Medical Pharmacology Chapter 43:  Adult Cardiac Procedures

 Thallium Scans

• "Rationale

  •   ²⁰¹Tl is a monovalent cation which acts as a potassium analogue, concentrating in viable cells - not dead ones. It is distributed to all parts of the body, largely in proportion to regional blood flow.

  • The concentration of ²⁰¹Tl in the myocardium is proportional to coronary blood flow over a wide range of flows. This is important in exercise or pharmacological stress testing where flows can be as high as 4-5 times normal. Once inside the myocardial cell, the ²⁰¹Tl ion does not stay there. There is constant and continuous thallium exchange between the myocardial cell and the extracellular space. 

    • This unique property is taken advantage of clinically during ²⁰¹Tl stress testing and is the basis for the `redistribution' or late 4 hour image.

• Imaging

  • Thallium scanning requires a gamma or scintillation camera which detects gamma ray emissions and converts them into an image of the tracer's distribution.

  • Planar images are recorded in the anterior, 45½ and 70½ left anterior oblique views.

    • These are quick and simple, but have overlying activity in them and vessel territory identification is less accurate.

    • Tomography, known as Single Photon Emission Computed Tomography or SPECT, gives 3-dimensional information and is now the method of choice.

      • The camera rotates around the left chest and the image is reconstructed by the computer to give 3 planes through the left ventricle horizontal, long and short axis slices.

      • SPECT has the advantage of removing overlying activity and making vessel territory identification much easier.

    • SPECT can also assess the severity of a coronary stenosis, or stenoses, by the degree to which ²⁰¹Tl uptake is decreased (mild, moderate, severe) in a perfusion defect.

      • The severity and extent of disease can be semiquantified using comparative data from the SPECT ²⁰¹Tl scans of a cohort of normal subjects.

      • The result is displayed as a polar map or bull'seye plot of the heart, showing the uptake of ²⁰¹Tl, in varying colours, outside 2.5 standard deviations from normal.

      • The three vessel territories (left anterior descending, left circumflex and right coronary arteries) are ascribed areas on the bull's-eye and their contributions to perfusion can be calculated as a percentage.

• Exercise Stress Testing

  • Exercise is the most frequently used form of stress in the testing of cardiac function.

    • During exercise, myocardial blood flow normally increases by up to 3-4 fold to supply the increase in oxygen demand. ²⁰¹Tl scanning measures this flow reserve.

    • If coronary flow is limited by a stenosis, there will be less ²⁰¹Tl extracted from the blood by the myocardium in the territory of that artery compared with the normal, surrounding myocardium.

      • This will cause a perfusion defect.

    • Over 2-4 hours, this perfusion defect will redistribute, or `fill in', as ²⁰¹Tl washes out of the normally perfused cells and some more ²⁰¹Tl is extracted by the hypoperfused myocardial cells. Hence, in ischaemia, the ²⁰¹Tl perfusion defect disappears with time and is called a reversible defect. By contrast, areas of scar tissue or infarction do not change with time and appear as fixed defects.

• Pharmacological Stress Testing

  • In the patient who cannot exercise for various reasons, e.g. those with amputations or peripheral vascular disease, pharmacological `stress' may be used.

    • This form of testing is also used in patients with left bundle branch block which frequently causes false positive septal defects after exercise testing.

    • Intravenous dipyridamole is used as it is a potent coronary vasodilator increasing myocardial blood flow by 4-5 times normal - even higher than exercise.

    • A higher flow, and therefore higher ²⁰¹Tl extraction by normal myocardium compared to that supplied by a stenosed artery, reveals an apparent perfusion defect. 

• Prognostic value

  •   ²⁰¹Tl stress testing has prognostic value in a variety of patients with suspected or known CAD.

    • Patients with one or more reversible perfusion defects on a ²⁰¹Tl stress test have a 6-12 fold increased risk of future cardiac events - cardiac death or acute myocardial infarction (MI) - compared with normal.

    • The best predictor of future cardiac events is the number of reversible ²⁰¹Tl defects.

    • Conversely, a normal ²⁰¹Tl scan, even in the presence of angiographically proven CAD, has a very high predictive value for a low (approaching the normal population) rate of future cardiac events.

    • Increased lung uptake of ²⁰¹Tl and transient ventricular dilatation are markers of severe CAD. Both are signs of exercise-induced ischaemic left ventricular dysfunction which has serious prognostic implications.

      • In one series, uptake of ²⁰¹Tl by the lungs was the best predictor of cardiac events among all clinical and angiographic variables tested.

    •   Following MI, ²⁰¹Tl stress testing has an important role in risk stratification. It is superior to sub-maximal exercise ECG testing alone because it has increased sensitivity for detecting multivessel disease, localises vascular territories, distinguishes ischaemia from infarct and identifies exercise induced left ventricular dysfunction.

      • Dipyridamole ²⁰¹Tl stress is better than sub-maximal exercise ²⁰¹Tl stress and is the only significant predictor of late cardiac death and recurrent MI. The incidence of cardiac events, post-MI, in patients with dipyridamole induced ischaemia is 64% compared with 8% if there is no ischaemia."

• Reference:  Diagnostic Testing, Andrew F. McLaughlin, Nuclear Medicine Physician, Sydney (Aust Prescr 1994; 17; 3; 57-60)