The effect of frequency (64-498 MHz) on specific absorption rate adjacent to metallic orthopedic screws in MRI: A numerical simulation study

BACKGROUND

The imaging of patients with implanted electrically-conductive devices via magnetic resonance imaging at ultra-high fields is hampered by uncertainties relating to the potential for inducing tissue heating adjacent to the implant due to coupling of energy from the incident electromagnetic field into the implant. Existing data in the peer-reviewed literature of comparisons across field strengths of tissue heating and its surrogate, the specific absorption rate (SAR), is scarce and contradictory, leading to further doubts pertaining to the safety of imaging patients with such devices.

PURPOSE

The radiofrequency-induced SAR adjacent to orthopedic screws of varying length and at frequencies of 64 to 498 MHz was investigated via full-wave electromagnetic simulations, to provide an accurate comparison of SAR across MRI field strengths.

METHODS

Dipole antennas were used for RF transmission to achieve a uniform electric field tangential to the screws located 120 mm above the antenna midpoints, embedded in a bone-mimicking material. The input power to the antennas was constrained to achieve the following targets without the screw present: (i) E = 100 V/m, (ii) B1 +  = 2 μT, and (iii) global-average-SAR = 3.2 W/kg. Simulations were performed with a spatial resolution of 0.2 mm in the volume surrounding the screws, resulting in 76-137 MCells, noting the maximum 1 g-averaged SAR value in each case. Simulations were repeated at 128 and 297 MHz for screws embedded in muscle tissue.

RESULTS

The peak SAR, occurring at the resonant screw length, substantially increased as the frequency decreased when the input power to the dipole antenna was constrained to achieve constant electric field in background tissue at the screws' locations. A similar pattern was observed when constraining input power to achieve constant B1 + and global-average-SAR. The dielectric properties of the tissue in which the screws were embedded dominated the SAR comparisons between 297 and 128 MHz.

CONCLUSIONS

The study design allowed for a direct comparison to be performed of SAR across frequencies and implant lengths without the confounding effect of variable incident electric field. Lower frequencies produced substantially larger SAR values for implants approaching the resonant length for the worst-case uniform incident electric field along the screws' length. The data may inform risk-benefit assessments for imaging patients with orthopedic implants at the new clinical field strength of 7 Tesla.

Computational modeling of the thermal effects of flow on radio frequency-induced heating of peripheral vascular stents during MRI

Purpose. The goal of this study was to develop and validate a computational model that can accurately predict the influence of flow on the temperature rise near a peripheral vascular stent during magnetic resonance imaging (MRI).Methods. Computational modeling and simulation of radio frequency (RF) induced heating of a vascular stent during MRI at 3.0 T was developed and validated with flow phantom experiments. The maximum temperature rise of the stent was measured as a function of physiologically relevant flow rates.Results. A significant difference was not identified between the experiment and simulation (P > 0.05). The temperature rise of the stent during MRI was over 10 °C without flow, and was reduced by 5 °C with a flow rate of only 58 ml min-1, corresponding to a reduction of CEM43from 45 min to less than 1 min.Conclusion. The computer model developed in this study was validated with experimental measurements, and accurately predicted the influence of flow on the RF-induced temperature rise of a vascular stent during MRI. Furthermore, the results of this study demonstrate that relatively low flow rates significantly reduce the temperature rise of a stent and the surrounding medium during RF-induced heating under typical scanning power and physiologically relevant conditions.

RF-induced heating of interventional devices at 23.66 MHz

OBJECTIVE

Low-field MRI systems are expected to cause less RF heating in conventional interventional devices due to lower Larmor frequency. We systematically evaluate RF-induced heating of commonly used intravascular devices at the Larmor frequency of a 0.55 T system (23.66 MHz) with a focus on the effect of patient size, target organ, and device position on maximum temperature rise.

MATERIALS AND METHODS

To assess RF-induced heating, high-resolution measurements of the electric field, temperature, and transfer function were combined. Realistic device trajectories were derived from vascular models to evaluate the variation of the temperature increase as a function of the device trajectory. At a low-field RF test bench, the effects of patient size and positioning, target organ (liver and heart) and body coil type were measured for six commonly used interventional devices (two guidewires, two catheters, an applicator and a biopsy needle).

RESULTS

Electric field mapping shows that the hotspots are not necessarily localized at the device tip. Of all procedures, the liver catheterizations showed the lowest heating, and a modification of the transmit body coil could further reduce the temperature increase. For common commercial needles no significant heating was measured at the needle tip. Comparable local SAR values were found in the temperature measurements and the TF-based calculations.

CONCLUSION

At low fields, interventions with shorter insertion lengths such as hepatic catheterizations result in less RF-induced heating than coronary interventions. The maximum temperature increase depends on body coil design.

Radio-frequency induced heating of intra-cranial EEG electrodes: The more the colder?

Many neurological disorders are analyzed and treated with implantable electrodes. Many patients with such electrodes have to undergo MRI examinations - often unrelated to their implant - at the risk of radio-frequency induced heating. The number of electrode contact sites of these implants keeps increasing due to improvements in manufacturing and computational algorithms. Electrode grids with multiple receive channels couple to the RF fields present in MRI, but, due to their proximity, a combination of leads has a coupling response which is not a superposition of the individual leads' response. To investigate the problem of RF-induced heating of coupled multi-lead implants, temperature mapping was performed on a set of intra-cranial electroencephalogram (icEEG) electrode grid prototypes with increasing number of contact sites (1-16). Additionally, electric field measurements were used to investigate the radio-frequency heating characteristics of the implants in different media combinations, simulating the device being partially immersed inside the patient. MR measurements show RF-induced heating up to 19.6 K for the single electrode, reducing monotonically with larger number of contact sites to a minimum of 0.9 K for the largest grid. The SAR calculated from temperature measurements agrees well with electric field mapping: The same trend is visible for different insertion lengths, however, the energy dissipated by the whole implant varies with the grid size and insertion length. Thus, in the tested circumstances, a larger electrode number either reduced or had a similar risk of RF induced heating, indicating, that the size of electrode grids is a design parameter, which can be used to change an implants RF response and in turn to reduce the risk of RF induced heating and improve the safety of patient with neuro-implants undergoing MRI examinations.

Artifact quantification of venous stents in the MRI environment: Differences between braided and laser-cut designs

PURPOSE

To quantify B₀- and B₁-induced imaging artifacts of braided venous stents and to compare the artifacts to a set of laser-cut stents used in venous interventions.

METHODS

Three prototypes of braided venous stents with different geometries were tested in vitro. B₀ field distortion maps were measured via the frequency shift Δf using multi-echo imaging. B₁ distortions were quantified using the double angle method. The relative amplitudes B₁^rel were calculated to compare the intraluminal alteration of B₁. Measurements were repeated with the stents in three different orientations: parallel, diagonal and orthogonal to B₀.

RESULTS

At 1.5 T, the braided stents induced a maximum frequency shift of Δfx<100Hz. Signal voids were limited to a distance of 2 mm to the stent walls at an echo time of 3 ms. No substantial difference in the B₀ field distortions was seen between laser-cut and braided venous stents. B₁^rel maps showed strongly varying distortion patterns in the braided stents with the mean intraluminal B₁^rel ranging from 63±18% in prototype 1 to 98±38% in prototype 2. Compared to laser-cut stents the braided stents showed a 5 to 9 times higher coefficient of variation of the intraluminal B₁^rel.

CONCLUSION

Braided venous stent prototypes allow for MR imaging of the intraluminal area without substantial signal voids due to B₀-induced artifacts. Whereas B₁ is attenuated homogeneously in laser-cut stents, the B₁ distortion in braided stents is more inhomogeneous and shows areas with enhanced amplitude. This could potentially be used in braided stent designs for intraluminal signal amplification.