Has cardiac magnetic resonance replaced nuclear imaging?  

No, says Juhani Knuuti,

Turku University Hosptial, Turku - Finland

There is no doubt that CMR has become an important imaging tool for cardiac patients. The number of CMR studies is continuously increasing. In some centres CMR has indeed become a very popular means of studying various cardiac diseases. 
 
However, statistics from clinical routine in Europe do not support the view that CMR has replaced nuclear imaging tests - or indeed any other tests. The 2009 European survey of nuclear cardiology showed that the average rate of examinations was around 2500 studies per million population, a rate steadily increasing from the 2005 and 2007 surveys.
 
In comparison, the number of CMR studies ranged from 50 to 320 per million population in 2004, only about 10% of the numbers of nuclear perfusion imagings. Even taking into account Germany, where CMR has been exceptionally popular, the message is still the same: about 200,000 nuclear perfusion tests but fewer than 30,000 CMR tests.
 
So why has CMR not replaced nuclear testing? The first explanation is that most nuclear tests are used for detection of myocardial ischemia. For this indication nuclear imaging is well established, with very strong evidence of accuracy and prognostic value. ESC guidelines suggest nuclear perfusion imaging or stress echocardiography for ischemia detection. However, the evidence about CMR is weaker in this indication. 
 
In all major hospitals the expertise, experience and technology to perform appropriate nuclear testing does exist. However, this is not the case with CMR. Thus, in practice it would be impossible to perform the number of CMR tests necessary for ischemia imaging. Moreover, the use of ischemia imaging is clearly underutilised in Europe. Based on the recent ESC guidelines, ischemia should be documented before revascularisation; however, some European patients are revascularised today without evidence of ischemia.
  
One of the driving forces to promote a specific imaging modality has been the fact that today most imaging experts do not have expertise in and access to other modalities - which fortunately is being addressed by a strong trend of cross-modality training and access. By working in an environment in which all imaging modalities are available, the political issues are less relevant and each modality is used according to its strengths.
 It thus becomes clear that all our current imaging modalities - echocardiography, cardiac CT, nuclear, CMR and recently PET - have their special strengths and unique indications. And in my opinion the main strengths of CMR lie not just in the detection of myocardial ischemia. There are several unique characteristics of CMR which no other imaging technique can offer. CMR is strong in the imaging of complicated cardiac structures, and in valvular diseases when echocardiography has limitations.
  
But what will happen in the future? Will CMR replace any of the other imaging modalities, or vice versa? There is no question that echocardiography remains the main imaging technique for our patients. The rapid development of cardiac CT made the field of imaging much tighter, especially for CMR. However, the strengths of CT are not the same as CMR or other imaging modalities. Cardiac CT is unique in its ability to perform robust non-invasive coronary angiography. Although some potential CT applications are directly competing with CMR, it is not obvious that these techniques will overtake the role of CMR.
 
Instead, it seems that the current trend is actually to combine the modalities. CT combined with SPECT or PET is now readily available (Figure 1), and systems with combined MRI and PET are being introduced (Figures 2). It is unlikely that these novel techniques will replace our existing established methods, but they do provide completely new possibilities. The imaging of molecular targets (such as neural function and receptors) as well as fibrosis, gene therapies and stem cells, could all be the potential applications of these new techniques.