Magnetic Resonance Materials in Physics, Biology and Medicine 13 (2002) 158 – 163 Recent advances in image reconstruction, coil sensitivity calibration, and coil array design for SMASH and generalized parallel MRI  Daniel K. Sodickson a,c, *, Charles A. McKenzie b , Michael A. Ohliger c , Ernest N. Yeh c , Mark D. Price c a Department of Medicine, Cardioascular Diision, Beth Israel Deaconess Medical Center and Harard Medical School, One Autumn Street, Fifth Floor, Boston, MA 02215, USA b Department of Radiology, Beth Israel Deaconess Medical Center and Harard Medical School, Boston, MA, USA c Harard -MIT Diision of Health Sciences and Technology, Boston, MA, USA Received 20 August 2001; received in revised form 2 October 2001; accepted 8 October 2001 Abstract Parallel magnetic resonance imaging (MRI) techniques use spatial information from arrays of radiofrequency (RF) detector coils to accelerate imaging. A number of parallel MRI techniques have been described in recent years, and numerous clinical applications are currently being explored. The advent of practical parallel imaging presents various challenges for image reconstruction and RF system design. Recent advances in tailored SiMultaneous Acquisition of Spatial Harmonics (SMASH) image reconstructions are summarized. These advances enable robust SMASH imaging in arbitrary image planes with a wide range of coil array geometries. A generalized formalism is described which may be used to understand the relations between SMASH and SENSE, to derive typical implementations of each as special cases, and to form hybrid techniques combining some of the advantages of both. Accurate knowledge of coil sensitivities is crucial for parallel MRI, and errors in calibration represent one of the most common and the most pernicious sources of error in parallel image reconstructions. As one example, motion of the patient and/or the coil array between the sensitivity reference scan and the accelerated acquisition can lead to calibration errors and reconstruction artifacts. Self-calibrating parallel MRI approaches that address this problem by eliminating the need for external sensitivity references are reviewed. The ultimate achievable signal-to-noise ratio (SNR) for parallel MRI studies is closely tied to the geometry and sensitivity patterns of the coil arrays used for spatial encoding. Several parallel imaging array designs that depart from the traditional model of overlapped adjacent loop elements are described. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Parallel MRI; SMASH; SENSE; RF coil arrays; Rapid imaging www.elsevier.com/locate/magma 1. Introduction Parallel magnetic resonance imaging (MRI) uses spa- tial information from an array of radiofrequency (RF) coils to substitute for information normally obtained using magnetic field gradients. Acquisition of some portion of image data in parallel rather than in a traditional sequential order allows acceleration of imag- ing beyond previous physical and physiologic limits associated with gradient switching and RF pulse appli- cation. The first proposals for parallel imaging [1 – 6] date back to relatively early in the development of MR multicoil arrays. The modern parallel imaging family includes the SiMultaneous Acquisition of Spatial Har- monics (SMASH) [7] and SENSitivity Encoding (SENSE) [8] techniques, as well as various new ap- proaches currently in development [9 – 13].  Summary of material presented at the 2001 ISMRM workshop on MRI hardware, Cleveland, OH, USA. * Corresponding author. Tel.: +1-617-632-7654; fax: +1-617-632- 7675. E-mail address: dsodicks@caregroup.harvard.edu (D.K. Sodick- son). 1352-8661/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S1352-8661(01)00144-2