Real-time H-scan ultrasound imaging using a Verasonics research scanner

Abstract

H-scan ultrasound (US) is a new imaging technique that relies on matching a model that describes US image formation to the mathematics of a class of Gaussian-weighted Hermite polynomials (GH). In short, H-scan US (where the 'H' denotes Hermite or hue) is a tissue classification technique that images the relative size of acoustic scatterers. Herein, we detail development of a real-time H-scan US imaging technology that was implemented on a programmable US research scanner (Vantage 256, Verasonics Inc, Kirkland, WA). This custom US imaging system has a dual display for real-time visualization of both the H-scan and B-scan US images. This MATLAB-based (Mathworks Inc, Natick, MA) system includes a graphical user interface (GUI) for controlling the entire US scan sequence including the raw radio frequency (RF) data acquisition parameters, image processing, variable control of a parallel set of convolution filters used to derive the H-scan US signal, and data (cine loop) save. The system-level structure used for software-based image reconstruction and display is detailed. Imaging studies were conducted using a series of homogeneous and heterogeneous tissue-mimicking phantom materials embedded with monodisperse spherical US scatterers of size 15-40 µm in diameter. Relative to H-scan US image measurements from a phantom with 15 µm-sized scatterers, data from phantoms with the 30 and 40 µm-sized scatterers exhibited mean intensity increases of 5.2% and 11.6%, respectively. Overall, real-time H-scan US imaging is a promising approach for visualizing the relative size and distribution of acoustic scattering objects.

Keywords: Acoustic scatterers; H-scan; Plane waves; Spatial angular compounding; Tissue characterization; Ultrasound.

Fig. 1.

Programming a Vantage ultrasound (US) research scanner essentially comprises of characterizing different framework parameters and attributes, and the sequence of events to be carried out by the Verasonics system hardware and software (MATLAB or the proprietary Verasonics Sequence eXecution, VSX) to collect requisite datatype (radiofrequency, RF, or inphase and quadrature, IQ, format) required for any US imaging mode.

Fig. 2.

Schematic diagraming our real-time H-scan US imaging scanner. This clinically-translatable and programmable MATLAB-based system incorporates a (A) graphical user interface (GUI) for controlling RF data acquisition and image processing including (B) variable control of the convolution filters used to derive the H-scan US signal. (C) System-level data flow chart details US image construction and dual (D) H-scan (left) and B-scan (right) display.

Fig. 3.

(A) Acoustic output measurements (mechanical index, MI) from a Vantage 256 US scanner equipped with a L11–4v linear array transducer with variable excitation at a fixed frequency of 5.2 MHz. Spatial maps detail a single plane wave transmission in both the (B) transverse and (C) elevational directions. Note that image amplitude reflects the measured mechanical index (MI) at a spatial resolution of 0.3 mm.

Fig. 4.

Example of an H-scan US image and RF data signal in a highly attenuating homogeneous tissue-mimicking phantom (A) before and (B) after attenuation correction using expected values. Image widths and depths are 20 and 40 mm, respectively. Note the improved H-scan US image quality and more uniform intensity at tissue depth.

Fig. 5.

H-scan US images (top) and corresponding power spectral density measurements (bottom) for visualizing any scaling adjustments made to the Hermite functions used for convolutional filtering and H-scan US image formation. The scale bar =5 mm. (A) Considerable overlap between the two filtering kernels compromises H-scan US image contrast, which can be alleviated by (B) increased kernel spacing.

Fig. 6.

H-scan US images from a series of tissue-mimicking phantom materials containing a homogeneous mixture of different-sized spherical microparticles, namely, (A) 15 μm scatterers, (B) 30 μm scatterers, and (C) 40 μm scatterers. The co-registered B-scan US images are provided for comparison and the white scale bar denotes 5 mm. Notably, as the acoustic scatterer size was progressively increased in the phantom materials, 3D histogram analysis of the entire US image reveals a marked increase in the red (R) channel signal used to generate the H-scan US images. These histograms summarize the color distribution for the images within a 3D RGB color space.

Fig. 7.

Repeat H-scan US images acquired in a homogeneous tissue-mimicking phantom material at three randomly selected spatial locations (A–C). The scale bar =5 mm. (D) Mean image intensity measurements illustrates reproducibility of H-scan US imaging results.

Fig. 8.

H-scan US images from a series of tissue-mimicking phantom materials containing a homogeneous mixture of two different monodisperse silica microparticles, namely, (A) 100% of 40 μm scatterers, (B) 75% of 40 μm and 25% of 15 μm scatterers, (C) 50% of 40 μm and 50% of 15 μm scatterers, (D) 25% of 40 μm and 75% of 15 μm scatterers, and (E) 100% of 15 μm scatterers. The scale bar = 5 mm. (F) Mean spatial analysis of the red and blue channel signals used to create the H-scan US images (denoting the relative size of small and large US scatterers, respectively) reveals that this new tissue characterization technique can detect subtle changes in the distribution of scatterer sizes.