Dwell time x Duty Factor 3. Vibration maintained without interruption - more efficient heating. Mechanical effects may be heightened with pulsed. High frequencies are more directional and more efficient at heating. The narrower the impact area, the more concentrated the ultrasound intensity, but conversely the more effective is the heat removal process.
Narrow focus can create heat lesions effectively in the brain by concentrating ultrasound energy.
In well vascularized tissue however the most efficient beam size for heating is moderately focused. The length of the pulse and the amount of time between pulses that the tissue has to recover relates to the damage that may be done. Long pulses with short recovery times are considered to have the greatest theoretical potential for bioeffects. Non-linear effects Conversion of energy into higher harmonics, i.e. change of energy from center frequency spectrum to higher harmonic frequencies. Non-linear effects increase with higher amplitude, higher frequency, and narrower focus.