PET and Cardiac MRI have been around for many years. What are the different contributions they make to assessing myocardial ischaemia and acute myocardial infarction?
Let me point out that PET is Positron Emission Tomography, which is a nuclear technique that has been around since the early eighties. Magnetic Resonance came around years after. So PET is a mature technique to assess fully quantitative myocardial blood flow and myocardial perfusion.
As to Magnetic Resonance, the assessment of quantitative myocardial perfusion is possible but it is still in its teens, so is mainly a tool for research.
As to myocardial ischaemia, there was a paper published and the first author is L. Gould in 2013, reporting around 5000 studies with positron emission tomography to assess myocardial ischaemia. And it is an expensive technique, but it is the best technique to assess myocardial blood flow and myocardial flow reserve.
Magnetic resonance is better suited to assess a previous silent myocardial infarction and to quantify the extension of the MI. So let’s say that they are two techniques that can be complimentary.
There are many papers on infarct size assessment with MRI, and the technique is very, very sensitive because you can detect two per cent of the LV mass infarction, and this has prognostic implications.
The same can be said for flow reserve assessed with PET. It has prognostic implications – mainly in diseases that are diffuse impairment of the microvasculature, like hypertrophic cardiomyopathy, be it primary, congential or hypertensive cardiomyopathy, and also in heart failure with reserved ejection fraction.
This is still a field that needs to be explored, because it’s still an ill-defined disease and we are trying to characterise it.
Are there also cost-effectiveness considerations that should be contemplated?
Cost is the Achilles Heel of positron emission tomography. So it’s a sort of third-level investigation. In Europe it is in-between €1500 and €2000 per investigation. So you need to highly select the patients. There is no point prescribing a PET scan if you just want to see if there is a significant stenosis. This is not the tool to use.
It is cost-effective in patients with microcirulatory dysfunction, because these are patients who undergo endless invasive investigations. And there is no significant stenosis, but you go back home and you still have pain and symptoms, and this is the best indication for this type of patient.
For magnetic resonance the cost is less, between €500 and €1000. It depends on what the investigation is. And it’s cost effective, again if you are looking for a better definition of infarction. If you want a qualitative assessment of ischaemic all in one session. I use both techniques.
I think we need to increase awareness of the prescribing cardiologists of the limitations of the technique, and what you want to obtain. They are not to be used as a shot in the dark – “I don’t know what this patient has, have a look and tell me what is going on.”