This study aims to demonstrate, using human cadavers the feasibility of energy-based adaptive focusing of ultrasonicwaves using magnetic resonance acoustic radiation force imaging (MR-ARFI) in the framework of non-invasive transcranial high intensity focused ultrasound (HIFU) therapy.Methods:
Energy-based adaptive focusing techniques were recently proposed in order to achieve aberration correction. The authors evaluate this method on a clinical brain HIFU system composed of 512 ultrasonic elements positioned inside a full body 1.5 T clinical magnetic resonance(MR)imaging system. Cadaver heads were mounted onto a clinical Leksell stereotactic frame. The ultrasonicwave intensity at the chosen location was indirectly estimated by the MR system measuring the local tissue displacement induced by the acoustic radiation force of the ultrasound(US) beams. For aberration correction, a set of spatially encoded ultrasonicwaves was transmitted from the ultrasonic array and the resulting local displacements were estimated with the MR-ARFI sequence for each emitted beam. A noniterative inversion process was then performed in order to estimate the spatial phase aberrations induced by the cadaver skull. The procedure was first evaluated and optimized in a calf brain using a numerical aberrator mimicking human skull aberrations. The full method was then demonstrated using a fresh human cadaver head.Results:
The corrected beam resulting from the direct inversion process was found to focus at the targeted location with an acoustic intensity 2.2 times higher than the conventional non corrected beam. In addition, this corrected beam was found to give an acoustic intensity 1.5 times higher than the focusing pattern obtained with an aberration correction using transcranial acoustic simulation-based on X-ray computed tomography(CT) scans.Conclusions:
The proposed technique achieved near optimal focusing in an intact human head for the first time. These findings confirm the strong potential of energy-based adaptive focusing of transcranial ultrasonic beams for clinical applications.
This work was partly funded by the Focused Ultrasound Surgery Fondation and by the French National Research Agency.
II. MATERIALS AND METHODS
II.A. High power therapeutic system
II.B. MR acquisitions
II.C. Energy-based adaptive focusing
II.D. Calf brain experiment using virtual aberration
II.E. Human brain experiment in head using the skull aberration
II.F. Finite differences simulations
II.G. Impact of the MR-ARFI SNR on adaptive focusing
III.A. Acoustic power
III.B. Adaptive focusing in calf brain
III.C. Adaptive focusing in human head
III.D. Impact of the MR-ARFI SNR on adaptive focusing
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