Purpose: To develop a novel approach for calculating the accurate sensitivity profiles of phased-array coils, resulting in correction of nonuniform intensity in parallel MRI. Materials and Methods:The proposed intensity-correction method estimates the accurate sensitivity profile of each channel of the phased-array coil. The sensitivity profile is estimated by fitting a nonlinear curve to every projection view through the imaged object. The nonlinear curve-fitting efficiently obtains the low-frequency sensitivity profile by eliminating the high-frequency image contents. Filtered back-projection (FBP) is then used to compute the estimates of the sensitivity profile of each channel. The method was applied to both phantom and brain images acquired from the phased-array coil.Results: Intensity-corrected images from the proposed method had more uniform intensity than those obtained by the commonly used sum-of-squares (SOS) approach. With the use of the proposed correction method, the intensity variation was reduced to 6.1% from 13.1% of the SOS. When the proposed approach was applied to the computation of the sensitivity maps during sensitivity encoding (SENSE) reconstruction, it outperformed the SOS approach in terms of the reconstructed image uniformity. Conclusion:The proposed method is more effective at correcting the intensity nonuniformity of phased-array surface-coil images than the conventional SOS method. In addition, the method was shown to be resilient to noise and was successfully applied for image reconstruction in parallel imaging.

Purpose: To develop a novel approach for high-resolution functional MRI (fMRI) using the conventional gradient-echo sequence. Materials and Methods:Echo-planar imaging (EPI) techniques have generally been used for fMRI studies due to their fast imaging time. However, it is difficult for studying brain function at the submillimeter level using this sequence. In addition, EPI techniques have some drawbacks, such as Nyquist ghosts and geometric distortions in the reconstructed images, and subsequently require additional postprocessing to reduce these artifacts. One way of solving these problems is to acquire fMRI data by means of a conventional gradient-echo imaging sequence instead of EPI. To provide a fast imaging time, the proposed method combines higher-order generalized series (HGS) imaging with a parallel imaging technique which is called the HGS-parallel technique. Results:The proposed HGS-parallel technique achieves a 12.8-fold acceleration in imaging time without the cost of spatial resolution. The proposed method was verified through the application of fMRI studies on normal subjects. Conclusion:This study suggests that the proposed method can be used for high-resolution fMRI studies without the geometric distortion and the Nyquist ghost artifacts compared to EPI.

Purpose:To reduce the artifacts due to pulsatile motion artifacts in diffusion-weighted imaging (DWI) with radial trajectories and to improve the image quality using projection data regeneration. Materials and Methods:The projection data is obtained by a radial spin-echo DWI (rSE-DWI) sequence, from which a temporary image is generated using the inverse Radon transform (IRT). The projection data include some degraded data due to cardiac pulsatile motion. The degraded data are then replaced with data that are regenerated using the Radon transform (RT) of the temporary image. The proposed method for image quality improvement is demonstrated through a computer simulation and in vivo images obtained by rSE-DWI. Results:In general, electrocardiograph (ECG) triggering is used to reduce the degradation of projection data in the DWI with radial trajectories, where the amount of degradation depends on the cardiac phase. Cardiac gating reduces the artifacts resulting from the cardiac pulsatile motion to a certain extent only. The proposed projection data regeneration method successfully improves image quality. Conclusion:The regeneration method based on back-projection reconstruction effectively uses the features of the degraded projections having lower signal intensity than the normal projections, resulting in image quality improvement without acquisition of additional data. While a number of MR imaging techniques have been developed for the improvement of DW image quality, the realization of a truly effective diffusion imaging technique has been technically difficult (5). Over time, the spin-echo diffusion-weighted echo-planar imaging (SE-DW-EPI) sequence (6,7) has gained popularity as it is reasonably immune to macroscopic motions. However, images obtained by SE-DW-EPI include artifacts related to magnetic susceptibility, and the spatial resolution is limited due to T 2 * decay, especially at high magnetic fields. If priority is given to the image quality over the total imaging time, the projection reconstruction (PR) technique can serve as an alternative to SE-DW-EPI. Although the acquisition time is long, the images produced by the PR method are fairly insensitive to magnetic susceptibility, bulk motion, and eddy currents (8 -10).Numerous diffusion imaging techniques that employ the PR technique have been presented since this approach was first proposed by Jung and Cho (11) and Gmitro and Alexander (10). More recent works include the radial turbo-spin-echo (rTSE) sequence, which reduces not only the imaging time but also image distortions caused by eddy currents and patient motion (12). This work shows that the images obtained with the PR technique can provide high-resolution images with better image quality than those obtained with the EPIbased methods. Although the PR technique is generally robust to motion-related artifacts, the artifacts caused by cardiac pulsatile motion cannot be fully resolved, especially when high diffusion gradients are applied.As the pulsatile motion due to an arterial expansion following a...