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it was quickly apparent that standard low-megahertz PZT composites did not scale well to high frequency because the grain size of the crystals is relatively large (5-10 μm). This made the material more difficult to machine and also led to material properties changing in ways that decreased perfor- mance at high frequency. Therefore, a great effort was placed on developing new piezoelectric materials for high-frequen- cy applications (Zhou et al., 2011).
Initial efforts to make HFU transducers focused on single-el- ement, large-surface-area transducers because the technical requirements were not as difficult as those for array-based transducers. Piezopolymer films such as polyvinylidene flu- oride (PVDF) and polyvinylidene-trifluoroethylene (PVDF- TrFE) of 9-50 μm thickness proved particularly useful be- cause they could be pressed into spherical shapes for focused transducers and their acoustic impedance was close to that of water, which reduced transmission and reception losses (Sherar and Foster, 1989). However, these piezoelectric films do not have particularly good electromechanical coupling (about 0.15%, where 100% would be perfect coupling with no losses), require high voltage to excite (>200 V peak-to- peak), and are not well suited to linear arrays where a small element size is required because their electrical impedance becomes quite high.
Composite materials were also pursued and good success was found with lithium niobate and lead magnesium nio- bate-lead titanate (PMN-PT). These materials require acous- tic matching layers to better match the material impedance to water, but their much better electromechanical coupling (50%) helps improve overall performance. Piezocomposite materials at high frequency are usually ground down to cre- ate the necessary thickness and, thus, are planar. To create a single-element transducer with a focused geometry requires fracturing a thin disc of the piezocomposite by pressing the material against a rigid ball and then setting the material in a backing epoxy (Cannata et al., 2003). Reliable techniques were developed to do this, but the process is really only ef- fective for single-element transducers.
As single-element transducers began to enter clinical use, a great deal of effort was being applied to building HFU lin- ear arrays. In addition, new fabrication techniques needed to be developed, particularly for cutting a piezoelectric ma- terial into individual elements (dicing) and for connecting elements to cabling (interconnects). Numerous prototypes were developed, particularly at the Transducer Resource Center of USC. The transducers were validated with proto-
type ultrasound imaging systems, but the main focus was research. The big commercial breakthrough came in 2007 when VisualSonics introduced a preclinical HFU system with arrays covering a range of center frequencies from 15 to 50 MHz (Foster et al., 2009). The release of the Visual- Sonics system represented the transition of high-frequency material development and array fabrication from a research topic to a commercially viable product.
The basic instrumentation approach for HFU systems dif- fers little from the low-megahertz clinical counterparts. The significant differences arise from the technical requirements related to the higher bandwidth of HFU systems. Although HFU instrumentation may simply be an extension of cur- rent clinical imaging systems, the mass-produced electronic components specifically designed for ultrasound array im- aging only went up to sampling rates of about 40-50 MHz, which limited the maximum center frequency for transduc- er operation to about 15 MHz. Early HFU systems, there- fore, were custom-made using components not necessarily designed for ultrasound imaging, and these systems were quite labor intensive to build as one-off prototypes (Hu et al., 2006). Once a system was ready to test, it was often al- ready obsolete. As analog-to-digital sampling components have dropped in cost, increased in bit depth, and increased in maximum sampling rate, the “front end” of HFU systems has become easier to design and manufacture and the tech- nical challenges of building an ultrasound system are now mostly related to transducer design, beamforming, and sig- nal processing.
Ophthalmic Imaging
Human ophthalmic imaging was one of the earliest HFU applications (Pavlin et al., 1990, 1991). Ophthalmic ultra- sound has unique requirements in that it needs to operate at or above 10 MHz and needs to be inexpensive so that it is affordable for an ophthalmologist. Low-megahertz array- based systems were not suitable for the eye because their im- age quality was poor and they were too expensive to use in ophthalmic practice. Thus, ophthalmic machines have been based on single-element transducers with an assortment of probes designed to image specific regions of the eye (Figure 2).
Ophthalmic ultrasound is split between anterior-segment (35- to 50-MHz) and posterior-segment (10- to 20-MHz) imaging. (The 35- to 50-MHz frequency range is often re- ferred to as “ultrasound biomicroscopy” [UBM].) The rea-
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