High MI Imaging (e.g. >0.5)
When bubbles are exposed to a high MI, they are disrupted almost immediately and this destruction creates a powerful signal that can be detected by the transducer. However, continuous imaging results in persistent bubble destruction which prevents detection of microbubbles in the myocardium. Thus the concept of intermittent (triggered) imaging was developed, in which ultrasound transmission is interrupted after once cycle for a few cycles (degree of triggering can be specified – e.g. 1:2, 1:4, 1:10 etc) to allow bubble replenishment before another high MI pulse is emitted.
The main advantage of this technique is the high sensitivity, as the harmonic signals generated by bubble destruction at high MI are rich and stronger than those emitted at a lower MI. The main disadvantage is that more contrast is used and real-time imaging is not feasible. As tissue produces strong harmonic signals at high MI, contrast-specific imaging modalities are necessary to suppress these signals (i.e. power Doppler, ultra-harmonics and power pulse inversion).
Both of these clips show normal perfusion at rest. These are high MI (thus triggered imaging) techniques, so bubble destruction is continuous. The frames are triggered to each 4th beat, as it takes up to 4 beats following bubble destruction for complete replenishment at rest. Thus the image on the right side displays normal perfusion before the ‘black myocardium’ frame (which confirms complete bubble destruction). The next frame then shows the 4th beat after perfusion (also shown on the left image) which confirms normal replenishment with no perfusion defects seen.
Low MI Imaging (e.g. 0.1-0.3)
Real-time imaging is possible if the mechanical index is reduced sufficiently such that microbubbles will resonate but not explode from continuous exposure to the ultrasound waves. Using such a low MI also generates virtually no tissue harmonic signal, so the use of background subtraction techniques is usually not necessary. The two biggest advantages of this method are firstly that real-time assessment of both wall motion and perfusion are possible and secondly, since there is less bubble destruction at low MI, smaller volumes of contrast are usually used. The disadvantage is that, at low MI, the signal emanating from the bubbles is weaker than at high MI and so the sensitivity is slightly lower.
One can also assess perfusion separately from all motion by triggered imaging at a certain point in the cardiac cycle. End-systolic triggering is preferred since:
- Myocardium is at its thickest in systole and thus the area of interest is largest at this time point
- At this time-point arterioles and venules are maximally compressed, so one is only imaging microbubble uptake by capillaries
- Reduced incidence of side/lateral artefacts
Given the advances in contrast-specific imaging techniques, we would recommend: Low MI imaging for myocardial contrast echocardiography (simultaneous assessment of myocardial function and perfusion).