Research Interest

HIFU Quality assurance

GR ter Haar, J Civale, S Kaiplavil, I Rivens; in collaboration with A Shaw, National Physical Laboratory (NPL), Teddington, UK
Source of funding:  EPSRC

There is an urgent need to develop equipment and methodology appropriate for quality-assurance testing of medical HIFU transducers and complete clinical systems. We are currently investigating existing, and potentially novel techniques relating to quality assurance of HIFU fields.

Towards this general aim we are currently working on the following:

1) Acoustic field measurements of HIFU beams
2) Acoustic power output measurements from HIFU transducers
3) Rapid visualisation techniques
4) Transducer electrical power input monitoring
5) In-line acoustic power measurement
6) Ultrasound imaging and HIFU beam registration

A substantial amount of progress has been made using the acoustic field measurement system to acquire free-field estimates of acoustic pressure parameters and focal dimensions. In addition a fibre-optic hydrophone has been used to plot measurements of phase which may be of use in determining the angle of propagation of ultrasound at a given field position, the relative alignment of the HIFU beam with respect to the coordinate system, and potentially, to provide an indication of  malfunction of the transducer. Further areas of research include the use of the fibre-optic hydrophone to perform scans on a spherical bowl surface a few millimetres from the surface of the transducer (Figure 1), thus detecting potential transducer element failure, and variable resolution 2D and 3D scanning designed to resemble the conical nature of HIFU beams in order to reduce scan time with minimal loss in spatial detail.

A new system for measuring acoustic power has been built.  This buoyancy method, based on a design from NPL, consists of a castor oil target immersed in a water tank. The target is large enough to absorb the majority of the energy in the ultrasound beam and is suspended from a digital balance. The radiation force exerted by the ultrasound beam on the target can be detected by the balance; absorbed energy in the ultrasound beam is also converted into heat causing the target to expand and leading to an apparent change in target weight. The radiation force and buoyancy change can both be measured and related to the total acoustic power. Early measurements using this system are showing promise, the radiation force measurement giving a sensitivity of ~50 mW which is a considerable improvement on our previous mechanical radiation force balance. Measurements using a pyro-electric sensor placed directly in front of a HIFU transducer are also showing promise as a way of estimating total acoustic power output without significant loss of transmitted power. Research in the other areas of this project are still ongoing.

Figure 1.  The acoustic pressure (MPa) measured from a spherical bowl scan using a fibre-optic hydrophone sensor ~3 mm from the surface of a 10 element HIFU transducer. The central imaging aperture (5.5 cm diameter) is clearly visible, as is a section of the transducer (red ellipse) which does not appear to be functioning as well as the rest of the device.