The field of High Frequency micro-ultrasound (ultrasound having centre frequencies greater than 15 MHz) is growing, with clinical and pre-clinical array based systems having centre frequencies of up to 50 MHz currently in production. Limitations in hydrophone technology related to the size of hydrophone apertures, limited bandwidth, and limited calibration range are problematic for clinical regulatory measurements. A method of making a membrane hydrophone with an active area as small as 10 um, having very low field fringing effects is presented. A new method of creating well aligned electrodes on opposite sides of a very thin polymer membrane have been developed and successfully implemented using proprietary FujiFilm VisualSonics Inc. (VSI) Excimer laser technology. Measurements made using newly developed 30 micron membrane hydrophones, including custom electronics and amplifiers, are presented. Calibration results to 110 MHz are presented along with a discussion of uncertainties and challenges related to measuring acoustic fields in water at 50 MHz centre frequency and high pressures.

Most institutions now have a suite of imaging tools to follow mouse models of human disease. Micro-ultrasound is one of these tools and is second after whole-mouse fluorescence or bioluminescent imaging, in terms of installed systems. We report in this paper the first commercially available array transducer-based ultrasound imaging system that enables micro-ultrasound imaging at center frequencies between 15 and 50 MHz. At the heart of the new scanner is a laser-machined high-frequency 256 element, linear transducer array capable of forming dynamic diffraction limited beams. The power of the linear array approach is embodied in the uniform high resolution maintained over the full field of view. This leads to greatly expanded scope for real-time functional imaging that is demonstrated in this paper. The unprecedented images made with the new imaging system will enable many new applications not previously possible. These include real-time visualization of flow in the mouse placenta, visualization of flow development in the embryo, studies of embryonic to adult cardiac development/disease, and studies of real-time blood flow in mouse models of tumour angiogenesis. (E-mail: Stuart.foster@ sunnybrook.ca) Crown Copyright (C) 2009 Published by Elsevier Inc. on behalf of World Federation for Ultrasound in Medicine & Biology.

A three-part finite element model is developed that characterizes the ultrasonic pulse produced by an electromagnetic acoustic transducer (EMAT). The model represents several significant improvements over previously published works, as follows: (a) spatial inhomogeneities in the magnetic flux density are calculated and then incorporated in the determination of body forces, (b) an improved model of the electromagnetic induction phenomenon is formulated, allowing a more accurate evaluation of the ultrasonic pulse launched by an EMAT transmitter and (c) results from the model are compared directly with experimental measurements, yielding discrepancies of the order of 15% in the amplitude of the ultrasonic pulse. The new model is used to optimize the design of the EMAT system. In particular, a parametric study was conducted on the effects of varying an EMAT's magnet-to-coil width ratio. For the EMAT configuration considered, significant improvements can be achieved in the ultrasonic beam amplitude and profile by increasing the ratio to about 1.2; further increases in magnet dimensions yield only marginal improvements in the ultrasonic beam, at the cost of excessive EMAT size.