Parallax error in long-axial field-of-view PET scanners—a simulation study

Abstract

There is a growing interest in the design and construction of a PET scanner with a very long axial extent. One critical design challenge is the impact of the long axial extent on the scanner spatial resolution properties. In this work, we characterize the effect of parallax error in PET system designs having an axial FOV of 198 cm (total-body PET scanner) using fully-3D Monte Carlo simulations. Two different scintillation materials were studied: LSO and LaBr3. The crystal size in both cases was 4×4×20 mm3. Several different depth-of-interaction (DOI) encoding techniques were investigated to characterize the improvement in spatial resolution when using a DOI capable detector. To measure spatial resolution we simulated point sources in a warm background in the center of the imaging FOV, where the effects of axial parallax are largest, and at several positions radially offset from the center. Using a line-of-response based OSEM reconstruction algorithm we found that the axial resolution in an LSO scanner degrades from 4.8 mm to 5.7 mm (FWHM) at the center of the imaging FOV when extending the axial acceptance angle (α) from ±12° (corresponding to an axial FOV of 18 cm) to the maximum of ±67°—a similar result was obtained with LaBr3, in which the axial resolution degraded from 5.3 mm to 6.1 mm. For comparison we also measured the degradation due to radial parallax error in the transverse imaging FOV; the transverse resolution, averaging radial and tangential directions, of an LSO scanner was degraded from 4.9 mm to 7.7 mm, for a measurement at the center of the scanner compared to a measurement with a radial offset of 23 cm. Simulations of a DOI detector design improved the spatial resolution in all dimensions. The axial resolution in the LSO-based scanner, with α = ±67°, was improved from 5.7 mm to 5.0 mm by incorporating a two-layer DOI detector. These results characterize the maximum axial blurring for a fully open 2 m long PET scanner and demonstrate that large sensitivity gains are possible with a modest reduction in resolution when using current clinical detector technology with no DOI capability.

Physical performance of a long axial field‐of‐view PET scanner prototype with sparse rings configuration: A Monte Carlo simulation study

“…Schmall et al showed~19% and <6% reduction in axial and transverse resolutions respectively when increasing the acceptance angle from AE12°to AE67°. 39 Similarly, our findings showed 18% and less than 6% reduction in axial and transverse spatial resolutions respectively where the acceptance angle was increased from AE15°(mCT) to AE28°( Ex-mCT). In a sparse detector-ring configuration, such in the case of the Ex-mCT, parallax effect will be enhanced in particular for small acceptance angles, due to the increased apparent crystal size; gamma photons penetrating through the physical gaps will hit the side (20 mm long) of a crystal.…”

Total-Body PET: Maximizing Sensitivity to Create New Opportunities for Clinical Research and Patient Care

“…A 40-fold increase in sensitivity means that a tracer could be followed for 5-6 additional half-lives (ignoring biologic clearance) above the approximately 3 half-lives possible on existing scanners. For labeled radiotracers, the implication is that imaging could be conducted as late as about 18 h for 18 F, 3 h for 11 C, and 18 min for 15 O. For a long-lived radiotracer such as 89 Zr, sufficient signal might still be available for imaging about 30 d after injection.…”

“…Existing time-of-flight PET detector technology used in current-generation PET/CT and PET/MRI scanners can in principle be used. Although it would be desirable to further improve timing resolution, and incorporate depth-ofinteraction encoding into the detectors to account for increased parallax errors caused by the long cylindric geometry (18), the largest increment in sensitivity and utility comes immediately from the geometry of the total-body PET concept, which covers the entire body and efficiently detects almost all the photons that leave the body, regardless of the direction in which they are emitted.…”

“…Taking a standard "eyes-to-thighs" wholebody 18 F-FDG cancer imaging protocol as a reference, we are anticipating that the first-generation EXPLORER scanner will be able to reduce imaging times by approximately a factor of 24. Thus, a 12-min multiple-bed-position study could be conducted on a total-body PET scanner in 30 s in a single bed position with the same image quality.…”

“…For applications in which the distribution of radiotracer in the entire body or multiple organ systems is of interest, this limitation leads to further inefficiencies and makes it difficult to acquire dynamic data from all the tissues of interest. If one takes whole-body PET scanning with 18 F-FDG as an example, the total efficiency with which pairs of coincidence photons that escape the body are detected is well under 1% even on today's best scanners. Simplistically, this number can be derived by considering that the average geometric sensitivity within the FOV of a typical clinical PET scanner is under 5% and that with an axial coverage of 20 cm, less than an eighth of the body is in the FOV at any one time.…”