Large elements and advanced beamformers for increased field of view in 2-D ultrasound matrix arrays

TL;DR

This work seeks to demonstrate an increased field-of-view using a reduced element count array design and demonstrates how larger matrix arrays could be constructed with larger elements, with resolution maintained by advanced beamformers.

Abstract

Three-dimensional (3D) ultrasound promises various medical applications for abdominal, obstetrics, and cardiovascular imaging. However, ultrasound matrix arrays have extremely high element counts limiting their field of view (FOV). This work seeks to demonstrate an increased field-of-view using a reduced element count array design. The approach is to increase the element size and use advanced beamformers to maintain image quality. The delay and sum (DAS), Null Subtraction Imaging (NSI), directional coherence factor (DCF), and Minimum Variance (MV) beamformers were compared. K-wave simulations of the 3D point-spread functions (PSF) of NSI, DCF, and MV display reduced side lobes and narrowed main lobes compared to DAS. Experiments were conducted using a multiplexed 1024-element matrix array on a Verasonics 256 system. Elements were electronically coupled to imitate a larger pitch and element size. Then, a virtual large aperture was created by using a positioning system to collect data in sections with the matrix array. High-quality images were obtained using a coupling factor of two, doubling the FOV while maintaining the same element count in the virtual large aperture as the original matrix array. The NSI beamformer demonstrated the best resolution performance in simulations and on the large aperture, maintaining the same resolution as uncoupled DAS for coupling factors up to 4. Our results demonstrate how larger matrix arrays could be constructed with larger elements, with resolution maintained by advanced beamformers.

Figures (11)

• Figure 1: The left column displays representations of (a) a block of coupled elements and (c) and a large element, with corresponding Fourier transforms representing their directivities in (b) and (d). (e) is the cross section of the 2D Fourier transforms at an elevation of 0. Note there are only small differences in the side lobes, meaning coupled elements are a good approximation of a large element, despite kerf gaps.
• Figure 2: Diagram of transmission angles used for simulations and virtual large aperture experiment. Fewer diagonal angles (i.e. those with both azimuth and elevation components) were included due to increased element width, and thus a narrowed directivity, on the diagonals.
• Figure 3: Example of element coupling for a coupling factor of four. Black dots represent the original element positions for the 32 x 32 array. Red boxes represent the larger elements made from coupling. Only coupling factors 1, 2 and 4 were used to divide 32x32 elements evenly and avoid coupling across panels.
• Figure 4: (a) Photograph of the positioning system setup with the general purpose phantom. (b) Diagram of the 4 quadrants making up the virtual large aperture. Each colored square represents one position of the Vermon probe, and its corresponding section of the virtual aperture.
• Figure 5: Max depth projection of simulated PSFs of data from an 8 x 8 matrix array with a pitch of 6.7 wavelengths and element size 6.5 wavelengths. (a) DAS, (b) NSI with DC = 0.5, (c) DCF, (d) MV with L = Q/2. (e) Lateral profile across elevation of 0 mm of each max projection. (f) Elevation profile across lateral position of 0 mm of each max projection. The red and yellow boxes in (a) illustrate the ROIs used for contrast estimates in Table 1. The lateral profiles were used for FWHM estimates.