Harmonic Source Wavefront Correction For Ultrasound Imaging

Abstract

<p>Aberration is a correctable phenomenon that degrades diagnostic quality in a significant number of ultrasound images. Previous aberration correction studies have focused on development of aberration estimation algorithms or on aberration reduction by using harmonic imaging. In the past, a major drawback of aberration estimation algorithms has been the assumptions required about the imaging target, assumptions that can limit clinical application where correction for multiple locations within a scan may be required. Harmonic imaging attempts to reduce the effect of aberration, without making assumptions about the imaging target, by using a lower-frequency transmit beam that is less prone to aberration. However, harmonic imaging does not correct for any aberration that may remain. This research presents a harmonic source wavefront correction technique allowing the generation of a point-like acoustic source at user-specifiable locations for use in aberration estimation and correction. Harmonic source wavefront correction utilizes the reduced aberration of harmonic imaging to create a known acoustical source to satisfy the assumptions of the aberration estimation algorithms, thus improving their clinical application.</p> <p>Generation of a point-like acoustical source in the presence of aberration is demonstrated using both spatially correlated and spatially uncorrelated electronic aberrators varying in strength from 0.25&pi; radians to 1.16&pi; radians. Beam properties of the 2.08 MHz fundamental, 4.16 MHz generated harmonic, and 4.17 MHz imaging beam were compared; in the presence of aberration, relative peak beam amplitude of the 4.16 MHz generated harmonic beam was up to 81% higher than the 4.17 MHz imaging beam, while -6 dB beam width indicated the 4.16 MHz generated harmonic beam was 88% narrower and more point-like than the 2.08 MHz fundamental beam.</p> <p>The practicability of harmonic source wavefront correction was demonstrated by correcting for spatially uncorrelated electronic aberrators in a water tank using a point target, specular reflector, and speckle region as correction targets. Harmonic source wavefront correction was paired with a cross-correlation algorithm to estimate corrective delays and was most effective in correcting peak amplitude of the 4.17 MHz imaging beam using a point target (up to 94% improvement), followed by use of a specular reflector (up to 83% improvement), followed by use of a speckle region (up to 47% improvement). Aberration correction is sensitive to signal-to-noise ratio (SNR), and correction utilizing the 2.08 MHz fundamental, which provided higher SNR, was more effective than correction utilizing the more point-like 4.16 MHz harmonic for the experimental setup used. A harmonic SNR of 14 dB was estimated as necessary for harmonic-based correction performance to equal or surpass fundamental-based correction, regardless of fundamental SNR.</p> <p>Finally, performance of harmonic source wavefront correction was quantified in a clinical setting. Correction of spatially correlated electronic aberrators was performed using both <italic>ex vivo</italic> porcine kidneys and the left kidneys of 11 human volunteers as correction targets. Correction utilizing porcine kidney resulted in 10 dB more improvement in peak beam amplitude than correction utilizing the left kidney of human volunteers. Body wall aberration present in the human volunteers was not accounted for during correction and was likely the cause of the disparity in correction performance. An average upper limit for body wall aberration for the human subjects was estimated at 65 ns (&plusmn;9 ns) RMS.</p>