From the Guidelines
Lower frequencies in ultrasound have a higher risk of thermal biological effects due to increased penetration depth and energy absorption in deeper tissues. This is supported by a systematic review of ultrasound-mediated drug delivery to the eye, which found that reducing frequency increases epithelial damage at higher pressures and durations 1. The study also notes that transscleral delivery penetration depth through sclera increased with decreasing frequency, indicating that lower frequencies have a greater potential for thermal biological effects due to deeper tissue absorption 1. Key points to consider include:
- The effect of changing ultrasound frequency on safety and delivery efficacy
- The relationship between frequency, penetration depth, and energy absorption in tissues
- The importance of controlling output parameters, such as frequency, to minimize thermal biological effects
- The use of the thermal index (TI) to monitor potential thermal effects during ultrasound procedures The evidence suggests that lower frequencies are associated with increased thermal biological effects, and this should be taken into account when using ultrasound in diagnostic and therapeutic applications, particularly in sensitive areas such as the eye 1.
From the Research
Thermal Biological Effects in Ultrasound
- The risk of thermal biological effects in ultrasound is a concern, particularly in sensitive organs and embryos/fetuses 2.
- Two basic mechanisms of ultrasound interaction with biological systems have been identified: thermal and non-thermal 2.
- The thermal index (TI) is used to assess the potential for thermal biological effects, and it can be used directly in the low-frequency range 3.
- However, the mechanical index (MI) becomes less reliable at lower frequencies, and an improved formulation for the MI is proposed for frequencies below 500 kHz 3.
- The ultrasound frequency spectrum can be divided into three bands: US(A) (17.8-500 kHz), US(B) (500 kHz-100 MHz), and US(C) (above 100 MHz) 4.
- In the US(B) band, temperature rise in tissues becomes the most important biological effect of exposure, suggesting that higher frequencies may have a higher risk of thermal biological effects 4.
- Studies have shown that pulsed ultrasound can produce elevations of temperature and damage in biological tissues in vivo, particularly in the presence of bone 5.
- The risk of thermal effects is also a concern in postnatal subjects, and the temperature increase threshold of tissue damage has been estimated to be around 18 degrees C for nonfetal tissue 6.
- The current thermal indices could be improved to reduce the deviations and scatter of computed maximum temperature rises, particularly for soft tissue and bone cases 6.