High-Frequency Ultrasonic Transducers and Arrays

Ultrasonic imaging is one of the most important and still growing diagnostic tools today. Ultrasound is more appealing as a clinical imaging modality compared with such modalities as magnetic resonance imaging (MRI), nuclear imaging, and x-ray computed tomography (CT) in that it is more cost-effective, noninvasive, capable of real-time operation, and portable while providing images of comparable quality and resolution. State-of-the-art ultrasonic scanners offer real-time gray scale images of anatomical detail with millimeter spatial resolution, which are superimposed on a map of Doppler blood flow, displaying the information in color thereby (Shung 2006). Clinical applications of these devices are still expanding, and the operating frequencies of these devices seem to inch higher and higher. High-frequency (HF) imaging (higher than 20MHz) yields improved spatial resolution at the expense of a shallower depth of penetration. There are a number of clinical problems that may benefit from high-frequency ultrasonic imaging (Lockwood et al. 1996). Intravascular imaging with probes mounted on catheter tips at frequencies higher than 20MHz with the highest frequency being 60MHz has been used to characterize atherosclerotic plaque and to guide stent placement and angioplasty procedures (Saijo and van der Steen 2003). Endoscopic imaging with probes mounted on the tip of an endoscope or a catheter at frequencies from 10 to 20MHz has been proven clinically beneficial in diagnosing esophageal, gastrointestinal, and urinary lesions (Liu and Goldberg 1995). Medical efficacy of ultrasonic imaging of anterior segments of the eye at frequencies higher than 50MHz in diagnosing glaucoma and ocular tumors and in assisting refractive surgery has been demonstrated (Pavlin and Foster 1995). The availability of a noninvasive imaging tool for dermatological applications could reduce the number of biopsies that are associated with patient discomfort and could better demarcate tumor involvement. An additional benefit is that the results are known immediately or shortly after the examination unlike biopsy where a substantial time lag is likely to occur between the examination time and the report of histological analysis. Small-animal imaging is another frontier of HF ultrasound. Small-animal imaging is of intense interest recently because of the utilization of small animals in drug and gene therapy research.MicroMR, microCT, and microPET have all been developed to meet this need. Ultrasound thus far has only played a very limited role.