Contrast modalities (high power vs. low power, real time vs. triggered, imaging technology)

Figure 4: A schematic illustrating the various contrast-specific imaging modalities available for low MI and high MI techniques.

Real time imaging – LOW Mechanical Index

Real-time imaging uses a MI low enough to generate little tissue signal whilst generating sufficient signal from microbubbles. This allows continuous imaging as the low MI avoids significant bubble destruction. Microbubbles can be intentionally destroyed by a “flash” of high MI ultrasound pulses and contrast replenishment is then observed to allow qualitative & quantitative assessment of myocardial perfusion. 

Key advantages of low MI imaging:

• Simultaneous assessment of function and perfusion possible
• Effective ‘automatic background subtraction’ of tissue signal as low MI generates minimal tissue harmonics.
• Minimal bubble destruction with lower MI → less contrast utilisation

Key disadvantage of low MI imaging:

• Reduced oscillation of bubbles at low MI → reduced acoustic signal generated → less sensitive than high MI imaging

Triggered / intermittent imaging – HIGH Mechanical Index

In high MI imaging techniques, microbubbles are intentionally destroyed by high power signals sent intermittently (continuous high power would destroy all bubbles and provide little value!) triggered to the ECG (e.g. 1:4 – every 4th cycle).
The time between pulses allows replenishment of bubbles. With each destructive pulse, a high amplitude backscatter rich in harmonics is returned to the transducer, enabling static images of myocardial perfusion.
By incrementally increasing the triggering intervals (continuous → 1:1 → 1:2 → 1:4 → 1:8 etc), the rate of replenishment of the ultrasound beam over time can be assessed both qualitatively and quantitatively.

Key advantages of high MI imaging:

• Increased sensitivity to contrast (bubble destruction results in greatest amplitude backscatter)

Key disadvantages of high MI imaging:

• Lacks simultaneous assessment of function
• Requires reliable ECG triggering & image acquisition
• Can be time-consuming & is more technically challenging
• Increased bubble destruction → more contrast use → increased cost
• Increased bubble destruction → more near-field artefacts → risk of false perfusion defects