Error of uptake in dual energy acquisition with 201Tl and 123I labeled radiopharmaceuticals.

In myocardial scintigraphy, simultaneous injection of two radionuclides and dual energy acquisition are potentially useful in the assessment of regional perfusion and metabolism. The feasibility of dual radionuclide study with 201Tl and 123I labeled radiopharmaceuticals (meta-iodobenzylguanidine I 123, in this study) was investigated in phantoms and patients. The crosstalk of one radionuclide to the other pulse height window was defined as the ratio of the 201Tl count in the 123I and 201Tl windows, R[I/Tl], and the ratio of the 123I count in the 201Tl and 123I windows, R[Tl/I]. The ratios were determined in planar images and SPECT studies. In clinical studies with whole body scintigraphy (n = 8), the value of R differed significantly in various organs. In the SPECT study (n = 13), R was not uniform and varied with time and location, resulting in significant errors in uptake and regional count ratio. Thus, dual energy acquisition with 201Tl and 123I labeled radiopharmaceuticals is generally not recommended because of varying amounts of crosstalk interference. Possible solutions to this problem are discussed. The feasibility of dual radionuclide study must be confirmed in human as well as in phantom studies.