Positron emission tomography (PET) is a nuclear medicine imaging technique that produces a three-dimensional image of functional processes in the body by detecting pairs of gamma rays emitted indirectly by a positron emitting radiotracer.
This is a non-invasive imaging technique that evaluates blood flow to the heart muscle. The blood flow to the heart muscle is reduced in the narrowed/disease coronary arteries. This will lead to reduction of oxygen supply to the involved heart muscle and producing symptoms like chest discomfort or shortness of breath.
A small amount of radioactive medication will be administered through your IV line during the test. The amount of radiation exposure is extremely low, as compared to the quality of information gained from having the PET scan performed. Your doctor will explain the benefits and any risks prior to the exam.
Rubidium-82 is a positron emitting radiotracer with a physical half-life of 75seconds. It is administered into the human body intravenously. Following intravenous administration, Rb-82 is an analog to potassium and rapidly taken up by heart muscle. In human studies, Rb-82 activity was noted in the heart within the first minute of injection. The uptake of Rb-82 by the heart muscle is related to the blood flow. Therefore, areas of the heart with adequate blood flow would have more Rb-82 activity in comparison to those areas with compromise blood flow. A PET scanning camera takes three dimensional images of Rb-82 uptake by the heart. Further analysis of these images helps to identify the location, severity and extent of reduced blood flow to the heart muscle (ischaemia).
The whole procedure including the rest/stress scans and the pharmacological stress test of the heart takes less than 60 minutes.
Please arrive at 15minutes prior to your appointment so that we can have adequate time to prepare to the Rb-82 and the medication for pharmacological stress test. If you are not able to come for the procedure, please inform us in advance.
Please take note of the following:
Studies performed in 855 patients have shown a high sensitivity of 95% and specificity of 95%.
Radiation exposure is minimal due to short half-life of Rb-82. (72 seconds) However, this procedure should not be done on a pregnant woman. Please inform us if you think you are pregnant.
We use pharmacologic stress with Dipyridamole for patients. We do a set of images while a patient is at rest and subsequently stress them with Dipyridamole infusion. At peak vasodilation, we give them another dose of the isotope and compare what the perfusion is at stress versus rest. If the stress image shows a defect that isn’t there at rest, then that is ischemia. If it’s abnormal at rest, it could be a scar from a previous heart attack.
A calcium score will be done after the stress scan. Coronary calcium is a marker for plaque (fatty deposits) in a blood vessel or atherosclerosis (hardening of the arteries). The presence and amount of calcium detected in a coronary artery by CT scan, indicates the presence and amount of atherosclerosis plaque.
A calcium score is computed based upon the volume and density of the calcium deposits. It does not correspond directly to the percentage of narrowing in the artery but does correlate with the severity of the underlying coronary atherosclerosis.
The blockage is important and now they are doing CT angiography to see the blood vessels. But the anatomy isn’t always the answer. The physiology is even more important because if somebody has a partial blockage, but there is no evidence of ischemia on a good test physiologically, like SPECT or PET, then they don't need any intervention because whatever the degree of anatomic change, it isn’t really causing the patient a perfusion abnormality.
There was the END study (Economics of Noninvasive Diagnosis) that was published several years ago. It looked at several thousand patients with chest pain. Half of them went right to the cath/Angiography lab, and the other half had a SPECT/MIBI study. The decision of whether to have the invasive angiography hinged on the results of your mibi scan. The patients who had the invasive test didn’t do any better than those that had the scan first. So, there’s no improved outcome by taking the patient right to the cath lab, and the expense on the patients that have the angiogram was far greater.
We send patients to the catheterization/angiogram lab if they showed they have moderate amount of inducible ischemia. The point of it was that it makes more sense to do the non-invasive test; patients do just as well, and the test helps predict whether they need coronary angiography and any other test down the line.
Dr Nico Pijls (Catharina Hospital, Eindhoven, the Netherlands). "If you have a lesion that isn't causing ischemia, the intrinsic risk of the stenosis is very low, lower than the risk of placing the stent," Because interventionalists can't discriminate by angiogram which lesions are causing ischemia, "we place stents everywhere, and the benefits of placing the stent in the right position is countered by the damage we do by placing stents where it is not necessary."
Dr William Boden (Buffalo General Hospital, NY), told heartwire that the results show that a physiologic assessment of stenosis is much better than what the eye can detect."There are some arteries that don't look that narrow, which you can avoid," he said. "Conversely, there are other borderline lesions that turn out to be functionally significant because there is a gradient with FFR. This is important, because it will help to really streamline practice, to help avoid unnecessary PCI in some vessels that shouldn't be fixed. It will also selectively target the really diseased vessels that would benefit from PCI."
Dr Ajay Kirtane (Columbia University, New York, NY) told heartwire that the findings emphasize treating the patient and the ischemia and leaving the other vessels alone. "Treating lesions that don't need to be treated is not a good thing and can harm people, but treating lesions that need to be treated is also beneficial over leaving them alone," said Kirtane. In the COURAGE nuclear substudy, for example, patients with flow-limiting lesions left to medical therapy fared worse than those treated with PCI, findings that are supported by the DEFER study.