Department of Cardiology, Henri Mondor University Hospital, Féderation de Cardiologie, Créteil, France
* Correspondence to: Jérôme Garot, Department of Cardiology, Henri Mondor University Hospital, Féderation de Cardiologie, 51 Avenue du Maréchal de Lattre de Tassigny, 94010 Créteil, France. Tel.: +33149812331; fax: +33149812883 (E-mail: jgarot{at}free.fr).
This editorial refers to "Persistent diastolic dysfunction despite complete systolic functional recovery after reperfused acute myocardial infarction demonstrated by tagged magnetic resonance imaging" by C.F. Azevedo et al. on
page 1419
Over the last 15 years, tagged magnetic resonance imaging (MRI) of the heart has emerged as an accurate non-invasive technique that provides detailed quantitative assessment of myocardial strain or deformation.1,2 In particular, it has been used to precisely characterise regional systolic intrinsic contractility of the left ventricle (LV) in experimental models of acute myocardial ischaemia3 and in patients with coronary artery disease.4 Recently, for the detection of myocardial ischaemia during pharmacological stress testing, the superiority of tagged MRI over visual assessment of untagged cine-MRI has been established.5 Also, tagged MRI has proved to be very accurate in acute coronary syndrome patients for the quantitative analysis of the recovery of segmental LV function after revascularisation.6 Besides, the availability of fast automated post-processing techniques has also made tagged MRI of the heart attractive and ready for clinical use.7 Altogether, MRI myocardial tagging has become the non-invasive method of reference for the study of segmental systolic LV function. However, because of inherent limitations such as fading of tagged planes over time and/or sub-optimal temporal resolution, tagged MRI has so far been mainly limited to the study of myocardial deformation throughout systole. The assessment of LV myocardial deformation during diastole by cardiac tagging has primarily focused on LV untwisting during early diastole.8
In this issue of the Journal, Azevedo et al.9 report the results of an experimental study aimed at characterising not only the systolic but also diastolic, mechanical properties of the ischaemically injured LV myocardium in dogs, through the use of the innovative tagged MRI strain rate analysis technique. Similar to echocardiography, strain rate analysis measures the rate at which the LV myocardium deforms during systole and diastole. The main findings of the study confirm that peak systolic strain and strain rate are reduced in infarcted segments vs. remote, with a greater decrease in transmural vs. sub-endocardial infarcts. Interestingly, the authors report that peak diastolic strain rate is decreased in segments with transmural and sub-endocardial infarcts, and that a greater decrease is observed when microvascular obstruction is present at the tissue level after reopening of the coronary artery. Additionally, risk regions, as defined by the combination of reduced tissue perfusion during coronary occlusion (radioactive microspheres) and absence of infarct on TTC stain, display significant diastolic dysfunction 24 h after reflow, whereas regional systolic contractility has recovered at this particular time point.
The study by Azevedo et al. represents a successful attempt to characterise the diastolic deformation of the LV myocardium by tagged MRI after an experimental ischaemic insult in detail. Through the combined assessment of peak diastolic strain rate and early LV untwisting, the method enables unique detailed and comprehensive assessment of LV diastolic mechanics. The demonstration by tagged MRI of subtle and persistent alterations of diastolic function 24 h after reflow, despite complete recovery of systolic function, has patho-physiological significance. First, it corroborates the concept that there is a meaningful diastolic component to post-ischaemic LV dysfunction. Second, it supports the idea that post-ischaemic diastolic dysfunction may develop as a result of a lower ischaemic threshold and/or may persist longer than systolic dysfunction after the ischaemic insult. In the infarcted area, the recognition by MRI of exacerbated alterations of diastolic mechanical properties within regions of microvascular obstruction is novel and important. It may help us improve our understanding of the potential relation between alterations of segmental LV diastolic function after acute myocardial ischaemia and subsequent LV remodelling, which is in turn closely related to patient outcome.
Technical aspects
The imaging sequence used by Azevedo et al. is based on a fast spoiled gradient-recalled echo with spatial modulation of magnetisation (SPAMM) tagging sequence to generate a grid tag pattern. Importantly, the sequence has been designed to provide 22 ms temporal resolution, which yields lower frame rates than those obtained by echocardiography, but allows for the assessment of LV diastolic mechanics with 1.5x.5 mm2 spatial resolution in the imaging plane. Six to 16 frames could be acquired during diastole at the cost of
25 s breath-hold per studied location, depending mainly on the heart rate of the animals (range 76160 bpm). Tagged cardiac images were analysed by the Harmonic Phase (HARP) technique, which confirms its accuracy and ability to detect subtle alterations of systolic contractility, but also demonstrates its applicability for quantitative assessment of diastolic myocardial deformation. In the study, the analysis was confined to circumferential shortening strain and strain rate, but Harmonic Phase MRI also allows for the quantitative measurement of radial thickening strain, principal strains, and the angle of the eigenvector.7 Remarkably, the assessment of strain and strain rate can be performed at three different myocardial layers in a given myocardial segment, which is crucial to enhance our understanding of complex cross-fibre interactions within the wall. Ultimately, this high level of detail is likely to improve precise characterisation of various ischaemic myocardial substrates.
What do we need to get there in humans?
Because of lower heart rates in humans, the imaging sequence used in the study would require unrealistic breath-hold times in order to achieve comparable temporal resolution in patients. In view of the work by Azevedo et al., there is clearly a need for improved MRI pulse sequences that allow for shorter breath-holds, while increasing temporal resolution and signal-to-noise. More recent MRI sequences such as the steady-state free precession (ssfp),10 or an interleaved spiral imaging sequence,11 can be used in combination with complementary SPAMM tagging, and thereby improve persistence of tag planes, temporal resolution and signal. This should make complete coverage of the cardiac cycle by tag lines available shortly in the clinical arena with friendly breath-hold times and high frame rates. With such sequences, high temporal resolution myocardial tagging should provide precise detection of patho-physiologically relevant myocardial motion such as the post-systolic shortening phenomenon.12 In patients we will then need to formally evaluate the robustness and reproducibility of standardised sequences and protocols designed for routine clinical use of tagged MRI during diastole. Besides, and this remark applies to all imaging modalities, we will have to improve our knowledge of the potential misleading effect of sub-optimal signal-to-noise on quantitative measurements of myocardial strain and strain rate extracted from the analytic techniques.
Overall, this work by Azevedo et al. adds innovative technical aspects of applying myocardial tagging during diastole, as well as exciting new patho-physiological concepts. It therefore represents a landmark study for the comprehensive detailed assessment of diastolic mechanical properties of the LV by MRI after acute myocardial ischaemia.
Footnotes
References
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