What is a myocardial perfusion (MPI) and viability scan?
Despite narrowing of the blood vessels of the heart, the actual implication for future risk of heart attack cannot be determined from conventional imaging (angiogram) e.g. a severe blockage may still be adequately able to deliver nutrients and oxygen to the heart muscle. Alternatively, new blood vessels may have formed. Reversing these using angioplasty or bypass surgery may be riskier than not.
In patients that have undergone these procedures it may be useful to see if the procedure has been effective. Even though a stent has been placed, it may have caused very little improvement in delivery of nutrients and oxygen to the heart muscle.
Patients with impaired delivery may be at risk for future heart attacks or heart failure or even death.
By injecting a chemical that is small enough (Sestamibi and Thallium) to flow through the narrowest of occlusions and that only gets taken up by living tissue, we can see a) how much gets delivered, if any and b) whether the tissue is alive or not. By attaching the Sestamibi to a radioactive substance we are able to form an image using a special machine called a gamma camera.
The Thallium molecule used is already radioactive.
Only in extreme occlusions where there is no uptake, the cold spot on the scan may represent a heart attack or inactive heart tissue that is hibernating in order to preserve itself. In this case another modality named PET-CT offers a tracer that is a glucose or sugar analogue that will take up even in hibernating heart muscle. If this study still demonstrates no uptake, then the diagnosis is definitely dead heart tissue or heart attack.
Alternatively, one of the tracers used for the MPI (201Thallium) can demonstrate slow redistribution
and will ultimately collect in hibernating tissue. Viability can thus also be detected by doing a repeat scan after 4 hours. This method, however, is less sensitive than the PET-CT scan.
The former is the myocardial perfusion scan and the latter is the viability scan.
What can I expect to happen next?
Fasting is required for both MPI and viability studies.
You will need to refrain from taking coffee or smoking on the day of the study.
Medications that need to be stopped for MPI include beta-blockers, calcium channel blockers, beta-blockers, nitrates and aminophylline derivatives. Please speak to you doctors about stopping these safely.
Furthermore, since the test itself requires either exercise or injection of another chemical that mimics heart stimulation or stressing, you will be assessed carefully for any contra-indication to doing one or the other stress test. Please inform the radiographer if you have an irregular heartbeat, a history of blood clots or dissecting aneurysms, disabilities, asthma or emphysema or anything else you may think is of concern. Also let us know if you unable to exercise.
You will either be given exercise to perform on a treadmill with increasing difficulty with the objective being to elicit the pain or discomfort you usually experience from your heart.
Alternatively, your will be given one or other chemical that stimulates the effect that your heart undergoes when stressed.
During the exercise or chemical stress period the radiotracer will be injected. Imaging will be performed 45-60 minutes after the test and lasts approximately 25 minutes.
You will have to return for a second test (more often on another days) to assess you baseline perfusion. In this case you will simply be injected with the radiotracer and then imaged for 35 minutes after a 90-minutewaiting period.
The nuclear physician will compare your stress study to your rest study and decide if there is dead tissue and whether or not there is heart muscle that takes strain during stress because of the occlusion. If a hibernating (but living) heart muscle is suspected, then a viability study will be recommended.
The viability study requires no cessation of medicine and is usually a lot shorter but will have to be performed on a separate day with PET-CT. If PET-CT is to be used you will be required to fast overnight. Glucose-rich (sugar-rich) meal is administered upon arrival to force the heart to use glucose instead of fatty-acids for energy.
Acipimox® (a substance that prevents free fatty acid availability in the blood) and Aspirin® (to prevent allergic response to the Acipimox®) are administered. If the patient is allergic to Aspirin® Acipimox® can be administered alone. One hour later another dose of Acipimox® is given. The tracer is then administered. Forty minutes after 18FDG injection, a scan is performed lasting approximately 30 minutes. After the study the patient can eat again.
In a diabetic patient multiple insulin injection may be administered to achieve a desired blood glucose concentration since high blood glucose can cause spurious scan results. When suitably controlled the radiotracer will be injected. Forty minutes afterwards a scan of 30 minutes is performed.
After the study, the patient receives a meal
If viability is performed using the redistribution method (201Tl), you will be required to have another scan 4-hours after the MPI lasting approximately 20-30 minutes.
