Novel acoustic coupling bath using magnetite nanoparticles for MR‐guided transcranial focused ultrasound surgery

Abstract

Purpose Acoustic coupling baths, nominally composed of degassed water, play important roles during transcranial focused ultrasound surgery. However, this large water bolus also degrades the quality of intraoperative magnetic resonance (MR) guidance imaging. In this study, we test the feasibility of using dilute, aqueous magnetite nanoparticle suspensions to suppress these image degradations while preserving acoustic compatibility. We examine the effects of these suspensions on metrics of image quality and acoustic compatibility for two types of transcranial focused ultrasound insonation regimes: low‐duty cycle histotripsy procedures and high‐duty cycle thermal ablation procedures. Methods Magnetic resonance guidance imaging was used to monitor thermal ablations of in vitro gel targets using a coupling bath composed of various concentrations of aqueous, suspended, magnetite nanoparticles in a clinical transcranial transducer under stationary and flowing conditions. Thermal deposition was monitored using MR thermometry simultaneous to insonation. Then, using normal degassed water as a coupling bath, various concentrations of aqueous, suspended, magnetite nanoparticles were placed at the center of this same transducer and insonated using high‐duty cycle pulsing parameters. Passive cavitation detectors recorded cavitation emissions, which were then used to estimate the relative number of cavitation events per insonation (cavitation duty cycle) and the cavitation dose estimates of each nanoparticle concentration. Finally, the nanoparticle mixtures were exposed to low‐duty cycle, histotripsy pulses. Passive cavitation detectors monitored cavitation emissions, which were used to estimate cavitation threshold pressures. Results The nanoparticles reduced the MR signal of the coupling bath by 90% in T2‐ and T2*‐weighted images and also removed almost all imaging artifacts caused by coupling bath motion. The coupling baths caused <5% changes in peak temperature change achieved during sonication, as observed via MR thermometry. At low duty cycle insonations, the nanoparticles decreased the cavitation threshold pressure by about 15 ± 7% in a manner uncorrelated with nanoparticle concentration. At high duty cycle insonations, the 0.5 cavitation duty cycle acoustic power threshold varied linearly with nanoparticle concentration. Conclusions Dilute aqueous magnetite nanoparticle suspensions effectively reduced MR imaging artifacts caused by the acoustic coupling bath. They also attenuated acoustic power deposition by <5%. For low duty cycle insonation regimes, the nanoparticles decreased the cavitation threshold by 15 ± 7%. However, for high‐duty cycle regimes, the nanoparticles decreased the threshold for cavitation in proportion to nanoparticle concentration.

The Potential of Adjusting Water Bolus Liquid Properties for Economic and Precise MR Thermometry Guided Radiofrequency Hyperthermia

“…In this case, a WB is used for coupling of acoustic waves and cooling. Removal of WB signal will result in similar improvements in MRT precision as we have shown in this study as shown in [22]. Still, the effect of these compounds on acoustic properties and their effect on losses in the WB should investigated.…”

“…Three different compounds were selected [14,21,22] The EM and MRI properties in this range is relatively stable and changes less than 1% [25,26]. Therefore, all the measurements were made at 21 • C, at the room temperature.…”

“…Manganese chloride (MnCl 2 ) was used by Delannoy et al [14] for hyperthermia and Chopra et al [21] for MR guided high intensity focused ultrasound (HIFU). Allen et al have proposed the use of suspending iron oxide nanoparticles (SPIO) to suppress water bath signal for MRgFUS surgery and reported that SPIO doped water weakly attenuates acoustic waves [22]. Although these studies showed promising results, no data are available on their effect on electromagnetic material properties, which are pivotal for efficient MR guided RF hyperthermia.…”

A preclinical study of diffusion‐weighted MRI contrast as an early indicator of thermal ablation

“…The transducer’s construction, along with acoustic coupling water bath, introduced features that exacerbated these sensitivities, including a geometry incompatible with standard imaging coils, a conductive transducer surface that generates B 1 RF standing waves, and eddy currents 35 . Finally, motion in the water bath caused by strong vibrations induced by the imaging gradients could also pose a problem to image quality 36 . In response, we constructed a custom, 25‐cm transmit/receive surface coil that shifted the standing wave nodes, used a twice‐refocused, adiabatic spin‐echo scheme, 37‐39 and prescribed the selected slice to intersect with as little of the water bath as possible.…”

“…35 Finally, motion in the water bath caused by strong vibrations induced by the imaging gradients could also pose a problem to image quality. 36 In response, we constructed a custom, 25-cm transmit/receive surface coil that shifted the standing wave nodes, used a twice-refocused, adiabatic spin-echo scheme, [37][38][39] and prescribed the selected slice to intersect with as little of the water bath as possible. The strong eddy currents, potential corruption by the water bath, and B 1 inhomogeneities precluded us from using the single-shot DWI-EPI method that is standard in many diffusion studies in the brain.…”