Orthogonal Plane-Wave Transmit-Receive Isotropic-Focusing Micro-Ultrasound (OPTIMUS) with Bias-Switchable Row-Column Arrays

TL;DR

This work tackles the challenge of isotropic high-quality 3D ultrasound imaging with row-column arrays by proposing OPTIMUS, a scheme that leverages bias-switchable TOBE arrays and Hadamard-encoded readout to achieve near-isotropic transmit-receive focusing over a large volume. The method decouples transmit and receive focusing: transmit patterns are varied across plane waves while receive data are encoded and decoded to form a virtual 2D aperture, enabling full spherical receive focusing. Simulations and phantom/ex-vivo experiments demonstrate superior resolution and contrast (gCNR) for OPTIMUS compared with HERCULES, VLS, and TPW, including imaging beyond the shadow of the aperture, albeit at the cost of a high number of transmit-receive events. The primary limitation is the acquisition rate, which may be mitigated by motion compensation or partial decoding; nonetheless, OPTIMUS represents a viable route to rich structural volumetric information for static or slowly varying tissues, with potential applications in tumor-margin assessment and tissue characterization.

Abstract

High quality structural volumetric imaging is a challenging goal to achieve with modern ultrasound transducers. Matrix probes have limited fields of view and element counts, whereas row-column arrays (RCAs) provide insufficient focusing. In contrast, Top-Orthogonal-to-Bottom-Electrode (TOBE) arrays, also known as bias-switchable RCAs can enable isotropic focusing on par with ideal matrix probes, with a field of view surpassing conventional RCAs. Orthogonal Plane-Wave Transmit-Receive Isotropic-Focusing Micro-Ultrasound (OPTIMUS) is a novel imaging scheme that can use TOBE arrays to achieve nearly isotropic focusing throughout an expansive volume. This approach extends upon a similar volumetric imaging scheme, Hadamard Encoded Row Column Ultrasonic Expansive Scanning (HERCULES), that is even able to image beyond the shadow of the aperture, much like typical 2D matrix probes. We simulate a grid of scatterers to evaluate how the resolution varies across the volume, and validate these simulations experimentally using a commercial calibration phantom. Experimental measurements were done with a custom fabricated TOBE array, custom biasing electronics, and a research ultrasound system. Finally we performed ex-vivo imaging to assess our ability to discern structural tissue information.

Figures (6)

• Figure 1: Summary of the OPTIMUS imaging scheme for an arbitrarily sized array of N rows and N columns with N$\times$M transmit events. In a) we use the rows of a Hadamard matrix to determine the biasing and transmit polarities that are applied on the rows as shown in b), with each column of the matrix corresponding to a different row of the array. c) shows a render of the effective layout of the element electrodes on the transducer, and an effective transmit waveform. In d) we can see the effective bias pattern between each transmit event. For a set of N transmits the transmitted waveform will not change, but the received waveform will, and decoding will result in the receive aperture shown in e). f) We will repeat those N transmits for M different plane-wave angles and obtain the effective imaging aperture shown in g).
• Figure 2: Setup for experimental imaging. We image with a Vantage Ultrasound Research platform, controlled by a host computer that also coordinates with the Central Control Unit of custom biasing electronics. The Central Control Unit controls the high-voltage biasing cards ($\pi$Cards), while waiting on triggers from the Vantage Unit to determine when to change the biasing pattern. The DC signals from the $\pi$Cards are mixed with AC signals from the Vantage and coupled on a bias tee and delivered to the TOBE array. An unhoused array is also shown.
• Figure 3: Simulations of a grid of point scatters imaged with OPTIMUS. a-b) are maximum intensity projections over a 5 mm slice, centered around a single plane of points in the XZ, YZ & XY planes, respectively. c) is a volume render of the 3D grid of scatters.
• Figure 4: Experimental volume imaging of a commercial calibration phantom (CIRS ATS 539). a-d) show the center slice of the volumes, and e-h) show cut-outs. i) is a measurement of the gCNR of the largest cysts. The HERCULES, VLS & TPW volumes were created using 128 emissions, while the OPTIMUS volume was created using 1152 emissions.
• Figure 5: Experimental comparisons of OPTIMUS with standard RCA imaging methods using an equal number of emissions. All 3 volume were created using 1152 emissions. The gCNR of the largest cysts are plotted in d).