Imaging Blood Flow to the Heart Muscle
The heart beats approximately 70 times/minute throughout your entire life. In order for you heart to perform this amazing function, the muscles in your heart need a constant supply of blood. Coronary artery disease occurs when the arteries that supply blood to your heart get blocked.
There are several conditions, such as heartburn, that can have symptoms similar to coronary artery disease. Imaging can help your doctor determine if your symptoms are due to coronary artery disease or something else. Once a patient is diagnosed with coronary artery disease, imaging may also be used to pinpoint the blockage and guide treatment.
There are many imaging tools for diagnosing coronary artery disease. The most common is x-ray imaging using a dye (also called contrast). This test creates images of all the arteries and can show where potential blockages are.
Coronary artery disease can also be diagnosed using a radioactive tracer. This tracer is carried by the blood into the heart muscle where it sticks. A part of the heart muscle that has a low blood supply will not get much tracer, so it will show up as a dark spot on the heart image. When the radioactive part of the tracer decays, it emits an x-ray that we can see with a scanner. PET/CT and SPECT are two types of scanners used for this application.
It may seem like the x-ray contrast method and the radioactive tracer method give more-or-less the same results. But this is not the case. In principle the radioactive tracer method, is superior because it is directly measuring what is happening in the heart muscle. The x-ray contrast method is measuring what is happening in the “pipes” before the muscle and then guessing what is actually happening in the muscle. For arteries that are completely blocked, the two methods give the same results. However, most of the time, the arteries are not completely blocked. With the x-ray contrast method it is difficult to know if the partial blockage is enough to be clinically significant.
One way to think about this is to imagine a farmer’s field in a dry climate that requires an irrigation system to keep the plants healthy. The easiest way to see if the plants are getting enough water is to look at them. This is equivalent to the radioactive tracer method. With the x-ray contrast method, the farmer does not get to look at the plants or the actual water coming out. All the farmer is allowed to look at is the irrigation equipment. He/she can inspect each sprinkler nozzle. Most will have some calcium build-up. The farmer will have to guess if the build-up is enough to be significant.
PET/CT vs SPECT for Cardiac Blood Flow Imaging
SPECT is the most common radioactive tracer method for measuring the blood flow in the heart. This is because it is less expensive and the equipment is often owned by the cardiologists themselves.
PET/CT is a more powerful and accurate radiotracer method. Scans done on this equipment can be completed in about 30 minutes as opposed to 4 hours for SPECT. There are fewer false positives and false negatives. PET/CT heart blood flow imaging is reimbursed by many insurance companies. There is no reason to have a SPECT blood flow image if you have access to PET/CT.
Heart Muscle Viability Imaging
If your doctor finds that there is a part of your heart muscle that is not getting enough blood flow, then the next step is to determine what to do about it. Many of the procedures that can be used to restore blood flow, such as bypass surgery, are risky and expensive. So the question is whether or not that part of the heart that is not getting enough blood flow is alive and struggling or is dead. If it is dead than restoring blood flow will expose the patient to significant risk for no benefit.
PET/CT is the most accurate tool for determining if the part of the heart with low blood flow is viable. The way PET/CT does this is it switches from a tracer that measures blood flow to a tracer that measures glucose metabolism.
You can think of the heart link a diesel engine – it can burn just about any kind of fuel to get energy. When the heart has plenty of oxygen (carried by the blood) it prefers to get its energy from burning fat. However, when there is not enough oxygen, the heart switches to glucose for energy and burns it very inefficiently. This means that a part of the heart that is alive, but low on blood supply, will take up a lot of the tracer. And if the tissue is dead it will not take the tracer up at all.
If healthy heart does not take up glucose, and dead heart tissue does not take up glucose, then how can you tell the difference? From the blood flow study. Healthy tissue will have normal blood flow. While dead tissue will have almost no blood flow.
In summary, if the blood flow scan is normal, then a scan of glucose metabolism is not necessary. If a section of the heart muscle has low blood flow, then a glucose metabolism scan is indicated. If the glucose metabolism scan shows high uptake where there is low blood flow, then the tissue would likely benefit from restoring blood flow. While if the glucose metabolism scan shows low glucose uptake where there is low blood flow, then the tissue is probably dead and would not benefit from restoring blood flow.