The assessment of myocardial viability has relevant clinical implications. The presence of viable myocardium, in fact, is strictly connected with prognosis and has an important role in driving the therapeutic strategy (1).
To fulfil clinical expectations, the ideal method should reveal the presence and the extension of viable myocardium in dysfunctioning ventricular segments as well as the capability of recovering the mechanical function once the balance between blood flow supply and metabolic need has been recruited, spontaneously or following therapeutic procedures.
Nuclear medicine and echocardiography are commonly used to reach this target. However, the complexity of the mechanisms underlying the pathophysiology of myocardial viability and the intrinsic limitations of these techniques call for an alternative imaging approach. Nuclear medicine suffers from a suboptimal spatial resolution and implies the administration of large amounts of ionising radiations while echocardiography leads to an indirect evaluation of the metabolic status.
Magnetic Resonance (MR) seems to overcome these limitations, thereby opening a new comprehensive approach to the topic.
For clinical purposes, at present, there are four different ways of using MR for the assessment of myocardial viability. The measurement of end-diastolic wall thickness, the evaluation of contractile reserve during inotropic stimuli, the evaluation of regional myocardial perfusion, and the contrast based depiction of scar tissue.
End-diastolic thicknesses of less than 5.5 mm and systolic thickenings of less than 2 mm have been correlated with poor functional outcomes after revascularisation (2).
The evaluation of contractile reserve with MRI, during pharmacological inotropic stress, has been shown to have a diagnostic accuracy similar to echocardiography with the advantage of a superior level of image quality (3).
The evaluation of regional myocardial perfusion is a point of strength of MRI, for it allows the differentiation between post infarcted areas with severe impairment of perfusion and areas where a preserved perfusion is indicative of a positive prognostic outcome (4).
More recently, gadolinium-based contrast agent has been shown to enhance the necrotic tissue with high contrast relatively to viable tissue. This approach uses an Inversion Recovery Gradient Echo Technique and the delayed acquisition of images (12-20 minutes after contrast agent administration). The deposition of contrast agent within the necrotic tissue is influenced by several local factors, such as the presence of an abnormal collagen content, the enlargement of the interstitial space, etc.
This latter technique has the advantage of being easy and risk-free; it reliably allows the evaluation of necrotic tissue extension, as assessed at post-mortem analysis, and of viable tissue as assessed by nuclear techniques in human beings (5-11)
Fig. 1
Imaging of the heart in horizontal long axis. Patient with a recent antero-spetal myocardial infarction. The image obtained by the inversion recovery Gradient Echo sequence 15 minutes after Gd-based contrast agent (0.2 mmol/kg) shows a strong increase in the signal at the level of the necrotic tissue (white in the left panel and red in the right panel) while the non-necrotic (viable) myocardium remains hypointense.
(RA = right atrium, RV = right ventricle, LA = left atrium, RA = right atrium, AO = descending aorta)
With respect to the other imaging techniques, contrast enhanced MRI has the clear advantage of offering a higher spatial resolution that allows the detection of subendocardial necrosis.
The transmural extent of necrosis, as expressed in a percentage of wall thickness, is a sensitive predictor of functional recovery. Measurements are relatively easy to do, thanks to the high contrast between necrotic and viable myocardium (up to 500%). The main technical limitation seems to be the lack of standardisation in the proper setting of the scanner, which is crucial for contrast amplification between viable tissue and scar. However, when the proper expertise has been assured, it appears to be an easy task and many groups have shown consistently high quality images.
Comparing MRI with Echocardiography and Nuclear Medicine from an economical point of view, there is great cost variability from one country to the other. However, in general, because Echocardiography has the lowest cost, the cost of the other technologies available can be roughly expressed as its multiples. This means that SPECT costs are about two-fold those of echocardiography, MRI four-fold -when considering the contrast enhanced technique alone- or eight-fold when both contrast enhanced and functional assessment are required. Finally, figures drastically rise for PET with costs approximately fourteen-fold those of Echocardiography. Nevertheless, the non ionising nature and the possibility of the comprehensive evaluation of necrosis extent and functional reserve lead to a possibly convenient cost-effectiveness profile for MRI. In many cases an MRI study requested for other purposes (such as assessment of myocardial perfusion) can additionally provide the evaluation of viability at no extra cost, thus representing an adjunctive value of the diagnostic procedure.
While the methodological issues are settled, multicenter studies are running worldwide to establish the pros and cons of this new approach. The available results make us confident on the use of this approach on a routine basis, possibly associated during the same session with the evaluation of contractile reserve under inotropic stimulus.
The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.
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