What information should patients know before undergoing a Myocardial Imaging with Technetium-99m-sestamibi (MIBI) scan?

Patient Information for MIBI Scan (Myocardial Perfusion Imaging with Technetium-99m-sestamibi)

What the Test Is and Why It's Done

MIBI scanning is a nuclear medicine test that evaluates blood flow to your heart muscle to diagnose coronary artery disease, assess heart attack risk, and guide treatment decisions. The test involves injecting a small amount of radioactive tracer (technetium-99m sestamibi) that is taken up by healthy heart muscle in proportion to blood flow 1.

• The tracer distributes throughout viable heart muscle and allows imaging to detect areas with reduced blood flow (ischemia) or prior heart damage (infarction) 1.
• Unlike older tracers like thallium-201, MIBI does not redistribute after injection, which means images taken hours later still reflect the blood flow pattern at the time of injection 2, 3.
• The test has high diagnostic accuracy, with sensitivity around 90% for detecting coronary artery disease 1.

What to Expect During the Procedure

You will receive two separate injections—one during cardiovascular stress and one at rest—with imaging sessions separated by 3-4 hours, though total patient contact time is approximately 45 minutes per session 1.

Stress Component

• If you can exercise, you'll perform treadmill testing to achieve maximal cardiovascular stress 1.
• If you cannot exercise adequately (applies to one-third to one-half of patients), you'll receive pharmacological stress using either a vasodilator like dipyridamole or adenosine, or a medication like dobutamine 1, 4.
• The tracer is injected during peak stress, and you may need to continue stress briefly afterward 1.

Imaging Process

• A specialized camera rotates around you for 10-20 minutes to acquire three-dimensional images of your heart 1.
• Images are displayed using color scales to show tracer uptake in different heart regions 1.
• The injection must be given through an indwelling catheter or butterfly needle with saline flush to ensure complete dose delivery 5.

Radiation Exposure

The radiation dose from MIBI scanning is small and comparable to other nuclear medicine tests, with modern techniques allowing dose reduction while maintaining diagnostic quality 1.

• Technetium-99m has a physical half-life of only 6.02 hours, meaning the radioactivity decreases rapidly 5.
• In pediatric patients, doses as low as 3.5-5.2 mCi have been successfully used with high-efficiency cameras 1.
• Adult doses typically range from 10-20 mCi per injection 5.

Potential Risks and Adverse Events

Serious adverse events from MIBI scans are extremely rare, but asystole (cardiac arrest) can occur with dipyridamole stress, requiring immediate treatment with aminophylline 6.

• One case report documented two separate asystolic episodes following dipyridamole, with the second occurring 40 minutes after the first, indicating the need for prolonged cardiac monitoring 6.
• Aminophylline is the antidote for dipyridamole and should be readily available during and after the procedure 6.
• Common minor side effects from dipyridamole include headache, flushing, chest discomfort, and shortness of breath, which typically resolve quickly 4.

Interpreting Your Results

A normal MIBI scan indicates excellent prognosis with annual cardiac event rates below 1%, while abnormal scans identify patients requiring further evaluation or treatment 1.

What Results Mean

• Normal scan (homogeneous uptake): Indicates normal heart muscle and blood flow, essentially ruling out significant coronary disease with a negative predictive value approaching 100% 1.
• Reversible defect (abnormal during stress, normal at rest): Indicates inducible ischemia from a significant coronary artery blockage requiring treatment 1.
• Fixed defect (abnormal during both stress and rest): Indicates prior heart attack with loss of viable muscle 1.

Prognostic Value

• Patients with normal MIBI scans have annual cardiac event rates of approximately 0.5-1.5% 1.
• Patients with abnormal scans have annual event rates averaging 6.7%, with rates as high as 14.3% in some studies 1.
• The extent and severity of ischemia are the most important predictors of future cardiac events 1.

Special Clinical Applications

MIBI scanning is particularly valuable in emergency department evaluation of chest pain when the ECG is normal or non-diagnostic 1, 7.

• In emergency settings, MIBI can be injected during chest pain and images acquired 1-2 hours later when clinically stable, with the images still reflecting perfusion at the time of injection 1, 7.
• Studies show 93-100% sensitivity for detecting myocardial infarction in emergency department patients with chest pain and non-diagnostic ECGs 1.
• A normal emergency MIBI scan has a negative predictive value of 99-100% for ruling out acute cardiac events 1, 7.
• Among emergency department patients with typical angina and normal ECG, only 1 of 70 patients (1.4%) with normal MIBI scans had cardiac events, compared to 71% of those with abnormal scans 7.

Preparation and Logistics

You should avoid caffeine for 24 hours before the test if dipyridamole or adenosine stress will be used, as caffeine blocks the effect of these medications 4.

• The test can be performed as a same-day protocol with rest imaging first (low dose 7 mCi) followed 2 hours later by stress imaging (higher dose 25 mCi), which is preferable to stress-rest sequencing 4.
• Bring comfortable clothing and walking shoes if you'll be exercising on a treadmill 1.
• Plan for approximately 4-5 hours total time at the facility, though you'll have a break between the two imaging sessions 1.