Baudouin Denis de Senneville

Real‐time adaptive methods for treatment of mobile organs by MRI‐controlled high‐intensity focused ultrasound

Focused ultrasound (US) is a unique and noninvasive technique for local deposition of thermal energy deep inside the body. MRI guidance offers the additional benefits of excellent target visualization and continuous temperature mapping. However, treating a moving target poses severe problems because 1) motion‐related thermometry artifacts must be corrected, 2) the US focal point must be relocated according to the target displacement. In this paper a complete MRI‐compatible, high‐intensity focused US (HIFU) system is described together with adaptive methods that allow continuous MR thermometry and therapeutic US with real‐time tracking of a moving target, online motion correction of the thermometry maps, and regional temperature control based on the proportional, integral, and derivative method. The hardware is based on a 256‐element phased‐array transducer with rapid electronic displacement of the focal point. The exact location of the target during US firing is anticipated using automatic analysis of periodic motions. The methods were tested with moving phantoms undergoing either rigid body or elastic periodical motions. The results show accurate tracking of the focal point. Focal and regional temperature control is demonstrated with a performance similar to that obtained with stationary phantoms. Magn Reson Med 57:319–330, 2007.

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.

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.

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.

Real-time 3D target tracking in MRI guided focused ultrasound ablations in moving tissues

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, 2010.

Acceleration and validation of optical flow based deformable registration for image-guided radiotherapy

Materials and methods. Two registration methods based on optical flow estimation have been programmed to run on a graphics programming unit (GPU). One of these methods by Horn & Schunck is tested on a 4DCT thorax data set with 10 phases and 41 landmarks identified per phase. The other method by Cornelius & Kanade is tested on a series of six 3D cone beam CT (CBCT) data sets and a conventional planning CT data set from a head and neck cancer patient. In each of these data sets 6 landmark points have been identified on the cervical vertebrae and the base of skull. Both CBCT to CBCT and CBCT to CT registration is performed. Results. For the 4DCT registration average landmark error was reduced by deformable registration from 3.5±2.0mm to 1.1±0.6mm. For CBCT to CBCT registration the average bone landmark error was 1.8±1.0mm after rigid registration and 1.6±0.8mm after deformable registration. For CBCT to CT registration errors were 2.2±0.6mm and 1.8±0.6mm for rigid and deformable registration respectively. Using GPU hardware the Horn & Schunck method was accelerated by a factor of 48. The 4DCT registration can be performed in 37seconds. The head and neck cancer patient registration takes 64seconds. Discussion. Compared to image slice thickness, which limits accuracy of landmark point determination, we consider the landmark point accuracy of the registration acceptable. The points identified in the CBCT images do not give a full impression of the result of doing deformable registration as opposed to rigid registration. A larger validation study is being planned in which soft tissue landmarks will facilitate tracking the deformable registration. The acceleration obtained using GPU hardware means that registration can be done online for CBCT.

Real-time volumetric MRI thermometry of focused ultrasound ablation in vivo: a feasibility study in pig liver and kidney.

MR thermometry offers the possibility to precisely guide high‐intensity focused ultrasound (HIFU) for the noninvasive treatment of kidney and liver tumours. The objectives of this study were to demonstrate therapy guidance by motion‐compensated, rapid and volumetric MR temperature monitoring and to evaluate the feasibility of MR‐guided HIFU ablation in these organs. Fourteen HIFU sonications were performed in the kidney and liver of five pigs under general anaesthesia using an MR‐compatible Philips HIFU platform prototype. HIFU sonication power and duration were varied. Volumetric MR thermometry was performed continuously at 1.5 T using the proton resonance frequency shift method employing a multi‐slice, single‐shot, echo‐planar imaging sequence with an update frequency of 2.5 Hz. Motion‐related suceptibility artefacts were compensated for using multi‐baseline reference images acquired prior to sonication. At the end of the experiment, the animals were sacrificed for macroscopic and microscopic examinations of the kidney, liver and skin. The standard deviation of the temperature measured prior to heating in the sonicated area was approximately 1°C in kidney and liver, and 2.5°C near the skin. The maximum temperature rise was 30°C for a sonication of 1.2 MHz in the liver over 15 s at 300 W. The thermal dose reached the lethal threshold (240CEM43) in two of six cases in the kidney and four of eight cases in the liver, but remained below this value in skin regions in the beam path. These findings were in agreement with histological analysis. Volumetric thermometry allows real‐time monitoring of the temperature at the target location in liver and kidney, as well as in surrounding tissues. Thermal ablation was more difficult to achieve in renal than in hepatic tissue even using higher acoustic energy, probably because of a more efficient heat evacuation in the kidney by perfusion. Copyright

Real time monitoring of radiofrequency ablation based on MR thermometry and thermal dose in the pig liver in vivo

To evaluate the feasibility and accuracy of MR thermometry based on the thermal dose (TD) concept for monitoring radiofrequency (RF) ablations, 13 RF ablations in pig livers were performed under continuous MR thermometry at 1.5 T with a filtered clinical RF device. Respiratory gated fast gradient echo images were acquired simultaneously to RF deposition for providing MR temperature maps with the proton resonant frequency technique. Residual motion, signal to noise ratio (SNR) and standard deviation (SD) of MR temperature images were quantitatively analyzed to detect and reject artifacted images in the time series. SD of temperature measurement remained under 2°C. Macroscopic analysis of liver ablations showed a white zone (Wz) surrounded by a red zone (Rz). A detailed histological analysis confirmed the ongoing nature of the coagulation necrosis in both Wz and Rz. Average differences (±SD) between macroscopic size measurements of Wz and Rz and TD predictions of ablation zones were 4.1 (±1.93) mm and −0.71 (±2.47) mm, respectively. Correlation values between TD and Wz and TD and Rz were 0.97 and 0.99, respectively. MR thermometry monitoring based on TD is an accurate method to delineate the size of the ablation zone during the RF procedure and provides a clinical endpoint.

Motion correction in MR thermometry of abdominal organs: a comparison of the referenceless vs. the multibaseline approach.

Reliable temperature and thermal‐dose measurements using proton resonance frequency shift‐based magnetic resonance (MR) thermometry for MR‐guided ablation of abdominal organs require a robust correction of artefacts induced by the target displacement through an inhomogeneous and time‐variant magnetic field. Two correction approaches emerged recently as promising candidates to allow continuous real‐time MR‐thermometry under free‐breathing conditions: The multibaseline correction method, which relies on a pre‐recorded correction table allowing to correct for periodic phase changes, and the referenceless method, which depends on a background phase estimation in the target area based on the assumption of a smooth spatial variation of the phase across the organ. This study combines both methods with real‐time in‐plane motion correction to permit both temperature and thermal‐dose calculations on the fly. Subsequently, the practical aspects of both methods are compared in two application scenarios, a radio frequency‐ablation and a high‐intensity focused ultrasound ablation. A hybrid approach is presented that exploits the strong points of both methods, allowing accurate and precise proton resonance frequency‐thermometry measurements during periodical displacement, even in the presence of spontaneous motion and strong susceptibility variations in the target area. Magn Reson Med, 2010.

Improvement of MRI-Functional measurement with automatic movement correction in native and transplanted kidneys

To improve 2D software for motion correction of renal dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI) and to evaluate its effect using the Patlak–Rutland model.