Tl was introduced in the early 70-ties replacing experimental K isotopes for imaging of myocardial perfusion. When injected iv, Tl is distributed according to cardiac output, and taken up by all living cells. The first pass extraction fraction varies among tissues, for the myocardium it is about 85%. After 2-3 circulations there will be very little left in the blood. Only about 5% of the injected dose goes to the myocardium because, only 5% of the cardiac output goes to the myocardium. Within an organ like the heart the regional uptake is proportional to regional flow, thus areas with high flow will have high uptake, areas with low flow will have low uptake, and areas with no flow will have no uptake.

A Stress - Redistribution protocol was introduced: 2-3 mCi was injected during peak stress, and stress imaging was performed as quickly as possible. Redistribution imaging was performed 4 h later, when redistribution presumably had taken place, and these images were supposed to represent rest perfusion. This protocol was standard all over the world in millions of myocardial perfusion studies until 1989.

The problem with Tl is, that redistribution defined as disappearance of stress induced hypoperfusion (alias ischemia, alias low uptake) is unpredictable:
In some patient with severe stress induced ischemia (low uptake) this inhomogenity equilibrated within a few hours.
In other patients with similar stress induced ischemia or much less severe ischemia, the equilibration takes much longer time, 12 hours or even 24 hours.

In about 40% of patients the stress - 4-hour redistribution protocol suffered from incomplete redistribution, which leads to false "rest or fixed" perfusion defects, ie. infarction and missed ischemia.

24 h Imaging. When patients were imaged 24 h post stress injection many of the "rest or fixed" perfusion defects seen in 4 h redistribution images were gone (sic !!).

Re-Injection: An alternative to 24 h imaging using re-injection was introduced and found compatible with 24 h imaging, much more convenient, and showed better image quality. Re-injection protocols comes in many flavors, the most commonly used are:

The overall cost for Tl, Sestamibi and Tetrophosmin is similar - however local differences may exist.

A highly significant advantage with the Tc-based tracers is the high count statistics (5 to 10 times higher than Tl), which makes it possible to perform gated SPECT, thus adding functional information to the conventional perfusion information.