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R. H. Silverman, H. O. Lloyd, T. Raevsky, H. H. Kim, K. K. Shung, D. J. Coleman; Non-Linear Ultrasonic Imaging of the Eye With a Dual-Frequency Transducer. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4016.
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Tissue harmonic imaging is a valuable method for improving signal to noise and resolution in ultrasound imaging. It is based on non-linear propagation of the acoustic wave through the tissue medium, forming multiples of the emitted pulse frequency. Differential harmonic imaging is a recently developed method in which a single transducer is excited at two frequencies. Under these conditions, not only are multiples of these two frequencies formed but also their sums and differences. This technique has been reported to provide superior images, but is constrained by the limited bandwidth of a single transducer. This problem can be circumvented, however, by using a transducer with two independent elements of different frequencies.
The probe consisted of two lithium niobate piezoelectric elements, an outer ring 12-mm in outer diameter, and a central element of 5-mm diameter. The outer ring had a center frequency of 20 MHz and the inner ring a frequency of 40 MHz. The two elements shared a common focus, with a focal length of 35 mm. We conducted imaging experiments on the anterior and posterior segments of ex vivo pig eyes using the following modes: 20 MHz send and receive; 20 MHz send, 40 MHz receive; 20 MHz + 40 MHz send and receive, with excitation by normal and inverted monocycle at 20, 25 and 30 MHz. Post-processing was performed by digital bandpass, spectral parameterization and the pulse inversion technique.
Harmonic images generated by 20 MHz emission with receive by the central 40 MHz element showed great improvement in signal/noise and resolution compared to 20 MHz send and receive due to greater sensitivity for reception of harmonics. Spectra generated from combined excitation of both elements were complex, allowing generation of images by bandpass of various frequency ranges and by pulse inversion (which suppresses the linearly propagated fundamental).
The novel transducer design used in this study is ideal for tissue harmonic imaging with high frequency ultrasound, providing improved sensitivity and resolution compared to conventional ultrasound techniques. This is especially useful for high-resolution imaging of the retina and choroid, where conventional linear methods preclude use of higher frequencies due to two-way absorption. The differential harmonic technique offers further potential improvements. Characterization of transducer output is planned to allow clinical studies in the near future.
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