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 (Table 1). 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 exerciseinduced 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."

   

Diagnostic Testing, Andrew F. McLaughlin, Nuclear Medicine Physician, Sydney (Aust Prescr 1994; 17; 3; 57-60); http://www.australianprescriber.com/magazines/vol17no3/thallium_scanning.htm