Ultrasonic Cavitation

Ultrasound Safety

This week we’re participating in a selection of imaging with non-ionising radiation workshops. One focus is ultrasound safety. Measuring power outputs, performing QC and understanding the clinical requirements of ultrasound vs patient safety.

2 indices are used to describe ultrasound safety:

Thermal Index

Thermal indices describe the potential for tissue temperature increasing during scanning. There are 3 specific ones:

TIS- Thermal Index Soft Tissue

Used when ultrasound only insonates soft tissue e.g. obstetric scanning for <10 week pregnancies.

TIB- Thermal Index Bone

Used for foetal scans >10 weeks for when bone is at or near the focal region.

TIC- Thermal Index Transcranial Bone

Used for neonatal skull transcranial scanning.

BMUS Guidelines

The British Medical Ultrasound Society have prepared guidelines for the safe use of diagnostic ultrasound equipment which provide guidance on the thermal index limits that should be used during ultrasound scanning.

A TI value of 1 corresponds to half the worst case scenario temperature elevation i.e. 2°C . Studies have shown that a temperature elevation of a foetus of 4 °C for above 5 minutes can be potentially hazardous with a logarithmic relationship between temperature elevation and maximum time to be scanned.

Temperature elevation (°C)	Max time for scanning (minutes)
5	1
4	4
3	16
2	64
1	256

In practice, these maximum times for scanning have been rounded from 64 and 256 to 30 and 60 minutes respectively due to the lack of knowledge about possible subtle bioeffects associated with prolonged moderate temperature exposure.

Mechanical Index

As well as the thermal index, the mechanical index is used to describe ultrasound safety. Mechanical Index is a measure of acoustic power and describes adverse bioeffects induced by non-thermal mechanisms e.g. cavitation. The mechanical index is inversely proportional to the square root of the frequency of the emitted ultrasound wave.

Cavitation

Cavitation is the expansion and contraction or collapse of bubbles because of the acoustic pressure of the ultrasound beam. The most cavitation sensitive tissues are gas-filled organs such as the lung and intestine. Ultrasound sonographers keep an eye on the mechanical index during scanning to mitigate risk to the patient of damage to their lung or intestines or risk of cavitation with contrast agents.

Imagine my surprise then when in researching cavitation, I started being advertised “Ultrasound Fat Cavitation” by various body treatment sites and spas.

“Cavitation is a new, non-surgical and non-invasive fat removal procedure”.

“Cavitation is the perfect treatment for people who want to contour their body and regain their confidence and figure.”

A paper in the International Open Access Journal of the American Society of Plastic Surgeons write:

Despite the overall satisfaction of patients and surgeons for liposuction for body shaping purposes, there is an increasing demand for noninvasive fat reduction methods that are similarly effective, yet comfortable and safe, with minimal downtime.

Bani, D., Quattrini Li, A., Freschi, G., & Russo, G. L. (2013). Histological and Ultrastructural Effects of Ultrasound-induced Cavitation on Human Skin Adipose Tissue. Plastic and reconstructive surgery. Global open, 1(6), e41.

For noninvasive fat reduction, ultrasound cavitation causes adipocyte rupture and triglyceride release at the “fat droplet-watery cytoplasm interface”. However little is known about the possible noxious effects of ultrasound on adipocytes, blood vessels and mast cells and the neighbouring tissues crossed by the ultrasound beam (epidermis and dermis).

There is no doubt that ultrasound cavitation induces a marked decrease in the size of lipid vacuoles in adipocytes due to a statistically significant release of triglycerides from the adipose tissue into the interstitial fluid. And this study noted that cavitation induced effects seem to be restricted to adipocytes, while blood microvascular cells showed normal features.

I am not sure why I am so surprised that this treatment exists.

Perhaps because as medical physicists we do so much to assure the safety of patients with the equipment we use and yet the beauty industry seem much more laissez-faire with their approach using the same equipment?!

A programme to review the qualifications required for the delivery of non-surgical cosmetic interventions is underway. Currently non-surgical laser treatments are not regulated by the UK government meaning there are no restrictions on who can provide things such as laser tattoo removal or what equipment/premises they use. Similarly ultrasound cavitation treatments are not regulated.

For the time being, the only thing to do is be vigilant! Follow the CQC’s advice on how to choose a cosmetic surgery practitioner https://www.cqc.org.uk/help-advice/help-choosing-care-services/choosing-cosmetic-surgery and ask a medical physicist how they’d go about picking a laser hair removal specialist!