IEC TR 62649:2010 pdf – Requirements for measurement standards for high intensity therapeutic ultrasound (HITU) devices
IEC TR 62649:2010 pdf – Requirements for measurement standards for high intensity therapeutic ultrasound (HITU) devices.
Zhai of el, 2004) or the modulation of a laser beam via the acousto-optic effect in a water bath In a fibre-optic hydrophone, a laser is directed along an optical fibre and the end of the fibre is placed in the ultrasound field. The reflection from the end is modulated by the acoustic pressure which changes the refractive index of the water adjacent to the exposed end. The internal reflection off the end of the fibre is monitored and can be quantitatively related to the pressure around the end. Devices of this sort were developed initially for use in lithotripter fields but have also been used recently in HITU fields (Parsons of &, 2006; Khokhlova, 2005). The reported advantages relate to the small diameter of the fibres, uniform directional response, broad bandwidth, and the low cost of the optical fibre (which can be damaged by cavitation but is easily repaired by re-cleaving). Disadvantages relate to poor signal-to-noise ratio, non-uniform magnitude and phase response in the frequency domain, and the effects of outgassing near the fibre tip. Alternative approaches also using optical fibres (Wilkens and Koch, 1999; Beard of a?, 2000; Zhou of a?, 2006; Lewin at al, 2005; Morris of al, 2009) have also been suggested and these may offer advantages over the Eisenmenger technique:
however, they have not yet been tested In HITU fields.
Local density variations induced by the passage of an ultrasonic wave in water give rise to local perturbations in the refractive index. This acousto-optic effect can be utilised in a number of ways to perform optical measurements of the acoustic field. If the optical beam is allowed to enter a water tank, traverse the acoustic field and reflect back through the acoustic field from a mirror external to the tank, the interferometer will respond to the change in path length or the rate of change in path length. This acousto-optic effect is the basis of schlieren imaging but can also be used more quantitatively to perform non-invasive measurement of acoustic pressure using interferometric measurement techniques (Harland at a?, 2002:
Theobold ot a?, 2004). It is, however, a ‘line-of-sight’ technique so does not directly measure the pressure at a poInt, the pressure field must be reconstructed tomographically.
This report concentrates on the requirements and potential future requirements for International Standards related to HITU. Only standards related to manufacture, market- access. Quality Assurance and safety are considered since these are generally the aspects considered by the standards bodies. Other Important aspects such as standards for clinical practice are not addressed: these are more properly the preserve of professional clinical bodies. Many of the areas discussed in this clause will require substantial further research — either rnetrological or biological — before useful standards can be written. Clearly this research Is essential and It should generally take place In the academic and commercial arenas and not within standards bodies. However, there can be benefit In producing ‘pro- standard’ publications such as Technical Reports or Technical Specifications, since these can focus attention on specific questions where a clear consensus is needed before advancing to a validated and widely-accepted standard.
6.2 Measurement of power
In ultrasound fields at megahertz frequencies, output power is typically determined by measuring the force on a target using a radiation force balance. However, the relationship between the radiation force and the output power i affected by the focusing or other geometrical aspects of the field, by the type and shape of the target, by the distance of the target from the transducer, by absorption (including ‘shock-loss) in the water path, and by acoustic streaming currents. Highly focused fields created by large physical apertures and short focal distances typical in HITU applications require a more thorough accounting of angular contributions than those described in existing standards. Whilst many of these effects are small for typical diagnostic or physiotherapy ultrasound fields, they cannot generally be ignored for HITU fields. Furthermore, in HITU, the quantity of interest is the power incident on the patient rather than the output power at the transducer face. Since it is common for there to be a water stand-off between the transducer and the patient, attenuation and shock-loss in the water path may be significant and will vary depending upon the chosen distance.