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The localization of passive marker systems in interventional MRI is necessary to monitor the position and orientation of medical instruments that do not emit an MR signal. In this work an algorithm is presented that automatically detects a given marker system in an MR image with a precision better than one pixel. Therefore, a combination of a phase-only cross correlation algorithm with a subsequent center-of-mass analysis is utilized. The algorithm was evaluated in simulations and phantom experiments with respect to precision, noise sensitivity as well as the influence of unwanted signal amplitudes. Above a signal-to-noise ratio (SNR) of 4.5 a localization precision significantly better than the pixel dimension could be achieved. For SNR values of 6 and more the influence of unwanted signals on the localization could not be detected in the simulations. In phantom experiments the predicted precision of the marker localization could be realized which results for typical measurement parameters in a maximal deviation of the needle tip in an MR-guided needle injection of 0.6 mm.

MR-guided intravascular interventions require image update rates of up to 10 images per second, which can be achieved using parallel imaging. However, parallel imaging requires many coil elements, which increases reconstruction times and thus compromises real-time image reconstruction. In this study a dynamic coil selection (DCS) algorithm is presented that selects a subset of receive coils to reduce image reconstruction times. The center-of-sensitivity coordinates and the relative signal intensities are determined for each coil in a prescan. During the intervention m coils are selected for reconstruction using a coil ranking based on the distance to the current slice or catheter position. In a phantom experiment for m = 6, an optimal signal-to-background ratio (SBR) was achieved and foldover artifacts were avoided. In three animal experiments involving catheter manipulation in the aorta and the right heart chamber, the anatomy was successfully visualized at frame rates of about 5 Hz using active catheter tracking., ((c) 2006 Wiley-Liss, Inc.)

Objective: Integration of transformers into transmission lines suppresses radiofrequency (RF)-induced heating. New figure-of-eight-shaped transformer coils are compared to conventional loop transformer coils to assess their signal transmission properties and safety profile., Materials and Methods: The transmission properties of figure-of-eight-shaped transformers were measured and compared to transformers with loop coils. Experiments to quantify the effect of decoupling from the B1 field of the MR system were conducted. Temperature measurements were performed to demonstrate the effective reduction of RF-induced heating. The transformers were investigated during active tracking experiments., Results: Coupling to the B1 field was reduced by 18 dB over conventional loop-shaped transformer coils. MR images showed a significantly reduced artifact for the figure-of-eight- shaped coils generated by local flip-angle amplification. Comparable transmission properties were seen for both transformer types. Temperature measurements showed a maximal temperature increase of 30 K/3.5 K for an unsegmented/segmented cable. With a segmented transmission line a robotic assistance system could be successfully localized using active tracking., Conclusion: The figure-of-eight-shaped transformer design reduces both RF field coupling with the MR system and artifact sizes. Anatomical structure close to the figure-of-eight-shaped transformer may be less obscured as with loop-shaped transformers if these transformers are integrated into e.g. intravascular catheters.

To examine whether the fission yeast Mam1 ABC transporter can be used for secretion of heterologous proteins, thereby bypassing the classical secretion pathway, we have analyzed chimeric forms of the M-factor precursor. It was demonstrated that GFP can be exported when fused to both the amino-terminal prosequence from mfm1 and a CaaX motif. This secretion was dependent on the Mam1 transporter and not the classical secretion pathway. The secretion efficiency of GFP, however, was relatively low and most of the reporter protein was trapped in the vacuolar membranes. Our findings suggest that the Mam1 ABC protein is a promiscuous peptide transporter that can accommodate globular proteins of a relatively large size. Furthermore, our results help in defining the sequences required for processing and secretion of natural M-factor.