Super-resolution ultrasound imaging using a phase-coherent MUSIC method with compensation for the phase response of transducer elements.

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Time-reversal with multiple signal classification (TR-MUSIC) is an imaging method for locating point-like targets beyond the classic resolution limit. In the presence of noise, however, the super-resolution capability of TR-MUSIC is diminished. Recently a new method, phase-coherent MUSIC (PC-MUSIC), was developed. This algorithm modifies TR-MUSIC to make use of phase information from multiple frequencies to reduce noise effects and preserve the super resolution. PC-MUSIC however, ignores the phase response of the transducer elements. In this paper, we account for the phase response of the transducer elements in the derivation of the PC-MUSIC algorithm. Unfortunately, the phase response of the transducer elements may not be known beforehand. We develop an experimental method to estimate this response using measured signals scattered from a glass microsphere embedded in a tissue-mimicking phantom with a homogeneous background medium of a known sound speed. We use numerical simulations to illustrate that the maximum resolution achieved with PC-MUSIC is limited by the transducer bandwidth and the signal-to-noise ratio. We perform experiments on tissue-mimicking phantoms and compare images obtained with different imaging modalities, including X-ray mammography, synthetic-aperture ultrasound imaging, TR-MUSIC, and PC-MUSIC. We demonstrate the significantly improved resolving power of PC-MUSIC.