A less commonly used PET perfusion tracer is 15O-water. 15O is produced by a cyclotron and has a physical half-life of 122 seconds. Theoretically, 15Owater is an ideal perfusion tracer because it is freely diffusible and metabolically inert. Its tissue accumulation is almost exclusively a function of flow and it does not underestimate flow at high flow rates. However, its freely diffusible state and very short physical half-life result in poor image contrast resolution (Fig. 5). Visual analysis is generally not feasible and quantification of absolute MBF must be performed, which somewhat limits its use for clinical purposes owing to longer processing times and less diagnostic and prognostic data. 15O-water differs from 13N-ammonia and 82Rb in that gating is not possible with 15O-water and it is
neither approved by the FDA nor reimbursed by Medicare for the assessment of myocardial perfusion.