Myocardial Perfusion Imaging - Risks Vs. Benefits

Risks Vs. Benefits

Critics have written that myocardial perfusion imaging is associated with an increased risk of cancer due to high radiation doses that are not justified by randomized, controlled studies demonstrating benefit. However, radiation doses received during CT angiography and conventional coronary angiography are higher than those received during myocardial perfusion imaging done with 99m-Technetium labelled agents. The psychological block associated with radioactive materials may be responsible for these fears. (says who? citation needed)

In a study of patient exposure to low-dose ionizing radiation, myocardial perfusion imaging had the highest average effective dose (15.6 millisieverts) and the highest percentage (22.1%) of all effective doses to the entire patient population from all major radiological procedures, including computer tomographic studies. Older patients, 60 to 64 years old, had the highest doses, with 5.27% getting a high dose (>20 to 50 mSv/year) and 0.57% getting a very high dose (>50 mSv/year) from all sources.

Experimental and epidemiologic evidence has linked exposure to low-dose ionizing radiation with up to 2% of solid cancers and leukemia. Workers are monitored and limited to 100mSv every 5 years, but medical patients are not typically monitored. In one study of CT in abdominal and flank pain, "less than 50% of radiologists and only 9% of emergency department physicians reported even being aware that CT was associated with an increased risk of cancer."

Interestingly, the concern over radiation hazard has undermined the risk associated with the allergic potential of radiocontrast (dyes) used in CT angiography and coronary angiography. In myocardial perfusion imaging, radioisotopes are used in nanomole quantities, practically devoid of any risk of allergy with normal saline being used as the vehicle and no known adverse reaction to the chemical molecules (sestamibi or tetrofosmine).

From 1993-2001, myocardial perfusion scans increased >6%/y with "no justification," according to a commentary by Lauer. Mycardial perfusion imaging scans are "powerful predictors of future clinical events," and in theory may identify patients for whom aggressive therapies should improve outcome. But this is "only a hypothesis, not a proof," wrote Lauer. There are no randomized, controlled trials to demonstrate any benefits, and there is a small but cumulative danger from radiation.

However, radioisotope MPI is considered to be the most comprehensive test, providing information about criticality of coronary stenosis, area suffering from ischemia, severity of ischemia, total mass of viable myocardium, and the pump function of the heart, along with objective parameters such as end systolic volume, end diastolic volume, stroke volume and ejection fraction. The negative predictive value of the test is as high as 98%, offering excellent prognostic value.

New radionuclides such as rubidium-82 reduce the radiation dose to the patient by a factor of 10 compared to technetium-99m. In the future, therefore, a complete myocardial perfusion exam may be achievable while maintaining a patient dose under 3 mSv. Stress-only protocols may also prove to be effective at reducing costs and patient exposure.