Self-Navigated Interleaved Spiral (SNAILS): Application to High-Resolution Diffusion Tensor Imaging Chunlei Liu, 1,2 Roland Bammer, 1 Dong-hyun Kim, 1 and Michael E. Moseley 1* A fat-saturated twice-refocused spin echo sequence was im- plemented on a GE Signa 1.5-T whole-body system for diffu- sion-weighted imaging. Data were acquired using an analyti- cally designed interleaved variable-density (VD) spiral readout trajectory. This flexible design algorithm allowed real-time pre- scription on the scanner. Each interleaf of the VD spiral over- sampled the center of k-space. The oversampling provided an inherent motion compensation capability. The resultant diffu- sion-weighted images showed good quality without any retro- spective motion correction. An iterated motion correction algo- rithm was developed to further reduce the signal cancellation artifact caused by motion-induced phase error. In this algo- rithm, a low-resolution phase map was estimated using the oversampled data in the center of k-space in order to correct for phase error in image space. In vivo diffusion tensor imaging (DTI) studies were performed on the brains of healthy volunteers. High-quality isotropic dif- fusion-weighted images, trace maps, and FA maps from axial, sagittal, and coronal slices were obtained using a VD spiral readout trajectory with matrix size 256  256. To our knowl- edge, this was also the first time in vivo 512  512 DTI results were reported. Magn Reson Med 52:1388 –1396, 2004. © 2004 Wiley-Liss, Inc. Key words: magnetic resonance imaging; high resolution; dif- fusion; diffusion tensor imaging; variable density spiral; inter- leaved; motion correction Diffusion-weighted imaging (DWI) is a unique technique for studying random molecular motion in biologic tissues. Over the past decade, DWI has found routine applications in medical diagnosis, especially in detecting acute cerebral ischemia (1). Most diffusion-weighted images are cur- rently acquired using a single-shot echo-planar imaging (EPI) technique. Single-shot EPI has the advantage of rapid image acquisition and insensitivity to phase error caused by subject motion because the entire k-space is acquired with a single rapid train of gradient echoes. Despite the rapid image formation, single-shot EPI lasts long enough that T 2 *-decay limits image resolution and off-resonant spins can still cause serious image degradation. To shorten the readout time, multishot sequences can be used (2,3); however, they generally suffer from view-to- view phase variations caused by motion during the period when the diffusion-sensitizing gradients are turned on. One approach to correct these variations is to acquire additional navigator data that can be used to resolve the phase error (4 –7). The navigator can be implemented to correct for either one-dimensional or two-dimensional phase error. The navigator data are intended to provide a direct measure of the motion-induced phase variations. Under the assumption of rigid body motion, the data can be subsequently corrected for small amounts of motion. A few studies have recently explored the self-navigating capability of the spiral readout trajectory in multishot DWI (8,9). Magnetic resonance imaging (MRI) based on spiral readout has been found to be effective in various applica- tions, including functional neuroimaging (10) and spec- troscopy (11). The spiral trajectory has the merit of mo- ment-nulling motion compensation (12) and efficient use of gradient power (13). Conventional spiral readout trajec- tories have a limited potential for self-navigation because there are insufficient data in the central k-space, which effectively allows only zero- and (to some extent) first- order compensation. In order to improve the navigating capability, it is necessary to increase the sampling density at the center of k-space. With a recently developed analyt- ical variable-density (VD) spiral design technique, the k- space sampling density can easily be manipulated and prescribed on the scanner hardware in real time (14). With this technique, each spiral trajectory serves effectively as a self-navigator that corrects for motion induced view-to- view phase variations. There continues to be an increased demand for higher spatial resolution and diminished artifacts in diffusion- weighted imaging, which will eventually provide better lesion delineation in acute ischemic stroke and high-fidel- ity data for diffusion-tensor-based tractography. The pur- pose of this work is therefore to develop a reliable multi- shot diffusion-weighted VD spiral sequence and a recon- struction and navigation algorithm that effectively reduces motion artifacts. Applications to high-resolution (both 256  256 and 512  512) diffusion tensor imaging (DTI) are also addressed. METHODS Data Acquisition A twice-refocused spin echo (TRSE) sequence was imple- mented (8) because of its well-documented ability to re- duce eddy current distortions (15). A fat saturation pulse is added to reduce the off resonance effect of the fat signal. Diffusion-encoding was achieved by applying a pair of bipolar field gradients around two 180 o refocusing pulses. 1 Lucas MRS/I Center, Department of Radiology, Stanford University, Stan- ford, California. 2 Department of Electrical Engineering, Stanford University, Stanford, Califor- nia. Grant sponsor: National Institute of Health; Grant number: NIH-1R01NS35959 and NIH-1R01EB2711; Grant sponsor: Center of Advanced MR Technology of Stanford; Grant number: NCRR P41 RR 09784; Grant sponsor: Lucas Foundation. *Correspondence to: Michael E. Moseley, Radiological Science Laboratory at the Richard Lucas MRS/I Center, Department of Radiology, Stanford Univer- sity, 1201 Welch Road, Stanford, CA 94305-5488. E-mail: moseley@stanford.edu Received 12 May 2004; revised 12 July 2004; accepted 27 July 2004. DOI 10.1002/mrm.20288 Published online in Wiley InterScience (www.interscience.wiley.com). Magnetic Resonance in Medicine 52:1388 –1396 (2004) © 2004 Wiley-Liss, Inc. 1388