Enhanced spatial resolution of dual-mode ultrasound arrays using coded excitation

Abstract

A prototype 64-element 1-MHz concave (ROC=100 mm) DMUA has been shown to produce conventional synthetic aperture (SA) images in a field of view extending by 6 cm axially and 4 cm laterally (around its geometric center). With an f♯=0.8 and a fractional bandwidth ≊40%, the lateral and axial resolutions of this prototype were 1.1 mm and 2.6 mm, respectively. We have investigated the use of coded excitation and pseudoinverse filtering for enhancing the axial resolution of the DMUA. SA images were formed from a 12 μs linear chirp transmit pulse covering the frequency band of 0.5–1.5 MHz. The beamformed data was compressed using matched and regularized pseudoinverse filtering with a colored noise model. Image data were collected from a 3 D quality assurance phantom with an egg-shaped contrast target using the DMUA. The contrast ratio (CR) was 3.35, 7.8, and 6.6 for conventional pulse-echo, matched filter compression and inverse filter compression, respectively. In addition, axial and lateral speckle cell size parameters were calculated for the three methods. The axial resolution was 2.6, 2.5, and 1.65 for conventional pulse-echo, matched filter and pseudoinverse filter, respectively. Thus one can see that approximately 33% improvement in resolution was achieved without significant loss in CR or change in lateral resolution.