The use of ultrasound technology, which employs high-frequency sound waves to image the body’s interior, is a routine procedure in modern medicine. This process relies on sending mechanical energy pulses into tissues and recording the returning echoes to create diagnostic images. A physical phenomenon inherent to this wave-tissue interaction is cavitation, which is the formation and activity of microscopic gas bubbles within a liquid medium, such as the fluid in biological tissue, caused by pressure changes from the sound waves. Cavitation represents the primary non-thermal interaction mechanism between ultrasound energy and human tissue. Understanding how this bubble activity is managed for safety during diagnostic scans and harnessed for therapeutic benefit is central to appreciating the role of ultrasound in healthcare.
How Ultrasound Creates Bubbles
Sound waves travel through tissue as alternating cycles of high pressure (compression) and low pressure (rarefaction). During the low-pressure phase, the sound wave pulls the liquid apart, causing any pre-existing gas nuclei dissolved in the tissue fluid to expand dramatically. When the wave returns to the high-pressure phase, the bubble is squeezed back down. The way these bubbles behave under the influence of the oscillating pressure field determines the type of cavitation produced.
Stable Cavitation
The two major types of cavitation are stable and inertial, differentiated by the intensity of the pressure changes. Stable cavitation occurs at lower acoustic pressures, where the bubbles oscillate in size around an equilibrium point without violently collapsing, similar to a balloon gently pulsating. This oscillation can create microstreaming, which is localized fluid movement around the bubble. This microstreaming induces mechanical stress on nearby cells.
Inertial Cavitation
Inertial cavitation, also known as transient cavitation, occurs when the negative pressure is sufficiently high to cause the bubbles to expand rapidly and unstably, reaching two to three times their resonant size. This massive expansion is followed by a violent, asymmetrical collapse during the compression phase of the sound wave. This collapse releases intense localized energy in the form of shock waves, microjets, and high temperatures. This extreme energy release is capable of causing mechanical disruption to nearby tissues and cells.
Management During Medical Imaging
Diagnostic ultrasound procedures are designed to operate at acoustic power levels that minimize the potential for harmful inertial cavitation, focusing instead on obtaining high-quality images. Manufacturers engineer diagnostic equipment to adhere to safety limits by controlling the maximum acoustic output. The goal in diagnostic imaging is to ensure that if any cavitation occurs, it remains within the lower-energy stable range, which is generally not associated with adverse biological effects in the absence of contrast agents.
Sonographers and technicians play a direct role in minimizing potential bioeffects by adhering to the ALARA principle, which stands for “As Low As Reasonably Achievable”. This practice mandates using the lowest power setting and shortest scan time necessary to produce a clinically sufficient image. Operators adjust settings like frequency and power to balance image quality with patient safety, constantly monitoring the output metrics displayed on the screen. This careful management ensures that the mechanical effects of the sound waves remain well below the threshold required to induce damaging inertial cavitation in soft tissues.
Intentional Use in Advanced Treatment
While diagnostic imaging seeks to avoid the effects of inertial cavitation, therapeutic applications of ultrasound actively harness controlled bubble activity for medical benefit. High-Intensity Focused Ultrasound (HIFU) represents a significant area where cavitation is utilized, often in conjunction with thermal effects, to treat conditions like solid tumors.
High-Intensity Focused Ultrasound (HIFU)
In HIFU, the intense focusing of sound waves at a specific target can deliberately induce controlled inertial cavitation. This enhances the mechanical destruction or ablation of tissue.
Sonoporation
Another technique that relies on controlled cavitation is Sonoporation, which uses ultrasound to temporarily increase the permeability of cell membranes. At lower therapeutic intensities, stable cavitation induces fluid movement that exerts shear stress, creating transient pores. At higher intensities, the microjets and shock waves produced by controlled inertial cavitation can also form temporary channels in the cell membrane. These channels allow large therapeutic molecules, such as drugs or genes, to be delivered directly into the cell.
This process is significantly enhanced by the introduction of gas-filled microbubbles, which act as cavitation nuclei to lower the acoustic pressure threshold needed to initiate bubble activity. These microbubbles, which are often used as ultrasound contrast agents, oscillate and collapse under the therapeutic sound field. This dramatically boosts the mechanical effects for targeted drug delivery or tissue disruption.
Patient Safety and Monitoring Metrics
Because cavitation represents the primary non-thermal risk associated with ultrasound, regulatory and governing bodies mandate the use of specific monitoring metrics to ensure patient safety. The most important of these is the Mechanical Index (MI), which is a numerical value displayed on the ultrasound screen that estimates the likelihood of inertial cavitation occurring. The MI is calculated based on the peak negative pressure of the ultrasound beam divided by the square root of the center frequency.
A higher MI value indicates a greater potential for inertial cavitation and subsequent mechanical bioeffects. For general diagnostic procedures, the U.S. Food and Drug Administration (FDA) mandates that the MI should not exceed 1.9. However, for sensitive tissues, such as in obstetric or ophthalmic applications, practitioners are often advised to keep the MI substantially lower, sometimes below 0.4 or 0.3, respectively. The continuous on-screen display of the MI allows medical professionals to adjust the device settings in real-time, ensuring that the acoustic output remains below regulatory thresholds, thereby preventing unintended mechanical damage from bubble activity.