This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. INTRODUCTION : Variable density (VD) spiral has been applied in SNAILS (self-navigated interleaved spiral) for the acquisition of high-resolution diffusion weighted images (1, 2). It has been shown that by oversampling the center of k-space, SNAILS provides the capability to correct for motion-induced phase errors. Furthermore, the increased image SNR resulting from oversampling center k-space improves the tolerance for residual motion-induced phase errors. On the other hand, excessive oversampling of the center of k-space increases readout time, which may increase off-resonance blurring associated with spiral trajectory and noise correlation. Here, a method is proposed to improve SNAILS by using a short spiral-in navigator to acquire the phase map. This extra navigator reduces the oversampling factor of the VD spiral and hence improves the acquisition efficiency, while still maintaining a moderate oversampling and redundancy rate to compensate residual phase errors. In vivo high-resolution diffusion tensor imaging (DTI) are demonstrated with this improved SNAILS. METHOD: In SNAILS, the oversampled center k-space data provide a low resolution phase map which can be used to correct for motion-induced phase errors prior to combining all interleaf data. Furthermore, the phase navigation combined with the conjugate gradient based phase correction method has enabled SENSE SNAILS (3) and has been used successfully to acquire high-resolution multi-shot diffusion-weighted images (Ref). For both phase correction and parallel imaging of SNAILS, an accurate phase map is important for high quality multi-shot DTI. In this work, to improve quality the self-navigated spiral waveform is now broken down in a navigator part (spiral-in) that is acquired prior to the formation of the spin echo and an accelerated imaging part (spiral-out) after the spin echo. Specifically, the spiral-in navigator is designed as a single-interleaf conventional spiral (3) that fully samples a Cartesian grid of a size 32x32. This implementation offers users the flexibility to specify the resolution of the spiral-in and the spiral-out images independently on the scanner in real time. The navigator data were not added to the finally reconstructed images. In vivo diffusion measurements were performed on healthy volunteers with an 8-channel head coil (MR Devices) on a GE SIGNA 1.5T scanner. The following parameters were used: TR/TE = 2.5s/56ms, Gmax = 50mT/m, b = 800 s/mm2, FOV = 24cm, acquisition matrix size = 256x256, BW = 125 kHz, NEX = 2, and 20 interleaves. No cardiac gating was used. In total, six diffusion gradient directions [(1 1 0), (1 0 1), (0 1 -1), (-1 1 0), (0 1 1), (1 0 -1)] were applied to acquire the diffusion tensor. The scan was repeated for two VD spiral pitch factors: ? = 4 and 2, i.e. the larger ? the slower is the progression of the trajectory towards the edge of k-space. For comparison, images were reconstructed both with and without the spiral-in navigator. (Add another image where you no correction at all to see the effect VD spiral averaging!) RESULTS: We compare a set of two typical diffusion-weighted images and the corresponding fractional anisotropy (FA) maps. In Figure 1, ? = 4, and Figure 2, ? = 2. In both figures, row (a) shows results obtained with the spiral-in navigator; row (b) shows results without the navigator, in which the center VD spiral data provide phase and sensitivity estimation. Both figures illustrate that the spiral-in navigator improves the image contrast and SNR, especially for smaller pitch factors. The improvement is most obvious in the FA maps. Additionally, by reducing the pitch factor . both the diffusion-weighted images and the FA maps appears much sharper. The improvement is obvious, for example, in the posterior end of sagittal striatum . ACKNOWLEDGMENTS: NIH-1R01NS35959, NIH-1R01EB002771, Lucas Foundation, Center of Advanced MR Technology of Stanford (NCRR P41 RR 09784) REFERENCES: 1) Liu C, et al. Magn Reson Med 2004; 52:1388-1396. 2) D.H. Kim, et al. Magn Reson Med. 2003, 50: 214-219. 3) Liu C, et al. Proc ISMRM. 2005, p10.
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