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Parallel imaging

Parallel acquisition techniques combine the signals of several coil elements in a phased array to reconstruct the image, the chief objective being:

• either to improve the signal-to-noise ratio
• or to accelerate acquisition and reduce scan time

Apart from improving gradients (intensity and speed), these techniques bring a novel approach to accelerating MRI acquisition with vast fields of application. A shorter scan time enables breath-hold sequences or improves the temporal resolution of dynamic scans. Parallel reconstruction techniques can be used to improve image quality for the same acquisition time as a sequence without parallel acquisition (better spatial resolution, reduced artifacts).

Parallel reconstruction algorithms can be divided into 2 main groups:

• Algorithms that reconstruct the global image from the images produced by each coil (reconstruction in the image domain, after Fourier transform): SENSE (SENSitivity Encoding), PILS (Partially Parallel Imaging with Localized Sensitivity), ASSET (Array Spatial Sensitivity Encoding Technique)
• Algorithms that reconstruct the Fourier plane of the image from the frequency signals of each coil (reconstruction in the frequency domain, before Fourier transform): GRAPPA (GeneRalized Auto-calibrating Partially Parallel Acquisition)