MRI-guided gas bubble enhanced ultrasound heating inin vivorabbit thigh

Abstract

In this study, we propose a focused ultrasound surgery protocol that induces and then uses gas bubbles at the focus to enhance the ultrasound absorption and ultimately create larger lesions in vivo. MRI and ultrasound visualization and monitoring methods for this heating method are also investigated. Larger lesions created with a carefully monitored single ultrasound exposure could greatly improve the speed of tumour coagulation with focused ultrasound. All experiments were performed under MRI (clinical, 1.5 T) guidance with one of two eight-sector, spherically curved piezoelectric transducers. The transducer, either a 1.1 or 1.7 MHz array, was driven by a multi-channel RF driving system. The transducer was mounted in an MRI-compatible manual positioning system and the rabbit was situated on top of the system. An ultrasound detector ring was fixed with the therapy transducer to monitor gas bubble activity during treatment. Focused ultrasound surgery exposures were delivered to the thighs of seven New Zealand while rabbits. The experimental, gas-bubble-enhanced heating exposures consisted of a high amplitude 300 acoustic watt, half second pulse followed by a 7 W, 14 W or 21 W continuous wave exposure for 19.5 s. The respective control sonications were 20 s exposures of 14 W, 21 W and 28 W. During the exposures, MR thermometry was obtained from the temperature dependency of the proton resonance frequency shift. MRT2-enhanced imaging was used to evaluate the resulting lesions. Specific metrics were used to evaluate the differences between the gas-bubble-enhanced exposures and their respective control sonications: temperatures with respect to time and space, lesion size and shape, and their agreement with thermal dose predictions. The bubble-enhanced exposures showed a faster temperature rise within the first 4 s and higher overall temperatures than the sonications without bubble formation. The spatial temperature maps and the thermal dose maps derived from the MRI thermometry closely correlated with the resulting lesion as examined by T2-weighted imaging. The lesions created with the gas-bubble-enhanced heating exposures were 2-3 times larger by volume, consistently more spherical in shape and closer to the transducer than the control exposures. The study demonstrates that gas bubbles can reliably be used to create significantly larger lesions in vivo. MRI thermometry techniques were successfully used to monitor the thermal effects mediated by the bubble-enhanced exposures.

Lesions of ultrasound-induced lung hemorrhage are not consistent with thermal injury

“…Shear stress (AIUM 2000b;Church et al 2006;Miller et al 2006) or heat and microjets arising from inertial cavitation (AIUM 2000b;Church et al 2006;Dalecki 2004) are the most likely physical forms of injury that could produce damage not detectable using transmission electron microscopy. Also, because lung is not typical of other soft tissues, and if inertial cavitation is indeed the responsible damage mechanism (although our experimental findings do not support this hypothesis), then it might be feasible to hypothesize that the enhanced heating reported when ultrasound interacts with ultrasound contrast agents (Holt and Roy 2001;Sokka et al 2003;Razansky et al 2006) is likewise applicable in the lung.…”

HIFU for Palliative Treatment of Pancreatic Cancer

“…The time-averaged intensity remains low, and the thermal dose delivered to the tissue is not sufficient to cause thermal damage. Cavitation can also promote heating if longer HIFU pulses or continuous ultrasound is used (30)(31)(32). The energy of the incident ultrasound wave is transferred very efficiently into stable oscillation of resonant-size bubbles.…”

Acoustic Emissions During 3.1 MHz Ultrasound Bulk Ablation In Vitro

“…Such activity can be exploited to visualize ablation effects (Sanghvi et al 1995;Rabkin et al 2005Rabkin et al , 2006 or to enhance tissue absorption (Melodelima et al 2001;Sokka et al 2003;Umemura et al 2005;Kaneko et al 2005), but can also complicate ultrasound energy deposition and the resulting spatial pattern of tissue coagulation (Watkin et al 1996;Chen et al 2003;Makin et al 2005;. Mechanisms for interactions between cavitation activity and ultrasound-induced heating have been clarified by several detailed numerical modeling studies (Hilgenfeldt et al 2000;Chavrier et al 2000;Yang et al 2004) and phantom experiments (Holt and Roy 2001;Khokhlova et al 2006).…”