September 28, 2006 — K. Kirk Shung, a professor of biomedical engineering and director of USC’s Ultrasonic Transducer Resource Center (UTRC), has been awarded a $5 million grant from the National Institutes of Health (NIH) to renew his center and to develop the next generation high frequency ultrasonic arrays and scanners.  The devices will greatly improve imaging of superficial structures, such as the cornea of the eye and subcutaneous layers of the skin.

The work at USC, which is a global leader in the development of high frequency (HF) ultrasound arrays, will build on current HF transducer imaging capabilities.  These linear arrays offer the highest frequency ranges possible today, ranging from 30 MegaHertz to 35 MegaHertz. 

“We have developed the only functional 30-35 MHz linear arrays and HF system with real-time imaging capabilities in the world,” Shung said, “but going to higher frequency is not a trivial matter. It makes the electronics trickier, as well as the design and fabrication of the probe.  So in the next five years, we want to concentrate on improving the electronics and introducing novel transducer materials and fabrication technologies.” 

Kirk Shung

USC’s Ultrasonic Transducer Resource Center is ideally suited for this type of work. It is the nation’s only NIH resource center for ultrasonic technology.  These HF systems are being used for medical diagnostic procedures in opthamology, dermatology, prenatal care and small animal imaging.

Different Shapes and Sizes
Ultrasound transducers come in different shapes and sizes and are used to scan different parts of the body.  Pulsed ultrashort sound waves can be aimed at specific targets, such as lesions, the size of an apple seed in the breast or liver.  But as researchers attempt to go to higher frequencies, the depth of penetration will decrease.  Consequently, HF scanning cannot be used to image the brain or the liver because the depth of penetration is limited to about 1 to 2 centimeters below the surface of the skin.    
 
Current commercial systems rely on mechanical scanning of single element transducers with a fixed focus.  “That means that the spatial resolution is not uniform throughout the field of view and can cause distortion of the image,” Shung said.  “It also means that the frame rate is limited to less than 80 frames per second with a limited field of view.”

Several manufacturers, most notably Visualsonics in Toronto, Canada, offer systems capable of imaging in the 40 MHz range, but the costs per unit are exorbitant at about $200,000 per unit. 

A 35 MHz array, about as long as the tip of a pencil.

Shung’s research is aimed at not only increasing the frequencies but solving the problem of moving parts in mechanical scanners. So he is developing linear arrays that can sweep across a surface electronically imaging tissue at 50 MHz and a frame rate of 30 frames per second or higher.

How Does It Work?
The arrays operate on the same principle as a movie projector. The transducer of a real-time scanner typically contains more than 64 elements.  Elements are “piezoelectric” devices that are arranged in a row so that groups of elements each emit ultrasound beam sin rapid succession. The scanner sweeps more than 30 times a second over a section of the skin, the eye, or an animal’s chest, creating  images of more than 30 frames per second and a moving picture that can be seen in real-time.  These electronically controlled arrays are being upgraded and refined to fit different scanner designs, according to Jesse Yen, assistant professor of biomedical engineering, who is overseeing the electronics work.

Beam density and dynamic range control technologies are also in development for different scanner designs to optimize the images.  Jon Cannata, a research assistant professor of biomedical engineering heading up the transducer project, wants to increase the array’s resolution by increasing the active number of elements used to form an image line.  This would increase the field of view and improve the spatial resolution.  The arrays currently use a 16-channel beamformer, but Cannata hopes to increase the total number of elements to 256 and the total number of active channels to 64.  

Jon Cannata, who heads up the transducer team.

The introduction of some unique new array materials is another part of the project.  Up until now, the standard material has been PZT (lead zirconate titanate), which emits sound when it is electrically excited and converts received echoes into electrical signals.  Qifa Zhou, who leads the materials team, is experimenting with single crystal materials such as PMN-PT and other novel materials . These materials show promise because they are more efficient at converting the mechanical energy into electrical energy.

Prototype Array
A prototype high-frequency linear array and a real-time imaging system have already been developed, Shung said.  The array is about as long as the tip of a pencil and will be ready for commercialization following product development.  The array is easy to use and can image at 35 MHz.  

This ultra high-frequency system is most likely to find its first niche in small animal imaging and ophthalmology, both of which require high-resolution imaging of very small structures.

"There are a few players in the commercial eye scanner market right now eager for something better,” Shung said, “but I think we will be able to build a 50 MHz system in the next two or three years that offers much finer spatial resolution and image quality at a competitive price.”

A multi-institutional team of researchers is working with Shung on the NIH project, including Tom Shrout and Wenwu Cao, both materials scientists from Penn State University.  At USC, the team includes Brian Francis, an opthhalmologist, and David Peng, a dermatologist,  E.S. Kim, a professor of electrical engineering, Ruibin Liu, a research associate, Antton Hu, a biomedical engineer, and Jay Williams, a senior technician.