oncurrent acquisition of morphologic and functional imaging information for the diagnosis of neurologic disorders has fueled interest in simultaneous PET and MRI (PET/MRI) (1). PET/MRI allows spatial and temporal registration of the two imaging data sets; therefore, the information derived from one modality can be used to improve the other (2,3). For dementia evaluation, PET/ MRI enables a single combined imaging examination (4). Amyloid PET has become useful as an adjunct to the diagnosis of Alzheimer disease-amyloid plaque accumulation is a hallmark pathologic finding of Alzheimer disease and can precede the onset of frank dementia by 10 to 20 years (5)-as well as for screening younger populations at high risk of Alzheimer disease in clinical trials of Alzheimer disease pharmaceuticals (6,7). PET image quality depends on collecting a sufficient number of coincidence events from annihilation photon pairs. However, the injection of radiotracers will subject patients who are scanned to radiation dose; motion during the prolonged data acquisition period results in a misplacement of the events in space, leading to inaccuracies in PET radiotracer uptake quantification (8,9). Thus, reducing collected PET counts either through radiotracer dose reduction (the focus of this work) or shortening scan time (ie, limiting the time for possible motion) while maintaining image quality would be valuable for increased use of PET/MRI. Convolutional neural networks (CNNs) have the ability to learn translation-invariant representations of objects (10). This has led to remarkable performance increases for image identification (11) and generation (12-14).

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Purpose In vivo measurement of the spatial distribution of neurofibrillary tangle pathology is critical for early diagnosis and disease monitoring of Alzheimer's disease (AD). Methods Forty-nine participants were scanned with 18 F-PI-2620 PET to examine the distribution of this novel PET ligand throughout the course of AD: 36 older healthy controls (HC) (age range 61 to 86), 11 beta-amyloid+ (Aβ+) participants with cognitive impairment (CI; clinical diagnosis of either mild cognitive impairment or AD dementia, age range 57 to 86), and 2 participants with semantic variant primary progressive aphasia (svPPA, age 66 and 78). Group differences in brain regions relevant in AD (medial temporal lobe, posterior cingulate cortex, and lateral parietal cortex) were examined using standardized uptake value ratios (SUVRs) normalized to the inferior gray matter of the cerebellum. Results SUVRs in target regions were relatively stable 60 to 90 min post-injection, with the exception of very high binders who continued to show increases over time. Robust elevations in 18 F-PI-2620 were observed between HC and Aβ+ CI across all AD regions. Within the HC group, older age was associated with subtle elevations in target regions. Mildly elevated focal uptake was observed in the anterior temporal pole in one svPPA patient. Conclusion Preliminary results suggest strong differences in the medial temporal lobe and cortical regions known to be impacted in AD using 18 F-PI-2620 in patients along the AD trajectory. This work confirms that 18 F-PI-2620 holds promise as a tool to visualize tau aggregations in AD.

Our study indicates that PET image artifacts are significantly mitigated with integration of TOF information in simultaneous PET/MR. The impact is predominantly seen in patients with significant artifacts due to metal implants.