What is SPECT?

What is SPECT?

SPECT stands for single photon emission computed tomography. A radioactive tracer like technetium sestamibi is injected into a vein and a nuclear scan using a gamma camera is obtained after about 20 minutes. This will show the amount of radioactive tracer in each part of the heart at rest.

After that those who can walk on a treadmill will be connected with electrodes for monitoring ECG during the test. Treadmill slope and speed are typically increased every three minutes and maximal exercise without angina or ECG changes is achieved. The test is terminated earlier in case of significant angina, other major symptoms, or ECG changes. The ECG printouts are analysed by the supervising physician.

At maximal exercise, one more injection of radioactive tracer is given. One more nuclear scan is done after waiting about 20 minutes with the gamma camera. The rest images are compared with the exercise images by the nuclear medicine physician.

Those who are unable to exercise on the treadmill may be given dipyridamole, adenosine or dobutamine infusion to stress the heart and nuclear stress images of the heart obtained as described above.

Caffeinated drinks are avoided for 24 hours prior to the procedure. Any medications containing caffeine has also to be avoided. Asthmatics on theophylline preparations will have to hold it for 48 hours. Nitrates are withheld on the day of the test.  All these are followed strictly as per the advice of the individual’s treating physician as the exact medical condition may mandate some modifications.

Advantages of single photon emission computed tomography

Important advantages of SPECT are that it has been extensively validated and has a good sensitivity, compared to other methods of assessment of myocardial viability. Cost of SPECT is lower than PET (positron emission tomography) imaging and is more widely available than PET in most regions. SPECT can be used in the presence of cardiac implantable electronic devices (CIED) while cardiac magnetic resonance (CMR) imaging has important limitations in this setting.

Disadvantages of SPECT

SPECT has a much higher cost compared to echocardiography and less availability compared to echo. Spatial resolution of SPECT is limited. There is a potential difficulty in interpreting SPECT results in patients with three vessel disease and balanced myocardial ischemia [1,2] as the SPECT interpretation depends on comparing the counts between normal and abnormal regions. Compared to CMR and echocardiography, SPECT has some radiation risk, though not very high.

Balanced myocardial ischemia

If all three major coronary branches have similar degree of stenosis, the radionuclide used for perfusion study will have equal uptake in all regions of the myocardium. Thus a balanced three vessel coronary artery disease can produce a false negative myocardial perfusion scan. This situation is called balanced myocardial ischemia. In most cases this is picked up by the treadmill ECG usually done along with stress myocardial perfusion imaging. Since the ECG findings of myocardial ischemia does not depend on the relative perfusion in the coronary tree, ischemic changes will be noted in treadmill ECG even when nuclear perfusion study shows false negative due to balanced myocardial ischemia.

In a case reported in Heart [3], adenosine technetium (Tc99) sestamibi scan showed no resting or stress induced perfusion defects in an elderly person with stable exertional chest pain. But ST segment depression was noted on the ECG. Coronary computed tomography (CT) angiogram showed severe calcified ostial left main lesion, which was further confirmed by invasive coronary angiography. The patient underwent successful coronary artery bypass surgery.

In another case report published in Heart International, there was triple vessel coronary artery disease including 95% distal left main stenosis, with markedly positive stress ECG and a negative SPECT 99m-sestamibi scintigraphy [2]. This patient also underwent successful coronary artery bypass surgery.

Other potential causes of false negative nuclear perfusion studies

Other potential causes of false negative nuclear perfusion study are lesions in the left circumflex coronary artery, use of antianginal agents prior to the test and low levels of exercise leading to low myocardial oxygen demand. Caffeine use can interfere with dipyridamole stress nuclear perfusion imaging due to drug interaction.

As a corollary, balancing of electrical vectors in opposite directions have been invoked to explain false negative exercise treadmill ECGs in patients with multivessel coronary artery disease [4].

References

1. Emad F, Javed F, Alviar CL, Herzog E. Triple vessel coronary artery disease presenting as a markedly positive stress electrocardiographic test and a negative SPECT-TL scintigram: a case of balanced ischemia. Heart Int. 2011 Sep 29; 6(2): e22.
2. Aziz EF, Javed F, Alviar CL, Herzog E. Triple vessel coronary artery disease presenting as a markedly positive stress electrocardiographic test and a negative SPECT-TL scintigram: a case of balanced ischemia. Heart Int. 2011 Sep 29; 6(2): e22.
3. Lesser JR, Bae R, Flygenring B, Sharkey SS, Lindberg J, Schwartz RS. Balanced myocardial ischaemia: a case of “normal” stress Tc99 sestamibi scan and diagnosis. Heart. 2005 Jul; 91(7): e53.
4. Madias JE, Mahjoub M, Valance J. The paradox of negative exercise stress ECG/positive thallium scintigram. Ischemic ST-segment counterpoise as the underlying mechanism. J Electrocardiol. 1996 Jul;29(3):243-8.