Ultrasound Research:

Development of a High-Resolution High-Frequency Ultrasound Imaging System

Most ultrasound imaging systems operate in the frequency range from 3 to 5 MHz. This frequency range provides an image resolution of approximately 1 mm. By increasing the frequency, the ultrasound wavelength is decreased and finer resolution can be obtained. Recently, several high-frequency (30 – 100 MHz) ultrasound systems have been developed for imaging the eye, skin, vascular system, and even small animals for biological studies. These high-frequency systems can resolve tissue structures ranging from 20 to 100 microns in size. In spite of improved resolution, high-frequency ultrasound systems are not routinely used in clinical practice because they are currently based on single-element geometrically shaped transducers. The fixed geometry of these devices introduces a trade-off between resolution and depth of field. In low-frequency ultrasound systems, a great improvement in image quality is achieved by replacing the single-element transducer with a transducer array and electronic beamformer. This combination allows the ultrasound energy to be focused at a wide range of depths within the tissue. Unfortunately, developing a suitable array and beamformer is complicated by the increased ultrasound frequency. Generally, as the frequency is increased, the components of an ultrasound system become miniaturized and more sophisticated.

Transducer arrays are usually fabricated by cutting kerfs into and through the transducer substrate to create a series of individual elements. On lower frequency devices, these grooves can be machined using a diamond cutter or a laser. The small dimensions of a high-frequency array make conventional machining methods extremely difficult. As an alternative to machining, fabricating the array by simply depositing an appropriate electrode pattern on the surface of the substrate is being investigated. These arrays are fabricated using various micro-fabrication techniques, which include photolithography to define the electrode pattern, and ultrasonic wire bonding to make the electrical contacts. Figure 1 shows a photograph of an annular transducer array taken through a microscope. It shows the electrode pattern and the ultrasonic wire-bonds. The outer diameter of the array is 2 mm and the space separating the array elements is 10 microns. The fundamental operating frequency of this transducer is 50 MHz.

The performance of the combined system (array + beamformer) is currently being tested by imaging in-vitro tissue phantoms, as well as in-vivo laboratory mice. Imaging small animals in biological studies is a rapidly growing application of high-frequency ultrasound as it is more frequently being used as a monitoring tool for genomic and cancer studies. Figure 3 shows an image of a CD-1 mouse embryo. The embryonic eye is visible near the centre of the image, and the uterine wall and amniotic sac are also clearly visible. The mouse images that we generated were the first images generated by a high-frequency multi-element ultrasound system.

Presently, we are working on developing an imaging system based on a high-frequency linear array transducer (figure 4). A major advantage to linear array transducers is the extremely high frame-rate that can be achieved since the aperture is translated electronically instead of mechanically. High frame-rate imaging is vitally important in pre-clinical studies of the vascular system. Recently, we have developed a prototype linear array and its performance currently being characterized.

       PUBLICATIONS:

· R.W. Deas, R. Adamson, L.L. Curran, F.M. Makki, M.L. Bance, J.A. Brown, “Audiometric thresholds measured with single and dual BAHA motors: the effect of phase,” International Journal of Audiology, Vol. 49(12), pp. 993-999, 2010

· J. Yin, M. Lee, J.A. Brown, E. Cherin, F.S. Foster, "Effect of Pillar Geometry On High Frequency Piezo-Composite Transducers," IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control, Vol. 57, pp. 957-967, 2010.

· R.J. Pennings, A. Ho, J.A. Brown, R.G. Van Wijhe, M. Bance, “Analysis of Vibrant Soundbridge Placement Against the Round Window Membrane in a Human Cadaveric Temporal Bone Model,” Otology and Neurotology, Vol. 31(6),  pp. 998-1003, 2010

· Z. Torbatian, R. Adamson, M. Bance, J.A. Brown, “A Split Aperture Transmit Beamforming Technique with Phase Coherence Grating Lobe Supression,” IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control, Vol. 57(11),  pp. 2588-2595, 2010

· R. Adamson, M. Bance, J.A. Brown, “A Piezoelectric Bone-Conduction Bending Hearing Actuator,” JASA, Vol. 128(4), pp. 2003-2008,  2010

· J.A. Brown, Z. Torbatian, R. Adamson, R. Van Wijhe, R.J.E. Pennings, G.R. Lockwood, M.L. Bance, High-Frequency Ex-Vivo Ultrasound imaging of the Auditory System, Ultrasound in Medicine and Biology, Vol. 35, pp. 1899-1907, 2009

· J.A. Brown, E. Cherin, J. Yin, F.S. Foster, Fabrication and Performance of High-Frequency Composite Transducers with Triangular Pillar Geometry, IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control, Vol. 56, pp. 827-836, 2009.

· E. Cherin, J.A. Brown, S. Masoy, H. Shariff, R. Karshafian, R. Williams, P. Burns, F.S. Foster, Radial Modulation Imaging of Microbubble Contrast Agents at High Frequency, Ultrasound in Medicine and Biology, Ultrasound in Medicine and Biology, Vol. 34(6), pp. 949-962, 2008.

·  J.A. Brown, F.S. Foster, A. Needles, E. Cherin, G.R. Lockwood, Fabrication and Performance of a 40 MHz Linear Array based on a 1-3 Composite with Geometric Elevation Focussing, IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control, Vol. 54, pp. 1888-1894, 2007.

·  C.E.M. Demore, J.A. Brown, G.R. Lockwood, Investigation of Cross-Talk in Kerfless Annular Arrays for High Frequency Imaging, IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control, Vol. 53, pp.1046-1056, 2006.

·  J.A. Brown, G.R. Lockwood, A Digital Beamformer for High-Frequency Annular Arrays, IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control, Vol. 52, pp.1262-1269, 2005.

·  J.A. Brown, C.E.M. Demore, G.R. Lockwood, Design and Fabrication of Annular Arrays for High-Frequency Ultrasound, IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control, Vol. 51, pp.1010-1017, 2004.

·  J.A. Brown, G.R. Lockwood, A Low-Cost, High-Performance Pulse Generator for Ultrasound Imaging. IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control, Vol. 49 pp.848-851, 2002.

· R.W. Deas, R. Adamson, P. Garland, M. Bance, J.A. Brown, Combining Auditory and Tactile Inputs to Create a Sense of Auditory Space, Proc. Acoustical Society of America Symposium, 2010

· C.Mcknight, R. Adamson, M. Bance, D. Doman, J.A. Brown, Three Dimensional Laser Doppler Vibrometry of the Dry Human Skull, Proc. Acoustical Society of America Symposium, 2010

·  Z. Torbatian, R.Adamson, M. Bance, J.A. Brown, Transmit Beamforming Techniques for Supressing Grating Lobes in Large Pitch Phased Arrays, Proc. SPIE Medical Imaging Symposium, 2011

·  R. Adamson, Z. Torbatian, M. Bance, J.A. Brown, A High-Frequency Beamformer Design Based on variable PMN-PT SAW delays, Proc. IEEE Ultrasonics Symposium, 2009.

·  Z. Torbatian, R. Adamson, R. Pennings, R. Van Wijhe, M.Bance, J.A. Brown, Imaging the Auditory System: A New Application of High-Frequency Ultrasound, Proc. IEEE Ultrasonics Symposium, 2009

·  J. Yin, M. Lee,  J.A. Brown, F.S. Foster, Geometry Effect on Piezo-Composite Transducers with Triangular Pillars, Proc. IEEE Ultrasonics Symposium, pp. 1421-1424, 2008.

·  J.A. Brown, E. Cherin, J. Yin, F.S. Foster, Fabrication and Performance of a High-Frequency Geometrically Focussed Composite Transducer with Triangular Pillar Geometry, Proceedings IEEE Ultrasonics Symposium. pp. 80-83, 2007.

·  E. Chérin, J.A. Brown, H. Shariff, R. Karshafian, R. Williams, P.N. Burns, F.S. Foster, S.E. Måsøy, Radial Modulation Imaging of Microbubbles at High Frequency, Proceedings IEEE Ultrasonics Symposium.pp. 888-891, 2007.

·  J.A. Brown, F.S. Foster, A. Needles, G.R. Lockwood, Photograph Through a Microscope of a 40 MHz Linear Array, IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control (Front Cover Image), Vol. 54, 2007.

·  J.A. Brown, F.S. Foster, A. Needles, G.R. Lockwood, A 40 MHz Linear Array based on a 1-3 Composite with Geometric Elevation Focussing, Proceedings IEEE Ultrasonics Symposium. pp. 256-259, 2006.

·  J.A. Brown, Design, Fabrication, and Performance of a High-Frequency, Annular-Array-Based Ultrasound Imaging System, Ph.D. Thesis, Queen’s University, 2005.

·  J.A. Brown, G.R. Lockwood, Design of Sparse Annular Arrays for High Frequency Imaging. Proceedings IEEE Ultrasonics Symposium, pp.125-128, 2005.

·  J.A. Brown, C.E. Morton-Demore, F.S. Foster, G.R. Lockwood, Performance of a 50 MHz AnnularArray Based Imaging System. Proceedings IEEE Ultrasonics Symposium, pp.760-763, 2004.

·  J.A. Brown, C.E.M. Demore, G.R. Lockwood, Photograph Through a Microscope of a 50 MHz Annular Array, IEEE Transactions on Ultrasonics Ferroelectrics & Frequency Control (Front Cover Image), Vol. 51, 2004.

·  J.A. Brown, C.E. Morton, G.R. Lockwood, Fabrication and Performance of 40-60 MHz Annular Arrays, Proceedings IEEE Ultrasonics Symposium, pp.869-872, 2003.

· J.A. Brown, G.R. Lockwood, Gate Array Beamformer for High Frequency Annular Arrays. Proceedings IEEE Ultrasonics Symposium, pp.1658-1661, 2002