Cerebral Blood Flow Imaging with 15O-H2O PET/MR- Review and Workflow Optimization

Objectives Cerebral Blood Flow (CBF) is a valuable perfusion metric for evaluating cerebrovascular disease and PET imaging with 15O-H2O (T1/2 = 122s) is considered the gold standard for quantifying CBF. Until recently, no simultaneous comparison between 15O-H2O PET and MRI-based arterial spin labeling (ASL) for measuring CBF was possible. With the arrival of integrated PET/MR systems, such a comparison becomes feasible. In this study, we developed an optimized protocol and workflow to study the reproducibility of CBF measurements between 15O-H2O PET and ASL MRI.

Methods An efficient workflow is integral in minimizing radiation exposure to technologists and exam time for participants. Eight participants, five healthy volunteers and three subjects with cerebrovascular disease (24-66 years old; mean SD: 35 +/- 14 yr.; 5 female, 3 male), were imaged on a 3T time-of-flight (TOF) GE PET/MR system (Signa, GE Healthcare, Waukesha, WI) in the supine position using a 32-channel head coil. PET bed position and MR sequences were prescribed based on a localizer and PET/MR imaging began once the overall PET detector count rate reached 5 kcps. Our protocol included administration of 3 doses of 15O-H2O, a total of 3 dynamic PET acquisitions (15 min.) were acquired per subject with simultaneous ASL MRI. Timing is crucial when imaging with short-lived radiopharmaceuticals, therefore, strict time markers were established to administer the prescribed dose. A timer is started at completion of synthesis and one person is required to meet the dose at the cyclotron, ensure a clear path for delivery to the PET/MR suite and track via spreadsheet key time points for dose assay and injection. Once the dose is delivered from the cyclotron facility, it is placed into a dose-drawing apparatus and assayed when it is ~80 mCi. At ~35 mCi the dose is taken into the PET/MR suite, with a target injection of 15-25 mCi. Following the second PET acquisition, 15mg/kg of acetazolamide (Diamox), which increases CBF by 15-20%, is administered as a 2-min IV infusion. Anatomical MR sequences were acquired during the approximate 10-15 min required for the CBF increase to plateau after Diamox. Following this, the post-Diamox PET & ASL data were acquired.

Results The average imaging time for the entire protocol was 119 +/- 8 min. In total, 23 15O-H2O injections were administered. One subject only received two tracer injections due to cyclotron issues. The average dose administered was 21.3 +/- 5.6 mCi. For quantification of CBF, acquiring the first-pass peak during dynamic PET acquisition is crucial. Therefore, we prescribed our MRI scan planes for all the sequences before the start of PET scan and automated the acquisition start based on PET detector count.

Conclusions 15O-H2O PET/MR imaging has been successful in its early stages at our facility and our initial workflow goals have been achieved. Further workflow and protocol optimization is required in order to ensure efficient dose synthesis, delivery and administration to subject. Future work will focus on further reduction of imaging time and increased patient comfort.