Magnetic resonance safety and compatibility of tantalum markers used in proton beam therapy for intraocular tumors: A 7.0 Tesla study

Oberacker, Eva; Paul, Katharina; Huelnhagen, Till; Oezerdem, Celal; Winter, Lukas; Pohlmann, Andreas; Boehmert, Laura; Stachs, Oliver; Heufelder, Jens; Weber, A.; Rehák, Matúš; Seibel, Ira; Niendorf, Thoralf

doi:10.1002/mrm.26534

Cited by 26 publications

(30 citation statements)

References 51 publications

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“…The biomedical literature contains multiple reports of elevated 23 Na content in tumors (19,20,51). Sodium imaging of the human eye in vivo at 7 T could be used as a complement to proton imaging data, which might help in the planning of radiation treatments for which the ultrahigh field MRI safety of ocular tantalum markers has been recently demonstrated (52). Another potential use would be as a follow-up to proton beam therapy of intraocular tumors.…”

Section: Discussionmentioning

confidence: 99%

Millimeter spatial resolution in vivo sodium MRI of the human eye at 7 T using a dedicated radiofrequency transceiver array

Wenz

Kuehne²,

Huelnhagen

et al. 2018

Magnetic Resonance in Med

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Three most important eye compartments in the context of sodium physiology were clearly delineated in all of the images: the vitreous humor, the aqueous humor, and the lens. Our results provide encouragement for further clinical studies. The implications for research into eye diseases including ocular melanoma, cataract, and glaucoma are discussed. Magn Reson Med 80:672-684, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

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Section: Discussionmentioning

confidence: 99%

Millimeter spatial resolution in vivo sodium MRI of the human eye at 7 T using a dedicated radiofrequency transceiver array

Wenz

Kuehne²,

Huelnhagen

et al. 2018

Magnetic Resonance in Med

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“…SAR 10g is used widely as a RF monitoring parameter in surface coils at 7 T, but was not designed to be used in volume coils and in tissues near metal implants . The reliability of SAR 10g near the hip implants was evaluated and compared with adSAR, calculated as m gram‐averaged SAR in a layer of tissues surrounding metal implants .…”

Section: Methodsmentioning

confidence: 99%

“…Although thermal simulations were not performed, they estimated that this would have caused temperature increases larger than 1°C. Limitations of this study are that SAR 10g was used to correlate with temperature increase, but the averaging mass of 10 g was shown to be unreliable in scenarios with metal implants . Therefore, the correlation between SAR 10g and temperature cannot be inferred.…”

Section: Introductionmentioning

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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“…The lack of correlation between SAR 1g and temperature near an ocular tantalum marker has been reported in phantoms by Oberacker et al [137], where a smaller SAR averaging-mass was used to have matching profiles. We investigate this idea further by comparing current mass-averaging using SAR 10g with a locally adapting SAR averaging-mass.…”

Section: Introductionmentioning

confidence: 81%

“…Another limitation of these studies is that they based they reduction of metal-induced heating methods on directly reducing the and SAR 10g might lead to overestimation or underestimation of the temperature. Oberacker et al [137] showed that in the case of an ocular tantalum tracker marker at 7T, SAR 0.01g was the best estimate of RF heating when using a single surface coil element. Before the potential of pTx can be fully exploited at HF and UHF, better knowledge of the correlation between RF heating and SAR is required.…”

Section: Safety Of Ptx Near Metal Implants 261 Based On Simulationsmentioning

confidence: 99%

Radiofrequency safety and shimming near metal hip prostheses at high and ultra-high field

Destruel¹

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Early assessment of image quality in the proximity of hip replacements indicates that magnetic resonance imaging (MRI) shows better results than CT in some scenarios. MRI could provide tools for earlier diagnosis of joint diseases and elucidate the need for revision due to implant wear, but MRI based measures would benefit from higher spatial resolutions. Image resolution in MRI depends on the amount of signal available and the signal-to-noise ratio (SNR) increases at least linearly with magnetic field strength.However, technical challenges limit the use of high-field (HF) MRI, at 3 Tesla, and ultra-high field (UHF) MRI, stronger or equal to 7 Tesla, in the presence of metal implants. The major limitations are due to the decreased homogeneity of the radiofrequency (RF) field and the approximately quadratic increase of the power deposition in human tissue, which are amplified by the presence of metal implants.These effects are often further enhanced by antenna effects in metal implants such as hip prostheses. In addition, RF power deposition may cause a temperature increase in tissue. The amount of RF energy deposition is regulated using specific limits of the specific absorption rate (SAR). In practice MRI scanners have a global SAR prediction which ensures that the total amount of RF energy is limited and will not cause significant temperature increases during the examination. At UHF, global SAR becomes less helpful because the constructive and destructive interferences in the electric component of the RF field can concentrate the heating into 'hot spots', where local SAR can be several times higher than average SAR. To prevent localised heating, 10g-averaged SAR (SAR 10g ) is used to limit the input power in the RF coil, as it has been shown to better correlate with temperature than global SAR. Unfortunately, SAR 10g cannot be measured directly in vivo therefore numerical simulations to calculate SAR 10g in realistic human models have become the accepted method to define limits for RF power deposition.Parallel transmission (pTx) techniques have been shown to significantly reduce RF inhomogeneities in MR images and to control RF energy deposition by independently controlling the phase and amplitudes of the transmit coil. Constructive interference patterns in the area of interest can be generated which result in greater SNR and improved homogeneity. Although pTx has shown promise to improve image quality there are safety concerns that limit the routine use of pTx. Furthermore, though pTx may be beneficial near metal implants, metal implants may further focus the electric field, introducing additional safety concerns. In the absence of specific regulations and knowledge of how to prevent metal-induced RF heating, patients with implants are often excluded from HF and UHF pTx studies.iii The aim of this thesis was to investigate RF heating induced by hip prostheses at 3T and 7T, and evaluate the reliability of global SAR and SAR 10g to limit the temperature increase when pTx coils were used. Numerical simulations wi...

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Magnetic resonance safety and compatibility of tantalum markers used in proton beam therapy for intraocular tumors: A 7.0 Tesla study

Cited by 26 publications

References 51 publications

Millimeter spatial resolution in vivo sodium MRI of the human eye at 7 T using a dedicated radiofrequency transceiver array

Millimeter spatial resolution in vivo sodium MRI of the human eye at 7 T using a dedicated radiofrequency transceiver array

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

Radiofrequency safety and shimming near metal hip prostheses at high and ultra-high field

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