21.3 Contrast Optimization: Mechanical Index, Destruction, Replenishment, and Artifacts

Match the bubble to the clinical question

CCI task E9 is to optimize contrast imaging. A commercial ultrasound-enhancing agent (UEA) contains encapsulated high-molecular-weight gas microbubbles small and stable enough to traverse the pulmonary capillary bed after intravenous administration. Contrast-specific imaging detects their nonlinear acoustic response to opacify the LV cavity, improve endocardial borders and Doppler signals, and, under an authorized protocol, assess myocardial perfusion. UEA is recommended when two or more contiguous LV segments are inadequately seen for LV function or wall-motion assessment.

Agitated saline is a different test. Its larger, short-lived bubbles normally opacify the RA and RV but do not pass the pulmonary capillary bed. It is used with injection-site and provocative-maneuver protocols to evaluate intracardiac or intrapulmonary right-to-left passage and venous connections. Agitated saline does not replace commercial UEA for routine LV opacification; commercial UEA does not replace a properly performed agitated-saline shunt study. Label the agent, route, injection site, timing, and maneuver.

Follow product labeling, local policy, trained-personnel scope, IV verification, dosing/delivery protocol, and monitoring requirements. Record the indication and any symptoms. Optimization never justifies improvising an unapproved preparation or bypassing a contraindication and emergency-response procedure.

Use mechanical index to control bubble behavior

Mechanical index is MI = peak rarefactional pressure / √transmit frequency in the system's standardized units. It is an index of acoustic output, not bubble dose. At very low MI, generally below 0.2, microbubbles oscillate nonlinearly and contrast-specific multipulse sequences suppress linear tissue signal while preserving bubbles. This provides high bubble-to-tissue contrast for real-time LV opacification and perfusion. Raising output increases oscillation and eventually shell disruption, gas release, and loss of signal.

Continuous intermediate or high MI causes ongoing microbubble destruction, apical swirling, and heterogeneous cavity signal and should not be the default for LVO. Start with the manufacturer-specific LVO or VLMI preset, focus near or below the target, keep depth and sector only as large as needed, and use the lowest MI and gain that maintain uniform borders. Excess tissue gain defeats tissue cancellation; too little contrast gain or too low a dose leaves incomplete opacification.

For intentional destruction, use a brief high-MI “flash,” commonly several frames near MI 0.8–1.2 under the specified protocol, then immediately return to VLMI. The flash clears microbubbles from the myocardium; continued imaging shows replenishment from the microcirculation. A fixed plane, stable blood-pool concentration, ECG, and end-systolic comparison are important. Normal refill with 2-D imaging is generally uniform within about 5 seconds at rest and 2 seconds during stress, but perfusion interpretation requires specific training and authorization and must consider attenuation.

Balance delivery and instrument settings

A continuous diluted infusion can provide stable concentration; a small bolus with a slow saline flush can be effective for LVO. A rapid or excessive bolus produces dense near-field signal and acoustic shadowing of basal segments. Too little agent, infiltration, a poor IV, a fast destructive MI, or a long delay produces weak or swirling signal. Use product- and laboratory-specific preparation and dose rather than transferring volumes among agents. Wait for or restore a steady state before comparing views or replenishment.

Narrow the sector and reduce unnecessary depth to preserve frame rate, but include the entire myocardium. Place focus at or just below the structure per the contrast preset; a high focal pressure near the apex can increase local destruction. Adjust contrast gain so the cavity is homogeneous and tissue is suppressed without blooming the border into myocardium. Acquire nonforeshortened apical four-, two-, and long-axis views with the same plane held through destruction and refill.

Recognize concentration, energy, and propagation artifacts

Basal attenuation or shadowing occurs when a dense apical cavity absorbs/scatters energy, leaving basal segments dark. Reduce dose or concentration, slow the flush, use VLMI, change view, and allow concentration to fall. A defect that moves with the acoustic path or affects the far field more than a coronary distribution suggests attenuation.

Apical destruction and swirling occur when MI/focal pressure is too high or delivery is uneven. Lower MI, move focus deeper, use a slower flush or stable infusion, and wait for homogeneous mixing. Do not call apical thrombus from a transient dark swirl. Blooming from high contrast gain expands cavity signal across endocardium, hides trabeculae, and can make volumes too small; reduce gain and compare unenhanced anatomy.

Near-field rib/lung shadow, reverberation, side-lobe duplication, and tissue-suppression failure remain possible. A linear contrast echo outside the cavity may be artifact, not a dissection flap or shunt. Change window, frequency, depth, gain, MI, and harmonic/contrast-specific mode and confirm in another plane. During a flash, global signal loss is expected immediately; persistent regional delay is interpreted only after the blood pool has replenished and attenuation has been excluded.

UEA can reveal apical aneurysm, noncompaction recesses, pseudoaneurysm, or thrombus, but do not trace microbubble swirling as myocardium or mistake normal trabeculation for a mass. For a suspected thrombus, save precontrast and optimized postcontrast loops; a true avascular mass remains unopacified while the surrounding cavity fills, but artifact and low-flow stasis remain alternatives. The report states agent, administration method, MI/preset, image quality, views, adverse events, and whether enhancement resolved or merely narrowed the limitation.