Impact of Time-of-Flight (TOF) reconstruction on 40-segmented brain volumes of (+/-) AD patients with short-lived 11C-Pittsburgh compound-B PET/MR Imaging

Introduction: Simultaneous PET/MRI (positron emission tomography-magnetic resonance imaging) is an advance in hybrid imaging, and the demand for such imaging systems is continuously growing. Early clinical studies showed comparable or improved results of PET/MRI in the diagnosis of neurodegenerative diseases like Alzheimer’s disease (AD) compared to PET/CT (computed tomography).For patients with AD, the use of PET/CT with ¹⁸F-FDG or, more recently, Short-lived ¹¹C-Pittsburgh compound-B (PiB) is widely used and showed enhanced diagnostic accuracy compared to prevailing functional or morphological imaging modalities alone. Newly industrialized simultaneous PET/MRI with short-lived ¹¹C-PiB studies yielded equal results in diagnosing AD compared to PET/CT. Further, PET/ MRI improved the tracer localization in ¹¹C-PiB, which could even improve the accuracy of local staging over multi-parametric MRI. The amalgamation of two high-end imaging modalities was benefitted from the development of newer detector systems for PET by featuring TOF in PET/MR systems. Previous experiments have shown that TOF information improves image quality in both PET/CT and PET/MRI. However, studies comparing TOF versus non-TOF PET/MRI addressing the key brain VOIs in short-lived ¹¹C-PiB are still rare. PET acquisition in ¹¹C-PiB is usually started at 40-60 minutes post-injection because ¹¹C-PiB distribution reaches a stable state at that time window which is at 2-3 times of the half-life for ¹¹C (t_1/2= 20.38min). Therefore, ¹¹C-PiB PET frequently works in a low-count condition and hence results in degraded image quality. Time-of-Flight (TOF) reconstruction (VUE-FX) provide more accurate SUV quantification due to improved time resolution of the SiPM detectors, MRAC based attenuation correction and reduction of partial volume effect (PVE) than the VUE-HD (non-TOF). Therefore, the objective of our study was to explore the impact of the TOF reconstruction on the different brain volumes of interests (VOIs) with the short-lived ¹¹C-PiB tracer in PET/MR brain images among (+/-) Alzheimer’s patients.

Methods: Patients who underwent ¹¹C-PiB PET/MR imaging were retrospectively included in the study. Each PET LIST mode data was reconstructed with and without the TOF reconstruction algorithm. Standard uptake values (SUVs) of 40 brain volume of interest (VOIs) were measured using PMOD version 3.906. TOF and non-TOF SUVs and SUVr were assessed by paired t-test.

Results: Total 26 patients (n=26; (+) AD=22 ;(-) AD=4) were included with the median age (years) 62.07±7.7 y and body mass index (BMI) 25.06±3.2 kg/m² in ¹¹C-PiB. Higher average SUV_max and SUV_mean values were observed in TOF compared to non-TOF reconstruction. Differences of SUV_max and SUV_mean were significant (p<0.05) in 35/40 and 39/40 brain VOIs.Significant enhancement (p<0.05) in SUVr _max and SUVr _mean were observed among 3/39 and 26/39 segmented brain VOIs respectively.

Conclusions: TOF leads to significantly improved SUVs than non-TOF among the 40 segmented brains VOIs in short-lived ¹¹C-PiB among (+/-) Alzheimer’s patients. However, SUVr _max was not significantly improved among 36/39 brain VOIs. Improved timing resolution aided the rapid decay rate of short-lived ¹¹C-PiB, and it shortened the scan time by increasing patient comfort and reducing motion artefact among AD/PD patients.