Imaging of Weak-Source Distributions in LSO-Based Small-Animal PET Scanners

Abstract

Lutetium oxyorthosilicate (LSO)-or lutetium-yttrium oxyorthosilicate (LYSO)-based PET scanners have intrinsic radioactivity in the scintillator crystals due to the presence of 176 Lu, which decays by b-emission followed by one or more prompt g-ray emissions. This leads to intrinsic true counts that can influence the image when scanning low levels of activity. An evaluation of the effects of this intrinsic activity for low levels of activity and different energy windows is performed on an LSO-based small-animal PET scanner. Methods: Intrinsic count rate and sensitivity were measured for a range of lower-level discriminators (LLDs) ranging from 100 to 750 keV. The noise equivalent count rate (NECR) as a function of LLD for activity levels from 100 Bq to 100 kBq was estimated using a combination of measurement and previously published data for this scanner. Phantom imaging was performed using three 68 Ge sources of strength 55, 220, and 940 Bq and LLD levels of 250, 350, and 400 keV. The images were assessed using a contrast-to-noise ratio (CNR) analysis and by comparing the observed ratio of source activities to the true ratio value. Results: The intrinsic true count rate is reduced from 940 counts per second (cps) for a 250-to 750-keV energy window to ,2 cps for a 400-to 750-keV window. There is a corresponding 2-fold drop in sensitivity for detected true events for external positron sources for these 2 energy windows. The NECR versus LLD curves showed a highly peaked shape, with the optimum LLD being approximately 425 keV. The phantom image results were dominated by the intrinsic true counts when an energy window of 250-750 keV was used. The intrinsic true counts were almost completely removed by raising the LLD to 400 keV. The CNR for each of the sources was higher for the narrow energy window and the 55 Bq could be easily visualized in images acquired with LLD levels of 350 and 400 keV but not when the 250-keV LLD was used. Images acquired with an LLD of 400 keV and reconstructed with 2-dimensional filtered backprojection were the most quantitatively accurate. Conclusion: It is possible to visualize sources of ,1 kBq in LSO-based animal PET systems by raising the LLD to 400 keV to exclude the majority of the counts due to the intrinsic activity present in the LSO. PET scanner performance is usually optimized for a specific imaging task based on the results of noise equivalent count rate (NECR) measurements (1-4). The usual measure reported is the peak NECR value and the activity at which the peak occurs and, in general, the energy window is chosen to maximize the value of the peak NECR. When the imaging task involves very low levels of injected activity, such as cell trafficking studies (5) or gene expression imaging (6), the scanner is operating at a count rate far less than the peak NECR value.When low activity levels are being imaged, there is a very limited number of true counts that can be acquired so that one would consider using a wider energy window to increase the sensitivity (7). However, the choice...

Performance evaluation of a high-sensitivity large-aperture small-animal PET scanner: ClairvivoPET

“…These results indicated a wider dynamic acquisition, whereas quantitative accuracy was preserved by injecting an optimized dose. On the other hand, intrinsic background coincidences in LYSO were expected to cause slight biases in quantifi cation under weak activity conditions [16]. Although background coincidences without an object were detected for 1.1 kcps in this system, quantifi cation and image contrast within the object were less affected in general animal studies, because only a portion of the background components were included within the object and were further attenuated by the object itself, owing largely to its low photon energy (below 307 keV).…”

National Electrical Manufacturers Association NU-4 Performance Evaluation of the PET Component of the NanoPET/CT Preclinical PET/CT Scanner

“…PH-60-00-41; Mediso Ltd.) with a 2.5-mm-inner-diameter line source positioned 17.5 mm off-center along the axis of the phantom was similarly filled with 60 MBq of 18 F. Dynamic scans were obtained in 1-5 coincidence mode, with 250-to 750-keV energy and 5-ns coincidence windows, until the activity of the line source decayed to below 23.5 kBq. With the same settings, an empty phantom was scanned for 54,000 s to calculate the background due to the intrinsic radiation of 176 Lu in the LYSO:Ce crystals (8). Every dataset was sorted into sets of 2D sinograms using the SSRB method (0.3-mm bin size and 0.585 slice thickness) with 250-to 750-keV energy and 5-ns coincidence window.…”

System design of a small OpenPET prototype with 4-layer DOI detectors

“…The performance of the DOI detector was tested for 511-keV uniform irradiation (26 cm 68 Ge-68 Ga line source, 0.34 MBq). The LGSO crystals have an intrinsic radioactivity of 176 Lu [18,19]. The 176 Lu emits three gamma photons simultaneously with the beta particles.…”