Abstract
Purpose: :
Ultrasound biomicroscopy (UBM) allows imaging of anterior segment structures with a resolution on the order of 30 microns. The low focal ratio of UBM transducers, however, results in a very limited depth of field. Our aim was to demonstrate a high–frequency annular array for quasi real–time ocular imaging that permits an increased depth of field versus a similarly dimensioned single element transducer.
Methods: :
A spherically curved high–frequency annular array, polyvinylidene difluoride (PVDF) transducer with a copper–clad polyimide backing layer was fabricated. The array had five rings of equal area, a total aperture of 6 mm and a geometric focus of 12 mm. The nominal center frequency of all array elements was 40 MHz. An experimental system was designed that permits array elements to be pulsed individually and echo data digitized simultaneously from all five array elements. Using this technique, digitized radio–frequency data were acquired for all 25 transmit/receive annuli combinations. This data was then post–processed using a synthetic focusing algorithm. A final B–mode composite image was generated from 41 focal zones (170 microns/zone). Transducer operation was tested by scanning a wire phantom consisting of 25 micron wires spaced diagonally at 1–mm increments laterally and 1–mm increments in depth. Imaging capabilities of the annular array were demonstrated in ex vivo bovine, in vivo rabbit and human cadaver eyes.
Results: :
The wire phantom scans verified the operation of the array and demonstrated a 6.0 mm depth of field as compared to the 1.0 mm depth of field of a conventional single element transducer (6–mm aperture, 12–mm focal length). B–mode images of ex vivo bovine, in vivo rabbit and human cadaver eyes before and after synthetic focusing showed the enhanced image resolution obtained with the annular array.
Conclusions: :
An annular array for ocular imaging has been demonstrated. The increased depth of field provided by this technology offers greater axial sensitivity and better lateral resolution compared to conventional single element fixed focus transducers. This technology represents an avenue towards improved diagnostic imaging of the anterior segment.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • anterior segment