Imaging the mechanical properties of tissue with ultrasound: An investigation of the response of soft tissue to acoustic radiation force

The mechanical characterization of tissues and lesions within tissues has been used by clinicians to determine states of disease. Several pathological states can lead to a change in the mechanical properties of diseased tissues versus healthy tissues: fibrous tissue deposition in breast lesions may allow for masses to be found by manual palpation, replacement of healthy hepatic tissue by fibrosis can lead to stiffening in a cirrhotic liver, and the purposeful destruction of tissue using radio-frequency (RF) ablation leads to the denaturing of proteins and an inflammatory response that stiffens the induced lesion. Additionally, cancers and desmoplastic masses in the colon, breast, and other soft tissues can potentially be distinguished from healthy tissue based on their mechanical properties.; The work presented in this dissertation investigates the thermal and mechanical response of soft tissue to a new ultrasonic method, called Acoustic Radiation Force Impulse (ARFI) imaging. ARFI imaging uses impulsive, high-intensity ultrasound pulses to generate acoustic radiation force and material-dependent displacement fields. Displacement magnitudes and dynamics are dependent on tissue stiffness and structure, which may differentiate healthy from diseased tissues.; In order to insure patient safety in clinical implementations of ARFI imaging, the thermal response of soft tissue to acoustic radiation force is investigated using Finite Element Method (FEM) models of tissue heating and thermocouple measurements in porcine muscle. The mechanical response of soft tissue to impulsive acoustic radiation force excitation is investigated using an FEM model of elastic materials with varying stiffnesses, densities, and acoustic attenuations with spherical inclusions of varying sizes and material properties. These mechanical responses are validated in gelatin and graphite-based tissue-mimicking phantoms, along with a commercial phantom. Finally, the impact of ultrasonically tracking the dynamic displacement fields is evaluated using the FEM models in conjunction with a linear acoustic field simulation package.; Thermal FEM models and thermocouple measurements indicate that ARFI imaging is safe to implement clinically, but the implementation of new probes and beam sequences requires additional evaluation. Mechanical studies have revealed that tissue dynamics in response to ARFI excitations are shear wave dominated, and the size and stiffness contrast of spherical inclusions dictates the dynamic response of imaging lesions. Using ultrasound to track the displacement fields introduces jitter and bias due to scatterer shearing immediately after excitation that can alter the interpretation of the dynamic displacement data.; These studies indicate that ARFI imaging is safe to perform in vivo and can be useful in characterizing material properties to differentiate healthy from diseased tissues.