Abstract
BackgroundWith the expanding applications of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU), there is an urgent need for a convenient, reliable, and fast acoustic pressure field measurement method to aid treatment protocol design, ensure consistent and safe operation of the transducer, and facilitate regulatory approval of new techniques. Herein, we report a method for acoustic pressure field characterization of MR-HIFU systems with multi-element phased array transducers. This method integrates fiber-optic hydrophone measurements and electronic steering of the ultrasound beam with MRI-assisted HIFU focus alignment to the fiber tip.MethodsA clinical MR-HIFU system (Sonalleve V2, Profound Medical Inc., Mississauga, Canada) was used to assess the proposed method. A fiber-optic hydrophone was submerged in a degassed water bath, and the fiber tip location was traced using MRI. Subsequently, the nominal transducer focal point indicated on the MR-HIFU therapy planning software was positioned at the fiber tip, and the HIFU focus was electronically steered around the fiber tip within a 3D volume for 3D pressure field mapping, eliminating the need for an additional, expensive, and MRI-compatible 3D positioning stage. The peak positive and negative pressures were measured at the focus and validated using a standard hydrophone measurement setup outside the MRI magnet room.ResultsWe found that the initial MRI-assisted HIFU focus alignment had an average offset of 2.23 ± 1.33 mm from the fiber tip as identified by the 3D pressure field mapping. MRI guidance and electronic beam steering allowed 3D focus localization within ~ 1 h, i.e., faster than the typical time required using the standard laboratory setup (~ 3–4 h). Acoustic pressures measured using the proposed method were not significantly different from those obtained with the standard laboratory hydrophone measurements.ConclusionsIn conclusion, our method offers a convenient, reliable, and fast acoustic pressure field characterization tool for MR-HIFU systems with phased array transducers.
Highlights
With the expanding applications of magnetic resonance-guided high-intensity focused ultrasound (MR-High-intensity focused ultrasound (HIFU)), there is an urgent need for a convenient, reliable, and fast acoustic pressure field measurement method to aid treatment protocol design, ensure consistent and safe operation of the transducer, and facilitate regulatory approval of new techniques
With expanding applications of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU), there is a need for HIFU acoustic pressure characterization tools to aid treatment protocol design, ensure consistent and safe device operation, and facilitate regulatory approval of new techniques [2]
While various methods have been proposed for MR-HIFU acoustic characterization, hydrophone measurements are considered the gold standard for direct acoustic pressure measurements [3]
Summary
With the expanding applications of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU), there is an urgent need for a convenient, reliable, and fast acoustic pressure field measurement method to aid treatment protocol design, ensure consistent and safe operation of the transducer, and facilitate regulatory approval of new techniques. Hydrophone-based characterization of a clinical phased array HIFU transducer is associated with two technical challenges: standard laboratory hydrophone equipment is not MRI-compatible, and the bore of a clinical MRI scanner has limited space to fit those pieces of equipment Owing to these challenges, in previous studies the HIFU patient table was moved outside the magnet fringe field, and a heavy, high-precision, 3D positioning stage was placed on the table for pressure measurements [9,10,11]. Designing a high-precision MRI-compatible positioning system is technically challenging and expensive
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