MRI can be used for monitoring temperature during a thermocoagulation treatment of tumors. The aim of this study was to demonstrate the suitability of a 3D steady-state free precession sequence (3D Fast Imaging with Steady-State Precession, 3D TrueFISP) for MR temperature measurement at 0.23 T, and to compare it to the spin-echo (SE) and spoiled 3D gradient-echo (3D GRE) sequences. The optimal flip angle for the TrueFISP sequence was calculated for the best temperature sensitivity in the image signal from liver tissue, and verified from the images acquired during the thermocoagulation of excised pig liver. The treatment of hepatic metastases by local heat delivery has drawn considerable interest over the last few years. The efficiency of thermoablation is improved if real-time monitoring of heat distribution is available during the treatment (1-3). At the moment, MRI is the only medical imaging modality that can provide noninvasive temperature information from tissue.Temperature monitoring with MRI is possible using the temperature sensitivity of the proton resonance frequency, i.e., chemical shift (4), diffusion (5), or longitudinal relaxation time T 1 and equilibrium magnetization M 0 (6 -8). The applicability of each method depends on the field strength of the MR scanner used, and on the target anatomy (9).Low-field open magnets are well suited for performing tumor ablations. They offer maximum access to the patient; dedicated tools for interventional MRI, such as needle tracking systems and in-room controls; less-intense needle artifacts; and lower-cost procedures. A disadvantage is that temperature-measuring methods utilizing the temperature sensitivity of the proton resonance frequency are difficult to use, as the change in chemical shift is directly proportional to field strength. For example, the sensitivity is 6.5 times less sensitive at 0.23 T than at 1.5 T.Motion artifacts hamper diffusion-based temperature measurement methods (10), making them impractical for use in the liver. With low-field open scanners, the easiest method for obtaining temperature data from the liver is to use the temperature dependence of T 1 and M 0 . Their temperature sensitivity is still high at lower magnetic field strengths, and they are easily measurable with standard sequences and hardware.The accuracy of temperature measurement with T 1 and M 0 depends strongly on the type of MR sequence used and on the choice of acquisition parameters, such as the repetition time (TR) and flip angle (␣). Temperature resolution may be affected by a factor of 5, depending on the type of sequence used for the measurement (11). The parameters need to be carefully chosen to optimize the temperature measurement while maintaining a reasonable temporal and spatial resolution.Spin-echo (SE) sequences, gradient-recalled acquisitions in the steady state (GRASS), and spoiled gradientrecalled acquisitions (SPGR) have been employed for monitoring temperature by using the temperature dependency of T 1 (7,8,11,12). A steady-state sequence (3D Fast Imagi...

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Adaptive temporal sensitivity encoding (TSENSE) has been suggested as a robust parallel imaging method suitable for MR guidance of interventional procedures. However, in practice, the reconstruction of adaptive TSENSE images obtained with large coil arrays leads to long reconstruction times and latencies and thus hampers its use for applications such as MRguided thermotherapy or cardiovascular catheterization. Here, we demonstrate a real-time reconstruction pipeline for adaptive TSENSE with low image latencies and high frame rates on affordable commodity personal computer hardware. For typical image sizes used in interventional imaging (128 ؋ 96, 16 channels, sensitivity encoding (SENSE) factor 2-4), the pipeline is able to reconstruct adaptive TSENSE images with image latencies below 90 ms at frame rates of Real-time MRI for therapy control is used to provide retroactive feedback information for interventional applications such as catheter guidance or MRI-controlled tissue ablation. Such interventions require several minutes of continuous imaging and are preferably carried out under free-breathing conditions, which presents the following challenges to MR acquisition and reconstruction.The data acquisition has to be rapid enough to resolve physiologic motion such as respiratory or cardiac motion and the signal changes from the interventional process itself. In addition, the employed data acquisition and reconstruction schemes must not introduce long image latencies because this degrades the value of the image data for feedback control.One of the significant developments in this field was the introduction of parallel imaging methods such as simultaneous acquisition of spatial harmonics (SMASH) (1), sensitivity encoding (SENSE) (2), or generalized auto-calibrating partially parallel acquisitions (GRAPPA) (3), which allows, on one hand, acceleration of the data acquisition and, on the other hand, shortening of readout echo-trains and thus reduces signal dropout and image distortions in regions of large susceptibility variations.Apart from variable-density methods, such as GRAPPA, which incorporate self-calibration by nature of the sampling pattern, these parallel imaging methods require coil sensitivity data for the image reconstruction, which depends on the coil position. Because of the long duration of interventional imaging procedures, displacements of the receiver coils are common and lead either to increased image artifacts over time or, in the cases in which the receiver coils are embedded in the interventional device itself, the complete unsuitability of these approaches. Temporal sensitivity encoding (TSENSE) (4) is an approach that overcomes this problem by collecting highresolution calibration data at a lower temporal resolution than that used for image encoding. However, the additional computational overhead leads to reconstruction times that often exceed the MR acquisition time, in particular if large coil arrays are used, and are therefore in practice hard to exploit for real-time therapy guidance.Gutt...

Purpose:To evaluate the feasibility of a new MR compatible optical tracking guided bone biopsy system. Materials and Methods:Wireless optical tracker elements were connected to a bone biopsy set. Biopsies of five different anatomic areas and lesions varying from 9 mm to 40 mm (mean 25 mm) in size, were performed. Results:We were able to perform the biopsies in all cases, and the samples allowed for a pathologic diagnosis. The procedure time was limited to less than 40 minutes, and no complications occurred. Conclusion:MR-guided bone biopsy with this new guiding system was a feasible and fast procedure that did not compromise the usability of the bone biopsy set.

Performing interventional procedures in the close proximity to an MR scanner widens the range of operations available for an optical tracking system. In order to gain the full benefits from both unrestricted use of surgical instruments outside the magnet and intraoperative imaging, a method for transferring the registration data of the optical navigator between two locations is required. An optical tracking system, which provides such a transfer method and tracks patient position during a surgical procedure, has been developed, tested, and demonstrated with two patient cases. J. Magn. Reson. Imaging 2001;13:93–98. © 2001 Wiley‐Liss, Inc.