Myocardial perfusion scan
Encyclopedia
Myocardial perfusion scan is a nuclear medicine
procedure that illustrates the function of the heart muscle (myocardium).
It evaluates many heart conditions from coronary artery disease (CAD) to hypertrophic cardiomyopathy
and myocardial wall motion abnormalities. The function of the myocardium is also evaluated by calculating the left ventricular ejection fraction
(LVEF) of the heart. This scan is done in conjunction with a cardiac stress test
.
Planar techniques, such as conventional scintigraphy
, are rarely used. Rather, SPECT
is more common in the US. With multihead SPECT systems, imaging can often be completed in less than 10 minutes. With SPECT, inferior and posterior abnormalities and small areas of infarction can be identified, as well as the occluded blood vessels and the mass of infarcted and viable myocardium.
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.
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
procedure that illustrates the function of the heart muscle (myocardium).
It evaluates many heart conditions from coronary artery disease (CAD) to hypertrophic cardiomyopathy
Hypertrophic cardiomyopathy
Hypertrophic cardiomyopathy is a disease of the myocardium in which a portion of the myocardium is hypertrophied without any obvious cause...
and myocardial wall motion abnormalities. The function of the myocardium is also evaluated by calculating the left ventricular ejection fraction
Ejection fraction
In cardiovascular physiology, ejection fraction is the fraction of Blood pumped out of the Right Ventricle of the heart to the Pulmonary Circulation and Left Ventricle of the heart to the Systemic Circulation with each Heart beat or Cardiac cycle...
(LVEF) of the heart. This scan is done in conjunction with a cardiac stress test
Cardiac stress test
Cardiac stress test is a test used in medicine and cardiology to measure the heart's ability to respond to external stress in a controlled clinical environment....
.
Planar techniques, such as conventional scintigraphy
Scintigraphy
Scintigraphy is a form of diagnostic test used in nuclear medicine, wherein radioisotopes are taken internally, and the emitted radiation is captured by external detectors to form two-dimensional images...
, are rarely used. Rather, SPECT
Single photon emission computed tomography
Single-photon emission computed tomography is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it is able to provide true 3D information...
is more common in the US. With multihead SPECT systems, imaging can often be completed in less than 10 minutes. With SPECT, inferior and posterior abnormalities and small areas of infarction can be identified, as well as the occluded blood vessels and the mass of infarcted and viable myocardium.
Major indications for a myocardial perfusion test
- Diagnosis of CAD and various cardiac abnormalities.
- Identifying location, criticality of existing coronary stenosis and degree of coronary artery disease (CAD) in patients with a history of CAD.
- Prognostication (risk stratification) and evaluation of patients that are at risk of having a myocardial or coronary incident. (ex: myocardial infarction, myocardial ischemia, coronary aneurysm, wall motion abnormalities)
- Assessment of viable myocardium in particular coronary artery territory following heart attacks to justify revascularization
- Post intervention revascularization (coronary artery bypass graft, angioplastyAngioplastyAngioplasty is the technique of mechanically widening a narrowed or obstructed blood vessel, the latter typically being a result of atherosclerosis. An empty and collapsed balloon on a guide wire, known as a balloon catheter, is passed into the narrowed locations and then inflated to a fixed size...
) evaluation of heart.
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.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.