| The primary hypothesis that this work was intended to test was that the difference in image quality between normal and harmonic images was related to the degree of phase aberration present in the imaged medium; we specifically postulated that increased phase aberration in the medium results in a higher quality harmonic image as compared to the normal image. This work used a unique 8 x 128 array (Tetrad Corp.) interfaced with the Siemens Elegra to collect coincident individual channel data for the normal and harmonic imaging modes. This array was used in order to obtain the higher quality aberration measurements achievable with elevation sampling of the aberrator. Two different clinically relevant comparisons of normal and harmonic imaging were performed: first, a comparison of normal and harmonic modes from a single transducer, with normal mode at 6.5MHz and harmonic mode with transmit at 3.8 MHz and receive at 7.2 IMHz; second, the emulation of two different transducers, with normal being transmit and receive at 8 MHz, and harmonic being transmit at 4 MHz and receive at 8 MHz. Phantom studies and clinical studies (primarily in vivo breast tissue) were performed. No statistically significant trend was found for either quantitative or qualitative measures of image quality with estimated aberrator strength. The complete lack of trend with any of the measures tested: mainlobe beamwidth, speckle brightness change, contrast, signal-to-noise ratio, and qualitative evaluation (forced-choice test by 3 clinicians and one ultrasound expert), as well as beamwidth spread and the first moment of the beam (results of which were not presented), indicate that while aberration certainly affects image quality, the degree of degradation in image quality may not be a direct function of aberrator strength even within a mode, as well as not being a predictor of relative image quality between normal and harmonic modes. |
