Abstract
Abstract: :
Purpose: Non–linear effects on propagating ultrasound pulses generate harmonics at frequencies that are multiples of the emitted fundamental frequency. By suppressing the fundamental and generating images based on the harmonic, this effect has been found to provide improved lateral resolution, improved signal–to–noise, and improved delineation of tissue structures. Furthermore, high–frequency harmonics need only travel one–way (e.g., from the retina to the transducer) rather than from the transducer to the tissue and back. This allows use of higher frequencies than would otherwise be possible. Methods: We measured the spectral properties of broadband 10 and 20 MHz transducers (35 mm focal length) using a needle hydrophone placed 1 mm from the face of the transducer and by recording reflections from a glass plate in the focal plane (70 mm path). The experiment was performed using both water and bovine vitreous as propagation media. We acquired scan data (immersion method) of human eyes both at the fundamental and the harmonic by insertion of an appropriate analog filter. We also generated harmonic images by digital filtering of full–bandwidth scan data. Results: A harmonic for the 10 MHz transducer appeared at a frequency of 22 MHz. The harmonic increased in amplitude as excitation energy was increased, reaching a maximum of 15 dB above baseline. Similar experiments with a broadband 20 MHz transducer showed a harmonic centered at approximately 30 MHz. Results obtained with water and vitreous as propagation media were virtually identical. We were able to readily generate images of the posterior segment at the 2nd harmonic either by analog or digital filtering. Conclusions: : Imaging of harmonics is feasible in ophthalmology. While tissue harmonic imaging has become commonplace in other specialties, where lower frequencies are utilized, this methodology has not been applied to ophthalmic imaging. In addition to providing improvements in resolution, the use of harmonics might be particularly useful to visualization of bloodflow, since scattering by blood cells increases relative to solid tissues as frequency is increased. The use of more sophisticated hardware and signal processing strategies (e.g., increased bit–depth, pulse–inversion) may allow further improvements over the current results.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)