Noninvasive Estimation of Tumor Interstitial Fluid Pressure from Subharmonic Scattering of Ultrasound Contrast Microbubbles

Abstract

The noninvasive estimation of interstitial fluid pressure (IFP) using ultrasound contrast agent (UCA) microbubbles as pressure sensors will provide tumor treatments and efficacy assessments with a promising tool. This study aimed to verify the efficacy of the optimal acoustic pressure in vitro in the prediction of tumor IFPs based on UCA microbubbles' subharmonic scattering. A customized ultrasound scanner was used to generate subharmonic signals from microbubbles' nonlinear oscillations, and the optimal acoustic pressure was determined in vitro when the subharmonic amplitude reached the most sensitive to hydrostatic pressure changes. This optimal acoustic pressure was then applied to predict IFPs in tumor-bearing mouse models, which were further compared with the reference IFPs measured using a standard tissue fluid pressure monitor. An inverse linear relationship and good correlation (r = -0.853, p < 0.001) existed between the subharmonic amplitude and tumor IFPs at the optimal acoustic pressure of 555 kPa, and pressure sensitivity was 1.019 dB/mmHg. No statistical differences were found between the pressures measured by the standard device and those estimated via the subharmonic amplitude, as confirmed by cross-validation (mean absolute errors from 2.00 to 3.09 mmHg, p > 0.05). Our findings demonstrated that in vitro optimized acoustic parameters for UCA microbubbles' subharmonic scattering can be applied for the noninvasive estimation of tumor IFPs.

Keywords: interstitial fluid pressure; subharmonic scattering; ultrasound contrast agent microbubbles.

Figure 1

In vitro acoustic attenuation experiment setup.

Figure 2

In vitro flow circulation system for acoustic scattering measurement.

Figure 3

The schematic of in vivo experiment. (a) Ultrasonic RF data acquisition and IFP measurement. (b–d) ROI selected from reconstructed B-mode image of tumors and subharmonic wave extraction (red box, ROI selected).

Figure 4

The RF data processing flow (red box, ROI selected).

Figure 5

Acoustic attenuation spectra of Sonazoid microbubbles’ suspensions at different injection times. (a) 1 min, (b) 3 min, (c) 5 min, (d) 9 min after injection.

Figure 6

The results of in vitro experiments. (a) The amplitude of subharmonic increases with the increased acoustic pressure at ambient pressures of 10 mmHg and 40 mmHg. (b) Subharmonic amplitudes vary with ambient pressure at acoustic pressures of 407 kPa (r = −0.883, p = 0.117), 555 kPa (r = −0.966, p = 0.034), and 663 kPa (r = −0.924, p = 0.076).

Figure 7

Correlation analysis between tumor IFP and subharmonic amplitudes. The dotted curves on either side of the fitted lines are limits of 95% confidence intervals.