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Refraction and time of flight corrections in 3D ultrasound computed tomography

Previous research showed that 2D refraction and time of flight (TOF) corrections in full angle spatial compounding (FASC) can improve image quality significantly. In this paper we focus on three dimensional beamline corrections (i.e. TOF and refraction) in 3D FASC. A tissue mimicking phantom was imaged using a conventional ultrasound scanner and a custom made mechanical applicator. The phantom contains a reservoir, filled with an NaCl solution to realize an inhomogeneous distribution of speed of sound (SOS). Two wires are placed inside the reservoir to verify the system's ability to image 3D structures with sub-wavelength dimensions. Pulse-echo-data was recorded fully around the object and in multiple cross-sectional planes. Via a reflector the SOS distribution in the imaged volume was reconstructed with the algebraic reconstruction technique (ART). In a second compounding process ultrasound beamlines were corrected in 3D for refraction at the surface and TOF inside the phantom using the reconstructed SOS distribution. Finally, corrected volume data sets from all viewing angles were superimposed to form a new FASC volume data set, corrected for refraction and TOF. With this method, spatial courses of thin structures can be imaged in 3D, with suppression of double line artifacts and correction of geometrical distortions.