No abstract

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 (d o ‫؍‬ 20 mm, d i ‫؍‬ 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. Key words: metal artifact; eddy current; implant; instrument; MRI Depending on the medical indication magnetic resonance imaging (MRI) may be performed on patients having nonferromagnetic metallic implants. MRI is also increasingly applied in radiologic interventions where metallic instruments are used.Most metals show distinctly different magnetic susceptibility compared to human tissue and perturbed B 0 homogeneity leads to typical related image artifacts. In gradientecho (GRE) imaging, signal loss due to spin dephasing dominates, whereas disturbed gradient linearity near the metal becomes visible as distortions in spin-echo (SE) images. Perturbed slice selection and perturbed spatial encoding in the read-out (RO) direction produce characteristic artifacts (1).In addition, metals can interact with the time varying magnetic fields of the MR scanner (RF magnetic field B 1 , gradient switching) due to their electrical conductivity, i.e., induction phenomena can occur. Concerning B 1 effects, the alteration of the amplitude adjacent to the metal (2,3) and shielding inside circular structures, i.e., vascular stents, has been investigated (4,5). Gradient switching effects from eddy currents, induced in conductive components of the scanner, are a well-known problem and have been intensively examined (6,7). However, apart from the very recent observations of Shenhav and Azhari (8) no examinations exist confirming artifact generation caused by gradient switching induced eddy currents in metal parts embedded in the sample, such as in the case of medical implants. A probable explanation seems to be that in vitro examination of specimens has been performed near isocenter (9,10) and that in vivo susceptibility artifacts dominate in most cases.The presented work demonstrates that gradient switching induced eddy currents can play an important role for artifact ...

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 Faraday's 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.

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.

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. An imaging protocol operating at first level mode was applied, allowing radio frequency (RF) irradiation up to the legal specific absorption rate (SAR) limits. An identical placebo protocol with active gradient switching but without RF transmission was used. Temperature changes were measured with a fiber-optic thermometer (FO) and an infrared camera (IR).Results: Temperature differences to the placebo were highest for imaging of the pelvis (FO: ⌬T ϭ 0.88 Ϯ 0.13°C, IR: ⌬T ϭ 1.01 Ϯ 0.15°C) as compared to the head (FO: ⌬T ϭ 0.46 Ϯ 0.12°C, IR: ⌬T ϭ 0.47 Ϯ 0.10°C) and the knee (FO: ⌬T ϭ 0.33 Ϯ 0.11°C, IR: ⌬T ϭ 0.37 Ϯ 0.09°C). The volunteers were able to discriminate between imaging and placebo for pelvic (P Ͻ 0.0001) and head (P ϭ 0.0005) imaging but not for knee imaging (P ϭ 0.209). No changes in heart rate or blood pressure were detected. Conclusion:The 3.0T MRI in the first operational mode may lead to measurable and perceptible thermal energy deposition. However, it may be regarded as safe concerning the thermoregulatory cardiovascular stress.