Purpose
The goal of this study was to explore the feasibility of using an array of electric dipole antennas for RF transmission in spine MRI at high fields.
Method
A two‐channel transmit array based ...on an electric dipole design was quantitatively optimized for 7T spine imaging and integrated with a receive array combining eight loop coils. Using
B1+ mapping, the transmit efficiency of the dipole array was compared with a design using quadrature loop pairs. The radiofrequency energy deposition for each array was measured using a home‐built dielectric phantom and MR thermometry. The performance of the proposed array was qualitatively demonstrated in human studies.
Results
The results indicate dramatically improved transmit efficiency for the dipole design compared with the loop excitation. A gain of up to 76% was achieved within the spinal region.
Conclusion
For imaging of the spine, electric dipole–based transmitters provide an attractive alternative to the traditional loop‐based design. Easy integration with existing receive array technology facilitates practical use at high fields. Magn Reson Med 74:1189–1197, 2015. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
Recent in vivo MRI studies at 7.0 T have demonstrated extensive heterogeneity of T(2)* relaxation in white matter of the human brain. In order to study the origin of this heterogeneity, we performed ...T(2)* measurements at 1.5, 3.0, and 7.0 T in normal volunteers. Formalin-fixed brain tissue specimens were also studied using T(2)*-weighted MRI, histologic staining, chemical analysis, and electron microscopy. We found that T(2)* relaxation rate (R(2)* = 1/T(2)*) in white matter in living human brain is linearly dependent on the main magnetic field strength, and the T(2)* heterogeneity in white matter observed at 7.0 T can also be detected, albeit more weakly, at 1.5 and 3.0 T. The T(2)* heterogeneity exists also in white matter of the formalin-fixed brain tissue specimens, with prominent differences between the major fiber bundles such as the cingulum (CG) and the superior corona radiata. The white matter specimen with substantial difference in T(2)* has no significant difference in the total iron content, as determined by chemical analysis. On the other hand, evidence from histologic staining and electron microscopy demonstrates these tissue specimens have apparent difference in myelin content and microstructure.
Abstract High signal-to-noise ratios (SNR) are essential for high-resolution anatomical and functional MRI. Phased arrays are advantageous for this but have the drawback that they often have ...inflexible and bulky configurations. Particularly in experiments where functional MRI is combined with simultaneous electrophysiology, space constraints can be prohibitive. To this end we developed a highly flexible multiple receive element phased array for use on anesthetized monkeys. The elements are interchangeable and different sizes and combinations of coil elements can be used, for instance, combinations of single and overlapped elements. The preamplifiers including control electronics are detachable and can serve a variety of prefabricated and phase matched arrays of different configurations, allowing the elements to always be placed in close proximity to the area of interest. Optimizing performance of the individual elements ensured high SNR at the cortical surface as well as in deeper laying structures. Performance of a variety of arrangements of gapped linear arrays was evaluated at 4.7 and 7T in high-resolution anatomical and functional MRI.
MRI at high magnetic field strength potentially allows for an increase in resolution and image contrast. The gains are particularly dramatic for T(2)(*)-weighted imaging, which is sensitive to ...susceptibility effects caused by a variety of sources, including deoxyhemoglobin, iron concentration, and tissue microstructure. On the other hand, the acquisition of high quality whole brain MRI at high field is hampered by the increased inhomogeneity in B(o) and B(1) fields. In this report, high-resolution gradient echo MRI was performed using an 8-channel detector to obtain T(2)(*)-weighted images over large brain areas. The high SNR achieved with the multi-channel array enabled T(2)(*)-weighted images of the brain with an unprecedented spatial resolution of up to 0.2 x 0.2 x 0.5 mm(3). This high resolution greatly facilitated the detection of microscopic susceptibility effects. In addition to the expected contrast between gray, white matter, cerebral spinal fluid, and veins, a large degree of heterogeneity in contrast was observed throughout the white matter of normal brain. The measured T(2)(*) values in white matter varied as much as 30% with some of the variation apparently correlating with the presence of large fiber bundles.
The tumor microenvironment is characterized by a highly reducing redox status, a low pH, and hypoxia. Anti-angiogenic therapies for solid tumors frequently function in two steps: the transient ...normalization of structurally and functionally aberrant tumor blood vessels with increased blood perfusion, followed by the pruning of tumor blood vessels and the resultant cessation of nutrients and oxygen delivery required for tumor growth. Conventional anatomic or vascular imaging is impractical or insufficient to distinguish between the two steps of tumor response to anti-angiogenic therapies. Here, we investigated whether the noninvasive imaging of the tumor redox state and energy metabolism could be used to characterize anti-angiogenic drug-induced transient vascular normalization.
Daily treatment of squamous cell carcinoma (SCCVII) tumor-bearing mice with the multi-tyrosine kinase inhibitor sunitinib resulted in a rapid decrease in tumor microvessel density and the suppression of tumor growth. Tumor pO2 imaging by electron paramagnetic resonance imaging showed a transient increase in tumor oxygenation after 2-4 days of sunitinib treatment, implying improved tumor perfusion. During this window of vascular normalization, magnetic resonance imaging of the redox status using an exogenously administered nitroxide probe and hyperpolarized (13)C MRI of the metabolic flux of pyruvate/lactate couple revealed an oxidative shift in tumor redox status.
Redox-sensitive metabolic couples can serve as noninvasive surrogate markers to identify the vascular normalization window in tumors with imaging techniques.
A multimodal imaging approach to characterize physiological, metabolic, and redox changes in tumors is useful to distinguish between the different stages of anti-angiogenic treatment.
Resting-state, low-frequency (<0.08 Hz) fluctuations of blood oxygenation level-dependent (BOLD) magnetic resonance signal have been shown to exhibit high correlation among functionally connected ...regions. However, correlations of cerebral blood flow (CBF) fluctuations during the resting state have not been extensively studied. The main challenges of using arterial spin labeling perfusion magnetic resonance imaging to detect CBF fluctuations are low sensitivity, low temporal resolution, and contamination from BOLD. This work demonstrates CBF-based quantitative functional connectivity mapping by combining continuous arterial spin labeling (CASL) with a neck labeling coil and a multi-channel receiver coil to achieve high perfusion sensitivity. In order to reduce BOLD contamination, the CBF signal was extracted from the CASL signal time course by high frequency filtering. This processing strategy is compatible with sinc interpolation for reducing the timing mismatch between control and label images and has the flexibility of choosing an optimal filter cutoff frequency to minimize BOLD fluctuations. Most subjects studied showed high CBF correlation in bilateral sensorimotor areas with good suppression of BOLD contamination. Root-mean-square CBF fluctuation contributing to bilateral correlation was estimated to be 29±19% (N=13) of the baseline perfusion, while BOLD fluctuation was 0.26±0.14% of the mean intensity (at 3 T and 12.5 ms echo time).
Purpose
We tested the feasibility of implementing parallel transmission (pTX) for high‐field MRI using a radiofrequency (RF) amplifier design to be located on or in the immediate vicinity of an RF ...transmit coil.
Method
We designed a current‐source switch‐mode amplifier based on miniaturized, nonmagnetic electronics. Optical RF carrier and envelope signals to control the amplifier were derived, through a custom‐built interface, from the RF source accessible in the scanner control. Amplifier performance was tested by benchtop measurements as well as with imaging at 7T (300 MHz) and 11.7 T (500 MHz). The ability to perform pTX was evaluated by measuring interchannel coupling and phase adjustment in a two‐channel setup.
Results
The amplifier delivered in excess of 44 W RF power and caused minimal interference with MRI. The interface derived accurate optical control signals with carrier frequencies ranging from 64 to 750 MHz. Decoupling better than 14 dB was obtained between two coil loops separated by only 1 cm. Application to MRI was demonstrated by acquiring artifact‐free images at 7 T and 11.7 T.
Conclusion
We propose an optically controlled miniaturized RF amplifier for on‐coil implementation at high fields that should facilitate implementation of high‐density pTX arrays. Magn Reson Med 76:340–349, 2016. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
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