Resolution estimation and bias reduction in acoustic radiation force impulse imaging

Pathological conditions give rise to mechanical changes in tissue that can be exploited for the purpose of diagnosis and treatment of disease. Elasticity imaging is a field developed to creating images of tissue stiffness by mechanically exciting tissue and tracking the tissue response. Acoustic Radiation Force Impulse (ARFI) imaging is one such modality that measures the micron-scale displacements induced in tissue by local acoustic radiation forces using a high intensity ultrasound pulses generated by a standard diagnostic ultrasound scanner. Ultrasound pulses track displacements that are quantified using conventional correlation-based speckle-tracking methods. Generated displacement images can exhibit improved contrast of diseased tissue than conventional ultrasound techniques.;In this thesis, the spatial resolution limits of ARFI imaging have been measured using novel simulation and experimental techniques. The full-width, half-maximum (FWHM) of the point-spread function (PSF), a measure of the resolution limit of an imaging system, was extracted by imaging a tissue-mimicking phantom composed of two bonded materials. The ARFI image of the material interface was an estimate of the step response of the system. The ARFI imaging resolution limit was further explored using FEM/acoustic field simulations and linear shift invariant (LSI) models. The ARFI imaging resolution limit was submillimeter, but was highly dependent on imaging parameters. ARFI axial resolution was limited by the correlation window length and tracking pulse parameters. When the correlation window length was less than 1 mm, FEM and LSI models suggest the mechanical response of the tissue influences the resolution, resulting in a larger FWHM than would be predicted by imaging and signal processing parameters alone. ARFI lateral resolution limit corresponded to the lateral two-way beamwidth of the tracking beam. Measuring ARFI imaging resolution capabilities on small phantom inclusions and tissue ablation lesions proved the validity of the step-response based estimated resolution limits on objects of relevant, circular geometry. ARFI imaging resolution was again primarily a function of imaging and signal processing parameters, in good agreement with modulus step phantom derived results. To improve the ability of ARFI imaging to resolve targets near bright boundaries, a method called envelope weighted normalization (EWN) was developed to reduce amplitude modulation of ultrasound signals, thereby reducing displacement estimation bias.