All ophthalmic clinical imaging is currently performed with single-element, mechanically scanned transducers that have a limited depth of field and a resulting limited depth range where a good image can be obtained. For the eye, this means that a limited region of the front or back of the eye, but not both simultaneously, can be viewed in a high quality image. Annular-array technology developed over the last 7 years at Riverside Research by Dr. Jeffrey Ketterling provides a major improvement in image quality versus current ophthalmic imaging technology. With only five active elements, annular arrays can exceed the performance of more-sophisticated linear-array devices that have hundreds of elements. The reduced number of elements in the annular array ensures a level of simplicity that makes integration into existing probe configurations feasible within the cost-sensitive, ophthalmic imaging market. However, annular arrays must be mechanically scanned to form an image, and motorized units to accomplish this are difficult to obtain premade or to make from scratch. The current prototype at Riverside Research uses a bulky, heavy, and rigidly mounted linear actuator to accomplish scanning. A clinically feasible annular-array system requires the integration of a compact, hand-held, mechanically-scanned probe capable of achieving frame rates greater than 6 per second. Therefore, Riverside Research, in collaboration with a commercial partner, invested in an Independent Research and Development (IR&D) project to integrate a hand-held probe into the Riverside Research annular-array prototype in order to establish feasibility of a hand-held system that is largely based on available technology.
To integrate the hand-held probe into the Riverside Research prototype system, modifications were made to the annular-array transducer, system software, and system hardware. A hand-held probe was provided with access to evenly spaced spatial triggers that are used to acoustically excite a transducer as it scans back and forth. A custom annular array was assembled based on the size restrictions of the hand-held probe and the annular array was mounted to the tip of the probe. A custom circuit was assembled that generated a frame-gate signal that indicated the start of an image frame and to pass the spatial triggers when the Riverside Research imaging system signaled that it was ready to capture a new image. In addition, the control software of the prototype system was modified to bypass the normal motion subsystem and to signal the custom circuitry to pass spatial triggers at the appropriate time. Finally, additional methods of acoustic excitation and synthetic-focus image reconstruction were added in the real-time imaging component of the software. The system performance was then validated using a special high-frequency anechoic sphere phantom with 0.53 mm spheres.
A custom annular array was successfully mounted in the hand-held probe and five wires were passed out that connected to a five-channel pulser/receiver unit. Control of the hand-held probe via USB interface was confirmed and the operation of the custom frame-gate circuit was verified. Images were generated from the reference phantom using a 65 dB dynamic range. Examples are shown with full 5-transmit/5-receive synthetic focusing (Figure, left) and 3-transmit/5-receive synthetic focusing (Figure, right). In both cases, the system was able to maintian a frame rate of 2.5 fps which corresponded to the cyclic rate of the hand-held probe movement. These results establish the feasibility of merging the Riverside Research annular-array imaging approach with a commercial hand-held probe. Further development is necessary to mount the annular array in compact fashion with adequate electrical shielding and to reduce some instability in the spatial triggers. However, the main technical obtacles have been surmounted and an operational hand-held system was achieved.