Evaluation of image signal-to-noise ratio in time-of-flight PET

In PET imaging Noise Equivalent Counts (NEC) is a common image quality index, derived to be proportional to image SNR 2 and used as an index of general system performance. Many studies have shown that TOF information leads to reduced noise, faster image convergence, and improved SNR. However, the original NEC formula does not account for any contribution of timing resolution to the final image quality, and modified versions of the formula have been proposed to account for the reduction in noise variance and increased sensitivity due to TOF information. In this study, we aim to investigate the relationship between NEC and image SNR in uniform phantoms when OSEM and TOF OSEM image reconstruction is applied. Two cylindrical uniform phantoms, 20 and 35 cm in diameter, were acquired over a wide range of activity levels on a Philips Gemini TF PET scanner. Multiple realizations of the original scans were obtained through bootstrapping and reconstructed with OSEM and TOF OSEM algorithms to obtain mean and standard deviation images. The ratio of the mean value in a central ROI over both images was taken as a measure of image SNR. NEC was calculated from the original data using both the classical and a TOF-adapted formula. The results show that Trues have a better proportionality with image SNR 2 than NEC and TOF NEC in the considered range of activities. Timing resolution and Random fraction appear to have a limited influence on image SNR, but the usage of a matched TOF kernel in reconstruction is found to be necessary to maximize the gain

NEMA NU 4-2008 Comparison of preclinical PET imaging systems

The National Electrical Manufacturers Association (NEMA) standard NU 4-2008 for performance measurements of smallanimal tomographs was recently published. Before this standard, there were no standard testing procedures for preclinical PET systems, and manufacturers could not provide clear specifications similar to those available for clinical systems under NEMA NU 2-1994 and 2-2001. Consequently, performance evaluation papers used methods that were modified ad hoc from the clinical PET NEMA standard, thus making comparisons between systems difficult. Methods: We acquired NEMA NU 4-2008 performance data for a collection of commercial animal PET systems manufactured since 2000: micro- PET P4, microPET R4, microPET Focus 120, microPET Focus 220, Inveon, ClearPET, Mosaic HP, Argus (formerly eXplore Vista), VrPET, LabPET 8, and LabPET 12. The data included spatial resolution, counting-rate performance, scatter fraction, sensitivity, and image quality and were acquired using settings for routine PET. Results: The data showed a steady improvement in system performance for newer systems as compared with first-generation systems, with notable improvements in spatial resolution and sensitivity. Conclusion: Variation in system design makes direct comparisons between systems from different vendors difficult. When considering the results from NEMA testing, one must also consider the suitability of the PET system for the specific imaging task at hand.This work was funded by the Natural Sciences and Engineering Research Council of Canada under Discovery Grant 341628-2007. No other potential conflict of interest relevant to this article was reported.En prens

Effect of angiotensin-converting enzyme inhibitor and angiotensin receptor blocker initiation on organ support-free days in patients hospitalized with COVID-19

IMPORTANCE Overactivation of the renin-angiotensin system (RAS) may contribute to poor clinical outcomes in patients with COVID-19. Objective To determine whether angiotensin-converting enzyme (ACE) inhibitor or angiotensin receptor blocker (ARB) initiation improves outcomes in patients hospitalized for COVID-19. DESIGN, SETTING, AND PARTICIPANTS In an ongoing, adaptive platform randomized clinical trial, 721 critically ill and 58 non–critically ill hospitalized adults were randomized to receive an RAS inhibitor or control between March 16, 2021, and February 25, 2022, at 69 sites in 7 countries (final follow-up on June 1, 2022). INTERVENTIONS Patients were randomized to receive open-label initiation of an ACE inhibitor (n = 257), ARB (n = 248), ARB in combination with DMX-200 (a chemokine receptor-2 inhibitor; n = 10), or no RAS inhibitor (control; n = 264) for up to 10 days. MAIN OUTCOMES AND MEASURES The primary outcome was organ support–free days, a composite of hospital survival and days alive without cardiovascular or respiratory organ support through 21 days. The primary analysis was a bayesian cumulative logistic model. Odds ratios (ORs) greater than 1 represent improved outcomes. RESULTS On February 25, 2022, enrollment was discontinued due to safety concerns. Among 679 critically ill patients with available primary outcome data, the median age was 56 years and 239 participants (35.2%) were women. Median (IQR) organ support–free days among critically ill patients was 10 (–1 to 16) in the ACE inhibitor group (n = 231), 8 (–1 to 17) in the ARB group (n = 217), and 12 (0 to 17) in the control group (n = 231) (median adjusted odds ratios of 0.77 [95% bayesian credible interval, 0.58-1.06] for improvement for ACE inhibitor and 0.76 [95% credible interval, 0.56-1.05] for ARB compared with control). The posterior probabilities that ACE inhibitors and ARBs worsened organ support–free days compared with control were 94.9% and 95.4%, respectively. Hospital survival occurred in 166 of 231 critically ill participants (71.9%) in the ACE inhibitor group, 152 of 217 (70.0%) in the ARB group, and 182 of 231 (78.8%) in the control group (posterior probabilities that ACE inhibitor and ARB worsened hospital survival compared with control were 95.3% and 98.1%, respectively). CONCLUSIONS AND RELEVANCE In this trial, among critically ill adults with COVID-19, initiation of an ACE inhibitor or ARB did not improve, and likely worsened, clinical outcomes. TRIAL REGISTRATION ClinicalTrials.gov Identifier: NCT0273570

A model of scintillation detector performance for positron emission tomography

This work investigates two new Anger-logic detector models to improve the performance of PET scanners. The first model investigates using a slotted front surface in a position-sensitive NaI(Tl) detector. The sensitivity of an unslotted detector increases with crystal thickness, but the spatial resolution worsens due to increased spreading of light. A slotted detector reduces the light spreading which leads to a reduction of pulse-pileup, thereby extending the count-rate capability of the PET scanner. Experimental measurements were performed with a 1&inches; thick, slotted Nal(TI) detector to validate the model developed through simulations, and optimize the tradeoff of the slot depth and spatial resolution. The count-rate performance of NaI(TI) detectors is also limited by the long decay time of NaI(T1) signal. A pulse shaping circuit was developed which narrows the NaI(T1) signal and improves the energy resolution at short integration times and high count-rate. A high count-rate simulation program predicts a doubling of the peak performance rate of the current whole-body scanner (CPET), using the slotted detector together with the pulse shaping circuit. For the second detector model, a new scintillator (GSO) with a high attenuation coefficient, good energy resolution, and short signal decay time was chosen. Detector simulations and measurements helped in designing a lightguide which optimizes the discrimination of 4 x 4 x 10 mm3 crystals. The pulse shaping circuit was modified for the GSO signal to achieve good signal sampling with the digitizers used in the electronics. High count-rate simulations show that a GSO-based brain scanner using this detector will result in a five fold increase in the peak performance rate over the current Nal(Tl)-based brain scanner (HPET). A brain scanner based upon the GSO Anger-logic detector has been almost completed. Initial results show that the image resolution is 3.5 mm with very little pulse pileup in the energy spectrum at high activity levels. These results are consistent with the simulation results for the scanner

A model of scintillation detector performance for positron emission tomography

This work investigates two new Anger-logic detector models to improve the performance of PET scanners. The first model investigates using a slotted front surface in a position-sensitive NaI(Tl) detector. The sensitivity of an unslotted detector increases with crystal thickness, but the spatial resolution worsens due to increased spreading of light. A slotted detector reduces the light spreading which leads to a reduction of pulse-pileup, thereby extending the count-rate capability of the PET scanner. Experimental measurements were performed with a 1&inches; thick, slotted Nal(TI) detector to validate the model developed through simulations, and optimize the tradeoff of the slot depth and spatial resolution. The count-rate performance of NaI(TI) detectors is also limited by the long decay time of NaI(T1) signal. A pulse shaping circuit was developed which narrows the NaI(T1) signal and improves the energy resolution at short integration times and high count-rate. A high count-rate simulation program predicts a doubling of the peak performance rate of the current whole-body scanner (CPET), using the slotted detector together with the pulse shaping circuit. For the second detector model, a new scintillator (GSO) with a high attenuation coefficient, good energy resolution, and short signal decay time was chosen. Detector simulations and measurements helped in designing a lightguide which optimizes the discrimination of 4 x 4 x 10 mm3 crystals. The pulse shaping circuit was modified for the GSO signal to achieve good signal sampling with the digitizers used in the electronics. High count-rate simulations show that a GSO-based brain scanner using this detector will result in a five fold increase in the peak performance rate over the current Nal(Tl)-based brain scanner (HPET). A brain scanner based upon the GSO Anger-logic detector has been almost completed. Initial results show that the image resolution is 3.5 mm with very little pulse pileup in the energy spectrum at high activity levels. These results are consistent with the simulation results for the scanner

Total-body PET is ready for prime time

This is an exciting time for PET technology where we anticipate major developments in instrumentation that might lead to breakthoughs in clinical and research applications. Over the years, different design trends have emerged, with PET scanners now available with a broad spectrum of features, from those available commercially for clinical applications to others designed primarily in research laboratories specifically for very high-resolution research applications. The latter category includes organ-specific (brain, breast, prostate) and small-animal imaging systems