Radiofrequency heating studies on anesthetized swine using fractionated dipole antennas at 10.5 T

Tier-based formalism for safety assessment of custom-built radio-frequency transmit coils

The purpose of this work is to propose a tier-based formalism for safety assessment of custom-built radio-frequency (RF) coils that balances validation effort with the effort put in determinating the safety factor. The formalism has three tier levels. Higher tiers require increased effort when validating electromagnetic simulation results but allow for less conservative safety factors. In addition, we propose a new method to calculate modeling uncertainty between simulations and measurements and a new method to propagate uncertainties in the simulation into a safety factor that minimizes the risk of underestimating the peak specific absorption rate (SAR). The new safety assessment procedure was completed for all tier levels for an eight-channel dipole array for prostate imaging at 7 T and an eight-channel dipole array for head imaging at 10.5 T, using data from two different research sites. For the 7 T body array, the validation procedure resulted in a modeling uncertainty of 77% between measured and simulated local SAR distributions. For a situation where RF shimming is performed on the prostate, average power limits of 2.4 and 4.5 W/channel were found for tiers 2 and 3, respectively. When the worst-case peak SAR among all phase settings was calculated, power limits of 1.4 and 2.7 W/channel were found for tiers 2 and 3, respectively. For the 10.5 T head array, a modeling uncertainty of 21% was found based on B1 + mapping. For the tier 2 validation, a power limit of 2.6 W/channel was calculated. The demonstrated tier system provides a strategy for evaluating modeling inaccuracy, allowing for the rapid translation of novel coil designs with conservative safety factors and the implementation of less conservative safety factors for frequently used coil arrays at the expense of increased validation effort.

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

PURPOSE

An MR thermometry (MRT) method with motion and field fluctuation compensation is proposed to measure non-invasively sub-degree brain temperature variations occurring through radiofrequency (RF) power deposition during MR exams.

METHODS

MRT at 7T with a multi-slice echo planar imaging (EPI) sequence and concurrent field monitoring was first tested in vitro to assess accuracy in the presence of external field perturbations, an optical probe being used for ground truth. In vivo, this strategy was complemented by a motion compensation scheme based on a dictionary pre-scan, as reported in some previous work, and was adapted to the human brain. Precision reached with this scheme was assessed on eight volunteers with a 5 minute-long low specific absorption rate (SAR) scan. Finally, temperature rise in the brain was measured twice on the same volunteers and with the same strategy, this time by employing a 20-minutes scan at the maximum SAR delivered with a commercial volume head coil.

RESULTS

In vitro, the root mean square (RMS) error between optical probe and MRT measurements was 0.02°C with field sensor correction. In vivo, the low SAR scan returned a precision in temperature change measurement with field monitoring and motion compensation of 0.05°C. The 20-minutes maximum SAR scan returned a temperature rise throughout the inner-brain in the range of 0-0.2°C. Brain periphery remained too sensitive with respect to motion to lead to equally conclusive results.

CONCLUSION

Sub-degree temperature rise in the inner human brain was characterized experimentally throughout RF exposure. Potential applications include improvement of human thermal models and revision of safety margins.

Progress in Imaging the Human Torso at the Ultrahigh Fields of 7 and 10.5 T

Especially after the launch of 7 T, the ultrahigh magnetic field (UHF) imaging community achieved critically important strides in our understanding of the physics of radiofrequency interactions in the human body, which in turn has led to solutions for the challenges posed by such UHFs. As a result, the originally obtained poor image quality has progressed to the high-quality and high-resolution images obtained at 7 T and now at 10.5 T in the human torso. Despite these tremendous advances, work still remains to further improve the image quality and fully capitalize on the potential advantages UHF has to offer.

RF heating measurement using MR thermometry and field monitoring: Methodological considerations and first in vivo results

PURPOSE

A MR thermometry (MRT) method with field monitoring is proposed to improve the measurement of small temperature variations induced in brain MRI exams.

METHODS

MR thermometry experiments were performed at 7 Tesla with concurrent field monitoring and RF heating. Images were reconstructed with nominal k-space trajectories and with first-order spherical harmonics correction. Experiments were performed in vitro with deliberate field disturbances and on an anesthetized macaque in 2 different specific absorption rate regimes, that is, at 50% and 100% of the maximal specific absorption rate level allowed in the International Electrotechnical Commission normal mode of operation. Repeatability was assessed by running a second separate session on the same animal.

RESULTS

Inclusion of magnetic field fluctuations in the reconstruction improved temperature measurement accuracy in vitro down to 0.02°C. Measurement precision in vivo was on the order of 0.15°C in areas little affected by motion. In the same region, temperature increase reached 0.5 to 0.8°C after 20 min of heating at 100% specific absorption rates and followed a rough factor of 2 with the 50% specific absorption rate scans. A horizontal temperature plateau, as predicted by Pennes bioheat model with thermal constants from the literature and constant blood temperature assumption, was not observed.

CONCLUSION

Inclusion of field fluctuations in image reconstruction was beneficial for the measurement of small temperature rises encountered in standard brain exams. More work is needed to correct for motion-induced field disturbances to extract reliable temperature maps.

In vivo human head MRI at 10.5T: A radiofrequency safety study and preliminary imaging results

PURPOSE

The purpose of this study is to safely acquire the first human head images at 10.5T.

METHODS

To ensure safety of subjects, we validated the electromagnetic simulation model of our coil. We obtained quantitative agreement between simulated and experimental B 1 + and specific absorption rate (SAR). Using the validated coil model, we calculated radiofrequency power levels to safely image human subjects. We conducted all experiments and imaging sessions in a controlled radiofrequency safety lab and the whole-body 10.5T scanner in the Center for Magnetic Resonance Research.

RESULTS

Quantitative agreement between the simulated and experimental results was obtained including S-parameters, B 1 + maps, and SAR. We calculated peak 10 g average SAR using 4 different realistic human body models for a quadrature excitation and demonstrated that the peak 10 g SAR variation between subjects was less than 30%. We calculated safe power limits based on this set and used those limits to acquire T₂ - and T 2 ∗ -weighted images of human subjects at 10.5T.

CONCLUSIONS

In this study, we acquired the first in vivo human head images at 10.5T using an 8-channel transmit/receive coil. We implemented and expanded a previously proposed workflow to validate the electromagnetic simulation model of the 8-channel transmit/receive coil. Using the validated coil model, we calculated radiofrequency power levels to safely image human subjects.

Evolution of UHF Body Imaging in the Human Torso at 7T: Technology, Applications, and Future Directions

The potential value of ultrahigh field (UHF) magnetic resonance imaging (MRI) and spectroscopy to biomedical research and in clinical applications drives the development of technologies to overcome its many challenges. The increased difficulties of imaging the human torso compared with the head include its overall size, the dimensions and location of its anatomic targets, the increased prevalence and magnitude of physiologic effects, the limited availability of tailored RF coils, and the necessary transmit chain hardware. Tackling these issues involves addressing notoriously inhomogeneous transmit B1 (B1) fields, limitations in peak B1, larger spatial variations of the static magnetic field B0, and patient safety issues related to implants and local RF power deposition. However, as research institutions and vendors continue to innovate, the potential gains are beginning to be realized. Solutions overcoming the unique challenges associated with imaging the human torso are reviewed as are current studies capitalizing on the benefits of UHF in several anatomies and applications. As the field progresses, strategies associated with the RF system architecture, calibration methods, RF pulse optimization, and power monitoring need to be further integrated into the MRI systems making what are currently complex processes more streamlined. Meanwhile, the UHF MRI community must seize the opportunity to build upon what have been so far proof of principle and feasibility studies and begin to further explore the true impact in both research and the clinic.

Computer-Vision Techniques for Water-Fat Separation in Ultra High-Field MRI Local Specific Absorption Rate Estimation

OBJECTIVE

The purpose of this paper is to prove that computer-vision techniques allow synthesizing water-fat separation maps for local specific absorption rate (SAR) estimation, when patient-specific water-fat images are not available.

METHODS

We obtained ground truth head models by using patient-specific water-fat images. We obtained two different label-fusion water-fat models generating a water-fat multiatlas and applying the STAPLE and local-MAP-STAPLE label-fusion methods. We also obtained patch-based water-fat models applying a local group-wise weighted combination of the multiatlas. Electromagnetic (EM) simulations were performed, and B1+ magnitude and 10 g averaged SAR maps were generated.

RESULTS

We found local approaches provide a high DICE overlap (72.6 ± 10.2% fat and 91.6 ± 1.5% water in local-MAP-STAPLE, and 68.8 ± 8.2% fat and 91.1 ± 1.0% water in patch-based), low Hausdorff distances (18.6 ± 7.7 mm fat and 7.4 ± 11.2 mm water in local-MAP-STAPLE, and 16.4 ± 8.5 mm fat and 7.2 ± 11.8 mm water in patch-based) and a low error in volume estimation (15.6 ± 34.4% fat and 5.6 ± 4.1% water in the local-MAP-STAPLE, and 14.0 ± 17.7% fat and 4.7 ± 2.8% water in patch-based). The positions of the peak 10 g-averaged local SAR hotspots were the same for every model.

CONCLUSION

We have created patient-specific head models using three different computer-vision-based water-fat separation approaches and compared the predictions of B1+ field and SAR distributions generated by simulating these models. Our results prove that a computer-vision approach can be used for patient-specific water-fat separation, and utilized for local SAR estimation in high-field MRI.

SIGNIFICANCE

Computer-vision approaches can be used for patient-specific water-fat separation and for patient specific local SAR estimation, when water-fat images of the patient are not available.

Open Source 3D Multipurpose Measurement System with Submillimetre Fidelity and First Application in Magnetic Resonance

Magnetic resonance imaging (MRI) is the mainstay of diagnostic imaging, a versatile instrument for clinical science and the subject of intense research interest. Advancing clinical science, research and technology of MRI requires high fidelity measurements in quantity, location and time of the given physical property. To meet this goal a broad spectrum of commercial measurement systems has been made available. These instruments frequently share in common that they are costly and typically employ closed proprietary hardware and software. This shortcoming makes any adjustment for a specified application difficult if not prohibitive. Recognizing this limitation this work presents COSI Measure, an automated open source measurement system that provides submillimetre resolution, robust configuration and a large working volume to support a versatile range of applications. The submillimetre fidelity and reproducibility/backlash performance were evaluated experimentally. Magnetic field mapping of a single ring Halbach magnet, a 3.0 T and a 7.0 T MR scanner as well as temperature mapping of a radio frequency coil were successfully conducted. Due to its open source nature and versatile construction, the system can be easily modified for other applications. In a resource limited research setting, COSI Measure makes efficient use of laboratory space, financial resources and collaborative efforts.