In the current design, a piezoelectric transducer - mounted in a probe that is inserted through the patient's rectum - generates a high-intensity beam of ultrasound. As the sound waves propagate through the body, some of the energy is absorbed and converted into heat, which destroys the tissue. The simple, hemispherical transducer now in use has a fixed focus, like a mirror in a telescope, Frizzell said. The probe is mounted on a rod that can be rotated from side to side and moved back and forth. To penetrate the prostate at different depths, however, the probe must be removed and the transducer replaced, which is both inconvenient and time-consuming.
"What's needed is a more versatile, phased-array transducer system," Frizzell said. "By splitting the transducer into separate elements, and then adjusting the timing of signals applied to those elements, the focus of the array can be moved electronically, without the need of mechanical scanning." To maximize the performance of such an array, Frizzell and graduate student Joseph Tan performed computational studies of various transducer configurations, including a truncated spherical annular array, a truncated spherical shell linear array and a "flat" cylindrical array.
"The spherical arrays performed well when electronically steered along the array axis, but they performed poorly when steered laterally along the length of the prostate," Frizzell said. "The flat cylindrical array performed much better along the length of the prostate, but poorly along the axis." However, a compromise design - a curved cylindrical array - showed acceptable performance for steering both in depth and along the length of the prostate. While this design would permit more flexible electronic scanning of the prostate, the array would be extremely difficult to fabricate.
"At present, a prototype of our best spherical, annular phased-array has been built and is undergoing tests," Frizzell said. "Although you still have to rotate the probe and slide it back and forth, you don't have to remove the transducer to change the focus. That's definitely a step in the right direction."
Collaborators on the project include Narendra Sanghvi, Ralf Seip and Jimmy Wu at Focus Surgery and Jeffrey Kouzmanoff at Labthermics Technologies in Champaign. The researchers describe their phased-array transducers in the June issue of the Journal of the Acoustical Society of America.