Parallel imaging exploits the multiple elements of a phased array coil. Each element is associated with a dedicated radiofrequency channel whose signals can be processed and combined together.
This improves the signal-to-noise ratio compared to a standard single-element coil covering the same explored volume.
The spatial data yielded by the array of coil elements can be used for partial phase encoding only, to speed up acquisition. The acceleration factors routinely employed at 1.5 T range from 2 to 3. At higher fields, these can be higher.
Reconstruction algorithms for the intermediary, undersampled phase images are divided into 2 groups:
Those which reconstruct the global image before Fourier transformation: these operate in the frequency domain (SMASH, GRAPPA)
Those which reconstruct the image after Fourier transformation, in the image domain (SENSE, mSENSE)
These algorithms require calibration data for the volume explored by each coil element and the signal produced. Calibration is carried out before or during the imaging sequence (self-calibration).
The objective is either to recreate the missing k-space lines (SMASH, GRAPPA), or to “unfold” the image due to aliasing (SENSE).
The advantages of these techniques are numerous: reduced acquisition time (fast imaging, breath-hold, temporal resolution, perfusion and functional imaging etc.) reduced TE of sequences with echo train, reduction of certain artifacts …
Their main drawback is a decrease in signal to noise ratio compared to a non-accelerated sequence (fewer measurements, g factor etc.)
Summary