Shear Wave Induced Phase Encoding Imaging with Enhanced Resolution

Conventional ultrasound backscatter imaging (B-scan) relies on geometric focusing of ultrasound echoes across an aperture. We have previously presented a new method, Shear Wave Induced Phase Encoding (SWIPE) imaging, in which lateral resolution of the target is obtained by using traveling shear waves to encode the lateral position of the scatters in the phase of the received echo. Here we extend the imaging model and demonstrate enhanced image resolution using ultrasound phase modulation harmonics generated by linear propagation of both shear and ultrasound waves. Using a Fourier series description of the phase modulated echo signal, we demonstrate that phase modulated signals at multiples of the shear wave frequency reveal target k-space data at multiples of the shear wavenumber. Data were acquired using a single element transducer, driven by a spike-excitation pulser-receiver. Shear waves were induced in a phantom using an electromagnetic shaker over a range of 100-1000Hz. Echo signals were recorded with a digital oscilloscope and processed off-line. Diffuse and wire targets were embedded in 5kPa shear modulus gelatin phantom. Images of embedded nylon wires were obtained at the fundamental and 2nd harmonic of the shear wave frequency. The expected doubling of resolution was observed. Phase distortion due to the frequency dependence of the shear wave source was identified and compensated.