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

Abstract

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 thermometry: Methodology, pitfalls and practical solutions

“…These errors might be caused by respiratory, cardiac, peristaltic or bulk motion with standard deviations of >±10°C even without any application of heat (25,73). Structural changes of the treatment area due to tissue coagulation during an ablation procedure (40) This library is used to link the phase measured during thermal therapy to the correct reference image by means of an acquired navigator echo (22), non-similarity coefficients (77) or intercorrelation coefficients (78,79). While robust to periodic motion, reference or baseline methods are prone to irregular physiological motion such as arrhythmia.…”

On-line 3Dmotion estimation using low resolution MRI

“…Motion throughout the imaging session was estimated with an implementation (Roujol et al 2010) of non-rigid registration based on optical flow. As derived by Horn & Schunck (1981), the optical flow's objective function is formulated as…”

Hybrid referenceless and multibaseline subtraction MR thermometry for monitoring thermal therapies in moving organs

“…Selecting the corresponding baseline image can be performed by determining the organ's position with a navigator echo, 11 or based on a similarity criterion, such as nonsimilarity coefficients 12 or intercorrelation coefficients. 13,14 The multibaseline methods require cyclic organ motion in order to acquire a full set of all possible baseline images and require additional setup time to assemble the baseline library. They are generally much more robust to motion than conventional baseline subtraction, but remain sensitive to susceptibility changes during therapy.…”