Biomedical subjects

Reiner Umathum

Publications and source records attributed to Reiner Umathum.

11 recordsLinked to original sources

PubMed2006

B1 field-insensitive transformers for RF-safe transmission lines.

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.

Equipment Design↗

PubMed2006

Manganese-enhanced magnetic resonance imaging for in vivo assessment of damage and functional improvement following spinal cord injury in mice.

In past decades, much effort has been invested in developing therapies for spinal injuries. Lack of standardization of clinical read-out measures, however, makes direct comparison of experimental therapies difficult. Damage and therapeutic effects in vivo are routinely evaluated using rather subjective behavioral tests. Here we show that manganese-enhanced magnetic resonance imaging (MEMRI) can be used to examine the extent of damage following spinal cord injury (SCI) in mice in vivo. Injection of MnCl2 solution into the cerebrospinal fluid leads to manganese uptake into the spinal cord. Furthermore, after injury MEMRI-derived quantitative measures correlate closely with clinical locomotor scores. Improved locomotion due to treating the detrimental effects of SCI with an established therapy (neutralization of CD95Ligand) is reflected in an increase of manganese uptake into the injured spinal cord. Therefore, we demonstrate that MEMRI is a sensitive and objective tool for in vivo visualization and quantification of damage and functional improvement after SCI. Thus, MEMRI can serve as a reproducible surrogate measure of the clinical status of the spinal cord in mice, potentially becoming a standard approach for evaluating experimental therapies.

Algorithms↗

PubMed2006

Active catheter tracking using parallel MRI and real-time image reconstruction.

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.

Algorithms↗

PubMed2006

Dynamic coil selection for real-time imaging in interventional MRI.

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.

Algorithms↗

PubMed2005

Influence of different breathing maneuvers on internal and external organ motion: use of fiducial markers in dynamic MRI.

PURPOSE: To investigate, with dynamic magnetic resonance imaging (dMRI) and a fiducial marker, the influence of different breathing maneuvers on internal organ and external chest wall motion. METHODS AND MATERIALS: Lung and chest wall motion of 16 healthy subjects (13 male, 3 female) were examined with real-time trueFISP (true fast imaging with steady-state precession) dMRI and a small inductively coupled marker coil on either the abdomen or thorax. Three different breathing maneuvers were performed (predominantly "abdominal breathing," "thoracic breathing," and unspecific "normal breathing"). The craniocaudal (CC), anteroposterior (AP), and mediolateral (ML) lung distances were correlated (linear regression coefficient) with marker coil position during forced and quiet breathing. RESULTS: Differences of the CC distance between maximum forced inspiration and expiration were significant between abdominal and thoracic breathing (p < 0.05). The correlation between CC distance and coil position was best for forced abdominal breathing and a marker coil in the abdominal position (r = 0.89 +/- 0.04); for AP and ML distance, forced thoracic breathing and a coil in the thoracic position was best (r = 0.84 +/- 0.03 and 0.82 +/- 0.03, respectively). In quiet breathing, a lower correlation was found. CONCLUSION: A fiducial marker coil external to the thorax in combination with dMRI is a new technique to yield quantitative information on the correlation of internal organ and external chest wall motion. Correlations are highly dependent on the breathing maneuver.

Abdomen↗

PubMed2005

[Measurements of respiratory motion using fast magnetic resonance imaging and inductively-coupled marker coils].

The respiratory motion of the thoracic wall provides indirect information about the breathing displacement of the inner organs. To analyze the correlation between thoracic wall and lung motion for applications in radiation therapy, the breathing displacement of the lung is visualized with a fast gradient echo pulse sequence (trueFISP) at a rate of 2-3 images/sec. For quantification of the motion, a small inductively-coupled marker coil is attached to the chest wall and detected with a fast projection technique. Since the marker coil generates a flip angle amplification (factor 15) in its interior, very small nominal flip angles of 2 degrees can be used during the projection measurements which do not affect the image quality of the trueFISP images. Volunteer studies with the marker coil showed a good agreement with simultaneously acquired breathing belt data and position information extracted from the MR images. Whereas the breathing belt provided reliable data only within a certain dynamic range, the marker coil could detect also extreme breathing excursions with a precision better than 2 millimeters.

Equipment Design↗

PubMed2004

MR-guided intravascular procedures: real-time parameter control and automated slice positioning with active tracking coils.

PURPOSE: To implement and optimize a real-time pulse sequence and user interface to perform intravascular interventions using active catheter tracking. MATERIALS AND METHODS: In magnetic resonance (MR)-guided interventions, small radio-frequency coils can be used to rapidly determine the device position (active tracking). In this work, active catheter tracking was combined with a dedicated real-time pulse sequence and user interface. The pulse sequence offered the imaging contrasts fast low angle shot (FLASH), true Fast imaging with steady state precession (TrueFISP), and projection MR digital subtraction angiography (MR-DSA), which could be selected by the radiologist from within the scanner room at any time during the intervention. Automatic slice positioning was added to the real-time pulse sequence so that the location of the tracking coils defined the image slice position and orientation. The technique was assessed in phantoms and animal experiments. RESULTS: At a reaction time of 24 msec and a frame rate of three images per second, the movement of an active intravascular catheter could be monitored in the aorta and the renal arteries of a pig. With interactive contrast and orientation changes, the renal vasculature could be assessed by a fully MR-guided catheterization in less than 10 minutes. CONCLUSION: With carefully designed active catheters, a dedicated user interface, and an optimized pulse sequence intravascular interventions can successfully be performed by a single operator from within the MR scanner room.

Animals↗

PubMed2004

MR coil design for simultaneous tip tracking and curvature delineation of a catheter.

In active catheter tracking, small RF coils are attached to the catheter for localization. For interactive catheter steering at vessel branchings, it is necessary to visualize not only a single point near the catheter tip but also the entire shape and orientation of the catheter's distal end. Therefore, a 35-mm-long twisted-pair RF coil was added to a 5 French intravascular catheter with a single tip-tracking coil. With the use of small nonmagnetic electronic components at the catheter tip, and a special switching circuitry outside the catheter, the coil assembly could be operated in two different modes. During MRI, the tip-tracking coil was detuned so that the MR signal was received by the visualization coil only. During tracking, detuning was switched off and the MR signal was predominantly received by the more sensitive tracking coil. The catheter was used in combination with a MR pulse sequence with automatic slice positioning so that the current imaging slice was always placed at the position of the catheter tip. Phantom and animal experiments showed that the catheter tip is better visualized with the combined approach than with a tracking coil alone.

Animals↗

PubMed2004

Semiquantitative fast flow velocity measurements using catheter coils with a limited sensitivity profile.

Flow measurements can be used to quantify blood flow during MR-guided intravascular interventional procedures. In this study, a fast flow measurement technique is proposed that quantifies flow velocities in the vicinity of a small RF coil attached to an intravascular catheter. Since the small RF coil receives signal from only a limited volume around the catheter, a spatially nonselective signal reception is employed. To enhance signal from flowing blood, and suppress unwanted signal contributions from static material, a slice-selective RF excitation is used. At a velocity sensitivity of 150 cm/s, a temporal resolution of 2 x TR = 10.2 ms can be achieved. The flow measurement is combined with an automatic slice positioning to facilitate measurements during interventional procedures. The influence of the catheter position in the blood vessel on the velocity measurement was analyzed in simulations. For blood vessels with laminar flow, the simulation showed a systematic deviation between catheter measurement and true flow between -15% and 80%. In four animal experiments, the catheter velocity measurement was compared with results from a conventional ECG-triggered 2D phase-contrast (PC) technique. The shapes of the velocity time curves in the abdominal aorta were nearly identical to the conventional measurements. A relative scaling factor of 0.69-1.19 was found between the catheter velocity measurement and the reference measurement, which could be partly explained by the simulation results.

Animals↗

PubMed2004

Renal embolization: feasibility of magnetic resonance-guidance using active catheter tracking and intraarterial magnetic resonance angiography.

RATIONALE AND OBJECTIVES: Magnetic resonance (MR)-guidance of endovascular interventions offers various advantages, including the absence of ionizing radiation, excellent soft tissue contrast, and multiplanar and functional imaging capabilities. The objective of this study was to assess the feasibility of MR-guided renal embolization using active catheter tracking with automatic slice positioning and intraarterial contrast-enhanced MR angiography (MRA). MATERIALS AND METHODS: MR-guided embolization of 16 kidneys was attempted in 15 pigs using real-time tracking of active 5-Fr. catheters. Embolization was monitored by selective intraarterial projection MRA. Intraarterial three-dimensional (3D) MRA was used for the assessment of embolization results. Additional pathologic correlation was available in 2 animals. The image quality of intraarterial 3D contrast-enhanced-MRA was rated by an independent radiologist who was not involved in the animal experiments. RESULTS: Active catheter tracking with automatic slice positioning allowed reliable catheter guidance and catheterization of the renal artery in all animals. Embolization was successful in all kidneys (11 left, 5 right), as verified by intraarterial 3D contrast-enhanced MRA (ce-MRA) and/or pathology. The image quality of intraarterial 3D ce-MRA was rated excellent in 10 animals, moderate in 4 animals, and poor in 1 animal. CONCLUSION: Renal embolization using active catheter tracking and intraarterial ce-MRA is feasible. Selective intraarterial ce-MRA allows the assessment of blood supply and organ perfusion before, during, and after therapeutic interventions, thereby complementing MR-guided endovascular interventions.

Animals↗

PubMed2003

[Automatic slice tracking in interventional magnetic resonance imaging].

Magnetic resonance imaging (MRI) is ideally suited to monitor minimally invasive operations with catheters or needles, since it offers both a superior soft-tissue contrast and the possibility to perform functional tests. In the present study, small radio-frequency coils were attached to the instruments in order to localize the MR-invisible instruments. The implementation of active instrument tracking is described on the basis of the example of active catheter tracking. In this case, the current position information of the instrument is used to automatically position the MRI slice at the catheter location. In combination with a user interface, the interventional radiologist is offered the possibility to perform vascular interventions from within the MR scanner room. At image update rates of approximately 3 Hz, tracking and placement of catheters in vascular structures are possible with interactive switching of slice orientation and image contrast. In an animal model, the technique was successfully used to selectively visualize the abdominal vessels and their branches under MRI guidance.

Animals↗