO block detectors

A positron emission tomography (PET) scanner with a reduced number of detectors per ring subtending two opposing arcs of 60 degrees and mounted on a circular support was developed. Projection data at all angles are acquired by rotating the detector assembly. The scanner has no septa, and data from the equivalent of 16 full rings of detectors are collected and sorted into 256 sinograms. The number of rotational positions at which data must be acquired depends on the size of the field of view (FOV) required. Reconstruction is performed using a fully 3-D reconstruction algorithm. The scanner has an absolute efficiency of 0.5% at the center of the FOV, which is the same as that of a full ring scanner with septa extended. The sensitivity measured with a 20-cm uniform cylinder is 175000 cps/ mu Ci/ml, and the transaxial and axial spatial resolution is the same as for a full ring scanner ( approximately 6 mm). The scatter fraction is 39% with a lower energy threshold at 250 keV. The maximum noise equivalent count rate estimated for a 15-cm-diameter cylinder is 42000 cps at a concentration of 0.5 mu Ci/ml. The camera was used for a number of /sup 18/FDG applications in neurology, cardiology, and oncology.<<ETX>>

FORE(J)+OSEM2D versus OSEM3D reconstruction for large aperture rotating LSO panel detector PET prototype

3D reconstruction on large aperture PET systems based on rotating LSO panel detectors is a challenge due to the large amount of data. When reconstruction time is critical, fast reconstruction methods are attractive despite the potential image quality loss due to rebinning approximation and the large polar aperture angle. This work validates a new data processing suite aimed at processing 3D data for two new PET scanner prototypes using two and five rotating LSO panel detectors, respectively. The processing includes /spl mu/-map reconstruction, generation of a 3D attenuation correction, scatter correction and image reconstruction with FORE(J)+OSEM2D and weighted OSEM3D schemes. The validation is based on reconstructing images for simulated wholebody FDG data with realistic random and scatter fractions and given statistics. The same processing suite was applied on experimental data acquired at intermediate statistics on a contrast sphere phantom and on a low statistics FDG wholebody study. Despite the large polar aperture angle, the image quality obtained with FORE(J)+OSEM2D was in excellent agreement with those obtained with time consuming methods such as OSEM3D. The loss in axial resolution from FOREJ to FORE was assessed from noiseless data. We also observed that, on low statistics wholebody data, FOREJ as implemented, required sinogram rebinning and smoothing to control the noise which offsets the benefits of the exact rebinning. The best images in terms of resolution and contrast were obtained with OSEM3D but they required precise normalization and deadtime corrections to avoid ring and banding artifacts.

Clinical time OSEM3D: infrastructure issues

In previous work we compared two parallel algorithms for calculating 3D forward and backprojection on a distributed-memory cluster computer. These two methods were used to develop an implementation of fully three-dimensional ordered subset expectation maximization iterative reconstruction for emission tomography (OSEM3D). It is, however, necessary to embed these computational kernels in an environment that supports efficient data movement and other infrastructural operations, such as process management. Here we briefly describe two particular components of the infrastructure: the I/O subsystem and the service demon. For the I/O subsystem, a fortuitous relationship between the traditional representation of the fully three-dimensional sinogram S/sub 4DO/(/spl theta/,z,/spl phi/,r) and the distributed representation for image space decomposition permits an efficient solution involving minimal data movement. The service demon is similar to others, but contains additional recovery-oriented mechanisms for minimizing mean time to repair. We conclude with performance benchmarks for fully 3D reconstruction for a scanner that produces a significant volume of data.