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High-Frequency Ultrasound Applications
 Figure 2. Representative human eye images using a Quantel Aviso clinical ultrasound system with a 20-MHz and a 50-MHz imaging probe. a: The 20-MHz single-element probe (positioned to left of the image) is designed to image the back of the eye in the region of the retina. The optic nerve (ON) is also visible, but the deep focus of the transducer means that the front of the eye cannot be resolved. b: The 50-MHz single-element probe (positioned at top of the image) is de- signed to image the anterior chamber (AC). The region with the iris is within the transducer focus (as can be seen by the slightly brighter intensity of the image at this depth) and the resolution quickly de- grades when moving away from this region. The top surface of the lens and the ciliary body (CB) are also visible. The cornea is partially resolved but only when the surface is close to perpendicular with the acoustic field.
a length of several millimeters and a width of approximately 0.5 mm that rotates at the end of the catheter. This trans- ducer rotates to create a 360° image in the plane normal to the catheter. Phased-array designs with elements wrapped in a cylindrical shape are also available (O'Donnell et al., 1997). Like ophthalmic ultrasound, the basic technology for IVUS has not changed much over time, and sophisticated array technology has not yet translated into clinical use.
One type of array transducer that sits between a single-ele- ment and a linear-array transducer is known as an annular array. Annular arrays are composed of a series of circular, concentric elements and they can have either a planar or spherical geometry. Annular arrays have long been known to provide superior image quality because of their radial beam symmetry, but they were not practical for general clin- ical use at low frequencies because they require mechanical scanning and real-time frame rates were not feasible over long scan lengths. However, HFU imaging, with the small scale of what is imaged, is well suited to annular-array imaging.
Linear arrays, though, still represent the ultimate trend for HFU because they do not need to be mechanically scanned and can be electronically focused throughout the field of view. The development of HFU linear arrays has been a slow, gradual process requiring new advances in piezoelectric ma- terials and fabrication processes. The National Institutes of Health (NIH)-sponsored Transducer Resource Center at the University of Southern California (USC) (http://www.usc. edu/dept/biomed/UTRC/) and Pennsylvania State Universi- ty has played an important role in exploring new transducer materials, developing probes, and building systems to test new imaging applications. On the commercial side, FUJI- FILM VisualSonics (Toronto, ON, Canada) has refined an industrial process to make HFU linear arrays, has been sell- ing a small-animal research system for nearly 10 years and, just recently, has released a system that is approved by the US Food and Drug Administration (FDA) for human use. In addition, companies such as Vermon (Tours, France) and Kolo Medical (San Jose, CA) now offer linear arrays at center frequencies up to 30 MHz.
Piezoelectric Materials
The key to any ultrasound transducer is the piezoelectric material from which it is made. The difficulty with HFU transducers is that the material thickness and element di- mensions are inversely proportional to frequency. A typical linear array operating at 40 MHz would have a thickness of 50 μm and a center-to-center element spacing of 40 μm, whereas a 5-MHz transducer would have a 400 μm thick- ness and a pitch of 240 μm. The high-frequency transducer clearly requires more sophisticated fabrication to achieve the necessary small-length scale.
Materials such as lead zirconate titanate (PZT) have been used for many years for ultrasound sources in the low- megahertz frequency range and work very well. However,
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