Browsing by Author "Nishimura, D."

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Open Access
Full‐brain coverage and high‐resolution imaging capabilities of passband b‐SSFP fMRI at 3T
(Wiley‐Liss, Inc., 2008) Lee, J. H.; Dumoulin, S.; Sarıtaş, Emine Ülkü; Glover, G.; Wandell, B.; Nishimura, D.; Pauly, J.
Passband balanced-steady-state free precession (b-SSFP)fMRI is a recently developed method that utilizes the passband(ﬂat portion) of the b-SSFP off-resonance response to measureMR signal changes elicited by changes in tissue oxygenationfollowing increases in neuronal activity. Rapid refocusing andshort readout durations of b-SSFP, combined with the relativelylarge ﬂat portion of the b-SSFP off-resonance spectrum allowsdistortion-free full-brain coverage with only two acquisitions.This allows for high-resolution functional imaging, without thespatial distortion frequently encountered in conventional high-resolution functional images. Finally, the 3D imaging compati-bility of the b-SSFP acquisitions permits isotropic-voxel-sizehigh-resolution acquisitions. In this study we address some ofthe major technical issues involved in obtaining passband b-SSFP-based functional brain images with practical imaging pa-rameters and demonstrate the advantages through breath-holding and visual ﬁeld mapping experiments. Magn ResonMed 59:1099 –1110, 2008.
Open Access
Reduced field-of-view diffusion imaging of the human spinal cord: comparison with conventional single-shot echo-planar imaging
(American Society of Neuroradiology, 2011) Zaharchuk, G.; Sarıtaş, Emine Ülkü; Andre, J.; Chin, C.; Rosenberg, J.; Brosnan, T.; Shankaranarayan, A.; Nishimura, D.; Fischbein, N.
BACKGROUND AND PURPOSE: DWI of the spinal cord is challenging because of its small size and artifacts associated with the most commonly used clinical imaging method, SS-EPI. We evaluated the performance of rFOV spinal cord DWI and compared it with the routine fFOV SS-EPI in a clinical population. MATERIALS AND METHODS: Thirty-six clinical patients underwent 1.5T MR imaging examination that included rFOV SS-EPI DWI of the cervical spinal cord as well as 2 comparison diffusion sequences: fFOV SS-EPI DWI normalized for either image readout time (low-resolution fFOV) or spatial resolution (high-resolution fFOV). ADC maps were created and compared between the methods by using single-factor analysis of variance. Two neuroradiologists blinded to sequence type rated the 3 DWI methods, based on susceptibility artifacts, perceived spatial resolution, signal intensity–to-noise ratio, anatomic detail, and clinical utility. RESULTS: ADC values for the rFOV and both fFOV sequences were not statistically different (rFOV: 1.01 ± 0.18 × 10−3 mm2/s; low-resolution fFOV: 1.12 ± 0.22 × 10−3 mm2/s; high-resolution fFOV: 1.10 ± 0.21 × 10−3 mm2/s; F = 2.747, P > .05). The neuroradiologist reviewers rated the rFOV diffusion images superior in terms of all assessed measures (P < 0.0001). Particular improvements were noted in patients with metal hardware, degenerative disease, or both. CONCLUSIONS: rFOV DWI of the spinal cord overcomes many of the problems associated with conventional fFOV SS-EPI and is feasible in a clinical population. From a clinical standpoint, images were deemed superior to those created by using standard fFOV methods.
Open Access
SNR dependence of optimal parameters for apparent diffusion coefficient measurements
(IEEE, 2011) Sarıtaş, Emine Ülkü; Lee, J.; Nishimura, D.
Optimizing the diffusion-weighted imaging (DWI) parameters (i.e., the b -value and the number of image averages) to the tissue of interest is essential for producing high-quality apparent diffusion coefficient (ADC) maps. Previous investigation of this optimization was performed assuming Gaussian noise statistics for the ADC map, which is only valid for high signal-to-noise ratio (SNR) imaging. In this work, the true statistics of the noise in ADC maps are derived, followed by an optimization of the DWI parameters as a function of the imaging SNR. Specifically, it is demonstrated that the optimum b -value is a monotonically increasing function of the imaging SNR, which converges to the optimum b -value from previously proposed approaches for high-SNR cases, while exhibiting a significant deviation from this asymptote for low-SNR situations. Incorporating the effects of T 2 weighting further increases the SNR dependence of the optimal parameters. The proposed optimization scheme is particularly important for high-resolution DWI, which intrinsically suffers from low SNR and therefore cannot afford the use of the conventional high b -values. Comparison scans were performed for high-resolution DWI of the spinal cord, demonstrating the improvements in the resulting images and the ADC maps achieved by this method.