4J-5 A 3D Elastography System Based on the Concept of Ultrasound-Computed Tomography for In Vivo Breast Examination

Elastography holds great promises for the additional characterization of lesions especially in the domain of breast cancer diagnosis. Most ultrasound based approaches have so far been limited to a one dimensional (1D) or at most two dimensional (2D) displacement estimation in one plane. This leads for the general case to sparse data which cannot be used to solve the full three dimensional (3D) wave equation in an unbiased manner. For instance contributions from the compressional wave cannot be removed via application of the curl operator. In order to overcome this limitation we developed an ultrasound based elastography system which uses the concept of computed tomography for data acquisition in combination with 2D vector displacement estimation within the plane of the ultrasound beam. The vector displacement estimation is achieved using the concept of adaptive subapertures during the receive beamforming process. The object of interest is scanned using a conventional ultrasonic probe (4 MHz, 128 elements) from different directions on a circular orbit. The transducer is translated perpendicular to the orbit (~10 times) for each angle which leads to several block datasets (~30 blocks) each containing 2D displacement information. Thereby, the displacement of each voxel within the object is measured several times from different directions. This provides high resolution volumic 3D displacement fields after regridding each dataset from polar to Cartesian coordinates. The data acquisition system is contained within a water tank underneath a standard breast biopsy table. This enables in vivo measurements with the patient in prone position. Thereby, the 3D acquisition as already developed in the area of magnetic resonance elastography (MRE), is brought to the ultrasonic field. Initial phantom experiments were conducted with steady state mechanical excitation at 150 Hz. Inclusions are clearly visible in the complex shear modulus as reconstructed from inverting the full 3D wave equation. Taking benefit of the ultrafast acquisition speed of our ultrasound system, the proposed method allows to measure volumic datasets within clinically acceptable time. The method provides for each voxel of the 3D volume the frequency dependence of the complex shear modulus which in turn is linked to the underlying rheology of the material. This represents the proof of concept for a spectroscopic approach of elastography suitable for clinical application. The system enables the study of rheological properties of tumors which should further extend the diagnostic gain of elastography