Hansjörg Graf

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. Materials and Methods:Contrast media concentrations between 0.01 and 16 mM in human plasma were used for relaxation measurements. The R1 and R2 relaxation rates and r1 and r2 relaxivities were determined. Results:Gd-BOPTA produced the highest relaxation rates and relaxivities at all field strengths. The r1 and r2 values for Gd-BOPTA were 107–131% and 91–244% higher than for Gd-DTPA, respectively, and 72–98% and 82–166% higher than for Gd-BT-DO3A. Higher field strengths resulted in lower values of R1, R2, and r1 for all contrast agents tested and of r2 for Gd-DTPA and Gd-BT-DO3A. A linear dependence of R1 and R2 on concentration was found for Gd-DTPA and Gd-BT-DO3A and a nonlinear dependence for Gd-BOPTA for concentrations larger than 1 mM. The r1 and r2 relaxivity of Gd-BOPTA increased with decreasing concentration. Conclusions:Gd-BOPTA demonstrates the highest longitudinal r1 at all field strengths, which is ascribable to weak protein interaction. The R2/R1 ratio increases at higher field strength only for Gd-BOPTA, hence very short echo times are required for Gd-BOPTA to benefit from the higher longitudinal relaxivity.

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

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.

Numerical modeling of needle tip artifacts in MR gradient echo imaging.

Exact determination of needle tip position is obsolete for interventional procedures under control of magnetic resonance imaging (MRI). Exact needle tip navigation is complicated by the paramagnetism of microsurgical instruments: Local magnetic field inhomogeneities are induced resulting in position encoding artifacts and in signal voids in the surrounding of instruments and especially near their tips. The artifacts generated by the susceptibility of the material are not only dependent on the material properties themselves and on the applied MRI sequences and parameters, but also on the geometric shape of the instruments and on the orientation to the static magnetic field in the MR unit. A numerical model based on superposition of induced elementary dipole fields was developed for studying the field distortions near paramagnetic needle tips. The model was validated by comparison with experimental data using field mapping MRI techniques. Comparison between experimental data and numerical simulations revealed good correspondence for the induced field inhomogeneities. Further systematic numerical studies of the field distribution were performed for variable types of concentric and asymmetric tip shapes, for different ratios between tip length and needle diameter, and for different orientations of the needle axis in the external static magnetic field. Based on the computed local inhomogeneities of the magnetic field in the surroundings of the needle tips, signal voids in usual gradient echo images were simulated for a prediction of the artifacts. The practically relevant spatial relation between those artifacts and the hidden tip of the needle was calculated for the different tip shapes and orientations in the external field. As needle tip determination is crucial in interventional procedures, e.g., in taking biopsies, the present model can help to instruct the physician prior to surgical interventions in better estimating the needle tip position for different orientations and needle tip shapes as they appear in interventional procedures. As manufacturing prototypes with subsequent measurements of artifacts in MRI are a costly procedure the presented model may also help to optimize shapes of needle tips and of other parts of MR-compatible instruments and implants with low expense prior to production if some shape parameters can be chosen freely.

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., vascular stents, B1 produces rf eddy currents in accordance to Faradays law; the B1-related electrical rf field E1 injects directly to elongated structures (e.g., wires). Locally, the rf magnetic field Bl,ind (induced B1) is superimposed onto the rf field from the transmitter coil, which near the metal can dominate spin excitation. Geometry and arrangement of the parts determine the polarization of B(1,ind). Components parallel to B0 are of special interest. A copper sheet (100 mm x 15 mm, 3 mm thick) and a 27 cm long copper wire were examined in a water phantom using the spin-echo (SE) technique. In addition to rf-amplitude amplification, rf-phase shift due to z components of B(1,ind) could be detected near the metallic objects. Periodic rf-amplitude instabilities had an amplified effect for phase-shifted regions. Phase-encoding artifacts occurred as distinct ghosts (TR=200 ms) or band-like smearing (TR=201 ms) from affected spin ensembles. SE phase imaging can potentially be used in interventional magnetic resonance imaging for background-free localization of metallic markers.

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. For the first time, a systematic analysis of related artifacts was performed. Current strength increases in conjunction with increasing size of the part, increasing electrical conductivity, distance from isocenter, and increasing gradient strengths. A xy‐plane oriented copper ring (do = 20 mm, di = 15 mm, 2 mm thick) was examined at isocenter and at x = 15 cm, y = z = 0. Comparisons of xy‐, xz‐, and yz‐slices, recorded for both possibilities to select encoding directions, revealed effects from ramp‐down of the slice‐selection and ramp‐up of the read‐out gradient. Near the metal part, temporary inhomogeneities were superimposed to the static field and spin‐dephasing signal loss resulted, despite using spin‐echo technique. Artifacts depended on excitation and read‐out bandwidth. For an equivalent titanium ring, conductivity related effects could not be ascertained but distinct susceptibility effects occurred. MR compatibility of implants/instruments therefore requires both low susceptibility and low conductivity. Magn Reson Med 54:231–234, 2005.

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. Artifacts depending on susceptibility arise in MR images due to incorrect spatial encoding and intravoxel dephasing and thereby hamper the surgeons view onto the region of interest. To overcome the artifact problem, compensation of the paramagnetic properties by diamagnetic coating or filling of the instruments has been proposed in the literature. We used a numerical modeling procedure to estimate the effect of compensation. Modeling of the perturbation of the static magnetic field close to the instruments reflects the underlying problem and is much faster and cost efficient than manufacturing prototypes and measuring artifact behavior of these prototypes in the MR scanner. A numerical model based on the decomposition of the susceptibility distribution in elementary dipoles was developed by us. The program code was written object oriented to allow for both maximum computational speed and minimum random access memory. We used System International units throughout the modeling for the magnetic field, allowing absolute quantification of the magnetic field disturbance. The field outside a simulated needlelike instrument, modeled by a paramagnetic cylinder (out of titan, chi =181.1) of length 8.0 mm and of diameter 1.0 mm, coated with a diamagnetic layer (out of bismuth, chi=-165.0) of thickness 0, 0.1, 0.2, 0.3, and 0.4 mm, was found to be best compensated if the cross-sectional area of the cylinder, multiplied by the absolute susceptibility value of the cylinder material, is equal to the cross-sectional area of the coating, multiplied by the absolute susceptibility value of the coating material. At the extremity of the coated cylinder an uncompensated field distortion was found to remain. We studied various tip shapes and geometries using our computational model: Suitable diamagnetic coating or filling of paramagnetic instruments clearly reduced tip artifacts and diminished the dependency of artifact size on orientation of the instrument with respect to B0 in the numerical studies. We verified the results of the simulations by measuring coated and uncoated titanium wires in a 1.5 T MR scanner.

Inductively coupled rf coils for examinations of small animals and objects in standard whole-body MR scanners.

Inductively coupled solenoid coils fitting to objects in the size of mice or rats were developed to adapt modem whole-body MR scanners featuring sufficient gradient strength for animal examinations with high spatial resolution. Homogenous receiver characteristics is achievable over almost the whole inner region of the solenoid coils. The SNR can be increased by a factor 2 to 6 with the adapting coils for examinations using the head coil as connected receiver. Standard sequences on clinical 1.5 T scanners can be applied with adapted transmitter voltages. For example, a SNR value of about 30 is achievable in a mouse liver after 10 minutes measuring time using a 2-D spin echo imaging sequence and a size of 0.3 x 0.3 x 0.8 mm3 for the picture elements.

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

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.

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

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.

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 0.2-Tesla MR-scanner. Assessment of treatment response was based on dynamic MR imaging at 1.5Tesla. The mean follow-up was 22.9 months. Primary technique effectiveness was assessed 4 months after ablation therapy. Secondary technique effectiveness was assessed 4 months after a facultative second ablation procedure. Primary and secondary technique effectiveness of CT-guided and MR-guided RF ablation was compared by using Chi-Square (likelihood ratio) test. RESULTS Primary technique effectiveness after a single session was achieved in 26/27 (96.3%) HCC after MR-guided RF ablation and 23/29 (79.3%) HCC after CT-guided RF ablation (Chi-Square: p=0.04). Secondary technique effectiveness was achieved in 26/27 (96.3%) HCC after MR-guided RF ablation and in 26/29 (89.7%) HCC after CT-guided RF ablation (Chi-Square: p=0.32). A local tumor progression was detected in 8/52 (15.4%) tumors after initial technique effectiveness. Major complications were detected after 3/53 (5.7%) ablation procedures. CONCLUSIONS CT-guided and MR-guided RF ablations are locally effective therapies in the treatment of HCC. Due to a higher rate of primary technique effectiveness MR-guided RF ablation may reduce the number of required sessions for complete tumor treatment.