Common SENSE (Sensitivity Encoding Using Hardware Common to All MR Scanners): A New Method for Single- Shot Segmented Echo Planar Imaging David W. Carmichael, 1 * Andrew N. Priest, 2,3 Enrico De Vita, 1,2 and Roger J. Ordidge 1 A new method is presented that enables image acquisition to be segmented into two readouts. This is achieved using a new pulse sequence that creates two components of magnetization with different spatial profiles. Each component of the magneti- zation is measured in one of the readouts. This produces two images with complimentary “sensitivity profiles” and near iden- tical contrast. The images can be acquired with a reduced data matrix that corresponds to shorter periods of data acquisition. The reduced matrix images are then combined to produce a full matrix image using reconstruction methods previously applied to images from multiple RF coils in the sensitivity encoding (SENSE) technique. The most promising application for this technique is in im- proving the performance of echo planar imaging (EPI) at high field. In this application, common SENSE obtains two segments of data in a single excitation of the magnetization (i.e., two readouts are performed per shot). The combination of these segments in image space avoids the difficulties normally asso- ciated with segmented EPI methods, namely, increased ghost- ing from discontinuities in the k-space data. The main advan- tages are a reduction in distortion and blurring. Common SENSE is compatible with parallel imaging and partial Fourier methods. Magn Reson Med 54:402– 410, 2005. © 2005 Wiley- Liss, Inc. Key words: EPI; SENSE; common SENSE; TRAIL; segmented EPI Echo-planar imaging (EPI) (1,2), one of the most rapid imaging sequences, is frequently used in the observation of dynamic systems. One such application is fMRI (3) where a local change in blood volume and oxygenation causes a change in T 2 * contrast, called BOLD (4). BOLD contrast is increased by the use of higher field magnets; however, EPI suffers from increased distortion and blurring with field strength. The challenge is to maintain EPI image quality while utilizing the increased sensitivity afforded by higher fields in applications such as fMRI and diffusion-weighted imaging (DWI). Distortion is the result of local susceptibility gradients and chemical shifts that cause spins from a particular spatial position to precess with a different frequency than expected in the presence of a linear magnetic gradient field. In an EPI acquisition, these local susceptibility gra- dients are significant in size when compared to the gradi- ents encoding along the phase direction, causing an error in the assignment of spin position. To reduce this problem, the bandwidth per point along the phase encoding axis must be increased. Blurring of image detail occurs because of T 2 * decay over the length of the acquisition; this is a greater problem at higher field where T 2 * decay is faster. Shortening the total readout length reduces this problem because the T 2 * decay has less time to act over the time period that the signal is sampled. Ghosting is produced by a mismatch between echoes formed by read gradients of opposite magnitude. Gradient inconsistencies, eddy cur- rents, and susceptibility gradients can all contribute to this mismatch. Hardware improvement and corrections can both reduce the artifact level (5). To improve distortion and blurring in EPI, the simplest method is to reduce the length of the readout. Due to physiologic constraints and gradient performance, the speed of k-space traversal is limited. Therefore, one option available is to reduce the number of phase encoding steps in the readout to reduce the length of the k-space trajectory (i.e., to increasing the phase encoding bandwidth without faster read gradient switching). This reduces the amount of data available to reconstruct an image, necessitating the recovery of a full set of image information by some other means. There are a number of different techniques to per- form this function that can be classified into two ap- proaches. First, there is interleaved segmented EPI (for two segments, every other line of the standard k-space cover- age is read out, and then the remaining lines are read out in a separate acquisition). Unfortunately, ghosting is often made worse when data from different experiments is com- bined in k-space due to discontinuities in signal phase and amplitude between segments. Various strategies have been suggested to read out the segments consecutively. One is to use a 45° RF pulse and acquisition followed directly by a 90° RF pulse and acquisition (6). This method suffers from ghosting due to discontinuities in signal phase and ampli- tude between segments that are difficult to eliminate and so has not found widespread application. A more robust strategy is to allow the magnetization to relax fully before acquiring each segment (7), although this entails a penalty in temporal resolution that would be unacceptable for many applications. Even when all these factors are care- fully chosen, patient motion and susceptibility gradients can still cause ghosting at an unacceptable level in seg- mented EPI images, especially for diffusion imaging (8). 1 University College London, Department of Medical Physics and Bioengineer- ing, London, United Kingdom. 2 UCL Hospitals NHS Trust, Department of Medical Physics and Bioengineer- ing, London, United Kingdom. 3 University Hospital Hamburg-Eppendorf, Department of Diagnostic and In- terventional Radiology, Hamburg, Germany. Grant sponsor: Wellcome Trust; Grant sponsor: EPSRC. *Correspondence to: David W. Carmichael, University College London, De- partment of Medical Physics and Bioengineering, Malet Place Engineering Building, Gower Street, London WC1E 6BT, United Kingdom. E-mail: d.carmichael@medphys.ucl.ac.uk Received 6 October 2004; revised 7 February 2005; accepted 15 March 2005. DOI 10.1002/mrm.20581 Published online in Wiley InterScience (www.interscience.wiley.com). Magnetic Resonance in Medicine 54:402– 410 (2005) © 2005 Wiley-Liss, Inc. 402