Measuring radiofrequency field‐induced temperature variations in brain MRI exams with motion compensated MR thermometry and field monitoring

Purpose The aim of this work is the development of a thermometry method to measure temperature increases in vivo, with a precision and accuracy sufficient for validation against thermal simulations. Such an MR thermometry model would be a valuable tool to get an indication on one of the major safety concerns in MR imaging: the tissue heating occurring due to radiofrequency (RF) exposure. To prevent excessive temperature rise, RF power deposition, expressed as specific absorption rate, cannot exceed predefined thresholds. Using these thresholds, MRI has demonstrated an extensive history of safe usage. Nevertheless, MR thermometry would be a valuable tool to address some of the unmet needs in the area of RF safety assessment, such as validation of specific absorption rate and thermal simulations, investigation of local peak temperatures during scanning, or temperature‐based safety guidelines. Methods The harmonic initialized model‐based multi‐echo approach is proposed. The method combines a previously published model‐based multi‐echo water/fat separated approach with an also previously published near‐harmonic 2D reconstruction method. The method is tested on the human thigh with a multi‐transmit array at 7 T, in three volunteers, and for several RF shims. Results Precision and accuracy are improved considerably compared to a previous fat‐referenced method (precision: 0.09 vs. 0.19°C). Comparison of measured temperature rise distributions to subject‐specific simulated counterparts show good relative agreement for multiple RF shim settings. Conclusion The high precision shows promising potential for validation purposes and other RF safety applications.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Multi‐echo MR thermometry in the upper leg at 7 T using near‐harmonic 2D reconstruction for initialization

Kikken

Steensma

Berg

et al. 2023

“…The RF power applied to the transmit coil deposits energy in the biological tissue, and this increases with field strength 4 . Due to the short wavelength, the interactions between the tissue and the electromagnetic (EM) field results in a spatially dependent energy deposition, which increases the potential for localized RF heating 21 . The energy deposition during routine exams is controlled by the specific absorption rate (SAR) limits, which is a measure of the RF power absorbed per unit mass.…”

Section: Introductionmentioning

confidence: 99%

“…4 Due to the short wavelength, the interactions between the tissue and the electromagnetic (EM) field results in a spatially dependent energy deposition, which increases the potential for localized RF heating. 21 The energy deposition during routine exams is controlled by the specific absorption rate (SAR) limits, which is a measure of the RF power absorbed per unit mass. Therefore, the ability to provide an efficient and homogeneous RF excitation while operating within the SAR constraints is one of the most important engineering requirements in designing RF transmit coils for UHF MRI.…”

Section: Introductionmentioning

confidence: 99%

Electromagnetic and RF pulse design simulation based optimization of an eight‐channel loop array for 11.7T brain imaging

Chu

Gras

Mauconduit

et al. 2023

Self Cite

Purpose: Optimization of transmit array performance is crucial in ultra-high-field MRI scanners such as 11.7T because of the increased RF losses and RF nonuniformity. This work presents a new workflow to investigate and minimize RF coil losses, and to choose the optimum coil configuration for imaging. Methods: An 8-channel transceiver loop-array was simulated to analyze its loss mechanism at 499.415 MHz. A folded-end RF shield was developed to limit radiation loss and improve the B + 1 efficiency. The coil element length, and the shield diameter and length were further optimized using electromagnetic (EM) simulations. The generated EM fields were used to perform RF pulse design (RFPD) simulations under realistic constraints. The chosen coil design was constructed to demonstrate performance equivalence in bench and scanner measurements. Results:The use of conventional RF shields at 11.7T resulted in significantly high radiation losses of 18.4%. Folding the ends of the RF shield combined with optimizing the shield diameter and length increased the absorbed power in biological tissue and reduced the radiation loss to 2.4%. The peak B + 1 of the optimal array was 42% more than the reference array. Phantom measurements validated the numerical simulations with a close match of within 4% of the predicted B + 1 . Conclusion:A workflow that combines EM and RFPD simulations to numerically optimize transmit arrays was developed. Results have been validated using phantom measurements. Our findings demonstrate the need for optimizing the RF shield in conjunction with array element design to achieve efficient excitation at 11.7T.

The restricted SAR protocol: A method to assess MRI coil prototypes in an unconditionally safe manner

Dudysheva,

Mauconduit,

Abdeddaim

et al. 2023

Self Cite

PurposeTesting an RF coil prototype on subjects involves laborious verifications to ensure its safety. In particular, it requires preliminary electromagnetic simulations and their validations on phantoms to accurately predict the specific absorption rate (SAR). For coil design validation with a simpler safety procedure, the restricted SAR (rS) mode is proposed, enabling representative first experiments in vivo. The goal of the developed approach is to accelerate the transition of a custom coil system from prototype to clinical use.MethodsThe restricted specific absorption rate (SAR) (rS) mode imposes a radical limitation on the transmitted RF power based on a worst‐case scenario of local RF power absorption. The limitations used are independent of the SAR spatial distribution, making this approach unconditionally safe. The developed rS protocol contains the sequences required for coil evaluation and satisfies the imposed rS conditions. It provides a quantitative characterization of the coil transmission and reception profiles and a qualitative evaluation of the anatomical images. Protocol validation was performed on commercial and pre‐industrial prototype coils on a small cohort of healthy volunteers.ResultsThe proposed rS protocol enables coil evaluation within an acquisition time compatible with common clinical protocol duration. The total time of all evaluation steps does not exceed 17 min. At the same time, the global SAR remains 100 times less than the International Electrotechnical Commission safety limit for played sequences.ConclusionThe rS protocol allows characterizing and comparing coil prototypes on volunteers without extensive electromagnetic calculations and phantom validations in an unconditionally safe way.