A radiofrequency coil configuration for imaging the human vertebral column at 7 T

We describe the design and testing of a quadrature transmit, eight-channel receive array RF coil configuration for the acquisition of images of the entire human spinal column at 7 T. Imaging parameters were selected to enable data acquisition in a clinically relevant scan time. Large field-of-view (FOV) scanning enabled sagittal imaging of the spine in two or three-stations, depending upon the height of the volunteer, with a total scan time of between 10 and 15 min. A total of 10 volunteers have been scanned, with results presented for the three subjects spanning the range of heights and weights, namely one female (1.6 m, 50 kg), one average male (1.8 m, 70 kg), and one large male (1.9 m, 100 kg).     

A purpose-built neck coil for black-blood DANTE-prepared carotid artery imaging at 7 T

Atherosclerotic plaques in the bifurcation of the carotid arteries can pose a significant health risk due to possible plaque rupture and subsequent stroke. The assessment of plaques, and evaluation of the risk they pose, can be performed with Black-Blood (BB) vessel wall magnetic resonance imaging. However, resolution at standard clinical field strengths (up to 3 T) is limited, hampering reliable assessment and diagnosis. The aim of this study was to investigate the benefits of 7 T MRI using a BB application that has been successful at clinical field strengths. Therefore, for BB imaging, each sequence was preceded with ‘Delay Alternating with Nutation for Tailored Excitation’ (DANTE) preparation pulses for blood signal suppression. A coil comprising a 4-channel Tx array was designed and built to provide the required excitation coverage for the DANTE train; and a 4-channel Rx array was constructed to target the carotid bifurcation. Human and phantom results showed satisfactory blood suppression and comparable SNR and CNR to 3 T, therefore demonstrating the feasibility of the application at 7 T. However, the imposed SAR restrictions led to long scan times and subsequent motion artifacts. Thus, more accurate local SAR supervision schemes are required which could lead to a further improvement of BB DANTE vessel wall imaging at 7 T.     

Optimization of a quadrature birdcage coil for functional imaging of squirrel monkey brain at 9.4T

A quadrature transmit/receive birdcage coil was optimized for squirrel monkey functional imaging at the high field of 9.4 T. The coil length was chosen to gain maximum coil efficiency/signal-to-noise ratio (SNR) and meanwhile provide enough homogenous RF field in the whole brain area. Based on the numerical simulation results, a 16-rung high-pass birdcage coil with the optimal length of 9 cm was constructed and evaluated on phantom and in vivo experiments. Compared to a general-purpose non-optimized coil, it exhibits approximately 25% in vivo SNR improvement. In addition to the volume coil, details about how to design and construct the associated animal preparation system were provided.     

Reduced field-of-view diffusion-weighted imaging of the brain at 7 T

Ventral and rostral regions of the brain are of emerging importance for the MRI characterization of early dementia, traumatic brain injury and epilepsy. Unfortunately, standard single-shot echo planar diffusion-weighted imaging of these regions at high fields is contaminated by severe imaging artifacts in the vicinity of air–tissue interfaces. To mitigate these artifacts and improve visualization of the temporal and frontal lobes at 7 T, we applied a reduced field-of-view strategy, enabled by outer volume suppression (OVS) with novel quadratic phase radiofrequency (RF) pulses, combined with partial Fourier and parallel imaging methods. The new acquisition greatly reduced the level of artifacts in six human subjects (including four patients with early symptoms of dementia).     

Surface coil imaging of rat spine at 7.0 T

An inductively coupled surface coil for imaging the rat spine at 7 T is described. This planar circular probe was made from microwave substrate to limit the size of the coil and to minimize the magnetic susceptibility. The surface coil was used as a single transmit/receive coil and as a receive-only coil with a birdcage body coil for excitation. The signal-to-noise ratio (SNR) of the probe was compared to a 5-cm birdcage coil and exceeded the birdcage coil's SNR by three to six times at superficial structures. The main advantages of the probe are an improved SNR for superficial structures and a simple design and use. Images with 50 × 50 × 500 μm voxels were obtained of the rat spine with excellent anatomical detail.     

GRAPPA-based susceptibility-weighted imaging of normal volunteers and patients with brain tumor at 7 T

Susceptibility-weighted imaging (SWI) is a valuable technique for high-resolution imaging of brain vasculature that greatly benefits from the emergence of higher field strength MR scanners. Autocalibrating partially parallel imaging techniques can be employed to reduce lengthy acquisition times as long as the decrease in signal-to-noise ratio does not significantly affect the contrast between vessels and brain parenchyma. This study assessed the feasibility of a Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA)-based SWI technique at 7 T in both healthy volunteers and brain tumor patients. GRAPPA-based SWI allowed a twofold or more reduction in scan time without compromising vessel contrast and small vessel detection. Postprocessing parameters for the SWI needed to be modified for patients where the tumor causes high-frequency phase wrap artifacts but did not adversely affect vessel contrast. GRAPPA-based SWI at 7 T revealed regions of microvascularity, hemorrhage and calcification within heterogeneous brain tumors that may aid in characterizing active or necrotic tumor and monitoring treatment effects.     

Phased array 3D MR spectroscopic imaging of the brain at 7 T

Ultra-high-field 7 T magnetic resonance (MR) scanners offer the potential for greatly improved MR spectroscopic imaging due to increased sensitivity and spectral resolution. Prior 7 T human single-voxel MR Spectroscopy (MRS) studies have shown significant increases in signal-to-noise ratio (SNR) and spectral resolution as compared to lower magnetic fields but have not demonstrated the increase in spatial resolution and multivoxel coverage possible with 7 T MR spectroscopic imaging. The goal of this study was to develop specialized radiofrequency (RF) pulses and sequences for three-dimensional (3D) MR spectroscopic imaging (MRSI) at 7 T to address the challenges of increased chemical shift misregistration, B1 power limitations, and increased spectral bandwidth. The new 7 T MRSI sequence was tested in volunteer studies and demonstrated the feasibility of obtaining high-SNR phased-array 3D MRSI from the human brain.     

Design and construction of an interchangeable RF coil system for rodent spinal cord MR imaging at 9.4 T

Rodent models of spinal cord injury (SCI) have been widely used in pre-clinical studies. Injuries may occur at different levels of the lumbar and thoracic cord, and the number of segments injured and their depths may vary along the spine. It is thereby challenging to build one universal RF coil that exhibits optimal performance for all spinal cord imaging applications, especially in an animal scanner with small in-bore space and limited hardware configurations. We developed an interchangeable RF coil system for a 9.4 T small animal MRI scanner, in which the users can select an optimal coil specialized for imaging specific parts of a rat spine. We also developed the associated animal management device for immobilization and positioning. The whole system allows ease of RF coil exchange, animal fixation, and positioning, and thus reduces the animal preparation time before the MRI scan significantly. Compared to a commercial general-purpose 2-cm-diameter coil that was used in our previous studies, the specialized coil optimized for Sprague-Dawley rat lumbar spinal cord imaging exhibits up to 2.4 times SNR improvement.     

Design of an interventional magnetic resonance imaging coil for cerebral surgery

In clinical magnetic resonance imaging(MRI),the design of the radiofrequency(RF) coil is very important.For certain applications,the appropriate coil can produce an improved image quality.However,it is difficult to achieve a uniform B1 field and a high signal-to-noise ratio(SNR) simultaneously.In this article,we design an interventional transmitter-and-receiver RF coil for cerebral surgery.This coil adopts a disassembly structure that can be assembled and disassembled repeatedly on the cerebral surgery gantry to reduce the amount of interference from the MRI during surgery.The simulation results and the imaging experiments demonstrate that this coil can produce a uniform RF field,a high SNR,and a large imaging range to meet the requirements of the cerebral surgery.     

A microstrip transmission line volume coil for human head MR imaging at 4T

A high-frequency RF volume coil based on the use of microstrip transmission line (MTL) has been developed for in vivo 1H MR applications on the human head at 4T. This coil is characterized by major advantages: (i) completely distributed coil circuit, (ii) high-quality factor (Q), (iii) simple coil structure, and (iv) better sensitivity and less signal-intensity variation in the MR image of the human head compared with an RF shielded birdcage coil of similar coil size. The proposed MTL volume coil does not require additional RF shielding for preventing Q degradation from radiation losses due to the unique MTL structure; thus, it provides a maximal useable space inside the volume coil when compared with most volume coils available at high fields with the same overall coil size. The intrinsic B(1) distribution of the MTL volume coil effectively compensates for the dielectric resonance effect at 4T and improves the signal homogeneity in human head MR images in the transaxial planes. The results of this study demonstrate that the MTL volume coil design provides an efficient and simple solution to RF volume coil design for human MR studies at high fields.     

Automatic tuned MRI RF coil for multinuclear imaging of small animals at 3T

We have developed an MRI RF coil whose tuning can be adjusted automatically between 120 and 128 MHz for sequential spectroscopic imaging of hydrogen and fluorine nuclei at field strength 3 T. Variable capacitance (varactor) diodes were placed on each rung of an eight-leg low-pass birdcage coil to change the tuning frequency of the coil. The diode junction capacitance can be controlled by the amount of applied reverse bias voltage. Impedance matching was also done automatically by another pair of varactor diodes to obtain the maximum SNR at each frequency. The same bias voltage was applied to the tuning varactors on all rungs to avoid perturbations in the coil. A network analyzer was used to monitor matching and tuning of the coil. A Pentium PC controlled the analyzer through the GPIB bus. A code written in LABVIEW was used to communicate with the network analyzer and adjust the bias voltages of the varactors via D/A converters. Serially programmed D/A converter devices were used to apply the bias voltages to the varactors. Isolation amplifiers were used together with RF choke inductors to provide isolation between the RF coil and the DC bias lines. We acquired proton and fluorine images sequentially from a multicompartment phantom using the designed coil. Good matching and tuning were obtained at both resonance frequencies. The tuning and matching of the coil were changed from one resonance frequency to the other within 60 s.     

Longitudinal brain imaging of five malignant glioma patients treated with bevacizumab using susceptibility-weighted magnetic resonance imaging at 7 T

Malignant glioma is a rare tumor type characterized by prominent vascular proliferation. Antiangiogenic therapy with the monoclonal antibody bevacizumab is considered as a promising therapeutic strategy, although the effect on tumor vascularization is unclear. High-field susceptibility-weighted imaging (SWI) visualizes the microvasculature and may contribute to the investigation of antiangiogenic therapy responses in gliomas. We prospectively studied five adult malignant glioma patients treated with bevacizumab-containing regimens. In each patient, we performed three 7-T SWI and T1-weighted imaging investigations (baseline and 2 and 4 weeks after the start of bevacizumab treatment). In addition, we imaged a postmortem brain of a patient with glioblastoma using 7-T SWI and performed detailed histopathological analysis. We observed almost total resolution of brain edema in three of five patients after initiation of bevacizumab therapy. In one case with rapid increase of the lesion size despite bevacizumab therapy, SWI showed progressive increase of irregular hypointense structures, most likely corresponding to increasing amounts of pathological microvasculature. In one case with progressive neurological decline, 7-T images showed multiple intratumoral microhemorrhages after the first bevacizumab application. Correlation of postmortem neuroimaging with histopathology confirmed that SWI-positive structures correspond to tumor vasculature. The experience from our case series indicates that longitudinal 7-T SWI seems to be an appropriate method for investigation of changes in brain tumor vascularization over time under antiangiogenic therapy.     

A practical multinuclear transceiver volume coil for in vivo MRI/MRS at 7 T

A practical multinuclear transceiver RF volume coil with improved efficiency for in vivo small animal ¹H/¹³C/²³Na MR applications at the ultrahigh magnetic field of 7 T is reported. In the proposed design, the coil's resonance frequencies for ¹H and ¹³C are realized by using a traditional double-tuned approach, while the resonant frequency for ²³Na, which is only some 4 MHz away from the ¹³C frequency, is tuned based upon ¹³C channel by easy-operating capacitive “frequency switches”. In contrast to the traditional triple-tuned volume coil, the volume coil with the proposed design possesses less number of resonances, which helps improve the coil efficiency and alleviate the design and operation difficulties. This coil design strategy is advantageous and well suitable for multinuclear MR imaging and spectroscopy studies, particularly in the case where Larmor frequencies of nuclei in question are not separate enough. The prototype multinuclear coil was demonstrated in the desired unshielded design for easy construction and experiment implementation at 7 T. The design method may provide a practical and robust solution to designing multinuclear RF volume coils for in vivo MR imaging and spectroscopy at ultrahigh fields. Finite difference time domain method simulations for evaluating the design and 7-T MR experiment results acquired using the prototype coil are presented.     

A novel rotating-field coil for sodium imaging

A novel radiofrequency coil design which is especially useful for NMR imaging of sodium in small samples is described. The structure is neither a saddle nor a birdcage coil type, but rather consists of a series of tuned loops. In principle, the loops are not electrically connected to each other and the main interaction between them is by electromagnetic induction. The coil has approximate circular symmetry and uses much smaller capacitors than does the birdcage design. It is shown theoretically how a rotating held can be created by detuning two modes of the coil, and that the resulting impedance-frequency relationship differs substantially from that usually found in resonant circuits. It is shown that it is possible to use the coil simultaneously as both a transmitter and a receiver with no external phase-shifting networks, by using independent inductively coupled feed loops for transmitting and receiving. The behavior of the coil was studied in some detail using computer simulation, and a working model has been constructed and used for in vivo imaging of endogenous sodium in rats at 1.9 T. The theoretical analysis and experimental results indicate very high spatial homogeneity of the field and improved signal-to-noise ratio.     

Elliptical magnetic resonance spectroscopic imaging with GRAPPA for imaging brain tumors at 3 T

Magnetic Resonance Spectroscopic Imaging (MRSI) is a technique for imaging spatial variation of metabolites and has been very useful in characterizing biochemical changes associated with disease as well as response to therapy in malignant pathologies. This work presents a self-calibrated undersampling to accelerate 3D elliptical MRSI and an extrapolation-reconstruction algorithm based on the GRAPPA method. The accelerated MRSI technique was tested in three volunteers and five brain tumor patients. Acceleration allowed larger spatial coverage and consequently, less lipid contamination in spectra, compared to fully sampled acquisition within the same scantime. Metabolite concentrations measured from the accelerated acquisitions were in good agreement with measurements obtained from fully sampled MRSI scans.     

Diffusion-weighted imaging of the brain at 7 T with echo-planar and turbo spin echo sequences: preliminary results

Ultra-high-field clinical MRI scanners (e.g., 7 T and above) are becoming increasingly prevalent and can potentially enhance diagnostic ability through higher contrast, resolution and/or sensitivity. Diffusion-weighted MRI is a highly valued component in today's radiological exam and may benefit from the enhanced signal-to-noise ratio provided by high field with the appropriate imaging strategy. The most common diffusion pulse sequence readout (echo-planar imaging (EPI)) has been widely employed for in vivo human 7 T diffusion tensor imaging (DTI). In this article, we present results of brain DTI at 7 T with two diffusion-weighted imaging pulse sequence readouts: echo-planar imaging (EPI-DTI) and turbo spin echo (TSE-DTI). Results indicate that analogous coverage, quality and resolution typical of lower field (2 mm) can be obtained by properly processed EPI-DTI at 7 T, and, with some reduction in efficiency and sharpness, TSE-DTI at 7 T. Furthermore, 7 T TSE-DTI shows promise in obtaining higher-resolution results in targeted acquisitions of specific brain areas.     

Design of a superconducting volume coil for magnetic resonance microscopy of the mouse brain

We present the design process of a superconducting volume coil for magnetic resonance microscopy of the mouse brain at 9.4T. The yttrium barium copper oxide coil has been designed through an iterative process of three-dimensional finite-element simulations and validation against room temperature copper coils. Compared to previous designs, the Helmholtz pair provides substantially higher B(1) homogeneity over an extended volume of interest sufficiently large to image biologically relevant specimens. A custom-built cryogenic cooling system maintains the superconducting probe at 60+/-0.1K. Specimen loading and probe retuning can be carried out interactively with the coil at operating temperature, enabling much higher through-put. The operation of the probe is a routine, consistent procedure. Signal-to-noise ratio in a mouse brain increased by a factor ranging from 1.1 to 2.9 as compared to a room-temperature solenoid coil optimized for mouse brain microscopy. We demonstrate images encoded at 10x10x20mum for an entire mouse brain specimen with signal-to-noise ratio of 18 and a total acquisition time of 16.5h, revealing neuroanatomy unseen at lower resolution. Phantom measurements show an effective spatial resolution better than 20mum.