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  • ALGORITHM  (3)
  • 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: Germany ; PROSTATE ; ALGORITHM ; imaging ; INFORMATION ; NUCLEAR-MEDICINE ; TIME ; MARKER ; MR ; MRI ; SEQUENCE ; IDENTIFICATION ; REGISTRATION ; sensitivity ; nuclear medicine ; BIOPSY ; radiology ; methods ; NUCLEAR ; technique ; USA ; CATHETERS ; phantom ; MEDICINE ; 1.5 T ; SCANNER ; PASSIVE TRACKING ; POSITION ; INTERVENTIONS ; MR-guided prostate biopsy ; passive device tracking
    Abstract: MR-guided transrectal prostate biopsy is currently a time-consuming procedure because the imaging slice is often manually realigned with the biopsy needle during lesion targeting. In this work a pulse sequence is presented that automatically follows a passive marker attached to a dedicated MR biopsy device holder, thus providing an alternative to existing active tracking methods. In two orthogonal tracking FLASH images of the marker the position of the needle axis is automatically identified using a phase-only cross-correlation (POCC) algorithm. The position information is then used to realign a trueFISP imaging slice in real time. In phantom experiments the sensitivity of this technique to initial misalignments of the marker and to the signal-to-noise ratio was evaluated. In several puncture experiments the precision of the needle placement was analyzed. The POCC algorithm allowed for a precise identification of the marker in the images even under severe initial misalignments of up to 45 degrees. At a frame rate 1 image/s a precision of the needle placement of 1.5 +/- 1.1 mm could be achieved
    Type of Publication: Journal article published
    PubMed ID: 18429016
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