Quality assurance for clinical high intensity focused ultrasound fields

Figures & data

Table 1. Parameters that should be measured for detailed description of HIFU fields.

Parameter/feature of HIFU field	Method of measurement	Rationale for its requirement	Frequency of measurement
Acoustic working frequency	Hydrophone/Oscilloscope	Important for determining therapeutic mechanisms	Prior to each treatment
P− at focus	Hydrophone	Relevant for cavitation events	At quarterly QA check, or when problem suspected
P+ at focus	Hydrophone	Relevant for heating and cavitation	At quarterly QA check, or when problem suspected
Pressure profile (FWHM)	Hydrophone	Relevant for HIFU lesion size and development	At quarterly QA check, or when problem suspected
Effect of beam steering on pressure profile at, and outside, focus	Hydrophone	Relevant for HIFU lesion size and safety	At quarterly QA check, or when problem suspected
Time averaged intensity, Ita	Hydrophone	Relevant for heating	At quarterly QA check, or when problem suspected
Total acoustic power	Radiation force or calorimetric balance	Relevant for heating and cavitation.	Prior to each treatment
		Provides easy rapid check of transducer output
Position and amplitude of extra-focal peaks	Hydrophone	Important for safe exposure of critical structures outside the target volume	At quarterly QA check, or when problem suspected
Exposure duration	Radiation force or calorimetric balance	Important for safe and effective treatments	At quarterly QA check, or when problem suspected
Pulsing regime	Hydrophone/oscilloscope	Relevant for heating, and cavitation	Prior to each treatment
Focus location and heating rate	Phantom/test object	Relevant for heating, and cavitation	Prior to each treatment, or when problem suspected
Alignment of imaging planes and focal region	Hydrophone phantom	Important for safe and effective treatments	Prior to each treatment

FWHM, full width at half maximum; QA, quality assurance.

Figure 1. Trans-axial beam plot of an 84-mm diameter spherical bowl transducer (15-cm focal length, 1.7-MHz driving frequency) at the focal plane, measured with an Onda HGL-0200 hydrophone. The peak positive, negative and RMS pressure profiles are indicated, together with their respective full width at half maximum (FWHM) measurements.

Figure 2. RMS voltage amplitude (mV) for a curved spherical bowl transducer surface scan using a fibre-optic sensor close (<3 mm) to the surface of a 64-mm diameter, 63-mm focal length, 1.7-MHz HIFU transducer. The scan reveals solder connections (circled) arranged hexagonally which have reduced output.

Figure 3. Example of acoustic power measurement (at 55 W) using a radiation force/buoyancy system showing the weight measured by a precision balance (Sartorius LA230S) for a 10-s sonication. The radiation force at onset (a), and offset (b) are indicated, together with the shift in baseline weight (c) due to the change in the buoyancy force.