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Over the last few years, the scope of EuroEcho-Imaging has widened—original a purely echocardiography (echo) congress, it now includes sessions on multimodal imaging (hence the name “EuroEcho-Imaging”). Congress News explores how two different modalities—cardiac magnetic resonance (CMR) imaging and nuclear imaging—are useful adjuncts to echo.
According to Doctor Chiara Bucciarelli-Ducci (Bristol Heart Institute, Bristol, UK), although echo and cardiac MR are “different techniques with different principles” (the first being based on ultrasound and second being based on a magnetic field), they are “quite complimentary”.
She notes that there are benefits and advantages with each modality and observes that echo is more portable, accessible, and is cheaper and widely available. By contrast, cardiac MR cannot be moved around, the equipment is expensive and requires dedicated space, as well as needing specialised training to perform and interpret the images. However, Dr. Bucciarelli-Ducci comments that currently, it is “the ultimate non-invasive cardiovascular test. It is becoming increasingly popular because it can add precision of the measurements made and the diagnosis provided.” For a range of cardiovascular diseases, cardiac MR can, therefore, provide further diagnostic and prognostic information to echo.
Some cardiac MR images can be similar to those with echo (the cine images, for example), whereas myocardial tissue characterisation can only be performed with cardiac MR. She explains that once administered, the contrast agent accumulates in areas of the heart that are damaged. “These areas of contrast accumulation are very clear on the images, and we can see abnormalities in the heart. It is very helpful to identify the presence and the pattern of these damaged areas, as different patterns correspond to different diseases. In that sense, cardiac MR can provide additional information over and above echo.”
Cardiac MR is being increasingly used in the diagnosis of ischaemic heart disease, but also in cardiomyopathies and heart failure. Current European Society of Cardiology (ESC) guidelines for investigating chest pain give an equal level of recommendation to four different procedures—stress MR, stress echo, single photon emission computed tomography (SPECT), and positron emission tomography (PET). That means, says Dr. Bucciarelli-Ducci, in practice, cardiologists have to use whatever technology and expertise is available, as these are tests that are equally indicated in clinical practice.
The ability of cardiac MRI to identify the underlying disease over and above that afforded by echo, means that a “good chunk” of patients in whom the diagnosis is uncertain will need a cardiac MR for further assessment, says Dr. Bucciarelli-Ducci. “If you do a cardiac MR on a patient presenting acutely with chest pain and troponin rise but unobstructed coronaries, cardiac MR will give the opportunity to identify whether the underlying condition might be acute myocarditis, myocardial infarction with spontaneous recanalisation/embolic infarction or TakoTsubo cardiomyopathy. According to recent studies, in this context cardiac MR can change the initial diagnosis and management in a non-negligible proportion of patients”. It is to some extent a game changer in the way we practice cardiology,” Dr. Bucciarelli-Ducci states.
Nuclear imaging is also a useful adjunct, according to Professor Oliver Gämperli (University Heart Center Zurich, Zurich, Switzerland), who says it is “a big add-on” to echocardiographic assessment.
The two main pillars of nuclear cardiology are SPECT and PET. Both use emission tomography, in which photons are emitted from the body after injecting a radioactive substance with a very low activity.
Prof. Gämperli says: “Echo is very important. It visualises anatomical structures and is a real-time assessment of the heart. But the resolution is not high enough for visualising stenoses in the coronary arteries. This is where nuclear emission is complementary to echo because it shows whether there is any perfusion abnormality in the myocardium related to narrowing of the coronary arteries.”
Furthermore, nuclear imaging is also useful for diagnosing inflammatory heart diseases such as myocarditis or sarcoidosis, and infiltrative disease like amyloidosis. He notes: “In sarcoidosis, PET-FDG [fluorodeoxyglucose] performed with radioactive glucose is very sensitive at detecting inflammatory areas of the myocardium, and may be complementary to clinical data and echocardiography. And in amyloidosis, where degenerative deposits of protein in the heart can affect its function, a special type of SPECT can be very useful to detect small deposits of this degenerative amyloid protein.”
The technology for both SPECT and PET is developing fast. In the past, the time required to produce a perfusion image using SPECT was around 20 minutes. New, more sensitive devices have shortened this to five minutes for each acquisition, which, Prof. Gämperli says, improves accuracy. He explains: “The shorter the time that a patient has to spend in the scanner, the more comfortable and less anxious they will be. This means that they will be less likely to move around. That improves image quality and accuracy.”
PET imaging is a more complex technique than SPECT, requiring an onsite cyclotron to produce the vast majority of radionuclides. For perfusion imaging, these have a very short half-life and they degrade within minutes; this, for now, restricts its availability. However, Prof. Gämperli describes PET as “very promising” technique and says that it has “many advantages over SPECT”. These include a higher spatial resolution and fewer artefacts. It also allows blood flow to the myocardium to be quantified in millilitres per minute per gram. He adds that these factors “improve the diagnostic accuracy for determining whether a patient has a narrowing of the coronary artery, both in the large arteries and in the small vessels. This cannot be seen with the naked eye.”
However, one area where nuclear imaging has less value is the investigation of structural heart disease. Prof. Gämperli comments: “Echo has a big advantage over nuclear techniques in the diagnosis of structural heart disease because of the high spatial resolution, which allows us to assess the movement of valves. That is something we cannot do with nuclear imaging.”
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