The coax monopole antenna: A flexible end-fed antenna for ultrahigh field transmit/receive arrays

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.

Simulation-based evaluation of SAR and flip angle homogeneity for five transmit head arrays at 14 T

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.

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

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.

The Coax Dipole: A fully flexible coaxial cable dipole antenna with flattened current distribution for body imaging at 7 Tesla

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.

SAR and temperature distributions in a database of realistic human models for 7 T cardiac imaging

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.

Evaluation of the radiofrequency performance of a wide-bore 1.5 T positron emission tomography/magnetic resonance imaging body coil for radiotherapy planning

Background and purpose

The restricted bore diameter of current simultaneous positron emission tomography/magnetic resonance imaging (PET/MRI) systems can be an impediment to achieving similar patient positioning during PET/MRI planning and radiotherapy. Our goal was to evaluate the B₁ transmit (B₁⁺) uniformity, B₁⁺ efficiency, and specific absorption rate (SAR) of a novel radiofrequency (RF) body coil design, in which RF shielded PET detectors were integrated with the specific aim of enabling a wide-bore PET/MRI system.

Materials and methods

We designed and constructed a wide-bore PET/MRI RF body coil to be integrated with a clinical MRI system. To increase its inner bore diameter, the PET detectors were positioned between the conductors and the RF shield of the RF body coil. Simulations and experiments with phantoms and human volunteers were performed to compare the B₁⁺ uniformity, B₁⁺ efficiency, and SAR between our design and the clinical body coil.

Results

In the simulations, our design achieved nearly the same B₁⁺ field uniformity as the clinical body coil and an almost identical SAR distribution. The uniformity findings were confirmed by the physical experiments. The B₁⁺ efficiency was 38% lower compared to the clinical body coil.

Conclusions

To achieve wide-bore PET/MRI, it is possible to integrate shielding for PET detectors between the body coil conductors and the RF shield without compromising MRI performance. Reduced B₁⁺ efficiency may be compensated by adding a second RF amplifier. This finding may facilitate the application of simultaneous whole-body PET/MRI in radiotherapy planning.

Conditional safety margins for less conservative peak local SAR assessment: A probabilistic approach

Purpose

The introduction of a linear safety factor to address peak local specific absorption rate (pSAR_10g) 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 pSAR_10g value, given the estimated pSAR_10g value.

Methods

The conditional probability density function can be estimated from previously simulated data. A representative set of true and estimated pSAR_10g 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 pSAR_10g 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 pSAR_10g 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 pSAR_10g correction allows more accurate local SAR assessment with much lower overestimation, while a predefined level of underestimation is accepted (eg, 0.1%).

A deep learning method for image-based subject-specific local SAR assessment

Purpose

Local specific absorption rate (SAR) cannot be measured and is usually evaluated by offline numerical simulations using generic body models that of course will differ from the patient's anatomy. An additional safety margin is needed to include this intersubject variability. In this work, we present a deep learning–based method for image‐based subject‐specific local SAR assessment. We propose to train a convolutional neural network to learn a “surrogate SAR model” to map the relation between subject‐specific B1+ maps and the corresponding local SAR.

Method

Our database of 23 subject‐specific models with an 8–transmit channel body array for prostate imaging at 7 T was used to build 5750 training samples. These synthetic complex B1+ maps and local SAR distributions were used to train a conditional generative adversarial network. Extra penalization for local SAR underestimation errors was included in the loss function. In silico and in vivo validation were performed.

Results

In silico cross‐validation shows a good qualitative and quantitative match between predicted and ground‐truth local SAR distributions. The peak local SAR estimation error distribution shows a mean overestimation error of 15% with 13% probability of underestimation. The higher accuracy of the proposed method allows the use of less conservative safety factors compared with standard procedures. In vivo validation shows that the method is applicable with realistic measurement data with impressively good qualitative and quantitative agreement to simulations.

Conclusion

The proposed deep learning method allows online image‐based subject‐specific local SAR assessment. It greatly reduces the uncertainty in current state‐of‐the‐art SAR assessment methods, reducing the time in the examination protocol by almost 25%.

Homogeneous B1+ for bilateral breast imaging at 7 T using a five dipole transmit array merged with a high density receive loop array

To explore the use of five meandering dipole antennas in a multi-transmit setup, combined with a high density receive array for breast imaging at 7 T for improved penetration depth and more homogeneous B₁ field. Five meandering dipole antennas and 30 receiver loops were positioned on two cups around the breasts. Finite difference time domain simulations were performed to evaluate RF safety limits of the transmit setup. Scattering parameters of the transmit setup and coupling between the antennas and the detuned loops were measured. In vivo parallel imaging performance was investigated for various acceleration factors. After RF shimming, a B₁ map, a T₁ -weighted image, and a T₂ -weighted image were acquired to assess B₁ efficiency, uniformity in contrast weighting, and imaging performance in clinical applications. The maximum achievable local SAR_10g value was 7.0 W/kg for 5 × 1 W accepted power. The dipoles were tuned and matched to a maximum reflection of -11.8 dB, and a maximum inter-element coupling of -14.2 dB. The maximum coupling between the antennas and the receive loops was -18.2 dB and the mean noise correlation for the 30 receive loops 7.83 ± 8.69%. In vivo measurements showed an increased field of view, which reached to the axilla, and a high transmit efficiency. This coil enabled the acquisition of T₁ -weighted images with a high spatial resolution of 0.7 mm³ isotropic and T₂ -weighted spin echo images with uniformly weighted contrast.

Feasibility of 7-T fluorine magnetic resonance spectroscopic imaging (19F MRSI) for TAS-102 metabolite detection in the liver of patients with metastatic colorectal cancer

Trifluridine/tipiracil (TAS-102) has shown a significant overall survival benefit in patients with heavily pre-treated metastatic colorectal cancer. However, predicting treatment response and toxicity in individual patients remains challenging. Fluorine (¹⁹F)-containing drugs can be detected with magnetic resonance spectroscopy (MRS) to determine the metabolic rates and the biodistribution of the drug in normal and tumour tissue, which are related to treatment efficacy and toxicity. This is the first study to investigate the potential of 7-T ¹⁹F-MRS to detect TAS-102 metabolites in humans. We demonstrate that, with the used setup, TAS-102 is not detectable in liver metastases of metastatic colorectal cancer patients on a normal treatment schedule. Therefore, ¹⁹F-MRS TAS-102 metabolite detection is not yet useful for the clinical early prediction of treatment response. As ¹⁹F-MRS is able to detect TAS-102 in phantom and murine models, the use of ¹⁹F-MRS remains a potential tool to noninvasively detect and possibly monitor the metabolism when higher dosages of TAS-102 are administered, for example in organoid and animal studies.

Comparing signal-to-noise ratio for prostate imaging at 7T and 3T

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 (SI_tumor+SI_zone)/( SI_tumor‐SI_zone).

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.

Design of a forward view antenna for prostate imaging at 7 T

PURPOSE

To design a forward view antenna for prostate imaging at 7 T, which is placed between the legs of the subject in addition to a dipole array.

MATERIALS AND METHODS

The forward view antenna is realized by placing a cross-dipole antenna at the end of a small rectangular waveguide. Quadrature drive of the cross-dipole can excite a circularly polarized wave propagating along the axial direction to and from the prostate region. Functioning of the forward view antenna is validated by comparing measurements and simulations. Antenna performance is evaluated by numerical simulations and measurements at 7 T.

RESULTS

Simulations of B₁⁺ on a phantom are in good correspondence with measurements. Simulations on a human model indicate that the signal-to-noise ratio (SNR), specific absorption rate (SAR) efficiency and SAR increase when adding the forward view antenna to a previously published dipole array. The SNR increases by up to 18% when adding the forward view antenna as a receive antenna to an eight-channel dipole array in vivo.

CONCLUSIONS

A design for a forward view antenna is presented and evaluated. SNR improvements up to 18% are demonstrated when adding the forward view antenna to a dipole array.

Selective proton-observed, carbon-edited (selPOCE) MRS method for measurement of glutamate and glutamine 13 C-labeling in the human frontal cortex

PURPOSE

¹³ C magnetic resonance spectroscopy (MRS) in combination with infusion of ¹³ C-labeled substrates has led to unique insights into human brain metabolism and neurotransmitter cycling. However, the low sensitivity of direct ¹³ C MRS and high radiofrequency power requirements has limited ¹³ C MRS studies to predominantly data acquisition in large volumes of the occipital cortex. The purpose of this study is to develop an MRS technique for localized detection of ¹³ C-labeling of glutamate and glutamine in the human frontal lobe.

METHODS

We used an indirect (¹ H-[¹³ C]), proton-observed, carbon-edited MRS sequence (selPOCE) for detection of ¹³ C-labeled metabolites in relatively small volumes located in the frontal lobe at 4 T. The SelPOCE method allows for selective and separate detection of glutamate and glutamine resonances, which significantly overlap at magnetic field strengths used for clinical MRI.

RESULTS

Phantom data illustrate how selPOCE can be tuned to selectively detect ¹³ C labeling in different metabolites. Three-dimensional specific absorption rate simulations of radiofrequency power deposition show that the selPOCE method operates comfortably within the global and local Food and Drug Administration specific absorption rate guidelines. In vivo selPOCE data are presented, which were acquired from a 45-mL volume in the frontal lobe of healthy subjects. The in vivo data show the time-dependent ¹³ C-labeling of glutamate and glutamine during intravenous infusion of [1-¹³ C]-glucose. Metrics describing spectral fitting quality of the glutamate and glutamine resonances are reported.

CONCLUSIONS

The SelPOCE sequence allows the detection of ¹³ C-labeling in glutamate and glutamine from a relatively small volume in the human frontal lobe at low radiofrequency power requirements. Magn Reson Med 80:11-20, 2018. © 2017 International Society for Magnetic Resonance in Medicine.