Phase aberration correction using ultrasound radiation force and vibrometry optimization

Diagnostic ultrasound images suffer from degradation due to wave propagation through tissues with inhomogeneities in their speed of sound causing phase shifts of the propagating waves. These phase shifts serve to defocus the ultrasound beam, reducing spatial resolution and image contrast of the resulting image. Many methods have been proposed over the past two decades to address this problem. A phase aberration correction method that uses the dynamic ultrasound radiation force to harmonically excite an object using amplitude modulated continuous wave ultrasound is described. The phase of each element of an annular array transducer is adjusted to maximize the radiation force and obtain optimal focus of the ultrasound beam. The optimization of the radiation force is performed by monitoring the amplitude of the velocity of the object used as a target Theory is presented that models the optimization process. Simulation results show the ability to regain a focused field after a phase screen with an RMS time delay of 96.8 ns is applied. Experimental validation is shown for correcting phase aberration caused by an acrylic lens aberrator placed near the face of annular array transducer. A stainless steel sphere with diameter 1.59 mm was used as a target. The radiation force magnitude was improved by 11.8 dB and resolution of the sphere at -6 dB was improved from 3.05 mm to 1.68 mm which compares well with the un-aberrated resolution of 1.51 mm. An ultrasound beam defocused by phase aberration can be focused using dynamic radiation force and monitoring the velocity of scatterers in the focal region. This method has a well-behaved cost function which allows efficient correction. Simulation and experimental results show that the focus of the beam can qualitatively and quantitatively improved with this method.