Increasing ultrasound field-of-view with reduced element count arrays containing large elements

TL;DR

The paper addresses the limited field-of-view in ultrasound by increasing the effective element width through coupling to mimic larger elements, while keeping element counts manageable. It analyzes how larger elements alter beam patterns via Fourier-based theory and evaluates non-linear beamformers NSI, SCF, and MV to restore resolution and suppress grating lobes. An experimental demonstration uses plane-wave transmissions to form a virtual 120 mm aperture with 128 elements, showing that MV beamforming achieves the best resolution with acceptable speckle behavior, albeit with higher computational cost. The work suggests a viable path to real-time, wide-FOV 2D ultrasound using large-square elements and advanced beamformers, outlining trade-offs in frame rate, contrast, and superficial image quality.

Abstract

Several applications of medical ultrasound can benefit from a larger imaging field of view (FOV). This study is aimed at increasing the FOV of linear array probes by increasing the element size rather than the element count. To investigate larger FOV, this study used coupled elements to imitate a larger element size. The effects of coupling on array beam patterns are examined with Fourier transforms of elements. The effects of coupling on resolution, contrast, and speckle signal-to-noise ratio are examined through phantom images and in-vivo images of a rabbit tumor reconstructed with plane-wave compounding. Furthermore, a positioning system was used to acquire data from a virtual large aperture with 120 mm FOV and 128 elements, collected in sections with a single probe. This study also investigates the Null Subtraction Imaging (NSI), Sign Coherence Factor (SCF), and Minimum Variance (MV) beamformers for regaining resolution lost by an increased F-number with large elements. The MV beamformer, while the most computationally expensive, was best for improving resolution without increasing speckle variance, decreasing Full-Width at Half-Max (FWHM) estimates of wire targets from 0.78 mm with DAS on a 2.5 wavelength element size to 0.54 mm with MV on a 5 wavelength element size.

Figures (12)

• Figure 1: Beam patterns for 16-element arrays with a pitch of 2.5 wavelengths and different kerfs. A kerf of 0 cancels all grating lobes, but it is not an array (blue). A kerf that is half the pitch cancels the second grating lobe but leaves very high first grating lobes (yellow). The minimum kerf that can be manufactured is optimal to minimize all grating lobes (orange).
• Figure 2: Array beam patterns for different element widths. The larger element size/narrower directivity raises the array F-number, resulting in a wider main lobe for the array beam pattern.
• Figure 3: Directivities of coupled elements (with kerf gaps) compared to directivities of corresponding large elements. (a) Coupled element by 2 (top) and large element (bottom) with corresponding directivities in (b). (c) Coupled element by 4 (top) and large element (bottom) with corresponding directivities in (d). The directivities of the coupled elements match closely to those of the large elements, meaning coupled elements are a good approximation of a large element.
• Figure 4: Example transmission profiles of uncoupled (blue), coupled by 2 (orange), and coupled by 4 (yellow) arrays for a positive plane-wave steering angle. Blocks of consecutive elements fire at the same time for coupling by 2 and 4, making the jagged steps in the transmit delay profile.
• Figure 5: Photograph of the imaging setup for acquiring data from a virtual large aperture. The probe was held in place while the phantom was moved on the sliding table beneath it.