Design of a Volumetric Imaging Sequence Using a Vantage-256 Ultrasound Research Platform Multiplexed With a 1024-Element Fully Sampled Matrix Array

Abstract

Ultrasound imaging using a matrix array allows real-time multi-planar volumetric imaging. To enhance image quality, the matrix array should provide fast volumetric ultrasound imaging with spatially consistent focusing in the lateral and elevational directions. However, because of the significantly increased data size, dealing with massive and continuous data acquisition is a significant challenge. We have designed an imaging acquisition sequence that handles volumetric data efficiently using a single 256-channel Verasonics ultrasound research platform multiplexed with a 1024-element matrix array. The developed sequence has been applied for building an ultrasonic pupilometer. Our results demonstrate the capability of the developed approach for structural visualization of an ex vivo porcine eye and the temporal response of the modeled eye pupil with moving iris at the volume rate of 30 Hz. Our study provides a fundamental ground for researchers to establish their own volumetric ultrasound imaging platform and could stimulate the development of new volumetric ultrasound approaches and applications.

Fig. 1.

(a) Schematic of system configuration with RF data and control path. Solid arrows: RF data path, Dashed arrows: Control path; GUI: Graphical User Interface, VSX: Verasonics Sequence eXecution software - sequence loader program, DMA: Direct Memory Access, RF: Radio Frequency, HW: Hardware, Tx/Rcv: Transmit/Receive, UTA: Universal Transducer Adapter, HV-Mux: High-Voltage Multiplexer. (b) The map of the elements in a matrix array. Each element size is 275 μm × 275 μm and the element pitch in both lateral and elevational directions is 300 μm. Each bank is a group of 32×8 elements (256 elements) and a blank row for electrical wire connection is located between each bank. The aperture size of the matrix array is 9.3 mm × 10.2 mm.

Fig. 2.

The received raw radio-frequency data of (a) non-switching case (the same bank is used for transmitting and receiving) and (b) switching case (the different banks are used for transmitting and receiving). Switching noise can be observed in the panel (b) between 4 and 5 μs. Signal within the first 1 μs in both panels is transmitted wave.

Fig. 3.

(a) Location of the 25 virtual sources for diverging waves with a diverging angle of 30°. The origin ‘0’ is located at the center of the matrix array surface and the virtual sources are uniformly distributed in a 5×5 grid over the spherical surface with a radius of 19 mm from the origin. The negative sign on the z-axis represents the virtual space above the transducer surface. (b) The corresponding measured lateral resolution (X-axis) and elevation resolution (Y-axis) is 447 μm and 418 μm, respectively.

Fig. 4.

The relationship between pulse repetition interval (PRI), and the number of virtual sources (N) for a given volume rate (30 Hz). Black open dots at N=3², N=5, and N=7² represent the available number of virtual sources and a red dot represents the parameters (N=5², PRI=83.25 μs) selected for this study. More than 12.2 GB of system memory was required to store raw RF samples for our study (N = 5², imaging depth of 2 cm, the total number of acquired compounded volumes is 100).

Fig. 5.

The flow chart for the developed sequence for volumetric data acquisition. Two imaging modes are provided. Live image reconstruction & visualization is omitted in the data acquisition mode. Tx: Transmit, Rcv: Receive, S: the number of super-volumes, N: the number of virtual sources per volumes, M: the number of volumes in a super-volume. S×M: the number of total volumes, S×M×N: the number of total Tx/Rcv acquisition events.

Fig. 6.

The event timing diagram in the acquisition mode with parameters listed in Table 1. Tx: Transmit, DMA: Direct Memory Access

Fig. 7.

Multi-planar imaging of an ex vivo porcine eyeball. (a) The long axis image at azimuthal angle of 0°, (b) 45°, (c) 90°, (d) The image on the coronal plane reconstructed by using the maximum intensity projection. (e) The color-coded depth information superimposed to the coronal image (d). and (f) The 3D volume visualization.

Fig. 8.

The fast volumetric ultrasound imaging using in vitro stainless steel iris diaphragm. Top panels represent the maximum intensity projection of C-plane images and bottom panels represent the 3-D visualization at the same time point. All images are taken at the frame rate of 30 Hz and also available in the supplementary video 1.