3-D elasticity imaging with a 2-D array

Purpose: Assess the performance of various motion tracking strategies applied to a 3‐D RF echo data set from an oil‐in‐gelatin phantom with spherical targets for a multi‐step deformation totaling about 15% axial strain. Discuss the prospects and preliminary experience of in vivo motion tracking. Method and Materials: A prototype 9‐MHz 2‐D CMUT array connected to a Siemens SONOLINE Antares was used to acquire RF echo data from a 100‐mm × 100‐mm × 70‐mm oil‐in‐gelatin phantom containing a 10‐mm diameter spherical inclusion that has a 5:1 elastic contrast with the background. This CMUT array images like a 1‐D linear array in generating a 2‐D image in the azimuthal plane, and it acquires a 3‐D volume by electronically stepping the 2‐D imaging plane in the elevational direction. A series of controlled compressions of 1.5–2% axial strain were applied. Phantom motion was tracked with off‐line data processing using different approaches including 2‐D, 2.5‐D and 3‐D axial guidance tracking. The method that performed the best was applied to a 3‐D in vivo data set obtained with the same transducer. Results: The contrast to noise ratios (CNR) and the cross correlation between the motion‐compensated RF and the reference RF for the four motion tracking approaches were used as metrics of performance. The CNR increased with increasingly sophisticated motion tracking with 3‐D axial guidance performing the best. The same trend was observed with the normalized cross correlation. Tracking in vivo data has proved more difficult. Conclusions: These results demonstrate the improvement in motion tracking available through 3‐D tracking. This work also demonstrates that volume data acquisition allows accurate motion tracking and axial strain image formation for an entire target (within the field of view). Volume data acquisition with 2‐D arrays will provide a major advancement in the capabilities of elasticity imaging systems. © 2007, American Association of Physicists in Medicine. All rights reserved.