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  • 1
    Abstract: Passive markers such as contrast-agent filled spheres provide a cost-effective and reliable method to localize devices in an MR system. In this work two real time methods are evaluated that track the position of a robotic assistance system with passive markers. A fully automatic localization algorithm with sub-pixel precision, a combination of a phase-only cross correlation with a center-of-mass algorithm, is compared to a direct coordinate exchange between robot and MR. Slice positions and orientations are evaluated in a phantom experiment yielding a similar angular precision of about 1.3 Grad for the needle axis.
    Type of Publication: Proceeding
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  • 2
    Keywords: SIMULATIONS ; COMBINATION ; Germany ; ALGORITHM ; SYSTEM ; SYSTEMS ; MARKER ; SIMULATION ; MR ; MRI ; SIGNAL ; LOCALIZATION ; PARAMETERS ; sensitivity ; TRACKING ; EXTENSION ; interventional MRI ; analysis ; correlation ; SIGNALS ; phantom ; German ; PRECISION ; INSTRUMENTS ; MR-guided interventions ; percutaneous interventions ; PHASE CORRELATION ; phase-only cross correlation ; robotic assistance system ; SUBPIXEL REGISTRATION
    Abstract: 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 he 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
    Type of Publication: Journal article published
    PubMed ID: 17879815
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  • 3
    Keywords: MRI ; ACQUISITION ; LOCALIZATION ; sensitivity ; INTERVENTIONAL DEVICES ; ENHANCEMENT ; PULSES ; INNER VOLUME ; percutaneous intervention ; fast MR imaging ; OF-VIEW ; outer volume suppression ; PROJECTION-RECONSTRUCTION ; restricted field of view ; SSFP ; steady state sequence
    Abstract: Magnetic resonance-guided percutaneous interventions with needles require fast pulse sequences with acquisition times less than 1 s to image the needle trajectory within moving organs. To guide the movement of a rigid instrument with high sampling rate, an magnetic resonance imaging method was developed that reduces the acquisition time down to a few hundred milliseconds by restricting the field of view to a small stripe around the instrument trajectory. To maintain the dynamic steady state, saturation pulses for outer volume suppression were inserted into additional repetition time-intervals. These saturation intervals were combined with three sequence variants: a spoiled gradient echo sequence, an echo-shifted steady state free precession and a balanced steady state free precession sequence. The magnetization dynamics were analyzed by means of numerical optimized simulations. Results were compared with phantom experiments and an average signal-to-suppression-ratio of 15.5 could be achieved. With a field of view reduction of up to 12.5% an update rate of six images per second could be achieved. Finally, animal experiments demonstrated the fast and reliable needle tip visualization during percutaneous magnetic resonance-guided interventions with the help of a robotic assistance system.
    Type of Publication: Journal article published
    PubMed ID: 21337422
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