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R. H. Silverman, S. Babar, H. H. Kim, J. M. Cannata, K. K. Shung, D. J. Coleman; Tissue-Harmonic Ultrasound Imaging With a Dual-Frequency Annular Transducer. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4947.
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Ultrasound tissue harmonic imaging (THI) provides improved resolution and visualization of tissue boundaries. This technique is based on non-linear propagation of ultrasound waves which results in generation of harmonics at multiples of the emitted frequency. A difficulty associated with this method involves the limited bandwidth of the emitting transducer, which results in reduced sensitivity in detection of harmonic band echoes. Our aim was to fabricate and test a transducer suitable for high-resolution ocular imaging consisting of two annular elements, one to transmit a 20 MHz fundamental and the other to receive the 40 MHz harmonic.
The transducer consisted of two elements, an outer 20 MHz annulus (10 mm outer diameter) and an inner 40 MHz element (5.4 mm diameter). The transducer elements were composed of lithium-niobate press-focused to a common 30-mm focal length. During operation, the outer ring transmitted while the inner element received. Radiofrequency data were digitized at a 250 MHz sample rate (12-bits/sample). We evaluated transducer performance by imaging tissue-mimicking phantoms, wire targets, ex vivo bovine eyes, and rabbit eyes in vivo. Finally, the probe was tested clinically.
The center frequencies of the outer and inner elements were found to be 18 MHz and 35 MHz, respectively, with approximately 60% bandwidth. Needle hydrophone measurements showed higher sidelobes present in the sound field generated by the 20 MHz annulus compared to that of a single-element transducer of comparable focus and frequency. THI, however, reduces side-lobes, and this effect was observed to more than compensate for this anticipated consequence of the central hole in the transmit element in images of wire targets. Images of bovine anterior segments, rabbit eyes and human posterior segment showed significant improvement in resolution compared to single-element 20 MHz probes.
TIS has become the imaging standard in non-ophthalmic ultrasound applications because of the improvement in resolution and signal-to-noise obtainable with this technique. We demonstrated that TIS can be performed at frequencies suitable for high-resolution imaging of the eye, and that use of a confocal dual-element probe can result in enhanced detection of the harmonic, with consequent improvement in obtainable resolution and sensitivity.
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