On the RF heating of coronary stents at 7.0 Tesla MRI

Winter, Lukas; Oberacker, Eva; Özerdem, Celal; Ji, Yiyi; Knobelsdorff‐Brenkenhoff, Florian von; Weidemann, G.; Ittermann, Bernd; Seifert, F.; Niendorf, Thoralf

doi:10.1002/mrm.25483

Cited by 61 publications

(65 citation statements)

References 56 publications

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“…Results were in agreement for all 6 shims, with errors between simulations and measurements up to 21% on average, potentially due to the unknown exact dimensions of the body coil. These errors are in agreement with other E‐field measurements found in the literature . These results confirm the trend found in the psSAR maps, with shims generating substantially larger E‐fields when channel 1 was dominant.…”

Section: Resultssupporting

confidence: 92%

A numerical and experimental study of RF shimming in the presence of hip prostheses using adaptive SAR at 3 T

Destruel

Fuentes

Weber

et al. 2019

Magnetic Resonance in Med

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Purpose Parallel transmission techniques in MRI have the potential to improve the image quality near metal implants at 3 T. However, current testing of implants only evaluates the risk of radiofrequency (RF) heating in phantoms in circularly polarized mode. We investigate the influence of changing the transmission settings in a 2‐channel body coil on the peak temperature near 2 CoCrMo hip prostheses, using adaptive specific absorption rate (SAR) as an estimate of RF heating. Methods Adaptive SAR is a SAR averaging method that is optimized to correlate with thermal simulations and limit the temperature to 39°C near hip implants. The simulated peak temperature was compared when using whole‐body SAR, SAR10g, and adaptive SAR as a constraint for the maximum allowed input power. Adaptive SAR was used as a fast estimate of temperature to evaluate the trade‐off between good image quality and low heating near the hip implants. Electromagnetic simulations were validated by simulating and measuring B1 maps and electric fields in a phantom at 3 T. Results Simulations and measurements showed excellent agreement. Limiting whole‐body SAR to 2 W/kg and SAR10g to 10 W/kg resulted in temperatures up to 49.3°C and 40.7°C near the hip implants after 30 minutes of RF exposure, respectively. Predictions based on adaptive SAR limited the temperature to 39°C, and allowed to improve the B1 field distribution while preventing peak temperatures near the hip implants. Conclusion Significant RF heating can occur at 3 T near hip implants when parallel transmission is used. Adaptive SAR can be integrated in RF shimming algorithms to improve the uniformity and reduce heating.

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Section: Resultssupporting

confidence: 92%

A numerical and experimental study of RF shimming in the presence of hip prostheses using adaptive SAR at 3 T

Destruel

Fuentes

Weber

et al. 2019

Magnetic Resonance in Med

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“…UHF sites tend to be cautious with respect to the higher resonance frequency, the various transmit radiofrequency (RF) coils, and the fact that 7T MRI currently is not medically indicated. Nevertheless, some UHF sites already have performed dedicated safety tests for passive implants or have included carefully selected subjects with implants . Because manufacturers only have verified MRI of patients with implanted aneurysm clips to be conditional for some clips in an MR environment up to 3T, a detailed compliance test for 7T is a prerequisite before any examination.…”

Section: Introductionmentioning

confidence: 99%

In vitro and in silico assessment of RF‐induced heating around intracranial aneurysm clips at 7 Tesla

Noureddine

Kraff

Ladd

et al. 2017

Magnetic Resonance in Med

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Purpose: To examine radiofrequency-induced tissue heating around intracranial aneurysm clips during a 7 Tesla (T) head MR examination. Methods: Radiofrequency (RF), temperature simulations, and RF measurements were employed to investigate the effects of polarization and clip length on the electric field (E-field) and temperature. Heating in body models was studied using both a conservative approach and realistic exposure scenarios. Results: Worst-case orientation was found for clips aligned parallel to the E-field polarization. Absolute tissue temperature remained below International Electrotechnical Commission regulatory limits for 44 of 50 clinical scenarios. No significant effect on heating was determined for clip lengths below 18.8 mm, and worst-case heating was found for clip length 51.5 mm. The conservative approach led to a maximum permissible E-field of 72 V/m corresponding to B þ 1 of 1.2 mT, and an accepted power of 4.6 W for the considered RF head coil instead of 38.5 W without clip. Conclusion: Safe scanning conditions with respect to RFinduced heating can be applied depending on the information about the clip gained during screening interviews. However, force and torque measurements in the MR system shall be conducted to give a final statement on the MR safety of aneurysm clips at 7T. Magn Reson Med 79:568-581,

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“…Although SAR has been routinely used as an indirect quantitative measurement for clinical MRI procedures when conductive implants are present , some studies have shown that reported whole‐body SAR values that lead to the same temperature rise in implants significantly varied across different MRI scanners . Moreover, local SAR values do not always directly translate to temperature rise because factors such as thermal conduction and perfusion should be taken into account . Consequently, when it comes to patient safety, temperature calculations/measurements should be considered instead of SAR monitoring alone.…”

Section: Resultsmentioning

confidence: 99%

Feasibility of using linearly polarized rotating birdcage transmitters and close-fitting receive arrays in MRI to reduce SAR in the vicinity of deep brain simulation implants

et al. 2016

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Purpose MRI of patients with deep brain stimulation (DBS) implants is strictly limited due to safety concerns, including high levels of local specific absorption rate (SAR) of radiofrequency (RF) fields near the implant and related RF-induced heating. This study demonstrates the feasibility of using a rotating linearly polarized birdcage transmitter and a 32-channel close-fit receive array to significantly reduce local SAR in MRI of DBS patients. Methods Electromagnetic simulations and phantom experiments were performed with generic DBS lead geometries and implantation paths. The technique was based on mechanically rotating a linear birdcage transmitter to align its zero electric-field region with the implant while using a close-fit receive array to significantly increase signal to noise ratio of the images. Results It was found that the zero electric-field region of the transmitter is thick enough at 1.5 Tesla to encompass DBS lead trajectories with wire segments that were up to 30 degrees out of plane, as well as leads with looped segments. Moreover, SAR reduction was not sensitive to tissue properties, and insertion of a close-fit 32-channel receive array did not degrade the SAR reduction performance. Conclusion The ensemble of rotating linear birdcage and 32-channel close-fit receive array introduces a promising technology for future improvement of imaging in patients with DBS implants.

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On the RF heating of coronary stents at 7.0 Tesla MRI

Cited by 61 publications

References 56 publications

A numerical and experimental study of RF shimming in the presence of hip prostheses using adaptive SAR at 3 T

A numerical and experimental study of RF shimming in the presence of hip prostheses using adaptive SAR at 3 T

In vitro and in silico assessment of RF‐induced heating around intracranial aneurysm clips at 7 Tesla

Feasibility of using linearly polarized rotating birdcage transmitters and close-fitting receive arrays in MRI to reduce SAR in the vicinity of deep brain simulation implants

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