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Parallel MR Imaging: A User’s Guide1
James F. Glockner, MD, PhD ● Houchun H. Hu, BME ● David W. Stanley, BS ● Lisa Angelos, PhD ● Kevin King, PhD Parallel imaging is a recently developed family of techniques that take advantage of the spatial information inherent in phased-array radiofrequency coils to reduce acquisition times in magnetic resonanceimaging. In parallel imaging, the number of sampled k-space lines is reduced, often by a factor of two or greater, thereby significantly shortening the acquisition time. Parallel imaging techniques have only recently become commercially available, and the wide range of clinical applications is just beginning to be explored. The potential clinical applications primarily involve reduction inacquisition time, improved spatial resolution, or a combination of the two. Improvements in image quality can be achieved by reducing the echo train lengths of fast spin-echo and single-shot fast spin-echo sequences. Parallel imaging is particularly attractive for cardiac and vascular applications and will likely prove valuable as 3-T body and cardiovascular imaging becomes part of standard clinicalpractice. Limitations of parallel imaging include reduced signal-to-noise ratio and reconstruction artifacts. It is important to consider these limitations when deciding when to use these techniques.
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RSNA, 2005

Abbreviations: FOV field of view, GRAPPA generalized autocalibrating partially parallel acquisition, SE spin echo, SENSE sensitivity encoding, SNR signal-to-noise ratio, SPGR spoiledgradient echo, 3D three-dimensional, TR repetition time, 2D two-dimensional RadioGraphics 2005; 25:1279 –1297 ● Published online 10.1148/rg.255045202 ● Content Codes:
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the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905 (J.F.G., H.H.H.); and GE Medical Systems, Milwaukee, Wis (D.W.S., L.A., K.K.). Recipient of a Certificate of Merit award for an educationexhibit at the 2003 RSNA Annual Meeting. Received November 10, 2004; revision requested January 4, 2005; revision received and accepted March 30. J.F.G. and H.H.H. have no financial relationships to disclose. Address correspondence to J.F.G. (e-mail: glockner.james@mayo.edu). RSNA, 2005

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RG f Volume 25

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Introduction
Therecent history of magnetic resonance (MR) imaging has in large part been devoted to finding ways to increase acquisition speed. Although impressive gains have been made and the acquisition times of many sequences have been reduced from minutes to seconds, some fundamental limitations have been reached due to technical and physiologic problems associated with rapidly switching gradients. Recently,parallel imaging has emerged as a technique that can surmount many of these obstacles. Instead of relying on faster and stronger gradients for improvements in acquisition time, the inherent spatial sensitivity of the phased-array coils is used to provide some of the spatial information in the image that would otherwise be obtained in the traditional manner of Fourier transform MR imaging. In otherwords, parallel imaging allows a reduction in the number of phase-encoding steps while still producing images of reasonable quality and spatial resolution. Parallel imaging increases the acquisition speed by factors of 1.5 to 3 in most commercially available applications. In theory, much higher gains are possible, but they are currently limited by artifact and signal-to-noise ratio (SNR)considerations. The most obvious clinical application of parallel imaging is simply to shorten the acquisition time of a sequence. This is particularly important for body and cardiovascular imaging, where breath holding is both frequently required and a frequent source of patient complaint. Certainly, the distinction between an acquisition beyond a patient’s endurance and one comfortably within his or her...