Quality assurance and field characterisation for MRgHIFU treatments: their need and the challenges presented
© ter Haar et al; licensee BioMed Central Ltd. 2015
Published: 30 June 2015
In our drive to increase the clinical recognition of HIFU treatments, it is important that we pay attention to other comparable, but more widely accepted, therapeutic techniques, and match their rigorous quality assurance and calibration practices.
Well validated Quality Assurance (QA) and field characterisation techniques are important in order that treatments can be planned and simulated, and so they may be compared between patients, between centres and between machines. There is still some discussion as to which the most relevant parameters for such comparisons are.
While the pressure distribution and total power can be measured with reasonable accuracy in the laboratory, the presence of the high magnetic fields in the vicinity of an MR scanner render many of the current equipment unuseable. It is therefore important to develop techniques appropriate for MRgHIFU systems that work within the restricted space available.
We are building an MR compatible acoustic power measurement system which is designed to allow measurements to be made in the magnet bore in a similar way to those made in the laboratory. This system consists of a castor oil target immersed in water, and is connected by fishing wire to a load cell which enables measurement of the weight of the buoy. The load cell has been calibrated and allows measurement of the ultrasonic radiation force and buoyancy force due to thermal expansion of the castor oil. The use of a load cell instead of the dedicated laboratory balance presents problems in terms of oscillations in the measured weight due to the rapid onset/offset of the acoustic radiation force. Comparative measurements have been made in the laboratory, using a non-MR compatible target, a Sartorius balance and the load cell. We have also designed and built a positioning system which uses electric motors that make use of the static magnetic field to operate. This system allows positioning of a hydrophone (Onda HGL-0200) as pressure field mapping.
Results and conclusions
JC is funded on the ICR Focused Ultrasound Foundation Centre of Excellence grant. IR & RS-T are funded by the UK Engineering & Physical Sciences Research Council.
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