Myocardial perfusion scans

Myocardial perfusion imaging indicates relative differences in the distribution of blood flow in the myocardium at rest enduring stress, which may be produced by exercise or by pharmacological means.

Myocardial a arterioles distal to an epicardial coronary stenosis are dilated by autoregulation to maintain myocardial  blood flow at rest.

Under stressful conditions there is significant vasodilation of normal vascular beds, but little additional dilatation in vascular beds distal to a significant coronary stenosis, leading to differences in perfusion which appear as defects in a myocardial perfusion image.

Standard image displays of the post stress and rest images or compared and transverse, vertical long axis and horizontal long axis.

An irreversible defect indicates infarction, whereas a perfusion defect seen after stress but reversible indicates myocardial ischemia.

Images are taken after a stress test and when the heart recovers to a restful status.

Exercise is the preferred stress method because it provides prognostic information.

Pharmacological stress is indicated for patients who are unable to achieve Adequate Exercise and in patients with left bundle branch block or ventricular pacing, which often show relative septal hypoperfusion accentuated by exercise and can be mistakenly interpreted as ischemia.

Utilize a radionuclide such as thallium 201 or technetium 99m-labeled sestamibi which are detected by nuclear imaging.

Often prepared by commercial nuclear pharmacies and distributed to hospitals and clinics.

Regadenoson pharmacological stress test is most commonly used at present and has similar sensitivity and specificity compared to exercise stress testing.

There are two main types of nuclear heart scanning, a single positron emission computed tomography (SPECT) and cardiac positron emission tomography (PET).

SPECT is the most well-established and widely used type, while PET is newer.

SPECT is the most common nuclear cardiac scanning test for diagnosing coronary heart disease.

The most commonly used tracers are thallium-201, technetium-99m sestamibi (Cardiolite), and technetium-99m tetrofosmin (Myoview).

A resting scan combined with a stress scan using the radioactive tracer technetium-99m sestamibi – averages 11.3 mSv. exposure,, about 500 times the dose that comes from a chest x-ray.

A rest-stress scan using the radioactive tracer technetium-99m tetrofosmin averages 9.3 mSv. !about 500 times the dose that comes from a chest x-ray.

Doses are much higher for nuclear stress tests that use the radioactive tracer thallium-201 – about 22 mSv with a single injection of thallium.

When thallium and technetium-99m sestamibi are combined, the radiation dose averages about 29.2 mSv.

PET nuclear cardiac scans provide more detailed pictures of the heart.

No clear cut advantage of using SPECT over PET scans exists.

PET takes a clearer picture through thick layers of tissues of the chest wall and is better than SPECT at showing whether coronary heart disease is affecting more than one coronary artery.

If a SPECT scan does not provide adequate information a PET nuclear heart scan can be considered