Improving magnetic resonance imaging with smart and thin metasurfaces.

Over almost five decades of development and improvement, Magnetic Resonance Imaging (MRI) has become a rich and powerful, non-invasive technique in medical imaging, yet not reaching its physical limits. Technical and physiological restrictions constrain physically feasible developments. A common solution to improve imaging speed and resolution is to use higher field strengths, which also has subtle and potentially harmful implications. However, patient safety is to be considered utterly important at all stages of research and clinical routine. Here we show that dynamic metamaterials are a promising solution to expand the potential of MRI and to overcome some limitations. A thin, smart, non-linear metamaterial is presented that enhances the imaging performance and increases the signal-to-noise ratio in 3T MRI significantly (up to eightfold), whilst the transmit field is not affected due to self-detuning and, thus, patient safety is also assured. This self-detuning works without introducing any additional overhead related to MRI-compatible electronic control components or active (de-)tuning mechanisms. The design paradigm, simulation results, on-bench characterization, and MRI experiments using homogeneous and structural phantoms are described. The suggested single-layer metasurface paves the way for conformal and patient-specific manufacturing, which was not possible before due to typically bulky and rigid metamaterial structures.

Quantitative Dynamic Oxygen 17 MRI at 7.0 T for the Cerebral Oxygen Metabolism in Glioma.

Background Altered metabolism is a characteristic of cancer. Because of a shift in glucose metabolism from oxidative phosphorylation to lactate production for energy generation, malignant tumors are characterized by increased glycolysis followed by lactic acid fermentation, even in the presence of abundant oxygen (the Warburg effect). Purpose To quantitatively investigate dynamic oxygen 17 (17O) MRI in healthy participants and participants with untreated glioma to understand altered cerebral oxygen metabolism in glioma. Materials and Methods In this prospective study conducted from September 2016 to June 2018, individuals with newly diagnosed previously untreated glioma (World Health Organization grade II-IV) and healthy volunteers were included. Dynamic 17O MRI was performed with a 7.0-T whole-body system. 17O2 gas inhalation enabled dynamic measurement of the cerebral metabolic rate of oxygen (CMRO2) consumption. In healthy volunteers and participants with glioma, CMRO2 values in gray matter and white matter volumes were compared by using Wilcoxon signed rank tests. In participants with glioma, the tumor volume and tumor subcompartments were compared with normal-appearing gray matter and white matter by using Friedman test followed by Holm-Sidak post hoc tests. Results Ten participants (mean age, 42 years ± 18 [standard deviation]; nine men) with glioma and three healthy volunteers (mean age, 44 years ± 21; all men) were evaluated. CMRO2 was higher in normal-appearing gray matter compared with white matter in both participants with glioma (2.36 µmol/g/min ± 0.22 vs 0.75 µmol/g/min ± 0.10, respectively) and healthy volunteers (2.38 µmol/g/min ± 0.15 vs 0.63 µmol/g/min ± 0.05, respectively) (P < .001 and P = .03, respectively). In the tumor region, CMRO2 was reduced (high-grade tumor CMRO2, 0.23 µmol/g/min ± 0.07; low-grade tumor CMRO2, 0.39 µmol/g/min ± 0.16; overall CMRO2, 0.34 µmol/g/min ± 0.16) compared with normal-appearing gray matter (P < .001) and normal-appearing white matter (P < .001) in accordance with the Warburg theorem. Conclusion Dynamic oxygen 17 MRI method at 7.0 T as a direct metabolic imaging technique in glioma enabled quantitative visualization of the Warburg effect. A general reduction in oxidative glycolysis was observed in accordance with the Warburg theorem. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Rapalino in this issue.

SERIAL transmit - parallel receive (STxPRx) MR imaging produces acceptable proton image uniformity without compromising field of view or SAR guidelines for human neuroimaging at 9.4 Tesla.

PURPOSE: Non-uniform B1+ excitation and high specific absorption rates (SAR) compromise proton MR imaging of human brain at 9.4 T (400.5 MHz). By combining a transmit/receive surface coil array using serial transmission of individual coils with a total generalized variation reconstruction of images from all coils, acceptable quality human brain imaging is demonstrated. METHODS: B0 is shimmed using sodium MR imaging (105.4 MHz) with a birdcage coil. Proton MR imaging is performed with an excitation/receive array of surface coils. The modified FLASH pulse sequence transmits serially across each coil within the array thereby distributing SAR in time and space. All coils operate in receive mode. Although the excitation profile of each transmit coil is non-uniform, the sensitivity profile estimated from the non-transmit receive coils provides an acceptable sensitivity correction. Signals from all coils are combined in a total generalized variation (TGV) reconstruction to provide a full field of view image at maximum signal to noise (SNR) performance. RESULTS: High-resolution images across the human head are demonstrated with acceptable uniformity and SNR. CONCLUSION: Proton MR imaging of the human brain is possible with acceptable uniformity at low SAR at 9.4 Tesla using this serial excitation and parallel reception strategy with TGV reconstruction.

Reproducibility of CMRO2 determination using dynamic 17 O MRI.

PURPOSE: To assess the reproducibility of 17 O MRI-based determination of the cerebral metabolic rate of oxygen consumption (CMRO2 ) in healthy volunteers. To assess the influence of image acquisition and reconstruction parameters on dynamic quantification of functional parameters such as CMRO2 . METHODS: Dynamic 17 O MRI data were simulated and used to investigate influences of temporal resolution (Δt) and partial volume correction (PVC) on the determination of CMRO2 . Three healthy volunteers were examined in two separate examinations. In vivo 17 O MRI measurements were conducted with a nominal spatial resolution of (7.5 mm)3 using a density-adapted radial sequence with golden angle acquisition scheme. In each measurement, 4.0 ± 0.1 L of 70%-enriched 17 O gas were administered using a rebreathing system. Data were corrected with a PVC algorithm, and CMRO2 was determined in gray matter (GM) and white matter (WM) compartments using a three-phase metabolic model (baseline, 17 O inhalation, decay phase). RESULTS: Comparison with the ground truth of simulations revealed improved CMRO2 determination after application of PVC and with Δt ≤ 2:00 min. Evaluation of in vivo data yields to CMRO2,GM = 2.31 ± 0.1 µmol/g/min and to CMRO2,WM = 0.69 ± 0.04 µmol/g/min with coefficients of variation (CoV) of 0.3-5.5% and 4.3-5.0% for intra-volunteer and inter-volunteer data, respectively. CONCLUSION: This in vivo 17 O inhalation study demonstrated that the proposed experimental setup enables reproducible determination of CMRO2 in healthy volunteers. Magn Reson Med 79:2923-2934, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Improved T*2 determination in 23Na, 35Cl, and 17O MRI using iterative partial volume correction based on 1H MRI segmentation.

OBJECTIVE: Functional parameters can be measured with the help of quantitative non-proton MRI where exact relaxometry parameters are needed. Investigation of [Formula: see text] is often biased by strong partial volume (PV) effects. Hence, in this work a PV correction algorithm approach was evaluated that uses iteratively adapted [Formula: see text]-values and high-resolution structural 1H data to determine transverse relaxation in non-proton MRI more accurately. MATERIALS AND METHODS: Simulations, a phantom study and in vivo 23Na, 17O and 35Cl MRI measurements of five healthy volunteers were performed to evaluate the algorithm. [Formula: see text] values of grey matter (GM), white matter (WM) and cerebrospinal fluid (CSF) were obtained. Data were acquired at B 0  = 7T with nominal spatial resolutions of (4-7 mm)3 using a density-adapted radial sequence. The resulting transverse relaxation times were used for quantification of 17O data. RESULTS: The conducted simulations and phantom study verified the correction performance of the algorithm. For in vivo measured [Formula: see text] values, the correction of PV effects leads to an increase in CSF and to a decrease in GM/WM (23Na MRI: long/short GM, WM [Formula: see text]: 36.4 ± 3.1/5.4 ± 0.2, 23.3 ± 2.6/3.5 ± 0.1 ms; 35Cl MRI: 8.9 ± 1.4/1.0 ± 0.4, 5.9 ± 0.3/0.4 ± 0.1 ms; 17O MRI: 2.5 ± 0.1, 2.8 ± 0.1 ms). Iteratively corrected in vivo [Formula: see text] values of the 17O study resulted in improved water content quantification. CONCLUSION: The proposed iterative algorithm for PV correction leads to more accurate [Formula: see text] values and, thus, can improve accuracy in quantitative non-proton MRI.

Nuclear magnetic resonance diffusion pore imaging: Experimental phase detection by double diffusion encoding.

Diffusion pore imaging is an extension of diffusion-weighted nuclear magnetic resonance imaging enabling the direct measurement of the shape of arbitrarily formed, closed pores by probing diffusion restrictions using the motion of spin-bearing particles. Examples of such pores comprise cells in biological tissue or oil containing cavities in porous rocks. All pores contained in the measurement volume contribute to one reconstructed image, which reduces the problem of vanishing signal at increasing resolution present in conventional magnetic resonance imaging. It has been previously experimentally demonstrated that pore imaging using a combination of a long and a narrow magnetic field gradient pulse is feasible. In this work, an experimental verification is presented showing that pores can be imaged using short gradient pulses only. Experiments were carried out using hyperpolarized xenon gas in well-defined pores. The phase required for pore image reconstruction was retrieved from double diffusion encoded (DDE) measurements, while the magnitude could either be obtained from DDE signals or classical diffusion measurements with single encoding. The occurring image artifacts caused by restrictions of the gradient system, insufficient diffusion time, and by the phase reconstruction approach were investigated. Employing short gradient pulses only is advantageous compared to the initial long-narrow approach due to a more flexible sequence design when omitting the long gradient and due to faster convergence to the diffusion long-time limit, which may enable application to larger pores.

In vivo assessment of cold stimulation effects on the fat fraction of brown adipose tissue using DIXON MRI.

PURPOSE: To evaluate the volume and changes of human brown adipose tissue (BAT) in vivo following exposure to cold using magnetic resonance imaging (MRI). MATERIALS AND METHODS: The clavicular region of 10 healthy volunteers was examined with a 3T MRI system. One volunteer participated twice. A cooling vest that was circulated with temperature-controlled water was used to expose each volunteer to a cold environment. Three different water temperature phases were employed: baseline (23°C, 20 min), cooling (12°C, 90 min), and a final warming phase (37°C, 30 min). Temperatures of the water in the circuit, of the body, and at the back skin of the volunteers were monitored with fiberoptic temperature probes. Applying the 2-point DIXON pulse sequence every 5 minutes, fat fraction (FF) maps were determined and evaluated over time to distinguish between brown and white adipose tissue. RESULTS: Temperature measurements showed a decrease of 3.8 ± 1.0°C of the back skin temperature, while the body temperature stayed constant at 37.2 ± 0.9°C. Focusing on the two interscapular BAT depots, a mean FF decrease of -2.9 ± 2.0%/h (P < 0.001) was detected during cold stimulation in a mean absolute volume of 1.31 ± 1.43 ml. Also, a correlation of FF decrease to back skin temperature decrease was observed in all volunteers (correlation coefficients: |r| = [0.51; 0.99]). CONCLUSION: We found that FF decreases in BAT begin immediately with mild cooling of the body and continue during long-time cooling. LEVEL OF EVIDENCE: 2 J. Magn. Reson. Imaging 2017;45:369-380.

(39) K and (23) Na relaxation times and MRI of rat head at 21.1 T.

At ultrahigh magnetic field strengths (B0 ≥ 7.0 T), potassium ((39) K) MRI might evolve into an interesting tool for biomedical research. However, (39) K MRI is still challenging because of the low NMR sensitivity and short relaxation times. In this work, we demonstrated the feasibility of (39) K MRI at 21.1 T, determined in vivo relaxation times of the rat head at 21.1 T, and compared (39) K and sodium ((23) Na) relaxation times of model solutions containing different agarose gel concentrations at 7.0 and 21.1 T. (39) K relaxation times were markedly shorter than those of (23) Na. Compared with the lower field strength, (39) K relaxation times were up to 1.9- (T1 ), 1.4- (T2S ) and 1.9-fold (T2L ) longer at 21.1 T. The increase in the (23) Na relaxation times was less pronounced (up to 1.2-fold). Mono-exponential fits of the (39) K longitudinal relaxation time at 21.1 T revealed T1 = 14.2 ± 0.1 ms for the healthy rat head. The (39) K transverse relaxation times were 1.8 ± 0.2 ms and 14.3 ± 0.3 ms for the short (T2S ) and long (T2L ) components, respectively. (23) Na relaxation times were markedly longer (T1 = 41.6 ± 0.4 ms; T2S = 4.9 ± 0.2 ms; T2L = 33.2 ± 0.2 ms). (39) K MRI of the healthy rat head could be performed with a nominal spatial resolution of 1 × 1 × 1 mm(3) within an acquisition time of 75 min. The increase in the relaxation times with magnetic field strength is beneficial for (23) Na and (39) K MRI at ultrahigh magnetic field strength. Our results demonstrate that (39) K MRI at 21.1 T enables acceptable image quality for preclinical research. Copyright © 2016 John Wiley & Sons, Ltd.

Development and investigation of a magnetic resonance imaging-compatible microlens-based optical detector.

A noncontact optical detector for in vivo imaging has been developed that is compatible with magnetic resonance imaging (MRI). The optical detector employs microlens arrays and might be classified as a plenoptic camera. As a resulting of its design, the detector possesses a slim thickness and is self-shielding against radio frequency (RF) pulses. For experimental investigation, a total of six optical detectors were arranged in a cylindrical fashion, with the imaged object positioned in the center of this assembly. A purposely designed RF volume resonator coil has been developed and is incorporated within the optical imaging system. The whole assembly was placed into the bore of a 1.5 T patient-sized MRI scanner. Simple-geometry phantom studies were performed to assess compatibility and performance characteristics regarding both optical and MR imaging systems. A bimodal ex vivo nude mouse measurement was conducted. From the MRI data, the subject surface was extracted. Optical images were projected on this surface by means of an inverse mapping algorithm. Simultaneous measurements did not reveal influences from the magnetic field and RF pulses onto optical detector performance (spatial resolution, sensitivity). No significant influence of the optical imaging system onto MRI performance was detectable.

In vivo visualization of mesoscopic anatomy of healthy and pathological lymph nodes using 7T MRI: a feasibility study.

PURPOSE: To evaluate whether inguinal lymph nodes (LNs) may be visualized in vivo using 7T magnetic resonance imaging (MRI) at high spatial resolution. MATERIALS AND METHODS: Twelve healthy controls and six patients with LN metastasis of melanoma were included. Examinations were performed using a 7T MRI and a transmit/receive loop coil. The protocol included a B0 -map, B1 -map, and T1 -weighted-3D-fast low-angle shot (FLASH), T1 w-Dixon-volumetric interpolated breath-hold examination (VIBE) and T2 w sequences lasting 34.4 ± 0.5 minutes. Signal- and contrast-to-noise of LNs, artery, muscle, and fat were quantified in controls. Metastatic features of LNs (hypervascularization, lymph vessels, fat hilus sign, tumor bulk, number of metastases, and size) were classified in patients. RESULTS: Mesoscopic LN architecture such as central blood vessels and peripheral lymph vessels were observed in healthy controls with 0.5 mm(3) isotropic resolution for T1 w and 0.2 × 0.2 × 2 mm(3) for T2 w sequences. Mean signal-to-noise using 3D FLASH, Dixon VIBE and T2 TSE of healthy LN (27.2 ± 7.5, 35.3 ± 11.9, 31.7 ± 11.1), muscle (17.6 ± 4.6, 31.5 ± 9.3, 7.3 ± 5.4), artery (37.7 ± 5.9, 42.7 ± 19.7, 3.7 ± 3.9), and saturated fat (3.7 ± 0.9, 5.4 ± 1.9, 9.3 ± 5.2) and mean contrast-to-noise LN/fat (24.4 ± 6.7, 39.6 ± 11.1, 23.3 ± 6.1) were adequate. In patients, multiple signs of metastasis could be clearly visualized. CONCLUSION: We present a protocol with which inguinal LNs and their mesoscopic anatomy may be visualized in vivo using 7T MRI.

In vivo 35Cl MR imaging in humans: a feasibility study.

PURPOSE: To implement chlorine 35 ((35)Cl) magnetic resonance (MR) at a 7-T whole-body MR system and evaluate its feasibility for imaging humans. MATERIALS AND METHODS: All examinations were performed with ethical review board approval; written informed consent was obtained from all volunteers. Seven examinations each of brain and muscle in healthy volunteers and four examinations of patients were performed. Two patients with histologically confirmed glioblastoma multiforme underwent brain imaging. (35)Cl MR and (35)Cl inversion-recovery (IR) MR were performed. Two patients with genetically confirmed hypokalemic periodic paralysis underwent calf muscle imaging. Seven multiecho sequences (acquisition time, 5 minutes; voxel dimension, 11 mm(3)) were applied to determine transverse relaxation time as affected by magnetic field heterogeneity (T2*) and chlorine concentration. (35)Cl and sodium 23 ((23)Na) MR were conducted with a 7-T whole-body MR system. (35)Cl longitudinal relaxation time (T1) and T2* of healthy human brain and muscle were determined with a three-dimensional density-adapted-projection reconstruction technique to achieve short echo times and high signal-to-noise ratio (SNR) efficiency. A nonlinear least squares routine and mono- (T1) and biexponential (T2*) models were used for curve fitting. RESULTS: Phantom imaging revealed 15-fold lower SNR and much shorter relaxation times for (35)Cl than (23)Na. In vivo T2* was biexponential and extremely short. Monoexponential fits of T1 revealed 9.2 and 4.0 milliseconds ± 0.7 (standard deviation) for brain and muscle, respectively. In glioblastoma tissue, increased Cl(-) concentrations and increased Cl(-) IR signal intensities were detected. Voxel dimension and acquisition time, respectively, were 6 mm(3) and 9 minutes 45 seconds ((35)Cl MR) and 10 mm(3) and 10 minutes ((35)Cl IR MR). In patients with hypokalemic periodic paralysis versus healthy volunteers, Cl(-) and Na(+) concentrations were increased. Cl(-) concentration of muscle could be determined (voxel size, 11 mm(3); total acquisition time, 35 minutes). CONCLUSION: MR at 7 T enables in vivo imaging of (35)Cl in human brain and muscle in clinically feasible acquisition times (10-35 minutes) and voxel volumes (0.2-1.3 cm(3)). Pathophysiological changes of Cl(-) homeostasis due to cancer or muscular ion channel disease can be visualized.

An MR-compatible stereoscopic in-room 3D display for MR-guided interventions.

BACKGROUND AND METHODS: A commercial three-dimensional (3D) monitor was modified for use inside the scanner room to provide stereoscopic real-time visualization during magnetic resonance (MR)-guided interventions, and tested in a catheter-tracking phantom experiment at 1.5 T. Brightness, uniformity, radio frequency (RF) emissions and MR image interferences were measured. RESULTS AND DISCUSSION: Due to modifications, the center luminance of the 3D monitor was reduced by 14%, and the addition of a Faraday shield further reduced the remaining luminance by 31%. RF emissions could be effectively shielded; only a minor signal-to-noise ratio (SNR) decrease of 4.6% was observed during imaging. During the tracking experiment, the 3D orientation of the catheter and vessel structures in the phantom could be visualized stereoscopically.

Tracking of an interventional catheter with a ferromagnetic tip using dual-echo projections.

Interventional devices with ferromagnetic components can be manipulated remotely using forces induced by the MRI gradients. To deflect the tip of an endovascular catheter, large ferromagnetic spheres of 2 mm diameter are required to exert sufficiently high magnetic forces; however, tracking of these devices is difficult due to the large image artifacts. In this study, a new dual-echo technique is proposed to improve the stability of localizing and tracking medical devices with ferromagnetic components. MR tracking methods with selective off-resonant excitation and phase compensation with a rephasing gradient can detect ferromagnetic spheres up to a diameter of 1 mm only. In this work, a dual-echo technique is used with two rephasing gradients to stabilize the off-set localization. With rephasing being applied in orthogonal directions, an SNR of 5 was achieved in the signal projections. Compared to a single-echo acquisition the dual-echo method reduces the position error in a phantom from 8 mm to 1.6 mm. In an in vivo study a tracking precision of 4 mm was measured without steering gradients at an image update rate of 2 images per second. Steering experiments were successfully performed with a prototype catheter with ferromagnetic sphere in an aorta phantom and in the vena cava of a pig.

In vivo 39K MR imaging of human muscle and brain.

PURPOSE: To implement potassium 39 ((39)K) magnetic resonance (MR) imaging with a 7-T MR imaging system and to evaluate its feasibility for in vivo imaging of human muscle and brain. MATERIALS AND METHODS: Three healthy volunteers were examined with approval of the ethical review board of Heidelberg University. Written informed consent was obtained from all volunteers. Because the available 7-T MR imaging system did not support (39)K, a frequency conversion scheme was developed and connected to the imaging unit. The standard X-nucleus frequency of lithium 7 (115 MHz) was converted to the frequency of (39)K at 7 T (14 MHz). Relaxation times of healthy thigh muscle and brain tissue were estimated by using multiple-echo and inversion-recovery sequences. Data analysis was conducted with a nonlinear least squares curve fitting tool. In vivo (39)K MR imaging of healthy human thigh muscle and brain was performed. RESULTS: With use of the custom-built frequency conversion scheme, (39)K MR imaging is feasible with a commercially available 7-T MR imaging system. Nominal spatial resolutions of 8 × 8 × 16 mm(3) and 9.5 × 9.5 × 9.5 mm(3) were achieved for human thigh muscle and brain, respectively. Acquisition time was 30 minutes for both muscle and brain tissue. The measured potassium concentration (uncorrected for fat fraction) of thigh muscle tissue (112-124 mmol/L) lies within the expected range. CONCLUSION: In vivo (39)K MR imaging in humans can be performed in clinically feasible measurement times (approximately 30 minutes) with voxel sizes of approximately 1 mL.

Initial in vivo experience with a novel type of MR-safe pushable coils for MR-guided embolizations.

OBJECTIVE: Conventional detachable embolization coils are made from platinum or stainless steel and may thus be a magnetic resonance (MR) safety hazard because of resonant device heating. The objective of this experimental study was to assess the feasibility of MR-guided embolization procedures with a novel type of nonmetallic and, therefore, intrinsically MR-safe pushable coil. MATERIALS AND METHODS: The embolization coils are made from a polymer and coated with a hydrogel, which expands during contact with liquids. Magnetic resonance-guided embolizations were performed in 6 healthy domestic pigs by deploying up to 3 polymer pushable coils via an active tracking catheter under real-time magnetic resonance imaging monitoring. To assess the renal perfusion deficit induced by the coil embolization, intra-arterial 3-dimensional contrast-enhanced magnetic resonance angiography (3D ce-MRA) data sets were acquired before and every 5 minutes after coil placement until complete vessel occlusion. RESULTS: The MR-guided embolizations were successful in 5 of the 6 animals. The 3D ce-MRA data sets indicated first perfusion deficits within 2 to 40 minutes after coil deployment. Complete vessel occlusion was achieved after 6 to 53 minutes. In 1 animal, no perfusion defect could be detected. Because our experiments were designed as a preliminary proof-of-concept study, different sizes and numbers of all-polymer hydrocoils were deployed at different anatomical positions, making the drawing of correlation between the size/number of deployed coils and the occlusion efficiency difficult. The all-polymer hydrocoils did not induce any artifacts on the MR images, either in the real-time MR images, which were recorded during the embolization, or in the subsequently acquired 3D ce-MRA images. CONCLUSIONS: Our results demonstrated that the novel all-polymer and intrinsically MR-safe pushable hydrocoils may become a promising tool for MR-guided embolization procedures.

7 Tesla compatible in-bore display for functional magnetic resonance imaging.

BACKGROUND AND METHODS: A liquid crystal display was modified for use inside a 7 T MR magnet. SNR measurements were performed using different imaging sequences with the monitor absent, present, or activated. fMRI with a volunteer was conducted using a visual stimulus. RESULTS AND DISCUSSION: SNR was reduced by 3.7%/7.9% in echo planar/fast-spin echo images when the monitor was on which can be explained by the limited shielding of the coated front window (40 dB). In the fMRI experiments, activated regions in the visual cortex were clearly visible. The monitor provided excellent resolution at minor SNR reduction in EPI images, and is thus suitable for fMRI at ultra-high field.

Coaxial waveguide MRI.

As ultrahigh-field MR imaging systems suffer from the standing wave problems of conventional coil designs, the use of antenna systems that generate travelling waves was suggested. As a modification to the original approach, we propose the use of a coaxial waveguide configuration with interrupted inner conductor. This concept can focus the radiofrequency energy to the desired imaging region in the human body and can operate at different Larmor frequencies without hardware modifications, as it is not limited by a lower cut-off frequency. We assessed the potential of the method with a hardware prototype setup that was loaded with a tissue equivalent phantom and operated with imaging areas of different size. Signal and flip angle distributions within the phantom were analyzed, and imaging at different Larmor frequencies was performed. Results were compared to a finite difference time domain simulation of the setup that additionally provides information on the spatial distribution of the specific absorption rate load. Furthermore, simulation results with a human model (virtual family) are presented. It was found that the proposed method can be used for MRI at multiple frequencies, achieving transmission efficiencies similar to other travelling wave approaches but still suffers from several limitations due to the used mode of wave propagation.

MR safety: simultaneous B0, dΦ/dt, and dB/dt measurements on MR-workers up to 7 T.

OBJECT: The EU directive on safety requirements (2004/40/EC) limits the exposure to time varying magnetic fields to dB /dt=200 mT/s. This action value is not clearly defined as it considers only the temporal change of the magnitude of B. Thus, only the translational motion in the magnet's fringe field is considered and rotations are neglected. MATERIALS AND METHODS: A magnetic field probe was constructed to simultaneously record the magnetic flux density B(x, y, z) with a 3-axis Hall sensor and the induced voltage due to movements with a set of three orthogonal coils. Voltages were converted into time-varying magnetic flux d Φ(x, y, z)/dt serving as an exposition parameter for both translations and rotations. To separate the two types of motion, d B/dt was additionally calculated on the basis of the Hall sensor's data. The calibrated probe was attached to the forehead of 8 healthcare workers and 17 MR physicists, and B and dΦ/dt were recorded during standard operating procedures at three different MR systems up to 7 T. RESULTS: The maximum percentage of the translational motion referring the data including both translations and rotations amounts to 32%. During volunteer measurements, maximum exposure values of dΦ/dt=21 mWb/s, dB/dt=1.40 T/s and |B|=2.75 T were found. CONCLUSION: The findings in this work indicate that both translations and rotations in the vicinity of an MR system should be taken into account, and that a single regulatory action level might not be sufficient.

A measurement setup for direct 17O MRI at 7 T.

An efficient breathing system was designed for direct (17)O MRI to perform oxygen metabolism studies of the human brain. The breathing system consists of a demand oxygen delivery device for (17)O(2) supply and a custom-built re-breathing circuit with pneumatic switching valve. To efficiently deliver the (17)O gas to the alveoli of the lungs, the system applies short gas pulses upon an inspiration trigger via a nasal cannula. During and after (17)O(2) administration, the exhaled gas volumes are stored and filtered in the re-breathing section to make the most efficient use of the rare (17)O gas. In an inhalation experiment, 2.2 ± 0.1 L of 70%-enriched (17)O(2) were administered to a healthy volunteer and direct (17)O MRI was performed for a total imaging time of 38 min with a temporal resolution of 50 s per 3D data set. Mapping of the maximum signal increase was carried out showing regional variations of oxygen concentration of up to 30% over the natural abundance of (17)O water.