A convenient, reliable, and fast acoustic pressure field measurement method for magnetic resonance-guided high-intensity focused ultrasound systems with phased array transducers

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.

Feasibility and safety assessment of magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU)-mediated mild hyperthermia in pelvic targets evaluated using an in vivo porcine model

“…The Sonalleve V2 system was used for ultrasound exposures [29,30]. In HT treatment mode, the system is comprised a patient table locked into the bore of an MRI scanner (Ingenia 1.5T, Philips, Best, the Netherlands).…”

Clinical Performance and Future Potential of Magnetic Resonance Thermometry in Hyperthermia

“…It can be seen that in recent years more studies have satisfied these minimum requirements. A total of 8 out of those 13 studies are feasibility or comparison studies (marked in grey in Table 5 ), which can be grouped into having investigated: Hardware: MR-HIFU for different treatment location applications [ 45 , 47 ] Thermometry method: MR spectroscopy to measure absolute temperature [ 64 ] Sequences and parameters for MRT [ 43 , 56 , 57 , 70 ] Performance of MRT at different anatomical sites [ 71 ] …”

Focused ultrasound for safe and effective release of brain tumor biomarkers into the peripheral circulation

“…The mice were treated using an MR-guided FUS system ( Fig 1A and 1B) at three different peak negative acoustic pressure (PNP) levels: (i) 0.59 MPa; (ii) 1.29 MPa; (iii) 1.58 MPa (n = 5 in each group). The acoustic pressure output of the FUS transducer was calibrated using a fiber-optic hydrophone following our previously published method ( Fig 1C) [ 33,34]. eGFP mRNA was selected as a representative tumor-specific nucleic acid biomarker because it is highly specific to our tumor model, mRNA is more abundant in the plasma than the more intensively investigated ctDNA [35], and mRNA can be detected using established polymerase chain reaction (PCR)-based approaches [36], which provides a reliable benchmark test to evaluate the performance of FUS-LBx.…”

“…It was also modified to perform pulsed FUS sonication at low-pressure levels used in this study by only turning on 128 elements. The output of the FUS transducer was controlled by setting the nominal acoustic power to be 10 W, 20 W, or 30 W. The acoustic fields generated by the FUS transducer was calibrated by a fiber optic hydrophone using our previously published methods ( Fig 1C) [33,34]. The measured PNPs for the three acoustic power settings were 0.59 MPa, 1.29 MPa, and 1.58 MPa, respectively.…”