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ing the organ of interest from outside the body. Or, to put it another way, HFU imaging is not the solution for all clinical imaging applications.
Although the methods required to instrument an ultra- sound system and to fabricate an ultrasound transducer are well established for commercial clinical ultrasound sys- tems, translating these methods to HFU is no simple task. The technology to build such systems pushed the limits in terms of piezoelectric transducer technology and digital components. HFU system complexity scales with frequen- cy; increasing frequency means decreased dimensions for piezoelectric elements, increased bandwidth for the digital sampling components, increased quantity of digital data to process, and increased computational demands. These tech- nological challenges are one reason HFU is not widely avail- able in the clinic today. Another reason is the limited market forces driving development because clinical applications of HFU are still quite specialized.
For these reasons, HFU technology generally lagged behind what would be considered standard clinical ultrasound tech- nology. By lagged, we mean that clinical applications of HFU were primarily based on mechanically scanned, single-ele- ment transducers rather than array transducers. The main clinical applications of HFU have been in ophthalmology and intravascular ultrasound (IVUS). HFU has also been extensively employed for preclinical, small-animal imaging. In fact, mouse mothers have been able to see their babies as ultrasound images for many years just as we humans have.
Equipment
Transducers
Ultrasound imaging probes come in many forms, with the simplest breakdown being between mechanically scanned, focused single-element probes and multielement transducer arrays. Single-element transducers offer simplicity and a well-defined sound field, whereas arrays have more complex sound fields and more complicated instrumentation.
Single-element transducers are still the most common transducers found in HFU instrumentation. There are sev- eral reasons that single-element transducers have persisted. First, HFU applications do not require very large lateral- imaging dimensions, so mechanical scanning is a practical way to form an image. Second, the methods and materials to make HFU transducers over 20 MHz were initially geared toward single-element transducers because this was the easi- est way to evaluate material performance. Third, single-ele-
Figure 1. Schematic of the front (anterior) and back (posterior) of the eye. Reprinted with permission from the National Eye Institute, National Institutes of Health.
ment transducers, although not very good for general imag- ing given their fixed focal length, are well suited to imaging specific regions of organs such as the front or back of the eye (Figure 1). Finally, HFU systems have to be reasonably priced, and mechanically scanned single-element approach- es were the only means to accomplish this.
Single-element transducers are either focused or unfocused. Ophthalmic and small-animal systems use focused trans- ducers. The −6 dB lateral beamwidth of a focused single- element transducer is F*λ, where F represents the transducer focal ratio (focal length divided by transducer diameter [F#]) and λ is the wavelength of sound in water. The depth of field, or range over which the image is in focus, is described by 9.68λF2. A compromise must be made between resolution and the depth of field. A small F# provides the best lateral resolution but only over a limited depth range. This works out fine in the eye, for instance, where HFU is used for imag- ing the anterior segment and a 3-mm depth of field, which would be provided by a typical 50-MHz F# = 3 probe, is suf- ficient to capture the structures of interest (Figure 2a). How- ever, a transducer designed to image the anterior segment is useless for imaging the retina and optic nerve at the back of the eye (Figure 2b).
Unfocused single-element transducers are not ideal for im- aging but are used in catheter-based IVUS probes. These probes need to be very small in order to be inserted into the vasculature. Placing electronics at the tip of a probe with a diameter of a few millimeters, let alone a multielement array transducer, is technically challenging. A simple solution is to use a small rectangular piece of piezoelectric material with
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