Clinicians, researchers, and technologists who are interested in parallel imaging and its application in body MRI.

· Understand parallel imaging methods and how they can be used to accelerate image acquisition.

· Identify potential artifacts and specific considerations for applying parallel imaging techniques to body MRI.

· Discuss clinical applications of parallel imaging.

· Review recent advances in parallel imaging techniques that could benefit body MRI protocols.

Review of Parallel Imaging Concepts

Regardless of the clinical application, MRI acquisition times are relatively long. These long scan times occur because there is an inherent link between scan duration, the image field-of-view, and image resolution. Large fields-of-view images at high spatial resolutions result in long scan times because of how spatial encoding is performed in MRI. This is particularly challenging in body MRI, where many protocols require much larger volumetric coverage with high spatial resolutions. Additionally, body MRI scans are frequently performed under breath hold to mitigate respiratory motion artifacts, which limits the total possible scan length. In order to successfully meet the various needs of body MRI scans, accelerated acquisition methods are frequently considered.

Many techniques have focused on accelerating MRI acquisitions by reducing the amount of spatial encoding needed to achieve the desired image. Parallel imaging is an important technique that aims to reduce the amount of spatial encoding by using spatial information provided by a multi-coil array. These methods were introduced in the 1990s and early 2000s (1–3), and have since become widely incorporated to reduce clinical scan times (4–6).

In parallel imaging acquisitions, data are accelerated by skipping phase encoding lines. Scan time is directly proportional to the number of phase encoding lines acquired. Thus, if half of the phase encoding lines are skipped when acquiring an image, scan time is reduced by a factor of two. Although this acquisition achieves the goal of reducing scan times, the increased spacing between neighboring k-space lines results in a reduced field-of-view, which causes aliasing in the phase encoding direction. Parallel imaging reconstruction methods utilize additional information provided from a multi-coil array to mitigate these aliasing artifacts. Each coil within the array simultaneously acquires a separate image that is weighted by the sensitivity of that coil. Because each coil has a different, spatially-localized sensitivity across the field-of-view, this additional spatial information can be used to reconstruct an unaliased image. There have been many successful parallel imaging reconstructions proposed, but this review will focus on two representative methods: SENSE (2) and GRAPPA (3). SENSE (Sensitivity Encoding) is applied in the image domain to unfold aliasing artifacts caused by the undersampling. GRAPPA (Generalized Autocalibrating Partially Parallel Acquisition) is applied in k-space to estimate the missing or skipped phase encoding lines. Regardless of your choice of parallel imaging reconstruction method, some additional information about the coil sensitivities is required to perform the image reconstruction, and the acquisition of this coil sensitivity information in body applications should be considered. Lastly, there are practical limitations to the acceleration factor used for parallel imaging reconstructions. It is important to note that all parallel reconstruction methods result in a reduced signal-to-noise ratio (SNR) in comparison to a fully-sampled image. In addition to this reduction of SNR, these methods suffer from residual aliasing artifacts at high acceleration factors. These limitations and potential artifacts will be covered in this review, and methods to reduce or avoid these artifacts will also be discussed.

Application of Parallel Imaging to Body MRI

The gain in imaging speed achieved by parallel imaging can be applied to body MRI (or any image acquisition) in several ways. Parallel imaging can be used to reduce the scan time for each scan, which would reduce breath hold times and improve patient comfort. Alternatively, the acceleration provided by parallel imaging could be used to improve image resolution or extend the field-of-view in the same scan duration. Lastly, there are some applications where accelerating with parallel imaging can improve image quality. For example, when echo planar imaging (EPI) is used for diffusion acquisitions, accelerating with parallel imaging can reduce distortion artifacts resulting from the readout (4).

Presentation Overview

This presentation will provide a review of parallel imaging techniques and how they can be used to accelerate data acquisition in body MRI. As mentioned previously, limitations and common artifacts of these techniques will also be covered. Importantly, there are several recently proposed methods that have improved upon or extended traditional parallel imaging techniques. These include using alternative sampling patterns (such as CAIPIRINHA), combining non-Cartesian trajectories with parallel imaging, combining view-sharing methods with parallel imaging, and combining compressed sensing methods with parallel imaging (7–12). These exciting new directions and their application in body MRI will be discussed.