32-Channel self-grounded bow-tie transceiver array for cardiac MR at 7.0T.

PURPOSE: Design, implementation, evaluation, and application of a 32-channel Self-Grounded Bow-Tie (SGBT) transceiver array for cardiac MR (CMR) at 7.0T. METHODS: The array consists of 32 compact SGBT building blocks. Transmission field ( B1+ ) shimming and radiofrequency safety assessment were performed with numerical simulations and benchmarked against phantom experiments. In vivo B1+ efficiency mapping was conducted with actual flip angle imaging. The array's applicability for accelerated high spatial resolution 2D FLASH CINE imaging of the heart was examined in a volunteer study (n = 7). RESULTS: B1+ shimming provided a uniform field distribution suitable for female and male subjects. Phantom studies demonstrated an excellent agreement between simulated and measured B1+ efficiency maps (7% mean difference). The SGBT array afforded a spatial resolution of (0.8 × 0.8 × 2.5) mm3 for 2D CINE FLASH which is by a factor of 12 superior to standardized cardiovascular MR (CMR) protocols. The density of the SGBT array supports 1D acceleration of up to R = 4 (mean signal-to-noise ratio (whole heart) ≥ 16.7, mean contrast-to-noise ratio ≥ 13.5) without impairing image quality significantly. CONCLUSION: The compact SGBT building block facilitates a modular high-density array that supports accelerated and high spatial resolution CMR at 7.0T. The array provides a technological basis for future clinical assessment of parallel transmission techniques.

Simultaneous T2 and T2∗ mapping of multiple sclerosis lesions with radial RARE-EPI.

PURPOSE: The characteristic MRI features of multiple sclerosis (MS) lesions make it conceptually appealing to pursue parametric mapping techniques that support simultaneous generation of quantitative maps of 2 or more MR contrast mechanisms. We present a modular rapid acquisition with relaxation enhancement (RARE)-EPI hybrid that facilitates simultaneous T2 and T2∗ mapping (2in1-RARE-EPI). METHODS: In 2in1-RARE-EPI the first echoes in the echo train are acquired with a RARE module, later echoes are acquired with an EPI module. To define the fraction of echoes covered by the RARE and EPI module, an error analysis of T2 and T2∗ was conducted with Monte Carlo simulations. Radial k-space (under)sampling was implemented for acceleration (R = 2). The feasibility of 2in1-RARE-EPI for simultaneous T2 and T2∗ mapping was examined in a phantom study mimicking T2 and T2∗ relaxation times of the brain. For validation, 2in1-RARE-EPI was benchmarked versus multi spin-echo (MSE) and multi gradient-echo (MGRE) techniques. The clinical applicability of 2in1-RARE-EPI was demonstrated in healthy subjects and MS patients. RESULTS: There was a good agreement between T2 / T2∗ values derived from 2in1-RARE-EPI and T2 / T2∗ reference values obtained from MSE and MGRE in both phantoms and healthy subjects. In patients, MS lesions in T2 and T2∗ maps deduced from 2in1-RARE-EPI could be just as clearly delineated as in reference maps calculated from MSE/MGRE. CONCLUSION: This work demonstrates the feasibility of radially (under)sampled 2in1-RARE-EPI for simultaneous T2 and T2∗ mapping in MS patients.

Transient enlargement of brain ventricles during relapsing-remitting multiple sclerosis and experimental autoimmune encephalomyelitis.

The brain ventricles are part of the fluid compartments bridging the CNS with the periphery. Using MRI, we previously observed a pronounced increase in ventricle volume (VV) in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis (MS). Here, we examined VV changes in EAE and MS patients in longitudinal studies with frequent serial MRI scans. EAE mice underwent serial MRI for up to 2 months, with gadolinium contrast as a proxy of inflammation, confirmed by histopathology. We performed a time-series analysis of clinical and MRI data from a prior clinical trial in which RRMS patients underwent monthly MRI scans over 1 year. VV increased dramatically during preonset EAE, resolving upon clinical remission. VV changes coincided with blood-brain barrier disruption and inflammation. VV was normal at the termination of the experiment, when mice were still symptomatic. The majority of relapsing-remitting MS (RRMS) patients showed dynamic VV fluctuations. Patients with contracting VV had lower disease severity and a shorter duration. These changes demonstrate that VV does not necessarily expand irreversibly in MS but, over short time scales, can expand and contract. Frequent monitoring of VV in patients will be essential to disentangle the disease-related processes driving short-term VV oscillations from persistent expansion resulting from atrophy.

Wideband Self-Grounded Bow-Tie Antenna for Thermal MR.

The objective of this study was the design, implementation, evaluation and application of a compact wideband self-grounded bow-tie (SGBT) radiofrequency (RF) antenna building block that supports anatomical proton (1 H) MRI, fluorine (19 F) MRI, MR thermometry and broadband thermal intervention integrated in a whole-body 7.0 T system. Design considerations and optimizations were conducted with numerical electromagnetic field (EMF) simulations to facilitate a broadband thermal intervention frequency of the RF antenna building block. RF transmission (B1+ ) field efficiency and specific absorption rate (SAR) were obtained in a phantom, and the thigh of human voxel models (Ella, Duke) for 1 H and 19 F MRI at 7.0 T. B1+ efficiency simulations were validated with actual flip-angle imaging measurements. The feasibility of thermal intervention was examined by temperature simulations (f = 300, 400 and 500 MHz) in a phantom. The RF heating intervention (Pin = 100 W, t = 120 seconds) was validated experimentally using the proton resonance shift method and fiberoptic probes for temperature monitoring. The applicability of the SGBT RF antenna building block for in vivo 1 H and 19 F MRI was demonstrated for the thigh and forearm of a healthy volunteer. The SGBT RF antenna building block facilitated 19 F and 1 H MRI at 7.0 T as well as broadband thermal intervention (234-561 MHz). For the thigh of the human voxel models, a B1+ efficiency ≥11.8 µT/√kW was achieved at a depth of 50 mm. Temperature simulations and heating experiments in a phantom demonstrated a temperature increase ΔT >7 K at a depth of 10 mm. The compact SGBT antenna building block provides technology for the design of integrated high-density RF applicators and for the study of the role of temperature in (patho-) physiological processes by adding a thermal intervention dimension to an MRI device (Thermal MR).

Cardiorenal sodium MRI in small rodents using a quadrature birdcage volume resonator at 9.4 T.

OBJECTIVE: Design, implementation, evaluation and application of a quadrature birdcage radiofrequency (RF) resonator tailored for renal and cardiac sodium (23Na) magnetic resonance imaging (MRI) in rats at 9.4 T. MATERIALS AND METHODS: A low pass birdcage resonator (16 rungs, din = 62 mm) was developed. The transmission field (B1+) was examined with EMF simulations. The scattering parameter (S-parameter) and the quality factor (Q-factor) were measured. For experimental validation B1+-field maps were acquired with the double-angle method. In vivo sodium imaging of the heart (spatial resolution: (1 × 1 × 5) mm3) and kidney (spatial resolution: (1 × 1 × 10) mm3) was performed with a FLASH technique. RESULTS: The RF resonator exhibits RF characteristics, transmission field homogeneity and penetration that afford 23Na MR in vivo imaging of the kidney and heart at 9.4 T. For the renal cortex and medulla a SNRs of 8 and 13 were obtained and a SNRs of 14 and 15 were observed for the left and right ventricle. DISCUSSION: These initial results obtained in vivo in rats using the quadrature birdcage volume RF resonator for 23Na MRI permit dedicated studies on experimental models of cardiac and renal diseases, which would contribute to translational research of the cardiorenal syndrome.

Cardiorenal sodium MRI at 7.0 Tesla using a 4/4 channel 1 H/23 Na radiofrequency antenna array.

PURPOSE: Cardiorenal syndrome describes disorders of the heart and the kidneys in which a dysfunction of 1 organ induces a dysfunction in the other. This work describes the design, evaluation, and application of a 4/4-channel hydrogen-1/sodium (1 H/23 Na) RF array tailored for cardiorenal MRI at 7.0 Tesla (T) for a better physiometabolic understanding of cardiorenal syndrome. METHODS: The dual-frequency RF array is composed of a planar posterior section and a modestly curved anterior section, each section consisting of 2 loop elements tailored for 23 Na MR and 2 loopole-type elements customized for 1 H MR. Numerical electromagnetic field and specific absorption rate simulations were carried out. Transmission field ( B1+ ) uniformity was optimized and benchmarked against electromagnetic field simulations. An in vivo feasibility study was performed. RESULTS: The proposed array exhibits sufficient RF characteristics, B1+ homogeneity, and penetration depth to perform 23 Na MRI of the heart and kidney at 7.0 T. The mean B1+ field for sodium in the heart is 7.7 ± 0.8 µT/√kW and in the kidney is 6.9 ± 2.3 µT/√kW. The suitability of the RF array for 23 Na MRI was demonstrated in healthy subjects (acquisition time for 23 Na MRI: 18 min; nominal isotropic spatial resolution: 5 mm [kidney] and 6 mm [heart]). CONCLUSION: This work provides encouragement for further explorations into densely packed multichannel transceiver arrays tailored for 23 Na MRI of the heart and kidney. Equipped with this technology, the ability to probe sodium concentration in the heart and kidney in vivo using 23 Na MRI stands to make a critical contribution to deciphering the complex interactions between both organs.

Magnetic resonance safety and compatibility of tantalum markers used in proton beam therapy for intraocular tumors: A 7.0 Tesla study.

PURPOSE: Proton radiation therapy (PRT) is a standard treatment of uveal melanoma. PRT patients undergo implantation of ocular tantalum markers (OTMs) for treatment planning. Ultra-high-field MRI is a promising technique for 3D tumor visualization and PRT planning. This work examines MR safety and compatibility of OTMs at 7.0 Tesla. METHODS: MR safety assessment included deflection angle measurements (DAMs), electromagnetic field (EMF) simulations for specific absorption rate (SAR) estimation, and temperature simulations for examining radiofrequency heating using a bow-tie dipole antenna for transmission. MR compatibility was assessed by susceptibility artifacts in agarose, ex vivo pig eyes, and in an ex vivo tumor eye using gradient echo and fast spin-echo imaging. RESULTS: DAM (α < 1 °) demonstrated no risk attributed to magnetically induced OTM deflection. EMF simulations showed that an OTM can be approximated by a disk, demonstrated the need for averaging masses of mave = 0.01 g to accommodate the OTM, and provided SAR0.01g,maximum = 2.64 W/kg (Pin = 1W) in OTM presence. A transfer function was derived, enabling SAR0.01g estimation for individual patient scenarios without the OTM being integrated. Thermal simulations revealed minor OTM-related temperature increase (δT < 15 mK). Susceptibility artifact size (<8 mm) and location suggest no restrictions for MRI of the nervus opticus. CONCLUSION: OTMs are not a per se contraindication for MRI. Magn Reson Med 78:1533-1546, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Experimental MRI Monitoring of Renal Blood Volume Fraction Variations En Route to Renal Magnetic Resonance Oximetry.

Diagnosis of early-stage acute kidney injury (AKI) will benefit from a timely identification of local tissue hypoxia. Renal tissue hypoxia is an early feature in AKI pathophysiology, and renal oxygenation is increasingly being assessed through T2*-weighted magnetic resonance imaging (MRI). However, changes in renal blood volume fraction (BVf) confound renal T2*. The aim of this study was to assess the feasibility of intravascular contrast-enhanced MRI for monitoring renal BVf during physiological interventions that are concomitant with variations in BVf and to explore the possibility of correcting renal T2* for BVf variations. A dose-dependent study of the contrast agent ferumoxytol was performed in rats. BVf was monitored throughout short-term occlusion of the renal vein, which is known to markedly change renal blood partial pressure of O2 and BVf. BVf calculated from MRI measurements was used to estimate oxygen saturation of hemoglobin (SO2). BVf and SO2 were benchmarked against cortical data derived from near-infrared spectroscopy. As estimated from magnetic resonance parametric maps of T2 and T2*, BVf was shown to increase, whereas SO2 was shown to decline during venous occlusion (VO). This observation could be quantitatively reproduced in test-retest scenarios. Changes in BVf and SO2 were in good agreement with data obtained from near-infrared spectroscopy. Our findings provide motivation to advance multiparametric MRI for studying AKIs, with the ultimate goal of translating MRI-based renal BVf mapping into clinical practice en route noninvasive renal magnetic resonance oximetry as a method of assessing AKI and progression to chronic damage.