Bruno Quesson

Magnetic resonance temperature imaging for guidance of thermotherapy.

Continuous thermometry during a hyperthermic procedure may help to correct for local differences in heat conduction and energy absorption, and thus allow optimization of the thermal therapy. Noninvasive, three‐dimensional mapping of temperature changes is feasible with magnetic resonance (MR) and may be based on the relaxation time T1, the diffusion coefficient (D), or proton resonance frequency (PRF) of tissue water. The use of temperature‐sensitive contrast agents and proton spectroscopic imaging can provide absolute temperature measurements. The principles and performance of these methods are reviewed in this paper. The excellent linearity and near‐independence with respect to tissue type, together with good temperature sensitivity, make PRF‐based temperature MRI the preferred choice for many applications at mid to high field strength (≥ 1 T). The PRF methods employ radiofrequency spoiled gradient‐echo imaging methods. A standard deviation of less than 1°C, for a temporal resolution below 1 second and a spatial resolution of about 2 mm, is feasible for a single slice for immobile tissues. Corrections should be made for temperature‐induced susceptibility effects in the PRF method. If spin‐echo methods are preferred, for example when field homogeneity is poor due to small ferromagnetic parts in the needle, the D‐ and T1‐based methods may give better results. The sensitivity of the D method is higher that that of the T1 methods provided that motion artifacts are avoided and the trace of D is evaluated. Fat suppression is necessary for most tissues when T1, D, or PRF methods are employed. The latter three methods require excellent registration to correct for displacements between scans. J. Magn. Reson. Imaging 2000;12:525–533.

Volumetric HIFU ablation under 3D guidance of rapid MRI thermometry.

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.

In vivo macrophage activity imaging in the central nervous system detected by magnetic resonance

Cell‐specific imaging has been proposed to increase the potential of magnetic resonance imaging (MRI) for tissue analysis. The hypothezis of the present work was that following intravenous injection of ultra‐small particle iron oxide, a contrast agent that accumulates in mononuclear phagocyte cells, macrophages with iron burden would be detectable by MRI within the central nervous system at sites of inflammatory cellular activity. In experimental autoimmune encephalomyelitis in Lewis rats (in which intense macrophage activity results from both hematogenous macrophages and activated microglia), lesions have been seen by MRI as low signal intensities related to magnetic susceptibility effects induced by iron particles. Electron microscopy has revealed the presence of such particles within the cytoplasm of cells that had the morphological aspect of macrophages. Macrophage activity imaging might increase MRI capability with regard to the in vivo pathophysiological aspects of central nervous system (CNS) diseases and might help in therapeutic trials in the numerous CNS diseases in which macrophages are involved. Magn Reson Med 41:329–333, 1999.

Local hyperthermia with MR-guided focused ultrasound: Spiral trajectory of the focal point optimized for temperature uniformity in the target region

The objective of hyperthermia treatment is to deliver a similar therapeutic thermal dose throughout the target volume within a minimum amount of time. We describe a noninvasive approach to this goal based on magnetic resonance imaging (MRI)‐guided focused ultrasound (FUS) with a spherical transducer that can be moved along two directions inside the bed of a clinical MR imager and that has an adjustable focal length in the third dimension. Absorption of FUS gives rise to a highly localized thermal buildup, which then spreads by heat diffusion and blood perfusion. A uniform temperature within a large target volume can be obtained using a double spiral trajectory of the transducer focal point together with constant and maximum FUS power. Differences between the real and target temperatures during the first spiral are evaluated in real time with temperature MRI and corrected for during the second spiral trajectory employing FUS focal point velocity modulation. Once a uniform temperature distribution is reached within the entire volume, FUS heating is applied only at the regions boundaries to maintain the raised temperature levels. Heat conduction, together with the design and timing of the trajectories, therefore ensures a similar thermal dose for the entire target region. Good agreement is obtained between theory and experimental results in vitro on gel phantoms, ex vivo on meat samples, and in vivo on rabbit thigh muscle. Edema in muscle was visible 1 hour after hyperthermia as a spatially uniform rise of the signal intensity in T2‐weighted images. J. Magn. Reson. Imaging 2000;12:571–583.

Magnetic resonance temperature imaging.

Continuous, real-time, 3D temperature mapping during a hyperthermic procedure may provide (i) enhanced safety by visualizing temperature maps in and around the treated region, (ii) improved efficiency by adapting local energy deposition with feedback coupling algorithms and (iii) therapy end-points based on the accumulated thermal dose. Non-invasive mapping of temperature changes can be achieved with MRI and may be based on temperature dependent MRI parameters. The excellent linearity of the temperature dependency of the proton resonance frequency (PRF) and its near-independence with respect to tissue type make the PRF-based methods the preferred choice for many applications, in particular at mid- to-high field strength (≥0.5 T). The PRF methods employ RF-spoiled gradient echo imaging methods and incorporate fat suppression techniques for most organs. A standard deviation of less than 1°C, for a temporal resolution below 1 s and a spatial resolution of ∼2 mm is feasible for immobile tissues. Special attention is paid to methods for reducing artifacts in MR temperature mapping caused by intra-scan and inter-scan motion and motion and temperature-induced susceptibility effects in mobile tissues. Real-time image processing and visualization techniques, together with accelerated MRI acquisition techniques, are described because of their potential for therapy guidance.

Three-dimensional spatial and temporal temperature control with MR thermometry-guided focused ultrasound (MRgHIFU)

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. The proposed temperature control ensures heating throughout the volume of interest of up to 1 ml composed of 287 voxels with 95% of the energy deposited within its boundaries and reducing the typical average temperature overshoot to 1°C. Magn Reson Med, 2009.

In vivo evaluation of remyelination in rat brain by magnetization transfer imaging

The aim of this work was to assess quantitatively and qualitatively the ability of magnetization transfer imaging to follow in vivo remyelination. Demyelination lesions were induced in rats by the injection of L-alpha-lysophosphatidylcholine stearoyl into the corpus callosum and imaging was performed in vivo on a 4.7-Tesla system at different time points. The percentage of magnetization transfer ratio (MTR) decrease was calculated for each animal. To evaluate the MTR findings for remyelination, myelin was quantitated by histological analysis of the lesion size and counting the number of remyelinating axons. An MTR decrease was observed when demyelination was present at 7 days after injection. During the remyelinating phase between day 30 and 40 after injection, contralateral values almost complete returned to normal, thus indicating remyelination. Histologically, at days 30 and 40 after injection, the lesion area was reduced in size and the axons were surrounded by a thin myelin sheath, indicating the remyelination process. Statistical analysis showed that the profile of MTR values was significantly correlated with the course of remyelination. All the MTR changes show a correlation with both myelin damage and repair. In conclusion, the study of the MTR profile in this myelin lesion model demonstrates in vivo the loss of myelin and the presence of spontaneous remyelination. This methodological approach which can also be applied to multiple sclerosis patients to show demyelination, should prove helpful to determine the degree of spontaneous and therapeutically induced remyelination in multiple sclerosis lesions, and thus to validate therapeutic treatments for myelin repair.

Real-time MR-thermometry and dosimetry for interventional guidance on abdominal organs

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.

Improved Volumetric MR-HIFU Ablation by Robust Binary Feedback Control

Volumetric high-intensity focused ultrasound (HIFU) guided by multiplane magnetic resonance (MR) thermometry has been shown to be a safe and efficient method to thermally ablate large tissue volumes. However, the induced temperature rise and thermal lesions show significant variability, depending on exposure parameters, such as power and timing, as well as unknown tissue parameters. In this study, a simple and robust feedback-control method that relies on rapid MR thermometry to control the HIFU exposure during heating is introduced. The binary feedback algorithm adjusts the durations of the concentric ablation circles within the target volume to reach an optimal temperature. The efficacy of the binary feedback control was evaluated by performing 90 ablations in vivo and comparing the results with simulations. Feedback control of the sonications improved the reproducibility of the induced lesion size. The standard deviation of the diameter was reduced by factors of 1.9, 7.2, 5.0, and 3.4 for 4-, 8-, 12-, and 16-mm lesions, respectively. Energy efficiency was also improved, as the binary feedback method required less energy to create the desired lesion. These results show that binary feedback improves the quality of volumetric ablation by consistently producing thermal lesions of expected size while reducing the required energy as well.

MR thermometry for monitoring tumor ablation

Local thermal therapies are increasingly used in the clinic for tissue ablation. During energy deposition, the actual tissue temperature is difficult to estimate since physiological processes may modify local heat conduction and energy absorption. Blood flow may increase during temperature increase and thus change heat conduction. In order to improve the therapeutic efficiency and the safety of the intervention, mapping of temperature and thermal dose appear to offer the best strategy to optimize such interventions and to provide therapy endpoints. MRI can be used to monitor local temperature changes during thermal therapies. On-line availability of dynamic temperature mapping allows prediction of tissue death during the intervention based on semi-empirical thermal dose calculations. Much progress has been made recently in MR thermometry research, and some applications are appearing in the clinic. In this paper, the principles of MRI temperature mapping are described with special emphasis on methods employing the temperature dependency of the water proton resonance frequency. Then, the prospects and requirements for widespread applications of MR thermometry in the clinic are evaluated.