Why Is Degassed Water Used As A Coupling Medium In Clinical Hifu? Essential Energy Transfer & Equipment Safety

Degassed water is utilized in High-Intensity Focused Ultrasound (HIFU) primarily to prevent acoustic cavitation and ensure efficient energy transfer. Because ordinary water contains dissolved gases that form microbubbles under high pressure, using degassed water creates a pure, low-loss transmission path that protects both the patient's treatment efficacy and the medical equipment.

Ordinary water contains microscopic bubbles that react violently to high-intensity sound waves. Degassed water removes this variable, preventing energy scattering and protecting the transducer from heat damage caused by bubble collapse.

The Physics of Acoustic Transmission

The Problem with Microbubbles

Ordinary water naturally contains dissolved gases. Under normal conditions, these are harmless and often invisible.

However, HIFU generates extreme pressure variations. In this high-intensity field, dissolved gases are pulled out of solution, forming active microbubbles.

Understanding Cavitation

When these microbubbles are subjected to the intense acoustic pressure of HIFU, they undergo a process called violent cavitation.

The bubbles expand and collapse rapidly. This chaotic activity disrupts the organized propagation of sound waves required for clinical treatment.

Preventing Acoustic Shadowing

Effective HIFU relies on focusing waves precisely onto a target, such as a tumor.

Microbubbles act as acoustic barriers. They scatter the ultrasonic energy in random directions rather than allowing it to converge on the focal point.

This scattering effect, known as acoustic shadowing, significantly reduces the intensity of the treatment delivered to the patient.

Protecting the Equipment

Thermal Damage Risks

The violent collapse of microbubbles during cavitation releases intense bursts of localized heat.

Because the coupling water is in direct contact with the ultrasound transducer, this heat generation poses a severe risk.

Preserving the Transducer Surface

HIFU transducers are precision instruments with sensitive surfaces.

If cavitation occurs near the transducer face, the resulting thermal spikes can erode or crack the surface. Using high-purity degassed water eliminates the source of this heat, preserving the lifespan of the hardware.

Operational Challenges and Considerations

The Instability of the Medium

While degassed water is acoustically superior, it is not chemically stable in an open environment.

Gases from the surrounding air will naturally attempt to re-dissolve into the water over time. This means the coupling medium cannot simply be prepared once and left indefinitely.

Requirement for Continuous Management

To maintain the "low-loss" characteristics described, the water supply must be carefully managed.

Clinical setups often require circulation systems or specific protocols to ensure the water remains at a high purity level throughout the duration of the procedure.

Ensuring Clinical Success

To achieve safe and effective HIFU outcomes, the quality of the coupling medium is just as critical as the calibration of the device.

If your primary focus is Treatment Efficacy: Prioritize degassed water to eliminate acoustic shadowing, ensuring the full dose of ultrasonic energy reaches the target tissue.
If your primary focus is Equipment Longevity: Strictly adhere to degassing protocols to prevent cavitation-induced heat, which is a primary cause of transducer surface damage.

By removing dissolved gases, you convert the coupling medium from an unpredictable variable into a reliable tool for precision medicine.

Summary Table:

Feature	Ordinary Water	Degassed Water
Dissolved Gas Content	High (contains microbubbles)	Low (pure transmission path)
Acoustic Transmission	High energy scattering	Efficient, focused energy flow
Cavitation Risk	High (violent bubble collapse)	Minimal to None
Transducer Impact	Risk of thermal damage/cracking	Prolongs equipment lifespan
Treatment Focus	Unpredictable (Acoustic shadowing)	Precise target accuracy

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