A NEW STRATEGY FOR THE ASSESSMENT OF VIABLE MYOCARDIUM AND REGIONAL MYOCARDIAL BLOOD-FLOW USING O-15-WATER AND DYNAMIC POSITRON EMISSION TOMOGRAPHY

Background. We have developed a new measure of myocardial viability, the water-perfusable tissue index (PTI), which is calculated from transmission, (CO)-O-15, and (H2O)-O-15 positron emission tomography (PET) data sets. It is defined as the proportion of the total anatomical tissue within a given region of interest (ROI) that is capable of rapidly exchanging water and has units g (perfusable tissue)/g (total anatomical tissue). The aim of this study was to assess the prognostic value of PTI in predicting improvement in regional wall motion after successful thrombolysis for acute myocardial infarction (AMI) and to measure the myocardial blood flow to the perfusable tissue (MBF(p), ml/min/g [perfusable tissue]). Furthermore, PTI was compared with (FDG)-F-18 metabolic imaging in patients with old myocardial infarction (OMI). Methods and Results. PET scans were performed in healthy volunteers (group 1, n = 8), patients with OMI (group 2, n=15), and in patients who were successfully thrombolysed after an AMI (group 3, n=11). Systolic wall thickening was measured by two-dimensional echocardiography within 2-4 days of AMI and after 4 months to assess contractile recovery. In the healthy volunteers, MBF(p) was 0.95+/-0.13 ml/min/g (perfusable tissue). PTI in these regions was 1.08+/-0.07 g (perfusable tissue)/g (total anatomical tissue), which was consistent with all normal myocardium being perfusable by water. In the OMI group, the ratio of the relative (FDG)-F-18 activity to the relative MBF(p) defect (metabolism-flow ratio) was calculated for each asynergic segment. Regions in which the metabolism-flow ratio was greater-than-or-equal-to 1.20 were considered reversibly injured, whereas those in which the ratio was <1.20 were deemed irreversibly injured. PTI in the former group of regions (n=9) was 0.75+/-0.14 g (perfusable tissue)/g (total anatomical tissue) and was significantly higher than in irreversibly injured regions (n=6) (0.53+/-0.12 g [perfusable tissue]/g [total anatomical tissue], p<0.01). Values of MBF(p) were similar in these segments. Seven of 12 segments in the AMI patients showed improved systolic wall thickening on follow-up. PTI in these recovery segments was 0.88+/-0.10 g (perfusable tissue)/g (total anatomical tissue) (p=NS versus control). PTI in the nonrecovery regions was 0.53+/-0.11 g (perfusable tissue)/g (total anatomical tissue), which was similar to the segments in group 2 in which (FDG)-F-18 uptake was absent. MBF(p) was similar in both the recovery and nonrecovery segments in the subacute phase. Conclusions. These data indicate that PTI may be a good prognostic indicator for the recovery of contractile function after successful thrombolysis and show that myocardial viability may be assessed by PET without metabolic imaging.