Adaptive imaging in the thyroid using fundamental and harmonic echo data

The authors acquired normal (9 MHz) and harmonic (9 MHz) element r.f. data for a 3/spl times/80 element array from the thyroids of ten subjects. Harmonic imaging data was acquired using the phase inversion technique using 4.5 HMz fundamental waveforms. The normal and harmonic imaging data was acquired simultaneously from large regions of tissue (3 cm deep, 10 mm wide) using both F/2 and F/4 transmit apertures. The ability to acquire channel data over extended regions allows the authors to evaluate the lateral stability of arrival time error profiles, and to reconstruct B-mode images using different beamforming techniques. Regions of the thyroid were selected for analysis from the reconstructed B-mode images. Arrival time measurements were made using the speckle lookback algorithm, which makes arrival time estimates relative to the summed signal from a group of previously corrected elements. The authors found that measured arrival time errors were very sensitive to local scattering artifacts such as specular echoes and strong off axis reflectors, making arrival time estimates from non-specific regions of intereset unreliable. The authors present illustrative examples of "reliable" arrival time estimates (those that improved image brightness away from the correction algorithms region of interest) as well as examples of the artifactual echoes. For the "reliable" examples, arrival time errors were very small (15 ns r.m.s.) and resulted in small changes in image brightness (5-10 percent) with larger changes in image brightness (10 percent) in the correction region. Normal and harmonic arrival times profiles were very similar, and the F/2 and F/4 transmit aperture sizes also produced similar arrival time error profiles. The harmonic and normal data did not produce significantly different spatial decorrelation curves. The F/2 and F/4 transmit apertures produces significantly different spatial decorrelation functions in a phantom but not in the thyroid. The data suggest that phase aberration is very mild in the thyroid, and that harmonic imaging reduces the effects of off-axis scattering. Furthermore, care must be taken in selecting the region of interest used to make the arrival time error measurements.