Ultrasound Contrast Agents

What is the advantage of using contrast?

The primary advantage of using Doppler contrast is its ability to massively increase the amplitude of the returning echo, thereby increasing the signal-to-noise ratio. Why does this happen? It relies on the enormous mismatch in acoustic impedance between the microbubble's gas core and the surrounding liquid blood/tissue. Recall that there is a huge acoustic impedance mismatch between gas and any other medium.  So, gas-filled microbubbles are highly efficient ultrasound reflectors that return much stronger echoes from the blood than the blood alone.

Microbubble Resonance & The Mechanical Index (MI)

When exposed to an ultrasound pulse, the microbubbles compress during the high-pressure (compression) phase and expand during the negative-pressure (rarefaction) phase. This behavior of the bubble depends entirely on the Mechanical Index (MI).

Here is how the MI affects the microbubbles:

• Low MI (< 0.2): The microbubble oscillates symmetrically and linearly. It reflects sound at the fundamental (transmitted) frequency.

• Medium MI (0.2 – 0.6): The microbubble undergoes non-linear, asymmetrical oscillation (expanding more than it contracts). The contrast microbubbles "resonate" in the ultrasound field, generating their own harmonic frequencies (multiples of the fundamental frequency).  This is called "contrast harmonics"  By tuning the receiver to listen for these higher harmonic frequencies, the system can produce images that have less noise and clutter, which improves sensitivity and reduces artifact.

• High MI (> 0.6): The bubble oscillations become unstable, leading to forceful collapse and microbubble destruction (inertial cavitation).

In summary: Low MI, the bubbles reflect the transmitted frequency.  Medium MI, the bubbles generate harmonic frequencies. High MI, the bubbles forcefully collapse.

Image from Innovative Surgical Sciences

Contrast-Specific Imaging Techniques

Because standard B-mode imaging cannot easily differentiate tissue from contrast, modern scanners use multi-pulse sequencing techniques to exploit non-linear bubble behavior while canceling out linear tissue signals.  These are two techniques that are employed.

• Pulse Inversion Harmonics: The transducer fires two sequential pulses down the same scan line, with the second pulse being 180° out of phase (inverted). Linear tissue echoes cancel each other out to zero, leaving a strong contrast signal.

• Power / Amplitude Modulation: The transducer fires two consecutive pulses of identical shape but different amplitudes. By scaling and subtracting the received echoes, linear tissue signals are eliminated.

In this way the strong contrast echoes within the blood vessels can be differentiated from the echoes from the tissue.

Note: as we have seen, firing multiple pulses to generate a scan line of data will cause some reduction in frame rate.

Safety & Bioeffects

Are ultrasound contrast agents safe?  In comparison to other contrast agents used in radiology, UCAs are among the safest diagnostic contrast agents. They are not nephrotoxic and do not affect the thyroid.  However, there are a couple of things to keep in mind.

• Acoustic Cavitation: High-MI imaging of microbubbles intentionally induces inertial (transient) cavitation—the violent collapse of bubbles. In a clinical setting, this can lead to unintended bioeffects like capillary rupture or premature ventricular contractions (PVCs). The MI needs to be monitored during contrast procedures

• Adverse Reactions: Severe anaphylactoid reactions are extremely rare (approx. 1 in 10,000). However, because they can occur, a resuscitation (crash) cart and trained personnel must always be readily available.

Common reasons for using ultrasound contrast agents

• Left Ventricular Opacification and Function Assessment: Enhancing the delineation of the left ventricular endocardial border to accurately calculate cardiac volumes and ejection fraction, which is especially useful for patients whose standard, unenhanced echocardiograms are suboptimal or technically difficult.

• Stress Echocardiography: Improving the visualization of regional wall motion abnormalities and wall thickening during exercise or pharmacological stress tests to reliably detect myocardial ischemia.

• Focal Liver Lesion Characterization: Differentiating benign liver masses (such as hemangiomas and focal nodular hyperplasia) from malignant tumors (such as hepatocellular carcinoma and metastases) by analyzing the real-time wash-in and wash-out dynamics of the microbubbles across the arterial, portal venous, and late phases.

• Aortic Endoleak Surveillance: Monitoring patients who have undergone endovascular aneurysm repair (EVAR) for abdominal aortic aneurysms to detect and classify endoleaks, which are areas of persistent blood flow outside the stent-graft.

• Carotid Plaque Vulnerability Assessment: Identifying high-risk features in carotid artery atherosclerosis, such as intraplaque neovascularization and surface ulcerations, to improve risk stratification for stroke and other cerebrovascular events.

• Intracardiac Thrombus Detection: Detecting or ruling out intracardiac masses and thrombi, particularly those located in the left ventricular apex that are often obscured by near-field artifacts on standard B-mode imaging.

• Pediatric Vesicoureteral Reflux (VUR) Diagnosis: Evaluating children for VUR using contrast-enhanced voiding urosonography (ceVUS), which involves intravesical administration of contrast into the bladder via a catheter to provide a highly sensitive, radiation-free alternative to traditional fluoroscopic voiding cystourethrograms.

• Renal Mass and Cyst Characterization: Distinguishing solid, vascularized renal tumors from complex cysts and evaluating areas of impaired renal perfusion (such as infarctions), which is highly beneficial because ultrasound contrast agents are not nephrotoxic.

• Blunt Abdominal Trauma Evaluation: Rapidly identifying solid organ injuries, including lacerations and hematomas in the liver, spleen, or kidneys, particularly in hemodynamically stable pediatric patients to reduce the need for CT scans and radiation exposure.

• Aortic Dissection Diagnosis: Visualizing the intimal flap and detecting the direction and presence of blood flow to differentiate between the true and false lumens in cases of suspected aortic dissection.

A liver lesion becomes much more conspicuous using microbubble contrast.  Left: with contrast.  Right: without contrast. 

(Image from Cincinnati Children's Hospital)