Implementation of vibro-acoustography on a clinical ultrasound system

Abstract

Vibro-acoustography is an ultrasound-based imaging modality that uses two ultrasound beams of slightly different frequencies to produce images based on the acoustic response caused by harmonic ultrasound radiation force excitation at the difference frequency between the two ultrasound frequencies. Vibro-acoustography has demonstrated feasibility and usefulness in imaging of breast and prostate tissue. However, previous studies have been performed either in controlled water tank settings or a prototype breast scanner equipped with a water tank. To make vibro-acoustography more accessible and relevant to clinical use, we report here on the implementation of vibro-acoustography on a General Electric Vivid 7 ultrasound scanner. In this paper, we will describe software and hardware modifications that were performed to make vibro- acoustography functional on this system. We will discuss aperture definition for the two ultrasound beams and beamforming using a linear-array transducer. Experimental results from beam measurements and phantom imaging studies will be shown. The implementation of vibro-acoustography provides a step toward clinical translation of this imaging modality for applications in various organs including breast, prostate, thyroid, kidney, and liver.

Figure 1

Block diagram of vibro-acoustography performed with a linear array transducer. The intersection of the two ultrasound beams is electronically swept in the azimuthal direction of the transducer and then mechanically scanned in the elevation direction of the transducer. Different planes can be imaged by changing the electronic focal depth. The interaction of the two ultrasound beams in the tissue produces an acoustic signal at the difference frequency, Δf, between the two ultrasound beams. The acoustic signal is detected by a nearby hydrophone and the signal is filtered, amplified, digitized, and processed for image formation and display.

Figure 2

Block diagram of vibro-acoustography implementation with the General Electric Vivid 7. The control and analysis computer communicates with the Vivid 7 via TCP/IP protocol. Once an instruction from the computer is sent to the Vivid 7, a synchronization trigger (Sync) was sent to the digitizer and a waveform generator used to shade the power amplifier’s output. The power amplifier’s output served as a power supply for the transmit board in the Vivid 7. The control and analysis computer also controlled the motion control system which operated the mechanical slider with the ultrasound transducer.

Figure 3

Waveform generation for vibro-acoustography. (a) The waveforms are defined by the parameters (N₁, K₁, L₁) and (N₂, K₂, L₂). F_C represents the transmitter generator system clock. (b) Simulated signals for (N₁, K₁, L₁) = (16, 6, 1) and (N₂, K₂, L₂) = (16, 6, 0). Each dot represents one cycle of the 80 MHz clock.

Figure 4

Aperture assignment for 128 active elements in a 192 element transducer. The number of beams transmitted is 192. Gray denotes elements assigned a signal with f₁, white are elements assigned a signal with f₂ and black are not used. For beams 1–64 and 129–192 both the subapertures are steered and for beams 65–128, the subapertures are translated.

Figure 5

Transmit signal chain. A function generator output is split into inverted and noninverted signals and amplified by a two-channel amplifier which provides the power supply voltages for the transmit board for the GE Vivid 7 scanner.

Figure 6

Motorized translation stage for moving the linear array transducer. The probe is held in a fixture attached to the linear motor stage. The linear motor stage is mounted on a plastic support frame.

Figure 7

Photographs of objects used for imaging. (a) Breast phantom, (b) Diagram of breast phantom to scale with front and side views. The front view is scaled to be 40 × 80 mm and the side view is scaled to be 40 × 43 mm. In the side view, the left edge is the front of the phantom and is closest to the transducer. The vertical dashed line in the side view illustration depicts the location of the focal plane, about 15 mm from the surface of the phantom. During imaging, the transducer was offset from the front edge by 10 mm. (c) Excised human prostate.

Figure 8

Pressure signals from vibro-acoustography beamforming. (a) Simulated pressure using signals from Fig. 3(b). Two cycles of modulated ultrasound with Δf = 51.5464 kHz are shown. (b) Measured pressure with a needle hydrophone for a full toneburst which was shaded by the raised 3 kHz signal depicted as a dashed line. Note that the time scales are different for each plot.

Figure 9

Experimentally measured azimuthal and axial profiles for the Spl configuration. The field is focused at x = 0 mm and z = 25 mm. The magnitudes are independently normalized. (a) Azimuthal, (b) Axial.

Figure 10

Experimentally measured pressure at location of spatial peak pressure. The field is focused at x = 0 mm and z = 25 mm.

Figure 11

Breast phantom images. The azimuthal direction of the transducer corresponds to the vertical axis of the image and the mechanical translation of the transducer in the elevation direction corresponds to the horizontal axis. The image is 38.4 × 80 mm and streak correction has been applied.

Figure 12

Excised human prostate images. (a) X-ray image of prostate gland. Calcium clusters are marked with the designation Ca in this panel and all other panels. Wire loops in upper right and lower left corners are fiducial markers. Image is 50.0 × 50.0 mm, (b) Original VA image, (b) VA image after streak correction. The azimuthal direction of the transducer corresponds to the horizontal axis of the image and the mechanical translation of the transducer in the elevation direction corresponds to the vertical axis. Images are 46.7 × 60 mm.