High-intensity focused ultrasound (HIFU) is an efficient noninvasive technique for local heating. Using MRI thermal maps, a proportional, integral, and derivative (PID) automatic temperature control was previously applied at the focal point, or at several points within a plane perpendicular to the beam axis using a multispiral focal point trajectory. This study presents a flexible and rapid method to extend the spatial PID temperature control to three dimensions during each MR dynamic. The temperature in the complete volume is regulated by taking into account the overlap effect of nearby sonication points, which tends to enlarge the heated area along the beam axis. Volumetric temperature control in vitro in gel and in vivo in rabbit leg muscle was shown to provide temperature control with a precision close to that of the temperature MRI measurements. High-intensity focused ultrasound (HIFU) is an efficient noninvasive technique to produce local heating deep inside the human body (1,2). MRI provides excellent visualization of anatomical structures and tumors for treatment planning (3). In addition, MRI can provide continuous temperature mapping based on the proton resonance frequency (PRF) shift of water with a good spatial and temporal resolution (4,5) for real-time therapy control. New MRI developments in high-field, rapid imaging and magnet field stability have contributed to the performance of MRI temperature mapping. In parallel, major improvement in ultrasound technology have been made in the field of phased-array MR-compatible transducers (6). These recent developments offer the possibility to treat several points simultaneously and in a minimal amount of time.Automatic control of the temperature (7) or the thermal dose effect (8) during HIFU heating has been shown to be feasible. Using rapid temperature MRI methods and realtime data processing, an automatic proportional, integral, and derivative (PID) temperature feedback loop has been presented to control the temperature in a single point, the HIFU focal spot (9). This temperature control method has been extended to several points (10) using a multispiral trajectory of the HIFU focal point within a single plane perpendicular to the beam axis. In the latter method, the energy deposition during the next multispiral trajectory is calculated upon completion of the previous one. The resulting volume treated in three dimensions is deduced from numerical simulations. The focal point trajectory remains in a plane perpendicular to the beam axis since the overlap of the acoustic field for each focal point location presents difficulties for volumetric temperature control along the beam axis.Here, a novel method is proposed for volumetric temperature regulation with temperature control for each voxel of the predefined volume by taking into account beam overlap for the different sonication points. The optimum focal point locations and HIFU intensities are determined following each volumetric temperature map, leading to a rapid and flexible method. The new method was ...
A method is proposed for estimating the perfusion rate, thermal diffusivity, and the absorption coefficient that influence the local temperature during high intensity focused ultrasound (HIFU) thermotherapy procedures. For this purpose, HIFU heating experiments (N = 100) were performed ex vivo on perfused porcine kidney (N = 5) under different flow conditions. The resulting spatio-temporal temperature variations were measured non-invasively by rapid volumetric MR-temperature imaging. The bio-heat transfer (BHT) model was adapted to describe the spatio-temporal evolution of tissue temperature in the cortex. Absorption and perfusion coefficients were determined by fitting the integrated thermal load (spatial integration of the thermal maps) curves in time with an analytical solution of the BHT equation proposed for single point HIFU heating. Thermal diffusivity was determined independently by analyzing the spatial spread of the temperature in time during the cooling period. Absorption coefficient and thermal diffusivity were found to be independent of flow, with mean and average values of 11.0 +/- 1.85 mm(3) x K x J(-1) and 0.172 +/- 0.003 mm(2) x s(-1), respectively. A linear dependence of the calculated perfusion rate with flow was observed with a slope of 9.20 +/- 0.75 mm(-3). The perfusion was found to act as a scaling term with respect to temperature but with no effect on the spatial spread of temperature which only depends on the thermal diffusivity. All results were in excellent agreement with the BHT model, indicating that this model is suitable to predict the evolution of temperature in perfused organs. This quantitative approach allows for determination of tissue thermal parameters with excellent precision (within 10%) and may thus help in quantifying the influence of perfusion during MR guided high intensity focused ultrasound (MRgHIFU).
The objective of this study was to evaluate the feasibility of integrating real-time ultrasound echo guidance in MR-guided high-intensity focused ultrasound (HIFU) heating of mobile targets in order to reduce latency between displacement analysis and HIFU treatment. Experiments on a moving phantom were carried out with MRI-guided HIFU during continuous one-dimensional ultrasound echo detection using separate HIFU and ultrasound imaging transducers. Excellent correspondence was found between MR- and ultrasound-detected displacements. Real-time ultrasound echo-based target tracking during MR-guided HIFU heating is shown with the dimensions of the heated area similar to those obtained for a static target. This work demonstrates that the combination of the two modalities opens up perspectives for motion correction in MRI-guided HIFU with negligible latency.
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