The quality of magnetic resonance imaging (MRI) critically depends on the radio-frequency (RF) receiver coils. While volume coils and surface coils provide a large field-of-view (FOV) and high signal-to-noise ratio (SNR) respectively, a coil array has been introduced to achieve both appealing features simultaneously by using carefully arranged surface coils and low-noise pre-amplifiers. The high SNR offered by a coil array can also be traded-off for spatiotemporal resolution enhancement using parallel MRI (pMRI) methods, where different spatial sensitivity among channels of a coil array is used to estimate the skipped k-space data in acquisition by either an image domain or a k-space reconstruction algorithm. While there are versatile choices of reconstruction methods, the quality of the final reconstructed pMRI is still predominantly affected by the performance of an RF coil array.
One way to optimize the coil array in order to achieve high spatiotemporal resolution of pMRI is increasing the number of channels. Without reaching the theoretical limit, increasing the channel of an RF coil array can improve the condition of the imaging encoding matrix targeted at a specific spatiotemporal resolution enhancement rate. To this end, dense coil arrays for head imaging consisting of 16, 32, 64, and 90 elements have been constructed. There is also a cardiac array using up to 128 receiver channels. The other approach to optimize the RF coil array design is to tailor its geometry to closely fit the imaging object such that the SNR can be maximized. This principle has been recently realized in, for example, a 32-channel lung array, an 8-channel wrist array, and 32-channel head coil arrays for pediatric imaging. Independently, it has also been suggested that surface coils separated by a gap between them instead of overlapping neighboring ones can improve the quality of acquired data. Another issue of the coil array design regarding accelerated imaging is aliasing artifact; a coil array arranged to provide the most disparate spatial information from RF coil sensitivities about the aliased image voxels in accelerated scans is expected to maximally suppress the aliasing artifacts due to sub-Nyquist sampling. Following this rationale, a linear array with up to 64 elements has been used to reconstruct a two-dimensional image from the single-echo acquisition.
In order to achieve the optimal performance of pMRI, the design of the locations and orientations of coil elements in an array should consider the acquisition slice/volume orientation and the k-space trajectory, particularly the phase/partition encoding directions in 2D/3D imaging respectively. With these considerations, a coil array can generate images with better SNR and less aliasing artifacts, and the spatial encoding efficiency can be enhanced. However, coil arrays designed today lack considerations for the above-mentioned aspects.
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