Diffusion-weighted echo planar MR imaging of the neck at 3 T using integrated shimming: comparison of MR sequence techniques for reducing artifacts caused by magnetic-field inhomogeneities.

Objective: Our objective was to compare available techniques reducing artifacts in echo planar imaging (EPI)-based diffusion-weighed magnetic resonance imaging MRI (DWI) of the neck at 3 Tesla caused by B0-field inhomogeneities.

Materials And Methods: A cylindrical fat-water phantom was equipped with a Maxwell coil allowing for additional linear B0-field variations in z-direction. The effect of increasing strength of this superimposed gradient on image quality was observed using a standard single-shot EPI-based DWI sequence (sEPI), a zoomed single-shot EPI sequence (zEPI), a readout-segmented EPI sequence (rsEPI), and an sEPI sequence with integrated dynamic shimming (intEPI) on a 3-Tesla system.

Technical Note: MR-visualization of interventional devices using transient field alterations and balanced steady-state free precession imaging.

Purpose: In interventional magnetic resonance imaging, instruments can be equipped with conducting wires for visualization by current application. The potential of sequence triggered application of transient direct currents in balanced steady-state free precession (bSSFP) imaging is demonstrated.

Methods: A conductor and a modified catheter were examined in water phantoms and in an ex vivo porcine liver.

Magnetic resonance visualization of conductive structures by sequence-triggered direct currents and spin-echo phase imaging.

Purpose: Instrument visualization in interventional magnetic resonance imaging (MRI) is commonly performed via susceptibility artifacts. Unfortunately, this approach suffers from limited conspicuity in inhomogeneous tissue and disturbed spatial encoding. Also, susceptibility artifacts are controllable only by sequence parameters.

Successful Salvage Chemotherapy with FOLFIRINOX for Recurrent Mixed Acinar Cell Carcinoma and Ductal Adenocarcinoma of the Pancreas in an Adolescent Patient.

Pancreatic tumors are rare in children and adolescents. Here, we report the case of a 15-year-old boy who presented with a mixed acinar cell carcinoma/ductal adenocarcinoma with blastomatous components. He received multimodal treatment including various chemotherapy regimens and multistep surgery including liver transplantation.

Image-guided radiofrequency ablation of hepatocellular carcinoma (HCC): is MR guidance more effective than CT guidance?

Objectives: The purpose of the study was to retrospectively compare technique effectiveness of computed tomography (CT)-guided versus magnetic resonance (MR)-guided radiofrequency (RF) ablation of hepatocellular carcinoma (HCC).

Materials And Methods: In 35 consecutive patients 53 CT-guided (n=29) or MR-guided (n=24) ablation procedures were performed in the treatment of 56 (CT: 29; MR: 27) HCC. The entire ablation procedure was performed at a multislice CT-scanner or an interventional 0.

Resolution adapted finite element modeling of radio frequency interactions on conductive resonant structures in MRI.

Prediction of interactions between the radiofrequency electromagnetic field in magnetic resonance scanners and electrically conductive material surrounded by tissue plays an increasing role for magnetic resonance safety. Testing of conductive implants or instruments is usually performed by standardized experimental setups and temperature measurements at distinct geometrical points, which cannot always reflect worst-case situations. A finite element method based on Matlab (The Mathworks, Natick, MA) and the finite element method program Comsol Multiphysics (Stockholm, Sweden) with a spatially highly variable mesh size solving Maxwell's full-wave equations was applied for a comprehensive simulation of the complete geometrical arrangement of typical birdcage radiofrequency coils loaded with small conductive structures in a homogenous medium.

RF tissue-heating near metallic implants during magnetic resonance examinations: an approach in the ac limit.

Purpose: State of the art to access radiofrequency (RF) heating near implants is computer modeling of the devices and solving Maxwell's equations for the specific setup. For a set of input parameters, a fixed result is obtained. This work presents a theoretical approach in the alternating current (ac) limit, which can potentially render closed formulas for the basic behavior of tissue heating near metallic structures.

Quantification of direct current in electrically active implants using MRI methods.

The aim of this study was to evaluate a variety of phase- and magnitude-based MRI methods at 1.5 T and 3 T regarding their sensitivity and accuracy with respect to the quantification of electrical direct current via the induced magnetic field inhomogeneity. For this, a phantom was constructed which was specially designed to reduce RF effects and which provided a one-dimensional electrical direct current in a thin copper conductor perpendicular to the static magnetic field of the scanner.

Magnetic resonance perfusion imaging without contrast media.

Purpose: Principles of magnetic resonance imaging techniques providing perfusion-related contrast weighting without administration of contrast media are reported and analysed systematically. Especially common approaches to arterial spin labelling (ASL) perfusion imaging allowing quantitative assessment of specific perfusion rates are described in detail. The potential of ASL for perfusion imaging was tested in several types of tissue.

Biopsy needle tips with markers--MR compatible needles for high-precision needle tip positioning.

Needle tip visualization is of high importance in magnetic resonance imaging (MRI) guided interventional procedures, for example for taking biopsies from suspicious lesions in the liver or kidney. The exact position of the needle tip is often obscured by image artifacts arising from the magnetic properties of the needle. The authors investigated two special biopsy needle tip designs using diamagnetic coatings.

Tissue warming and regulatory responses induced by radio frequency energy deposition on a whole-body 3-Tesla magnetic resonance imager.

Purpose: To quantify the B1-field induced tissue warming on a 3T-whole-body scanner, to test whether the patient is able to sense the temperature change, and to evaluate whether the imaging procedure constitutes a significant cardiovascular stress.

Materials And Methods: A total of 18 volunteers were divided into three equal groups for 3.0T MRI of the pelvis, the head, or the knee.

Heating of metallic implants and instruments induced by gradient switching in a 1.5-Tesla whole-body unit.

Purpose: To examine gradient switching-induced heating of metallic parts.

Materials And Methods: Copper and titanium frames and sheets ( approximately 50 x 50 mm(2), 1.5 mm thick, frame width = 3 mm) surrounded by air were positioned in the scanner perpendicular to the static field horizontally 20 cm off-center.

Numerical simulations of intra-voxel dephasing effects and signal voids in gradient echo MR imaging using different sub-grid sizes.

Signal void artifacts in gradient echo imaging are caused by the intra-voxel dephasing of the spins. Intra-voxel dephasing can be estimated by computing the field distribution on a sub-grid inside each picture element, followed by integration of all magnetization components. The strategy of computing the artifacts based on the integration of the sub-voxel signal components is presented here for different sub-grids.

Eddy-current induction in extended metallic parts as a source of considerable torsional moment.

Purpose: To examine eddy-current-provoked torque on conductive parts due to current induction from movement through the fringe field of the MR scanner and from gradient switching.

Materials And Methods: For both cases, torque was calculated for frames of copper, aluminum, and titanium, inclined to 45 degrees to B0 (maximum torque case). Conditions were analyzed in which torque from gravity (legal limit, ASTM F2213-02) was exceeded.

Effects on MRI due to altered rf polarization near conductive implants or instruments.

In magnetic resonance imaging near metal parts variations in radio frequency (rf)-amplitude and of receive sensitivity must be considered. For loop structures, e.g.

Relaxivity of Gadopentetate Dimeglumine (Magnevist), Gadobutrol (Gadovist), and Gadobenate Dimeglumine (MultiHance) in human blood plasma at 0.2, 1.5, and 3 Tesla.

Objectives: We sought to determine the relaxivity and accurate relaxation rates of Gd-DTPA, Gd-BT-DO3A, and Gd-BOPTA at 0.2, 1.5, and 3 T in human blood plasma.

Interaction between grounding pads used for RF ablation therapy and magnetic resonance imaging.

Objectives: To characterize artifacts and imaging problems in the presence of conductive grounding pads for RF ablation therapy as well as potential heating problems due to induction of eddy currents in the pads. Strategies for avoidance of those problems are developed.

Materials And Methods: Underlying principles of interactions between grounding pads and MR imaging are reported.

Magnetic susceptibility effects on the accuracy of MR temperature monitoring by the proton resonance frequency method.

Purpose: To evaluate the error of MR temperature assessment based on the temperature-dependent Larmor frequency shift of water protons, which can result from susceptibility effects caused by the radiofrequency (RF) applicator.

Materials And Methods: Local frequency shifts due to RF applicator displacements were simulated numerically by means of a three-dimensional elementary dipole model. Experimental examinations using a water tank phantom equipped with a high-precision screw thread were applied to examine temperature and movement effects for five commercially available, MR-compatible RF applicators.

Magnetic resonance-guided percutaneous radiofrequency ablation of renal cell carcinomas: a pilot clinical study.

Objective: The objective of this study was to assess the feasibility and efficacy of magnetic resonance imaging-(MRI) guided percutaneous radiofrequency (RF) ablation of renal cell carcinomas (RCC).

Subjects And Methods: Twelve patients with RCC (63 to 82 years old) were treated with RF ablation in an interventional 0.2-Tesla open MR unit.

Metal artifacts caused by gradient switching.

In metal parts, e.g., implants or instruments, eddy currents can be induced from gradient switching if positioned off-center inside the MR scanner.

Radio frequency versus susceptibility effects of small conductive implants--a systematic MRI study on aneurysm clips at 1.5 and 3 T.

Purpose: Metallic implants cause enlarged artifacts in magnetic resonance (MR) images at higher magnetic fields, B0, due to their magnetic susceptibility. Interactions of conductive material with radio frequency (RF) pulses also change for higher field strengths, B0, due to the frequency dependence of resonance conditions. Systematic measurements on commercial aneurysm clips and simplified copper models were performed in order to investigate both phenomena at 1.

RF artifacts caused by metallic implants or instruments which get more prominent at 3 T: an in vitro study.

Metallic devices with high electrical conductivity inside or adjacent to the body might lead to marked alterations of the RF amplitude B1 in the tissue under investigation, especially at increased RF frequency, and if specific conditions for electromagnetic resonance are fulfilled. RF-metal interaction effects were investigated systematically at B0=0.2, 1.

rf enhancement and shielding in MRI caused by conductive implants: dependence on electrical parameters for a tube model.

Radio frequency (rf) eddy-currents induced in implants made of conductive material might cause significant image artifacts in magnetic resonance imaging (MRI) such as shielding of the lumen of vascular stents. rf alteration near metal parts was assessed theoretically in the approximation of alternating current electrodynamics: The implant was modeled as tube with diameter d(o), resistance R, and reactance Y, constituting the secondary winding of a transformer. The transmitter coil of the scanner acted as primary winding and generated the linearly polarized rf field B1,app.

Compensation of magnetic field distortions from paramagnetic instruments by added diamagnetic material: measurements and numerical simulations.

In minimally invasive procedures guided by magnetic resonance (MR) imaging instruments usually are made of titanium or titanium alloys (e.g., nitinol), because other more MR-compatible materials often cannot provide sufficient mechanical properties.