Degassed water is the fundamental coupling medium in High-Intensity Focused Ultrasound (HIFU) systems because it eliminates the microscopic air bubbles that destroy the integrity of sound waves. Without this specific treatment, the dissolved gases found in ordinary water would disrupt the acoustic beam, compromising both patient safety and treatment efficacy.
Core Insight: The primary function of degassed water is to provide a "transparent" acoustic path. By removing microbubbles, the system prevents the scattering of ultrasound energy and "violent cavitation" near the source, ensuring that heat is generated only at the specific internal target rather than on the patient's skin or the device itself.
The Physics of Acoustic Transmission
Matching Biological Tissue
Degassed water possesses an acoustic impedance similar to human tissue.
This similarity is critical for the seamless transfer of energy. When the impedance matches, ultrasound waves pass from the device into the body with minimal resistance.
If the impedance were mismatched, significant portions of the ultrasound energy would reflect off the interface. This reflection reduces the energy delivered to the target and creates unpredictable interference patterns.
Preventing Energy Loss
Ordinary water contains dissolved oxygen and other gases.
These gases exist as microscopic bubbles, often referred to as "air nuclei." While invisible to the naked eye, they act as significant obstacles to high-frequency sound waves.
Degassing removes these obstacles to create a low-loss transmission path. This ensures the full intensity of the sonic energy reaches the intended treatment depth.
The Critical Role of Bubble Elimination
Mitigating Violent Cavitation
In high-intensity fields, microbubbles undergo violent cavitation.
When exposed to HIFU energy, existing microbubbles can expand and collapse explosively. This phenomenon absorbs the acoustic energy before it ever reaches the patient.
By using degassed water, the system eliminates the "seeds" for this cavitation. This prevents energy loss within the coupling medium itself.
Eliminating Acoustic Scattering
Bubbles cause the ultrasound beam to scatter.
Instead of converging on a precise focal point, the energy diffuses in random directions when it hits a bubble. This "defocusing" effect renders the treatment inefficient.
Degassed water ensures the beam remains coherent. This allows for the precise delivery of acoustic energy to deep target tissues without distortion.
Safety and Equipment Protection
Preventing Surface Burns
Scattered energy is a major safety hazard.
When bubbles scatter energy near the skin's surface, it causes unintended local overheating. This can lead to painful skin burns rather than treating the internal lesion.
Degassing concentrates the energy internally. It ensures the heat is generated at the lesion site, not at the coupling interface.
Protecting the Transducer
Cavitation bubbles can damage the hardware.
The heat and shockwaves generated by bubbling near the transducer face can erode or crack the piezoelectric (PZT) elements. This degrades the device's lifespan and performance stability.
Additionally, the circulating water acts as a coolant. It dissipates the heat generated during operation to maintain beam uniformity.
Understanding the Operational Trade-offs
The Risk of Re-saturation
Water naturally re-absorbs gas over time.
Degassed water is unstable in an open environment; it will aggressively pull oxygen back in from the air. If the system is not sealed or continuously degassed, the water will lose its effectiveness rapidly.
Complexity of Support Systems
Maintaining water purity requires active management.
HIFU systems often require circulation pumps and vacuum degassing units. This adds mechanical complexity and maintenance requirements compared to simple gel-based coupling methods.
Making the Right Choice for Your Goal
To ensure optimal HIFU performance, the quality of the coupling medium must be strictly maintained.
- If your primary focus is Patient Safety: Prioritize degassing to prevent acoustic scattering, which is the leading cause of unintended surface skin burns.
- If your primary focus is Treatment Efficacy: Ensure the water is free of air nuclei to maximize energy transmission and prevent beam defocusing.
- If your primary focus is Device Longevity: Maintain high-purity, degassed water to prevent cavitation damage to the delicate transducer surface.
In summary, degassed water is not just a fluid; it is a critical optical component that ensures ultrasound energy is delivered safely, precisely, and efficiently.
Summary Table:
| Feature | Degassed Water | Regular Tap Water |
|---|---|---|
| Acoustic Path | Transparent & Coherent | Scattered & Diffused |
| Energy Delivery | Precise Focal Accuracy | High Energy Loss |
| Safety Risk | Low (Internal Targeting) | High (Skin Surface Burns) |
| Device Impact | Extends Transducer Life | Causes Cavitation Damage |
| Impedance | Matches Human Tissue | Mismatched due to Air Nuclei |
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References
- J.Q. Chen, Wen‐Yih Isaac Tseng. Proton resonance frequency shift‐weighted imaging for monitoring MR‐guided high‐intensity focused ultrasound transmissions. DOI: 10.1002/jmri.22582
This article is also based on technical information from Belislaser Knowledge Base .
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