Noninvasive pressue estimation using ultrasound methods: Preliminary in vitro results

Abstract

Tumor interstitial pressure (TIP) inside a tumor is often high compared to normal tissues and can impede the delivery of chemotherapeutic drugs as well as decrease the effectiveness of radiation therapy. Recent efforts have focused on the development of a nanometer-sized phase-change contrast agent (PCCA) that can be activated from a liquid (nanodroplet) state using low energy ultrasound (US) pulses to form a larger highly echogenic microbubble (MB). Since PCCA activation is dependent on the hydrostatic pressure of the surrounding fluid, these same agents and custom US methods may permit TIP measurement in cancerous tissue, which is a new technology we have termed TIP estimation using US (TIPE-US). Lipid shelled decafluorobutane gas MBs were produced using mechanical agitation and then stable nanodroplets were produced by cooling and pressurizing the vial solutions. An US setup was used to deliver acoustic energy and activate nanodroplets from liquid to gaseous MB states. The effects of hydrostatic pressure on the US activation threshold of PCCA was studied by injecting PCCA into acoustically transparent sample holder preloaded with saline. Varying levels of hydrostatic pressure (0 to 39 mmHg) were applied by injecting air in the sample holders. Each sample was imaged using an US system while the acoustic pressure was progressively increased (0.3 to 3.6 MPa). From each image sequence, a modified contrast-to-tissue (CTR) metric was computed. The activation threshold required to convert PCCA from the liquid to gaseous state was taken as the US pressure needed to achieve a CTR at least two times that observed at baseline. PCCA sizes before and after US activation were measured using particle sizing systems. The greatest volume of liquid nanodroplets had a diameter of 150 nm whereas after activation into gaseous MBs the size of these same PCCA were considerably larger (majority of MBs > 1 µm) and suitable for US imaging. CTR values measured from US imaging studies of PCCA after exposure with varying levels of US energy clearly captured nanodroplet activation. Importantly, a strong correlation was found between the PCCA activation pressure and hydrostatic pressure on the nanodroplets (R2 = 0.99). This correlation could serve as a reference function for converting PCCA activation pressures to TIP estimates.