Simultaneous grayscale and subharmonic ultrasound imaging on a modified commercial scanner

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Abstract

Objective

To demonstrate the feasibility of simultaneous dual fundamental grayscale and subharmonic imaging on a modified commercial scanner.

Motivation

The ability to generate signals at half the insonation frequency is exclusive to ultrasound contrast agents (UCA). Thus, subharmonic imaging (SHI; transmitting at f0 and receiving at f0/2) provides improved visualization of UCA within the vasculature via suppression of the surrounding tissue echoes. While this capability has proven useful in a variety of clinical applications, the SHI suppression of surrounding tissue landmarks (which are needed for sonographic navigation) also limits it use as a primary imaging modality. In this paper we present results using a commercial ultrasound scanner modified to allow imaging in both grayscale (f0 = 4.0 MHz) and SHI (f0 = 2.5 MHz, f0/2 = 1.25 MHz) modes in real time.

Methods

A Logiq 9 ultrasound scanner (GE Healthcare, Milwaukee, WI) with a 4C curvilinear probe was modified to provide this capability. Four commercially available UCA (Definity, Lantheus Medical Imaging, North Billerica, MA; Optison, GE Healthcare, Princeton, NJ; SonoVue, Bracco Imaging, Milan, Italy; and Sonazoid, GE Healthcare, Oslo, Norway) were all investigated in vitro over an acoustic output range of 3.34 MPa. In vivo the subharmonic response of Sonazoid was investigated in the portal veins of four canines (open abdominal cavity) and four patients with suspected portal hypertension.

Results

In vitro, the four UCA showed an average maximum subharmonic amplitude of 44.1 ± 5.4 dB above the noise floor with a maximum subharmonic amplitude of 48.6 ± 1.6 dB provided by Sonazoid. The average in vivo maximum signal above the noise floor from Sonazoid was 20.8 ± 2.3 dB in canines and 33.9 ± 5.2 dB in humans. Subharmonic amplitude as a function of acoustic output in both groups matched the S-curve behavior of the agent observed in vitro. The dual grayscale imaging provided easier sonographic navigation, while the degree of tissue suppression in SHI mode varied greatly on a case by case basis.

Conclusions

These results demonstrate the feasibility of dual grayscale and SHI on a modified commercial scanner. The ability to simultaneously visualize both imaging modes in real time should improve the applicability of SHI as a future primary clinical imaging modality.

Highlights

★ A commercial ultrasound scanner has been modified to allow simultaneous imaging in grayscale and subharmonic imaging. ★ Ultrasound contrast agents showed an in vitro subharmonic amplitude variation of 44.1 ± 5.4 dB relative to the noise floor. ★ Sonazoid (GE Healthcare) was imaged in the liver of four canines and four patients with suspected portal hypertension. ★ In vivo subharmonic signal variations showed similar S-curve behavior to in vitro findings, but with reduced amplitudes. ★ A dual grayscale/subharmonic imaging mode should make subharmonic imaging more translatable to clinical practice.

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