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Myocardial Perfusion Imaging

Myocardial perfusion imaging allows us to view the blood flow of the heart during stress and at rest.  Combining myocardial perfusion imaging with stress testing (either treadmill or pharmacologic) increases the sensitivity for detection of coronary artery disease to > 90% with a specificity of 90%.  In most cases, we perform same day gated rest-stress imaging with the use of Technetium-99m sestamibi (Cardiolite®).  When myocardial viability is a specific concern, dual isotope imaging may be performed using thallium-201 in place of Technetium-99m sestamibi (Cardiolite®) for the rest image. All studies are performed with state of the art dual -headed SPECT (single photon emission computed tomography) cameras that employ a gated acquisition and allow for assessment of regional and global contractility as well as perfusion.

Rest/Stress Technetium-99m sestamibi SPECT Imaging:

• Dual Isotope Imaging
• Thallium Stress/Redistribution
• Thallium Rest/Redistribution
• Gated Myocardial Perfusion Imaging

Rest/Stress Technetium-99m sestamibi SPECT Imaging

For this test, patients receive Technetium-99m sestamibi at rest and are imaged approximately 1 hour later.  Patients then undergo stress (either exercise or pharmacologic) and receive a second larger injection of Technetium-99m sestamibi at peak stress.  Stress images are then obtained approximately 30-60 minutes later.  The total test takes approximately 3-4 hours.  This is the preferred protocol in our laboratory because the higher energy of technetium-99m compared with thallium-201 results in superior images, especially in obese patients and in women (in whom breast attenuation of thallium can be problematic).  In addition, the higher count rates associated with Technetium-99m sestamibi facilitate gated image acquisitions that facilitates simultaneous analysis of regional and global wall motion.

Dual Isotope Imaging

This procedure provides greater information about myocardial viability.  Patients receive an injection of thallium-201 at rest followed by rest imaging 2-4 hours later.  After the rest imaging they undergo stress (either treadmill or pharmacologic) and receive technetium-99m sestamibi (Cardiolite®) at peak stress.  They are then re-imaged 30 to 60 minutes later.   Alternatively, to maximize assessment of viability, dual imaging can be performed by giving the thallium-201 injection the evening before imaging so that the rest image, are obtained 12-24 hours after thallium injection.

Thallium Rest/Redistribution

This procedure is used for assessing myocardial viability. For this test, patients are injected with thallium-201 at rest and undergo imaging immediately.  They are then typically re-imaged after 4-24 hours (redistribution images).  While the rest/redistribution sequence is sensitive for assessing viability, it will underestimate provokable ischemia.  Thus, we recommend combing rest thallium-201 imaging with pharmacologic or exercise stress imaging to characterize the ishemic burden or jeopardized viable myocardium.

Gated Myocardial Perfusion Imaging

All myocardial perfusion images are acquired in a gated format.  This allows us to provide estimates of ejection fraction (EF), regional myocardial wall motion and thickening, and end-systolic and end-diastolic volumes in combination with perfusion imaging.  All measurements have been validated by comparision with other techniques and EF estimates are accurate within 5%.  Thus, gating provides valuable additional information about your patients.  Estimation of EF may be inaccurate when irregular rhythems (fibrillation with irregular ventricular response or frequent ventricular ectopy) affect gating.

Pharmacological Stress Tests

For patients unable to excerise on a treadmill or for those who cannot achieve at least 85% of their predicted maximal heart rate during exercise, we perform pharmacologic stress testing using dipyridamole, adenosine or dobutamine.

Vasodilator Stress

Dipyridamole and adenosine are potent coronary vasodilators and  our preferred agents for pharmacologic stress testing because they induce a reproducible hemodynamic and pharmacologic profile.

Specific exclusions (relative) for dipyridamole or adenosine include:

• High degree A-V block (i.e. 2nd or 3rd degree) in patients without a pacemaker
• Active bronchospasm or history of active asthma
• Patients on theophylline, aminophylline, or other methylxanthines (such as Cafergot)
• Patients with recent caffeine ingestion (within 24 hours).  The caffeine in coffee, teas, and in some sodas like colas, blocks adenosine receptors and decreases the vasodilator properties of dipyridamole and adenosine.  Please advise your patients to withhold all caffeine including coffee, tea, soda, or chocolate for at least 24 hours (even if they are having a treadmill test, just in case a pharmacologic test is indicated).  Please also note that “caffeine-free” coffee and tea contain low doses of caffeine (approximately 6 mg) that may block the vasodilator effects and should also be avoided.

Beta-adrenergic Stress

Dobutamine is used when a patient has a contraindication to dipyridamole or adenosine.

Specific exclusions for dobutamine include:

• Patients with ventricular tachyarrhythmias
• Patients with atrial fibrillation with rapid ventricular response
• Patients on beta blockers

Treadmill Testing Only

Treadmill testing without imaging is an appropriate initial procedure for many patients being evaluated for chest pain, palpitations, or arrhythmias.  The overall sensitivity of treadmill testing for the diagnosis of coronary artery disease is 60-70%, with an overall specificity of 70-80%.  Thus, physicians must recognize that there is both a relatively high false positive rate (which is higher in women than in men) and a high false negative rate. Accordingly, many physicians will choose to incorporate imaging (see below) with treadmill testing. We generally employ a standard Bruce protocol.  We perform symptom limited testing on all patients except those with a recent myocardial infarction.

Exclusion Criteria for Treadmill Testing (relative)

• Patients whose exercise capacity will only allow a sub-maximal test which is associated with a high false negative rate
• Patients with recent myocardial infarction
• Patients with true unstable angina
• Patients with significant left main coronary artery disease
• Patients with greater than moderate aortic stenosis
• Patients with left bundle branch block (this makes the ECG uninterpretable for ischemia)
• Patients with fixed rate pacemakers
• Patients with left ventricular hypertrophy (LVH) on the ECG or patients taking digoxin (these patients have a high rate of false positive ECG responses to exercise)
• Patients with stroke or orthopedic problems preventing them from walking
• Patients with limiting peripheral vascular disease
• Patients with severe asthma or COPD 

Radionuclide Ventriculography

Radionuclide ventriculography (RVG), also called equilibrium gated radionuclide angiography (ERNA), or multigated acquisition (MUGA), is the most accurate approach for the determination of ejection fraction when you do not need information about myocardial perfusion.  In our laboratory the 95% confidence interval for repeated measures of ejection fraction is 3% for the left ventricle (7% for the right ventricle).  This means, for example, a measured decrease in from 45% to 41% is highly likely to be a true change.  Thus, the test can reliably detect relatively small changes in ventricular function not possible with other techniques such as echocardiography or contrast ventriculography.  The technique has been proven to be valuable in assessing potential effects of cardiotoxic chemotherapeutic agents.  It may be equally valuable in ventricular function in patients with myocarditis, cardiomyopathy or myocardial infarction.

For this test, the patient's red blood cells are withdrawn, labeled with technetium-99m and injected back into the patient.  First pass imaging is obtained to measure right ventricular ejection fraction, then equilibrium imaging is performed.  Total imaging time is approximately 30 minutes.

Interpretation

For the treadmill tests, interpretation includes assessment of symptoms functional capacity, blood pressure, ECG and arrhythmias.  For pharmacologic stress test, the blood pressure, ECG and symptom response as well as arrhythmias are reported. All images are interpreted to characterize heart size, the size, intensity and reversibility of perfusion defects, ejection fraction, and regional wall motion.