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.
Three-dimensional (3D) ultrasound is an emerging and attractive technique in the medical imaging community because it can provide full-volume views of a region of interest while being portable, safe, and capable of real-time imaging. Volume imaging with ultrasound has various medical applications in obstetrics, musculoskeletal, and cardiovascular imaging to name a few examples [1]. However, 3D ultrasound imaging usually requires a fully populated 2D matrix array. Such arrays have a prohibitively high number of array elements, e.g. 1024 elements in a 32x32 system. With an array pitch typically around one wavelength or less to prevent grating lobes during beam steering, the field of view (FOV) of such probes can be severely limited. Also, most ultrasound scanners are not equipped with enough input channels to receive data from such a high element count, so multiplexers are often used to switch between sections of the array on different transmit/receive events [2]. However, in this case, frame rates are limited by the need for multiple transmit/receive events to acquire all aperture data. With a 4x1 multiplexer, anywhere between 4 -16 transmit events would be needed to acquire, for example, a single plane-wave angle [3].
To handle higher element counts, so called “microbeamformers” have been integrated into the handles of the transducers [4,5]. With microbeamforming, the delay-and-sum (DAS) operation is partially performed on ASICs inside the transducer handle on patches of elements. Then, only one wire per patch needs to go back to the scanner for the remaining time delay compensation. With this approach, probes have been designed with over 9,000 active elements [6]. However, the micro-beamformers introduce deviations from ideal time delays due to static focusing and quantization [7]. Micro-beamformed patches often have a fixed focal depth to simplify the hardware, which is not always aligned with the dynamic receive focus when beamforming across patches [7]. Delay quantization is also introduced by the micro-beamformer architecture, which might use a sample-and-hold method that can only delay signals by discrete time intervals determined by an input clock [8]. These quantizations can reduce the resolution and contrast of high-frame rate images, especially for larger micro-beamformed patches [9].
Other approaches allow the FOV to extend beyond the footprint of the array. These approaches include convex and phased arrays [10], as well as panoramas (also called extended field-of-view or EFOV) [11]. With convex and phased arrays, the field of view can extend beyond the probe footprint due to array curvature or beam steering, respectively. However, with this approach, maintaining resolution over the FOV is a challenge because scan lines become more spread out with depth. With panoramas, the field of view is extended by translating the probe to multiple locations, acquiring an image at each location, then applying some image registration method to align the different views [12,13]. The issue with panoramas is they can only acquire static images, never real-time video data, because of the need to translate the probe to multiple locations.
A few alternative approaches to array design allow for larger footprints, and thus larger FOVs, with a reduced element count. Extensive work has been done to explore sparse arrays as an option for reducing the element count of 2D ultrasound arrays [14]. With sparsity, element spacing is allowed to increase far beyond the usual 1/2 -1 wavelength in some kind of optimized pattern. The most serious drawback of sparse arrays is that they have reduced transmit power and SNR because of the low element count and small elements. Larger elements have been used in sparse, periodic arrays used for ultrasound localization microscopy (ULM) [15,16,17]. With very large elements, the directivity of elements raises the minimum F-number of the array, reducing resolution. For ULM, researchers overcame this issue by placing a diverging acoustic lens over each element to widen the directivity [17]. A wider directivity will introduce grating lobes into the point-spread function (PSF) of the imaging system when the array pitch is greater than one wavelength. The lens approach is successful with ULM because ULM displays the tracks of moving microbubbles over many thousands of frames, and the microbubble localization and tracking algorithms are able to ignore grating lobes. However, for B-mode imaging, grating lobes create artifacts that can obscure other image details [18,19], meaning a lens which allows grating lobes to appear would not be optimal.
Another example of a reduced element count array design is a row-column addressed (RCA) array [20]. These are fully populated arrays where instead of addressing individual elements, entire rows or entire columns are accessed at once. This effectively creates two orthogonal arrays of long, line elements, reducing the element count from N x N to N + N. Howe
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