Decrease in the ability to detect elevated lung thallium due to delay in commencing imaging after exercise

Post-exercise elevation of the lung/myocardial thallium ratio and a high lung clearance rate between initial and delayed images have been reported to be markers for exercise-induced left ventricular (LV) dysfunction associated with coronary artery disease (CAD). We performed thallium exercise tests on 60 patients, 42 with CAD, in order to determine the effect of delaying initial imaging on detection of elevated lung thallium. In addition to images obtained at 2 minutes and at 2 hours after exercise, 18-minute images were also obtained to simulate such a delay. Because of rapid isotope clearance in those with initially elevated lung activity, there was decreased sensitivity of both the initial lung/myocardial ratio and lung thallium clearance for detecting CAD, using the 18-minute image as the initial post exercise study. We conclude that initial imaging should be done in the anterior view early after exercise to optimize detection of elevated lung thallium.