Subharmonic Aided Pressure Estimation for Monitoring Interstitial Fluid Pressure in Tumors
Authors:   , Halldórsdóttir Valgerður Guðrún , Forsberg Flemming , Lewin Peter A. 
Published: July 2021

The purpose of this study is to evaluate whether the ultrasound contrast agent based method subharmonic aided pressure estimation (SHAPE) can be used to measure the interstitial fluid pressure in locally advanced breast cancer thereby allowing for noninvasive monitoring of tumor response to neoadjuvant chemotherapy. Vascular ultrasound contrast agents (UCAs) are gas-filled microbubbles that improve the depiction of vascularity in ultrasound (US) images by enhancing the difference in reflectivity between tissue and the agent. At higher acoustic outputs (> 200 kPa) contrast agents act as nonlinear oscillators producing harmonics, ultra- and subharmonics in the received signals. In subharmonic imaging (SHI) pulses are transmitted at a frequency f0 and the echoes are received at half that frequency (f0/2). SHI has been shown to be a feasible option for contrast enhanced breast imaging due to marked subharmonic generation by the bubbles relative to limited subharmonic generation in tissues. A novel technique, subharmonic aided pressure estimation or SHAPE, utilizing the subharmonic amplitude of the scattered signal from the microbubbles for pressure tracking was developed by our group and has been implemented for various applications such as cardiac and hepatic pressure measurements. It has been hypothesized that the level of interstitial fluid pressure (IFP) in a breast cancer tumor can be used to predict the response to systemic preoperative therapy. Thus, tumors with high IFP should show a poorer response to therapy than those with low IFP. We propose that by using SHAPE the IFP of locally advanced breast cancer tumors can be estimated noninvasively in order to monitor the response to systemic preoperative chemotherapy. This method would be a considerable improvement from the wick-in-needle method currently used for IFP measurements as it is noninvasive and would thus potentially increase the use of IFP as a biomarker for neoadjuvant chemotherapy. The possibility of using SHAPE to noninvasively estimate IFP was studied both in vitro and in vivo. In vitro an inverse linear relationship between hydrostatic pressure and subharmonic amplitude was observed with r2 = 0.63-0.95; p < 0.05, maximum amplitude drop 11.36 dB at 10 MHz and 8 dB, and r2 as high as 0.97; p < 0.02 (10 MHz transmit frequency and -4/-8 dB acoustic output power most promising), indicating that SHAPE may be useful in monitoring IFP. Moreover, in vivo proof-of-concept for SHAPE as a noninvasive monitor of IFP was provided in four swine with naturally occurring melanoma. SHAPE showed excellent correlation with IFP values obtained in normal tissues and in the tumor compared to needle-based pressure measurements (r2 = 0.67 - 0.96, p < 0.01) with optimal sensitivity for SHAPE at a transmission frequency of 10 MHz and acoustic output settings -4 or -8 dB. Further studies in a murine model with human breast cancer xenografts showed correlations (r: -0.60 to -0.74, p < 0.01) between IFP in tumor and tissue and subharmonic amplitude. Results suggested that calibration curves can be used to noninvasively estimate IFP using subharmonic data such that there is no statistically significant difference between the calculated and measured groups at a 115% threshold (p > 0.14). These results strongly indicate the feasibility of using SHAPE as a noninvasive pressure monitor of IFP in tumors.