Tissue velocity and attenuation inhomogeneities reduce ultrasound image quality in many patients. Over the years a
number of methods have been developed to estimate the corrective delays necessary for phase aberration correction.
Past methods were based on assumptions of the target or required a separate transducer acting as a transponder point
source. A method is proposed which creates a known acoustical source in the tissue suitable for wavefront correction without a priori assumptions of the target or requiring a point source transponder. This method was tested with multiple
electronically produced aberrations with RMS focusing errors of 0.25π radians, 0.44π radians, and 0.87π radians at 4.17
MHz. These aberrators were corrected using excised pork kidneys and on the left kidney of human volunteers as targets. Waveform correction on pork kidney led to an improvement in imaging beam amplitude and side-lobe level. Waveform correction on human subjects for a 0.87π radians RMS error aberrator led to a 15.4 dB improvement in imaging beam amplitude and an 11.8 dB improvement in side-lobe level. This method shows promise of overcoming the limitations of previous phase correction methods.
It is demonstrated that distortion of a non-linearly generated first harmonic transmit beam due to a near-field aberrator is
reduced as transmit pressure is increased. The first harmonic transmit beam is then used as a source for correction of
aberration. In the first experiment, pieces of Lucite 11 mm and 24 mm thick were used as near-field aberrators. Beam
plots of the fundamental and first harmonic were measured in a water tank with and without the aberrators present at
multiple transmit voltages. The Lucite aberrator was then removed and an electronic aberrator with RMS delay error of
138 ns was applied to the transmit and receive apertures. The first harmonic reflected from the tip of a hydrophone was
measured, and correcting delays were determined using a multi-lag least-means-squares cross-correlation method.
Corrections were applied to an imaging beam transmitted at twice the frequency of the fundamental beam, the same
frequency as the generated first-harmonic. Results from the Lucite experiments showed a -6 dB beam width
improvement of 1.8 degrees when transmit voltage was increased from 20 volts to 80 volts. Results from first harmonic
based correction of the electronic aberrator resulted in significant improvement in beam width and showed an average
improvement of 16.8 dB in transmit beam signal level and 31.9 dB improvement in transmit-receive beam signal level.