A volumetric sonication method is proposed that produces volume ablations by steering the focal point along a predetermined trajectory consisting of multiple concentric outward-moving circles. This method was tested in vivo on pig thigh muscle (32 ablations in nine animals). Trajectory diameters were 4, 12, and 16 mm with sonication duration depending on the trajectory size and ranging from 20 to 73 s. Despite the larger trajectories requiring more energy to reach necrosis within the desired volume, the ablated volume per unit applied energy increased with trajectory size, indicating improved treatment efficiency for larger trajectories. The higher amounts of energy required for the larger trajectories also increased the risk of off-focus heating, especially along the beam axis in the near field. To avoid related adverse effects, rapid volumetric multiplane MR thermometry was introduced for simultaneous monitoring of the temperature and thermal dose evolution along the beam axis and in the near field, as well as in the target region with a total coverage of six slices acquired every 3 s. An excellent correlation was observed between the thermal dose and both the nonperfused (R=0.929 for the diameter and R=0.964 for the length) and oedematous (R=0.913 for the diameter and R=0.939 for the length) volumes as seen in contrast-enhanced T1-weighted difference images and T2-weighted postsonication images, respectively. Histology confirmed the presence of a homogeneous necrosis inside the heated volumes. These results show that volumetric high-intensity focused ultrasound (HIFU) sonication allows for efficiently creating large thermal lesions while reducing treatment duration and also that the rapid multiplane MR thermometry improves the safety of the therapeutic procedure by monitoring temperature evolution both inside as well as outside the targeted volume.
The use of proton resonance frequency shift-based magnetic resonance (MR) thermometry for interventional guidance on abdominal organs is hampered by the constant displacement of the target due to the respiratory cycle and the associated thermometry artifacts. Ideally, a suitable MR thermometry method should for this role achieve a subsecond temporal resolution while maintaining a precision comparable to those achieved on static organs while not introducing significant processing latencies. Here, a computationally effective processing pipeline for two-dimensional image registration coupled with a multibaseline phase correction is proposed in conjunction with high-frame-rate MRI as a possible solution. The proposed MR thermometry method was evaluated for 5 min at a frame rate of 10 images/sec in the liver and the kidney of 11 healthy volunteers and achieved a precision of less than 2°C in 70% of the pixels while delivering temperature and thermal dose maps on the fly. The ability to perform MR thermometry and dosimetry in vivo during a real intervention was demonstrated on a porcine kidney during a high-intensity focused ultrasound heating experiment. Magn Reson Med 63:1080-1087, 2010. V C 2010 Wiley-Liss, Inc. Key words: MRI; thermometry; temperature; interventional; imaging; real time system; motion artifacts; proton resonance frequency shift; PRF MR thermometry relying on the water proton resonance frequency is gaining importance for monitoring and guiding thermal therapies such as radiofrequency (1), laser (2), or focused ultrasound thermal ablation (3-5). Typically, proton resonance frequency-based MR thermometry relies on the voxelwise evaluation of phase differences between sequentially acquired gradient echo images. However, for the use on abdominal organs, this renders the method very sensitive to motion artifacts and magnetic field changes. These motion artifacts can be coarsely classed into the two following types: intrascan motion artifacts and interscan motion artifacts. Intrascan motion artifacts are caused by displacement during the MR acquisition process and lead to image blurring and object ghosting. Commonly, this type of artifact is addressed using fast MR acquisition schemes or alternatively with respiratory-gated sequences that reduce the temporal resolution to the respiratory frequency. Interscan motion artifacts are due to organ displacement between the MR acquisitions and lead to a misregistration between subsequent phase images and thus to artifacts in the subtraction process. Furthermore, since any displacement or plastic deformation of the abdominal organs will in general also lead to a modified demagnetization field and thus to a change of the local magnetic field (6-8), additional phase artifacts are introduced.To overcome these problems, several correction strategies have been proposed, such as respiratory gating (9), navigator echoes (10), multibaseline acquisition to sample periodic changes (11,12), and referenceless phase corrections (13). Furthermore, the concept of the equivalent...
Magnetic resonance imaging-guided high intensity focused ultrasound is a promising method for the noninvasive ablation of pathological tissue in abdominal organs such as liver and kidney. Due to the high perfusion rates of these organs, sustained sonications are required to achieve a sufficiently high temperature elevation to induce necrosis. However, the constant displacement of the target due to the respiratory cycle render continuous ablations challenging, since dynamic repositioning of the focal point is required. This study demonstrates subsecond 3D high intensity focused ultrasound-beam steering under magnetic resonance-guidance for the real-time compensation of respiratory motion. The target is observed in 3D space by coupling rapid 2D magnetic resonance-imaging with prospective slice tracking based on pencil-beam navigator echoes. The magnetic resonance-data is processed in real-time by a computationally efficient reconstruction pipeline, which provides the position, the temperature and the thermal dose on-the-fly, and which feeds corrections into the high intensity focused ultrasound-ablator. The effect of the residual update latency is reduced by using a 3D Kalman-predictor for trajectory anticipation. The suggested method is characterized with phantom experiments and verified in vivo on porcine kidney. The results show that for update frequencies of more than 10 Hz and latencies of less then 114 msec, temperature elevations can be achieved, which are comparable to static experiments. Magn Reson Med 64:1704-1712,
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