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  • 1
    Keywords: Germany ; IMAGES ; imaging ; HEART ; RESOLUTION ; TIME ; MR ; MRI ; sensitivity ; TRACKING ; MAGNETIC-RESONANCE ANGIOGRAPHY ; RECONSTRUCTION ; 2D ; MATRIX ; CATHETER TRACKING ; active device tracking ; interventional MRI ; intravascular catheters ; COILS ; SENSE ; CATHETER ; cardiac MRI ; GRAPPA ; interactive real-time parallel MRI ; TSENSE ; ACTIVE CATHETER TRACKING
    Abstract: In this work active MR catheter tracking with automatic slice alignment was combined with an autocalibrated parallel imaging technique. Using an optimized generalized autocalibrating partially parallel acquisitions (GRAPPA) algorithm with an acceleration factor of 2, we were able to reduce the acquisition time per image by 34%. To accelerate real-time GRAPPA image reconstruction, the coil sensitivities were updated only after slice reorientation. For a 2D trueFISP acquisition (160 x 256 matrix, 80% phase matrix, half Fourier acquisition, TR = 3.7 ms, GRAPPA factor = 2) real-time image reconstruction was achieved with up to six imaging coils. In a single animal experiment the method was used to steer a catheter from the vena cava through the beating heart into the pulmonary vasculature at an image update rate of about five images per second. Under all slice orientations, parallel image reconstruction was accomplished with only minor image artifacts, and the increased temporal resolution provided a sharp delineation of intracardial structures, such as the papillary muscle
    Type of Publication: Journal article published
    PubMed ID: 16683261
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  • 2
    Keywords: Germany ; imaging ; SYSTEM ; SYSTEMS ; RESOLUTION ; TIME ; MARKER ; MR ; SEQUENCE ; SEQUENCES ; SIGNAL ; SUSCEPTIBILITY ; FIELD ; MAGNETIC-RESONANCE ; ACQUISITION ; RELAXATION ; TRACKING ; BIOPSY ; RE ; INTERVENTIONAL DEVICES ; INFUSION ; ENHANCEMENT ; SIZE ; COILS ; technique ; VIEW ; RARE ; CATHETERS ; DEFT ; device localization ; fast MRI ; INNER VOLUME ; inner volume imaging ; local look (LoLo) ; percutaneous intervention ; WIRES
    Abstract: Percutaneous MR-guided interventions with needles require fast pulse sequences to image the needle trajectory with minimal susceptibility artifacts. Spin-echo pulse sequences are well suited for reducing artifact size; however, even with single-shot turbo spin-echo techniques, such as rapid acquisition with relaxation enhancement (RARE) or half-Fourier acquisition single-shot turbo spin-echo (HASTE), fast imaging remains challenging. In this work we present a HASTE pulse sequence that is combined with inner-volume excitation to reduce the scan time and limit the imaging field of view (FOV) to a small strip close to the needle trajectory (targeted-HASTE). To compensate for signal saturation from fast repeated acquisitions, a magnetization restore pulse (driven equilibrium Fourier transform (DEFT)) is used. The sequence is combined with dedicated active marker coils to measure the position and orientation of the needle so that the targeted-HASTE image slice is automatically repositioned. In an animal experiment the coils were attached to an MR-compatible robotic assistance system for MR-guided interventions. Needle insertion and infusion via the needle could be visualized with a temporal resolution of 1 s, and the needle tip could be localized even in the presence of a stainless steel mandrel
    Type of Publication: Journal article published
    PubMed ID: 16795081
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  • 3
    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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  • 4
    Keywords: AGENTS ; Germany ; IMAGES ; SYSTEM ; LINES ; REDUCTION ; MR ; SIGNAL ; FIELD ; AMPLIFICATION ; DESIGN ; etiology ; LINE ; SAFETY ; TRACKING ; INTEGRATION ; INCREASE ; intravascular catheters ; methods ; COILS ; TEMPERATURE ; TRANSMISSION ; CATHETER ; MR-IMAGES ; CATHETERS ; Time Factors ; diagnostic use ; prevention & control ; adverse effects ; Equipment Design ; Equipment Failure Analysis ; Heat ; instrumentation ; Phantoms,Imaging ; Radiation Injuries ; Radiation Protection ; Radiation-Protective Agents ; Radio Waves
    Abstract: 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
    Type of Publication: Journal article published
    PubMed ID: 17115123
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