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.
Purpose
The introduction of a linear safety factor to address peak local specific absorption rate (pSAR10g) uncertainties (eg, intersubject variation, modeling inaccuracies) bears one considerable ...drawback: It often results in over‐conservative scanning constraints. We present a more efficient approach to define a variable safety margin based on the conditional probability density function of the effectively obtained pSAR10g value, given the estimated pSAR10g value.
Methods
The conditional probability density function can be estimated from previously simulated data. A representative set of true and estimated pSAR10g samples was generated by means of our database of 23 subject‐specific models with an 8‐fractionated dipole array for prostate imaging at 7 T. The conditional probability density function was calculated for each possible estimated pSAR10g value and used to determine the corresponding safety margin with an arbitrary low probability of underestimation. This approach was applied to five state‐of‐the‐art local SAR estimation methods, namely: (1) using just the generic body model “Duke”; (2) using our model library to assess the maximum pSAR10g value over all models; (3) using the most representative “local SAR model”; (4) using the five most representative local SAR models; and (5) using a recently developed deep learning–based method.
Results
Compared with the more conventional safety factor, the conditional safety‐margin approach results in lower (up to 30%) mean overestimation for all investigated local SAR estimation methods.
Conclusion
The proposed probabilistic approach for pSAR10g correction allows more accurate local SAR assessment with much lower overestimation, while a predefined level of underestimation is accepted (eg, 0.1%).
Purpose
The coax monopole antenna is presented for body imaging at 7 T. The antenna is fed at one end, eliminating the possibility of cable‐coil coupling and simplifying cable routing. Additionally, ...its flexibility improves loading to the subject.
Methods
Like the coax dipole antenna, an interruption in the shield of the coaxial cable allows the current to extend to the outside of the shield, generating a B1+ field. Matching is achieved using a single inductor at the distal side, and a cable trap enforces the desired antenna length. Finite difference time domain simulations are employed to optimize the design parameters. Phantom measurements are conducted to determine the antenna's B1+ efficiency and to find the S‐parameters in straight and bent positions. Eight‐channel simulations and measurements are performed for prostate imaging.
Results
The optimal configuration is a length of 360 mm with a gap position of 40 mm. Simulation data show higher B1+ levels for the coax monopole (20% in the prostate), albeit with a 5% lower specific absorbance rate efficiency, compared to the fractionated dipole antenna. The S11 of the coax monopole exhibits remarkable robustness to loading changes. In vivo prostate imaging demonstrates B1+ levels of 10–14 μT with an input power of 8 × 800 W, which is comparable to the fractionated dipole antenna. High‐quality images and acceptable coupling levels were achieved.
Conclusion
The coax monopole is a novel, flexible antenna for body imaging at 7 T. Its simple design incorporates a single inductor at the distal side to achieve matching, and one‐sided feeding greatly simplifies cable routing.
Abstract Purpose A previously published method for MRI‐based transfer function assessment makes use of the so‐called transceive phase assumption (TPA). This limits its applicability to shorter leads ...and/or lower field strengths. A new method is presented where the background electric field is determined from both ‐ and ‐field distributions, avoiding the TPA and making it more generally applicable. Theory and Methods These ‐distributions are determined from a spoiled gradient echo multiflip angle acquisition. From the separated ‐components the background electrical field and the induced current are computed. Further improvement is achieved by recasting the ‐field model as a “magnitude squared least squares” problem. The proposed reconstruction method is used to determine transfer functions of various copper wire lengths up to 40 cm inside an elliptical ASTM phantom. The method is first tested on EM‐simulated data and subsequently phantom and bench measurements are used to determine transfer functions experimentally. Results In silica reconstructions demonstrate the validity of the proposed ‐field model resulting in highly accurate reconstructed ‐fields, currents, incident electric fields and transfer functions. The experimental results show slight deviations in the field model, however, resulting transfer functions are accurately determined with high similarity to simulations and comparable to bench measurements. Conclusion A more generally applicable method for MRI‐based transfer function assessment is presented. The proposed method circumvents phase assumptions making it applicable for longer objects and/or higher field strengths. Additional improvements are implemented in the ‐mapping method and the solution algorithm.
Patients with medical implants often are deprived of magnetic resonance imaging examination because of safety risks. One specific risk is the enhancement of the radiofrequency fields around the ...medical implant potentially resulting in significant tissue heating and damage. The assessment of this enhancement is a computationally demanding task, with simulations taking hours or days to converge. Conventionally the source of the radiofrequency fields, patient anatomy, and the medical implant are simulated concurrently. To alleviate the computational burden, we reformulate a fast simulation method that views the medical implant as a small perturbation of the simulation domain without the medical implant and calculates the radiofrequency fields associated with this perturbation. Previously, this method required an extensive offline stage where the result is intractable for large simulation domains. Currently, this offline stage is no longer required and the method is completely online. The proposed method results in comparable radiofrequency fields but is orders of magnitude faster compared to standard simulation technique; the finite-difference time-domain, the finite-sums, and the finite element methods. This acceleration could enable patient-specific and potentially online radiofrequency safety assessment.
Purpose
The coax dipole antenna, a flexible antenna for body imaging at 7T is presented. Similar to the high impedance coil, this coaxial cable antenna is fed on the central conductor and through ...gaps in the shield, the current passes to the outside of the antenna to generate B1 field. This could achieve more favorable current distributions and better adaptation to the body curvature.
Methods
Finite difference time domain (FDTD) simulations are performed to optimize the positions of the gaps in the shield for a flat current profile. Lumped inductors are added to each end to reduce losses. The performance of a single antenna is compared to a fractionated dipole using B1 maps and MR thermometry. Finally, an array of eight coax dipoles is evaluated in simulations and used for in‐vivo scanning.
Results
An optimal configuration is found with gaps located at 10 cm from the center and inductor values of 28 nH. In comparison to the fractionated dipole antenna, in single antenna phantom measurements the coax dipole achieves similar B1 amplitude with 18% lower peak temperature. In simulations, the eight‐channel array of coax dipoles improved B1 homogeneity by 18%, along with small improvements in transmit efficiency and specific absorption rate (SAR). MRI measurements on three volunteers show more consistent performance for the coax dipoles.
Conclusion
The coax dipole is a novel antenna design with a flattened current distribution resulting in beneficial properties. Also, the flexible design of the coax dipoles allows better adaptation to the body curvature and can potentially be used for a wide range of imaging targets.
Background
In MRI, the signal‐to‐noise ratio (SNR) theoretically increases with B0 field strength. However, because of attenuation of the radiofrequency (RF) fields at 7T, it is not certain if this ...SNR gain can be realized for prostate imaging.
Purpose/Hypothesis
To investigate the SNR gain in prostate imaging at 7T as compared with 3T. It is expected that SNR will improve for prostate imaging at 7T compared with 3T.
Study Type
Prospective.
Subjects
Four healthy volunteers and one prostate cancer patient.
Field Strength/Sequence
All subjects were scanned at 3T and at 7T using optimal coil setups for both field strengths. For all volunteers, proton density‐weighted images were acquired for SNR analysis and actual flip angle imaging (AFI) B1+| maps were acquired for correction of measured SNR values. In the patient, a T2‐weighted (T2w) image was acquired at 3T and at 7T.
Assessment
SNR was calculated in the prostate region for all volunteers. SNR was normalized for flip angle, receiver bandwidth, and voxel volume. SNR was also calculated for different sensitivity encoding (SENSE) acceleration factors.
Statistical Testing
SNR values are represented as the arithmetic mean of SNR values in the prostate. Estimated SNR in the T2w image is calculated as the arithmetic mean of the signal intensity (SI) divided by the standard deviation of the SI in a specified zone. Tumor‐to‐tissue contrast is calculated as (SItumor+SIzone)/( SItumor‐SIzone).
Results
An increase in SNR ranging from 1.7‐fold to 2.8‐fold was measured in the prostate at 7T in comparison to 3T for four volunteers. At 7T, it is possible to achieve a 4‐fold SENSE acceleration in the left‐right direction with similar SNR to a nonaccelerated 3T image. T2w imaging was done at 3T and 7T in one patient, where improved tumor‐to‐tissue contrast was demonstrated at 7T.
Data Conclusion
SNR improves for prostate imaging at 7T as compared with 3T.
Level of Evidence: 2
Technical Efficacy: Stage 1
J. Magn. Reson. Imaging 2019;49:1446–1455.
Objective
To demonstrate imaging performance for cardiac MR imaging at 7 T using a coil array of 8 transmit/receive dipole antennas and 16 receive loops.
Materials and methods
An 8-channel dipole ...array was extended by adding 16 receive-only loops. Average power constraints were determined by electromagnetic simulations. Cine imaging was performed on eight healthy subjects. Geometrical factor (g-factor) maps were calculated to assess acceleration performance. Signal-to-noise ratio (SNR)-scaled images were reconstructed for different combinations of receive channels, to demonstrate the SNR benefits of combining loops and dipoles.
Results
The overall image quality of the cardiac functional images was rated a 2.6 on a 4-point scale by two experienced radiologists. Imaging results at different acceleration factors demonstrate that acceleration factors up to 6 could be obtained while keeping the average g-factor below 1.27. SNR maps demonstrate that combining loops and dipoles provides a more than 50% enhancement of the SNR in the heart, compared to a situation where only loops or dipoles are used.
Conclusion
This work demonstrates the performance of a combined loop/dipole array for cardiac imaging at 7 T. With this array, acceleration factors of 6 are possible without increasing the average g-factor in the heart beyond 1.27. Combining loops and dipoles in receive mode enhances the SNR compared to receiving with loops or dipoles only.
Introduction
Various research sites are pursuing 14 T MRI systems. However, both local SAR and RF transmit field inhomogeneity will increase. The aim of this simulation study is to investigate the ...trade-offs between peak local SAR and flip angle uniformity for five transmit coil array designs at 14 T in comparison to 7 T.
Methods
Investigated coil array designs are: 8 dipole antennas (8D), 16 dipole antennas (16D), 8 loop coils (8D), 16 loop coils (16L), 8 dipoles/8 loop coils (8D8L) and for reference 8 dipoles at 7 T. Both RF shimming and
k
T
-points were investigated by plotting L-curves of peak SAR levels vs flip angle homogeneity.
Results
For RF shimming, the 16L array performs best. For
k
T
-points, superior flip angle homogeneity is achieved at the expense of more power deposition, and the dipole arrays outperform the loop coil arrays.
Discussion and conclusion
For most arrays and regular imaging, the constraint on head SAR is reached before constraints on peak local SAR are violated. Furthermore, the different drive vectors in
k
T
-points alleviate strong peaks in local SAR. Flip angle inhomogeneity can be alleviated by
k
T
-points at the expense of larger power deposition. For
k
T
-points, the dipole arrays seem to outperform loop coil arrays.
Purpose
To investigate inter‐subject variability of B1+, SAR and temperature rise in a database of human models using a local transmit array for 7 T cardiac imaging.
Methods
Dixon images were ...acquired of 14 subjects and segmented in dielectric models with an eight‐channel local transmit array positioned around the torso for cardiac imaging. EM simulations were done to calculate SAR distributions. Based on the SAR distributions, temperature simulations were performed for exposure times of 6 min and 30 min. Peak local SAR and temperature rise levels were calculated for different RF shim settings. A statistical analysis of the resulting peak local SAR and temperature rise levels was performed to arrive at safe power limits.
Results
For RF shim vectors with random phase and uniformly distributed power, a safe average power limit of 35.7 W was determined (first level controlled mode). When RF amplitude and phase shimming was performed on the heart, a safe average power limit of 35.0 W was found. According to Pennes' model, our numerical study suggests a very low probability of exceeding the absolute local temperature limit of 40 °C for a total exposure time of 6 min and a peak local SAR of 20 W/kg. For a 30 min exposure time at 20 W/kg, it was shown that the absolute temperature limit can be exceeded in the case where perfusion does not change with temperature.
Conclusion
Safe power constraints were found for 7 T cardiac imaging with an eight‐channel local transmit array, while considering the inter‐subject variability of B1+, SAR and temperature rise.
14 subject specific dielectric body models were constructed based on dixon MRI scans. SAR and thermal simulations were performed in all these for a 7T body array of 8 fractionated dipole antennas to find distributions of peak SAR and temperature. Based on our model database, safe power limits were found for 7T cardiac imaging, while considering inter‐subject variation of SAR, temperature and B1+.
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