Page 46 - 2017Spring
P. 46

 Jeffrey A. Ketterling
Lizzi Center for Biomedical Engineering Riverside Research Institute 156 William Street New York, New York 10038 USA
Ronald H. Silverman
Department of Ophthalmology Harkness Eye Institute Columbia University Medical Center 635 W. 165th St. New York, New York 10032 USA
Clinical and Preclinical Applications of High-Frequency Ultrasound
High-frequency ultrasound technology is set to emerge as a common clinical tool after decades of niche applications.
Ultrasound is widely used for medical imaging, and many of us saw our kids for the first time as gray-scale images or 3-dimensional (3-D) surface renderings at the doctor’s office. Although clinical ultrasound made its debut a half century ago, enormous advances in technology and capability have taken place since then. Clinical ultrasound machines are now extremely powerful digital signal-process- ing engines, and tasks that may once have been done as postprocessing in a re- search lab are now performed in real time while a patient stares at a screen with a sense of awe (if not comprehension).
Early ultrasound imaging systems formed an image by mechanically scanning a focused, single-element transducer at limited frame rates. (In the optical world, a single-element ultrasound transducer would be analogous to a microscope objec- tive that is raster scanned to generate an image.) These systems generally used frequencies of 1-5 MHz, which provided good penetration but not fine resolution. The trend over time has been replacement of mechanically scanned single-ele- ment transducers with arrays that operate at higher frequencies and that have an increasing number of independent transmit/receive channels. Analog technology has long been replaced by digital technology that now provides real-time 3-D and even 4-D (3-D over time) visualization. Digital signal-processing methods can ex- tract information from the ultrasound echoes (e.g., blood flow and tissue stiffness) that goes well beyond the simple log-compressed, gray-scale images that were the hallmark of early ultrasound machines.
The field of high-frequency ultrasound (HFU) represents the evolution of clini- cal ultrasound technology to the present time and generally refers to frequencies above 20 MHz. The term HFU was initially intended to indicate a research field that emerged over time as advances in transducer technology permitted ultra- sound imaging at frequencies beyond common, low-megahertz clinical frequen- cies. Twenty megahertz is really an arbitrary cutoff because there is nothing fun- damentally different about HFU imaging or instrumentation versus conventional lower frequencies. HFU instrumentation must be specialized, and clinical (or re- search) applications must similarly be specialized to take advantage of the superb resolution of HFU but the limited penetration into tissue.
HFU is interesting because increased frequency results in a shorter acoustic wave- length that, in turn, results in improved spatial resolution. (The acoustic wave- length in the body at 1 MHz is 1.5 mm and at 20 MHz, it is 0.075 mm.) Acoustic attenuation, however, also increases with frequency, which means that increasing the frequency decreases the penetration depth. Because clinical ultrasound sys- tems are generally intended for fairly deep imaging in the body (e.g., cardiac or fetal imaging), there is a limit to how high the frequency can go while still imag-
44 | Acoustics Today | Spring 2017
| volume 13, issue 1 ©2017 Acoustical Society of America. All rights reserved.

   44   45   46   47   48