A focused ultrasound treatment system for moving targets (part I):generic system design and in-silico first-stage evaluation

Background Focused ultrasound (FUS) is entering clinical routine as a treatment option. Currently, no clinically available FUS treatment system features automated respiratory motion compensation. The required quality standards make developing such a system challenging. Methods A novel FUS treatment system with motion compensation is described, developed with the goal of clinical use. The system comprises a clinically available MR device and FUS transducer system. The controller is very generic and could use any suitable MR or FUS device. MR image sequences (echo planar imaging) are acquired for both motion observation and thermometry. Based on anatomical feature tracking, motion predictions are estimated to compensate for processing delays. FUS control parameters are computed repeatedly and sent to the hardware to steer the focus to the (estimated) target position. All involved calculations produce individually known errors, yet their impact on therapy outcome is unclear. This is solved by defining an intuitive quality measure that compares the achieved temperature to the static scenario, resulting in an overall efficiency with respect to temperature rise. To allow for extensive testing of the system over wide ranges of parameters and algorithmic choices, we replace the actual MR and FUS devices by a virtual system. It emulates the hardware and, using numerical simulations of FUS during motion, predicts the local temperature rise in the tissue resulting from the controls it receives. Results With a clinically available monitoring image rate of 6.67 Hz and 20 FUS control updates per second, normal respiratory motion is estimated to be compensable with an estimated efficiency of 80%. This reduces to about 70% for motion scaled by 1.5. Extensive testing (6347 simulated sonications) over wide ranges of parameters shows that the main source of error is the temporal motion prediction. A history-based motion prediction method performs better than a simple linear extrapolator. Conclusions The estimated efficiency of the new treatment system is already suited for clinical applications. The simulation-based in-silico testing as a first-stage validation reduces the efforts of real-world testing. Due to the extensible modular design, the described approach might lead to faster translations from research to clinical practice

A nonparametric temperature controller with nonlinear negative reaction for multi-point rapid MR-guided HIFU ablation

International audienceMagnetic resonance-guided high intensity focused ultrasound (MRgHIFU) is a noninvasive method for thermal ablation, which exploits the capabilities of magnetic resonance imaging (MRI) for excellent visualization of the target and for near real-time thermometry. Oncological quality of ablation may be obtained by volumetric sonication under automatic feedback control of the temperature. For this purpose, a new nonparametric (i.e., model independent) temperature controller, using nonlinear negative reaction, was designed and evaluated for the iterated sonication of a prescribed pattern of foci. The main objective was to achieve the same thermal history at each sonication point during volumetric MRgHIFU. Differently sized linear and circular trajectories were investigated ex vivo and in vivo using a phased-array HIFU transducer. A clinical 3T MRI scanner was used and the temperature elevation was measured in five slices simultaneously with a voxel size of 1 x 1 x 5 mm(3) and temporal resolution of 4 s. In vivo results indicated a similar thermal history of each sonicated focus along the prescribed pattern, that was 17.3 +/- 0.5 degrees C as compared to 16 degrees C prescribed temperature elevation. The spatio-temporal control of the temperature also enabled meaningful comparison of various sonication patterns in terms of dosimetry and near-field safety. The thermal build-up tended to drift downwards in the HIFU transducer with a circular scan

Real-time method for motion-compensated MR thermometry and MRgHIFU treatment in abdominal organs

Magnetic resonance-guided high-intensity focused ultrasound is considered to be a promising treatment for localized cancer in abdominal organs such as liver, pancreas, or kidney. Abdominal motion, anatomical arrangement, and required sustained sonication are the main challenges

An experimental model to investigate the targeting accuracy of MR-guided focused ultrasound ablation in liver

International audienceBackground: Magnetic Resonance-guided High Intensity Focused Ultrasound (MRgHIFU) is a hybrid technology that aims to offer non-invasive thermal ablation of targeted tumors or other pathological tissues. Acoustic aberrations and non-linear wave propagating effects may shift the focal point significantly away from the prescribed (or, theoretical) position. It is therefore mandatory to evaluate the spatial accuracy of ablation for a given HIFU protocol and/or device. We describe here a method for producing a user-defined ballistic target as an absolute reference marker for MRgHIFU ablations.Methods: The investigated method is based on trapping a mixture of MR contrast agent and histology stain using radiofrequency (RF) ablation causing cell death and coagulation. A dedicated RF-electrode was used for the marker fixation as follows: a RF coagulation (4 W, 15 seconds) and injection of the mixture followed by a second RF coagulation. As a result, the contrast agent/stain is encapsulated in the intercellular space. Ultrasonography imaging was performed during the procedure, while high resolution T1w 3D VIBE MR acquisition was used right after to identify the position of the ballistic marker and hence the target tissue. For some cases, after the marker fixation procedure, HIFU volumetric ablations were produced by a phased-array HIFU platform. First ex vivo experiments were followed by in vivo investigation on four rabbits in thigh muscle and six pigs in liver, with follow-up at Day 7.Results: At the end of the procedure, no ultrasound indication of the marker's presence could be observed, while it was clearly visible under MR and could be conveniently used to prescribe the HIFU ablation, centered on the so-created target. The marker was identified at Day 7 after treatment, immediately after animal sacrifice, after 3 weeks of post-mortem formalin fixation and during histology analysis. Its size ranged between 2.5 and 4 mm.Conclusions: Experimental validation of this new ballistic marker method was performed for liver MRgHIFU ablation, free of any side effects (e. g. no edema around the marker, no infection, no bleeding). The study suggests that the absolute reference marker had ultrasound conspicuity below the detection threshold, was irreversible, MR-compatible and MR-detectable, while also being a well-established histology staining technique

An Optimal Control Approach for {HIFU} Self-Scanning Treatment Planning

In noninvasive abdominal tumor treatment, research has focused on canceling organ motion either by gating, breath holding or tracking of the target. This paper is based on the novel self-scanning method which combines the advantages of the gated and the tracking method. This approach leverages the respiratory organ motion by holding the focal spot of the high intensity focused ultrasound (HIFU) device static for a given time, while it passively scans the tumor due to respiratory motion. This enables to use a lower-cost HIFU device. We present a planning method for such a system that is based on optimal control theory which optimizes the scanning path and the sonication intensities simultaneously. The method minimizes treatment time and ensures complete tumor ablation according to the thermal dose under free-breathing. To verify our method, we simulated a tumor in two dimensions. The achieved treatment time performs on par to the gold-standard tracking method. Moreover, we measured the temperature profile of the HIFU device in a tissue-mimicking phantom to verify our temperature model

Ultrasound/MR Hybrid Imaging: Truly Simultaneous Motion Monitoring in the Abdomen & Image Co-Registration

Simultaneous US and MR acquisition is a hybrid method that offers a complementary description of the investigated anatomy. The study on healthy volunteers presented here with simultaneous 4DMRI/dynamic 2D ultrasound showed that the technical set-up is appropriate for clinical use and no significant RF mutual interferences were detectable in the acquired images. Post-processing of dual modality data permitted the accurate registration of the same imaging plane in US and MR images, and to overlay the 4DMRI motion vectors on the simultaneous US images from the abdomen during free breathing

Simultaneous Ultrasound/MRI Motion Monitoring in the Abdomen

Ultrasound and MRI are complementary imaging modalities. US imaging provides high temporal resolution and direct visualization of acoustic obstacles, while MRI provides excellent tissue contrast and a confirmed method for near real time thermometry. Overall, the expected added value of combining these two modalities would consist of more complete description of the investigated anatomy, a more accurate targeting, efficient motion tracking, and reliable immediate assessment of the therapeutic results. The aim of the present work is to set-up an integrated environment for simultaneously ultrasound and MR image acquisition, satisfying clinical standards, and to evaluate the feasibility of performing hybrid imaging. A study on healthy volunteers is presented here with simultaneous 4DMRI/dynamic 2D ultrasound