Evaluation of shear wave generation methods for intraoperative magnetic resonance elastography in a clinical transcranial focused ultrasound system

Abstract

The current standard for FDA approved real-time monitoring of transcranial Focused Ultrasound (FUS) ablation is MR-thermometry. Ultrasound-elastography and Magnetic Resonance Elastography (MRE) have shown success in monitoring FUS ablation in various parts of the body. Introducing shear wave vibrations into the brain during transcranial-FUS becomes technologically challenging given the stringent constraints of the operating space in water filled transcranial phased array devices and since patients are fixed in stereotactic headgear during treatment. This study investigates the feasibility of using intraoperative-MRE during transcranial-FUS ablation in the ExAblate 4000 (InSightec, Ltd.) platform. Acoustic radiation force induced vibration using the ExAblate 4000 transducer and common MRE surface actuation methods are evaluated for shear wave generation. Elastic phantom experiments were used to gauge the acoustic power levels and time incurred during vibrations from MRE scanning while in the FUS transducer. These values were compared to FUS and MR parameters used in clinical transcranial-FUS treatments. MRE derived 3D displacement fields within the phantom materials are evaluated for all vibration methods. The cooling water in the ultrasound traducer influenced the MR signal using current MRE pulse sequences. Future work will involve integrating MRE into sequences less sensitive to cooling water media and minimizing intraoperative-MRE scan time.