For simultaneous dual-isotope studies, we prefer to utilize
a protocol in which the patient is first injected at
rest with 7 mCi of Tc-99m Tetrofosmin and then given
4.0 mCi of Tl-201 during exercise. This protocol
minimizes radiation exposure to the patient, maximizes
efficiency in time duration of the study and utilizes
Tl-201 to best advantage in terms of its ability to
detect ischemic heart disease. Our experience with
the 7 pinhole technique indicates that gated studies
utilizing this protocol can be performed in 20 minutes.
It should be emphasized that the combined nature of the acquisition protocol also eliminates changes in appearance due to the position and size of the heart. These variables need to be included as contributing factors in the origin of systematic errors in sequential imaging protocols.
The visual presentation of these images shows the contrast of the Tc-99m images to be slightly higher than the Tl-201 images. Other than that, these images appear to be identical. Again, our 7-pinhole experience leads us to utilize circumferential profile plots as the most precise modality for comparison purposes. Over the past 7 years, we have performed over 16,000 simultaneous dual-isotope studies by the 7-pinhole technique and consider +/-10% to represent the systematic error of that technique. Utilizing a 20% threshold level for identifying the presence of ischemia, the 7-pinhole embodiment of this protocol provides diagnostic accuracies of 95% for both sensitivity and specificity.