NUCLEAR CARDIOLOGY

Nuclear cardiology is the study of the heart using small amounts of radioactive tracers. It allows us to study the function of the heart non-invasively (without inserting any instruments into the heart itself).

The most commonly used test in nuclear cardiology is called myocardial perfusion imaging. This is a form of stress test which makes use of myocardial perfusion tracers and gamma camera imaging methods to assess the adequacy of blood flow to the heart, as well as to find out if how much of the heart muscle is viable (alive) and how much is scarred and damaged beyond repair. It is a diagnostic tool that is often used for the detection of coronary artery disease (narrowing of the heart arteries) and for the assessment of the severity and extent of its effects on blood flow to the heart and myocardial viability.

How it works:

During the stress nuclear test, a small amount of a radioactive perfusion tracer (such as thallium, sestamibi or tetrofosmin) is injected into a hand vein while the patient is undergoing stress testing (Figure 1). These tracers are taken up by heart muscle in proportion to blood flow. Areas of myocardium with impaired blood flow or viability will have reduced tracer uptake. The uptake of tracer in the heart is then assessed by imaging the patient with a gamma camera, which is capable of detecting the small amounts of radiation emitted by the tracer. By rotating the camera around the patient to take multiple views from different angles (Figure 2), a 3-dimensional image of the distribution of tracer in the heart can be reconstructed using computerised methods (Single photon emission computed tomography, or SPECT Figure 3). The uptake of tracer in the heart can then be assessed in a tomographic (3-dimensional) way, which avoids overlap between different parts of the heart and allows us to localise the area and extent of any abnormalities with greater accuracy (Figure 4).

This technique is more accurate for detecting abnormal blood flow and heart disease than the standard ECG stress test.

How to interpret the results:

A normal scan will show uniform uptake of tracer throughout the heart, indicating that blood flow during stress is increased evenly throughout the heart (Figure 5). The likelihood of significant narrowing of heart arteries (coronary artery disease) is low, the prognosis is good and no further operation (angioplasty or bypass surgery) is usually indicated, since there is no evidence of insufficient blood flow.

An abnormal scan will show one or more areas of reduced tracer uptake following stress injection, and this can either be due to reduced blood flow to the heart or damaged heart muscle due to a previous heart attack (or a mixture of the two) (Figure 6). A rest study (which may involve a second scan with or without a second injection of tracer at rest) is needed to distinguish between the two.

The detection of a large amount of heart muscle with insufficient blood flow during stress indicates that the patient is likely to benefit from balloon angioplasty or bypass surgery.

With the use of certain tracers, additional information on heart function can also be obtained by using a special method of imaging called ECG-gated SPECT (Figure 7).

When are stress nuclear scans used?

Stress nuclear scans are useful in the detection of coronary artery disease, particularly when standard ECG stress testing is not possible or non-diagnostic. For instance, when the patient is unable to exercise or achieves an inadequate level of exercise on a treadmill, when the resting ECG is difficult to intepret or when the result of stress testing is borderline, then a stress nuclear scan is helpful. The overall sensitivity of the stress nuclear scan (83% to 98%) is higher than that of the standard ECG stress test (average 68%).

For patients who are unable to exercise, stress testing can be performed using drugs such as dipyridamole, adenosine or dobutamine which have been shown to have high accuracy for detection of coronary artery disease when used in combination with nuclear cardiac imaging.

In patients with known coronary artery disease on angiography, nuclear scans are often used to assess the effect of a narrowing in the arteries and to help decide if bypass surgery or balloon angiogioplasty is needed.

A normal stress perfusion study indicates a good prognosis and a low likelihood of sudden heart attack (approximately <1% cardiac event rate per year), even in the presence of known disease of the heart arteries, and in most cases, allows the patient to avoid having an angiogram, which is a test which involves inserting a tube(catheter) into the heart and injecting dye to outline the arteries.

In patients with damaged hearts due to a previously heart attack (myocardial infarction), perfusion imaging is often used to assess the risk that the patient may develop further heart attacks and help decide whether an angiogram, bypass surgery or balloon angioplasty is indicated.

In patients with known or suspected heart disease who are due for general (non-heart) surgery, myocardial perfusion imaging using pharmacologic stress is also used for assessment of risk before operation, particularly in patients due for surgery on their leg arteries who are unable to exercise.

Functional information vs anatomic information

One big difference between coronary angiography and stress nuclear scans is that angiography primarily provides information about the presence and severity of a narrowing, whereas nuclear scans provide information about the effect of the effect of that narrowing on blood flow and heart muscle function. This blood flow and function information is important in deciding if angioplasty or bypass surgery is needed, but is not routinely available from angiography. This difference also explains why there is sometimes an apparent discrepancy between the results of angiography and nuclear tests. Since not all narrowings in the heart arteries cause impaired blood flow, not all patients with coronary artery disease will have abnormal nuclear scans. Thus not all patients with heart disease will have an abnormal stress nuclear scan, and the accuracy for detection of narrowing of arteries can never be 100%. This limitation of nuclear scans should be kept in mind. Nevertheless, in the presence of a normal scan, indicating normal blood flow, the likelihood of disease is low and the prognosis is good, so that further investigation is usually not warranted at that point in time.

Practical aspects of stress nuclear scanning

On the day of the test, patients are advised to bring jogging shoes and attire. For those who cannot exercise, pharmacologic (drug) stress will be employed. Patients are advised not to take caffeine containing drinks (coffee, tea, colas) on the day of the test, since this can interfere with the action of dipyridamole, which is often used in stress testing with drugs. A light breakfast is allowed. Patients who are taking medications such as beta-blockers are also advised to stop these for 3 to 5 days before the test, depending on the half-life of the drug, and provided their physicians are agreeable. Alternatively, the patient can still be tested, by using drugs. Patients are then either exercised on a treadmill or undergo stress testing using drugs such as dipyridamole, adenosine or dobutamine, and the tracer is injected into a hand vein at the time of peak stress. Imaging with the gamma camera is performed half to one hour later, and takes 20 minutes to complete. If a rest scan is needed, this is performed 3-4 hours later, or on another day, depending on convenience.

Safety and radiation concerns

The total radiation exposure for patients undergoing stress nuclear scans is well within the level of what is considered safe. Considering that the nuclear test is being used to determine if the patient needs to undergo an angiogram or an angioplasty (or bypass surgery), this low amount of radiation exposure from stress nuclear scans appears to be quite acceptable.

Summary

In summary, stress nuclear scans have a important role to play in the management of patients with coronary artery disease by providing more accurate detection of disease than the standard stress ECG and additional useful information about blood flow and function which is usually not available from the coronary angiogram alone. This information, particularly the accurate assessment of the severity and extent of abnormal blood flow, is often critical in deciding whether the patient should be undergo coronary angiography, bypass surgery or balloon angioplasty.

Dr Terrance Chua
Dept. of Cardiology
National Heart Centre