Quantitation of thallium 201 kinetics after dipyridamole in normal volunteers

Quantitative norms for thallium 201 scintigraphy after dipyridamole have not been established. The regional myocardial uptake and washout of ²⁰¹Tl in 33 young volunteers after intravenous dipyridamole (0.56 mg/kg) was studied. Standard planar imaging and tomography were performed in addition to serial recordings of ²⁰¹Tl activity in the heart, liver, and skeletal muscle between 5 and 50 minutes. The myocardial uptake of thallium was found to increase continuously by an average of 11.5% above the first-pass extraction level during the first 20 minutes. This increment varied in magnitude for different individuals from 0% to 26% and was apparently supplied from a substantial pool of radionuclide in liver that discharged rapidly during the same interval. Thus, the 4-hour fractional washout of thallium was highly variable for different individuals when referenced to initial images taken at 5 minutes after ²¹⁰Tl injection (mean, 43%; range, 0% to 70%) but gave more uniform values when referenced to images taken at 20 minutes (mean, 49%; range, 30% to 60%). We conclude that the optimum time for initial imaging of thallium 201 after dipyridamole infusion is 15 to 20 minutes after injection of the nuclide, when myocardial activity is maximal. The tighter range of normal washout values obtained with this approach should provide for better discrimination between normals and patients demonstrating slow clearance of thallium 201. Normal values for quantitative analysis of regional distribution and washout of radiothallium after dipyridamole are given for planar and single photon emission computed (SPECT) imaging.