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.

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Purpose: To examine gradient switching-induced heating of metallic parts. Materials and Methods:Copper and titanium frames and sheets (Ϸ50 ϫ 50 mm 2 , 1.5 mm thick, frame width ϭ 3 mm) surrounded by air were positioned in the scanner perpendicular to the static field horizontally 20 cm off-center. During the execution of a sequence (three-dimensional [3D] true fast imaging with steady precession [True-FISP], TR ϭ 6.4 msec) exploiting the gradient capabilities (maximum gradient ϭ 40 mT/m, maximum slew rate ϭ 200 T/m/ second), heating was measured with an infrared camera. Radio frequency (RF) amplitude was set to zero volts. Heating of a copper frame with a narrowing to 1 mm over 20 mm at one side was examined in air and in addition surrounded by several liters of gelled saline using fiber-optic thermography. Further heating studies were performed using an artificial hip made of titanium, and an aluminum replica of the hip prosthesis with the same geometry. Results:For the copper specimens, considerable heating (Ͼ10°C) in air and in gelled saline (Ͼ1.2°C) could be observed. Heating of the titanium specimens was markedly less (Ϸ1°C in air). For the titanium artificial hip no heating could be detected, while the rise in temperature for the aluminum replica was approximately 2.2°C. Conclusion:Heating of more than 10°C solely due to gradient switching without any RF irradiation was demonstrated in isolated copper wire frames. Under specific conditions (high gradient duty cycle, metallic loop of sufficient inductance and low resistance, power matching) gradient switching-induced heating of conductive specimens must be considered. ONE IMPORTANT ASPECT in MR safety testing of medical implants and instruments is the inspection of whether hazardous heating can occur in the electromagnetic environment of an MR scanner. In the standard ASTM F 2182-02a (1), heating resulting from the interaction with the radio frequency (RF) magnetic field is considered. Numerous examinations have been reported concerning this subject (e.g., Ref. 2-10). Regarding gradient switching, there are a few works that address the alteration of induced nerve stimulation near metallic implants (11,12). Two publications (13,14) ascribe heat sensations of patients with larger metallic implants to vibrations of these implants caused by gradient switching.In contrast to ASTM F 2182-02a and to those works, the present study is concerned with direct gradient switching-induced heating of medical implants or instruments made of electrically conducting material. Following Faraday's law, the change of the magnetic flux through such a device induces eddy currents in the device and the metal subsequently converts electric energy into thermal energy. This effect increases with distance from isocenter. To obtain insight into the order of magnitude of possible effects, quadratic wire frames and sheets made of copper and titanium were examined. Most experiments were performed in air, since the underlying cause for the heating is pure magnetic induction during gradien...

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 ...

Bifunctional SuperCD suicide gene expression is highly effective in a rat hepatoma model, thereby significantly improving both the therapeutic index and the efficacy of hepatocellular carcinoma killing by fluorocytosine.

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.

MRI-guided RF ablation of RCC in an interventional MR unit is safe and feasible. Fast MR imaging is a convenient method for rapid positioning of MR-compatible RF electrodes. MR monitoring of ablation procedure with T2-weighted imaging allows for immediate assessment of coagulation extent.