Abstract
The neuroimaging of humans using 7T magnetic resonance imaging (MRI) has been conducted using phased array (PA) coils with different numbers of receiving channels. PA coils with a high number of channels may offer parallel imaging (PI) with a high reduction (R)-factor, which is enabled via under-sampling and coil geometry (g) factor, increasing the radiofrequency signal sensitivity provided by a small coil. The goals of this study were to assess and validate the coil performance of PA coils with different numbers of receiver (Rx)-channels in and to propose the coil selection guidelines by visualizing 7T brain images. The combined magnetic flux density (||B1||) distributions of four configurations of PA coils—4-, 8-, 12-, and 16-channel Rx-only mode under the local transmit (Tx) mode of birdcage coils—were evaluated using electromagnetic (EM) calculations. These four configurations of PA coils and a local Tx coil were designed and built for a 7T MRI experiment. For 7T brain imaging experiments, all PA coils with (w/) and without (w/o) R-factors were compared in terms of signal-to-noise ratio (SNR) and spatial noise variation (SNV). EM simulation results clearly demonstrated that PA coils with a high number of Rx channels showed more homogeneously distributed ||B1|| fields than a PA coils with a low number of Rx coils. The results of this study demonstrate that a collection of smaller surface coils can contribute to high RF signal sensitivity in terms of the anatomical coverage of the brain and may facilitate PI. With further improvement in coil technology, researchers and clinicians will be provided with PA coils with different numbers of channels, which can ensure the optimum SNR and PI benefits for 7T brain MR imaging.
Highlights
Ultra-high-field (UHF, 7T) magnetic resonance imaging (MRI) is being continuously developed to obtain high-quality neuroimages
S/N can be improved by the application of stronger ||B0||-fields due to the property of MR signals that the acquired MR signals are proportional to the number of hydrogen (1H) protons entering the resonance state, which is in turn proportional to ||B0||-field strength [1, 2]
The characteristics of phased array (PA) coils of enabling the acquisition of unique images with different coil elements has been used for parallel imaging (PI) reconstruction techniques, such as Sensitivity Encoding (SENSE) [6], Simultaneous acquisition of spatial harmonics (SMASH) [7], generalized auto-calibrating partial parallel acquisition (GRAPPA) [8] and array coil spatial sensitivity encoding (ASSET) [9]
Summary
Ultra-high-field (UHF, 7T) magnetic resonance imaging (MRI) is being continuously developed to obtain high-quality neuroimages. Increase in signal to noise and susceptibility induced contrast at higher field strengths may be useful for imaging techniques that require a high signal-to-noise (SNR) and contrast-to-noise (CNR) ratio, such as functional imaging techniques (fMRI) and MR spectroscopy, as well as high resolution anatomical imaging. For this purpose, the MR scanner should provide a high signal-to-noise ratio (SNR) while maintaining the acquisition time short. The MR scanner should provide a high signal-to-noise ratio (SNR) while maintaining the acquisition time short To meet these requirements, the utilization of high-magnetic field (||B0||) strength is essential. The coils were tested by performing in vivo brain experiments with different R-factors in terms of SNR and spatial noise variation (SNV)
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