Pregnant women models analyzed for <scp>RF</scp> exposure and temperature increase in <scp>3T</scp><scp>RF</scp> shimmed birdcages

Murbach, Manuel; Neufeld, Esra; Samaras, Theodoros; Córcoles, Juan; Robb, Fraser; Kainz, Wolfgang; Kuster, Niels

doi:10.1002/mrm.26268

Cited by 49 publications

(76 citation statements)

References 28 publications

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“…In a recent paper , we showed that the tissues surrounding a metallic hip prosthesis exposed to a gradient field (GF) may undergo a nonnegligible heating (up to some degree), as a result of the electromagnetic energy deposited in the implant. Since then, we have realized that in the MR community these results are generally received with some skepticism, probably because of the fact that most of the scientific papers dealing with thermal problems in MRI focus on RF fields . Similarly, the attention of relevant standardization and regulatory bodies is focused solely on RF‐induced heating .…”

Section: Total Joule Losses P (In Watt) Developed Within the Spherementioning

confidence: 99%

The underestimated role of gradient coils in MRI safety

Zilberti

Arduino

Bottauscio

et al. 2016

Magnetic Resonance in Med

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MRI scanning of patients carrying implants is becoming a reality in many hospitals, because of the wide increase of population with implants, particularly in the orthopedic ones (1). The responsibility for this practice falls on the subscribing physicians or radiologists, who rely on guidance establishing safety and compatibility of implants in the MR environment (eg, (2)), which basically address issues related to magnetically induced mechanical actions and radiofrequency (RF) induced heating of tissues. Even if large-scale MR safety studies on orthopedic implants have been published that show no evidence of specific risks for patients' health (eg, (3)), additional analyses of the interaction between MR fields and metallic implants can contribute to support the scan decision on a firm scientific rationale.In a recent paper (4), we showed that the tissues surrounding a metallic hip prosthesis exposed to a gradient field (GF) may undergo a nonnegligible heating (up to some degree), as a result of the electromagnetic energy deposited in the implant. Since then, we have realized that in the MR community these results are generally received with some skepticism, probably because of the fact that most of the scientific papers dealing with thermal problems in MRI focus on RF fields (5-16). Similarly, the attention of relevant standardization and regulatory bodies is focused solely on RF-induced heating (2,17). This tendency is absolutely well-grounded when studying native tissues, where GFs are not able to develop significant thermal effects. However, in the presence of metallic foreign bodies the situation is different.Many papers show that a significant heating can be associated with the exposure of such bodies to RF fields, especially with wire-like structures, which may act as antennas (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30). In this case, RF typically deposits a relatively low power inside the metallic parts, but their presence may cause the enhancement of the specific absorption rate directly developed in the tissues anyway. What we would like to highlight here is that, in presence of bulky metallic prostheses, GFs can produce a nonnegligible heating too, because of the thermal power generated inside the metal (Joule effect), which involves the tissues indirectly, by diffusion ("indirect effect" (31)). The GF thermal effect, which becomes sizable only for prostheses far from the coils' isocenter, has been considered up until now by a few articles (4,32-34). To sustain these findings, we exploit an analytical solution (35) that is not affected by possible artifacts of numerical results. The solution provides the current density J (in complex notation) induced within a nonmagnetic metallic sphere, with radius R, conductivity s and permeability m 0 , immersed in a nonconductive background. When the sphere is radiated by a homogeneous, timeharmonic, magnetic flux density B (peak value) at frequency f, the current density in an internal point iswhere r is the distance from the sphere center, u is the cola...

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Section: Total Joule Losses P (In Watt) Developed Within the Spherementioning

confidence: 99%

The underestimated role of gradient coils in MRI safety

Zilberti

Arduino

Bottauscio

et al. 2016

Magnetic Resonance in Med

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“…A number of previous studies have investigated via electromagnetic (EM) simulations the SAR experienced both by the mother and the fetus at 1.5T and 3T . Hand et al and Pediaditis et al used a finite integration technique (FIT): the former considered a truncated model of a 28‐wk pregnant woman at 1.5T and 3T, while the latter looked at a whole‐body 30‐wk pregnant female model at fields between 0.3T and 4T.…”

Section: Introductionmentioning

confidence: 99%

“…Some studies have extended the SAR analysis to also estimate temperature increases within the mother and fetus using the Pennes Bioheat equation and the thermoregulation model presented by Laakso and Hirata . In the most recent simulation study, Murbach et al investigated the effect of using RF‐shimming on a dual‐transmit 3T system in terms of the SAR and temperature rise both in the mother and fetus at different gestational stages. Their general conclusions were that RF shimming can reduce the local SAR in the mother but conversely can increase the whole‐body SAR and peak temperature in the fetus.…”

Section: Introductionmentioning

confidence: 99%

A simulation study on the effect of optimized high permittivity materials on fetal imaging at 3T

Gemert

Brink

Remis

et al. 2019

Magnetic Resonance in Med

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Purpose One of the main concerns in fetal MRI is the radiofrequency power that is absorbed both by the mother and the fetus. Passive shimming using high permittivity materials in the form of “dielectric pads” has previously been shown to increase the B1+ efficiency and homogeneity in different applications, while reducing the specific absorption rate (SAR). In this work, we study the effect of optimized dielectric pads for 3 pregnant models. Methods Pregnant models in the 3rd, 7th, and 9th months of gestation were used for simulations in a birdcage coil at 3T. Dielectric pads were optimized regions of interest (ROI) using previously developed methods for B1+ efficiency and homogeneity and were designed for 2 ROIs: the entire fetus and the brain of the fetus. The SAR was evaluated in terms of the whole‐body SAR, average SAR in the fetus and amniotic fluid, and maximum 10 g‐averaged SAR in the mother, fetus, and amniotic fluid. Results The optimized dielectric pads increased the transmit efficiency up to 55% and increased the B1+ homogeneity in almost every tested configuration. The B1+‐normalized whole‐body SAR was reduced by more than 31% for all body models. The B1+‐normalized local SAR was reduced in most scenarios by up to 62%. Conclusion Simulations have shown that optimized high permittivity pads can reduce SAR in pregnant subjects at the 3rd, 7th, and 9th month of gestation, while improving the transmit field homogeneity in the fetus. However, significantly more work is required to demonstrate that fetal imaging is safe under standard operating conditions.

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“…BJR such as oesophagus atresia, should rather be examined at lower field strengths. Tissue heating is also increased at 3.0 T, 44 albeit this heating is not dangerous for the fetus, it is uncomfortable for the pregnant female.…”

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confidence: 99%

Fetal MRI at 3T—ready for routine use?

Weisstanner¹,

Gruber²,

Brugger³

et al. 2017

BJR

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Fetal MR now plays an important role in the clinical work-up of pregnant females. It is performed mainly at 1.5 T. However, the desire to obtain a more precise fetal depiction or the fact that some institutions have access only to a 3.0 T scanner has resulted in a growing interest in performing fetal MR at 3.0 T. The aim of this article was to provide a reference for the use of 3.0 T MRI as a prenatal diagnostic method.

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Pregnant women models analyzed for RF exposure and temperature increase in 3TRF shimmed birdcages

Cited by 49 publications

References 28 publications

The underestimated role of gradient coils in MRI safety

The underestimated role of gradient coils in MRI safety

A simulation study on the effect of optimized high permittivity materials on fetal imaging at 3T

Fetal MRI at 3T—ready for routine use?

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