Deadtime measurements in scintillation cameras under scatter conditions simulating quantitative nuclear cardiography

Deadtime performance of scintillation cameras is sensitive to such factors as scatter fraction and analyzer window width. Data from manufacturers and previous investigators do not predict counting-rate losses under clinical conditions. Scintillation cameras used with Tc-99m for quantitative nuclear cardiology should be evaluated for deadtime performance by the two-source method using a scatter phantom designed to simulate the spectrum from Tc-99m in the heart. Under these conditions, scintillation cameras were found to follow the paralyzable model; accurate estimates could be obtained for data losses and maximum useful counting rates in a clinical setting. A survey of 39 contemporary scintillation cameras yielded a range of paralyzing deadtime values of 4.3 to 10 musec, with a 20% window centered on the Tc-99m photopeak. For an average deadtime of 6 musec, counting rates should be maintained below 36,000 cps to avoid undue data losses in excess of 25%.