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The focal underdetermined system solver (FOCUSS) was originally designed to obtain sparse solutions by successively solving quadratic optimization problems. This article adapts FOCUSS for a projection reconstruction MR imaging problem to obtain high resolution reconstructions from angular undersampled radial k -space data. We show that FOCUSS is effective for projection reconstruction MRI, since medical images are usually sparse in some sense and the center region of the undersampled radial k -space samples still provides a low resolution, yet meaningful, image essential for the convergence of FOCUSS. Projection reconstruction (PR) with radial k-space trajectory was the first MRI k-space trajectory in MR history (1). However, cartesian k-space trajectory has replaced PR, mainly because of artifacts of PR that are related to B 0 inhomogeneity and to gradient nonlinearity (2). However, recent advances in MR hardware technology have overcome problems related to B 0 inhomogeneity and gradient nonlinearity, and interest in PR has thus been revived. PR has many advantages over the conventional cartesian k-space trajectory (3). Since no phase-encoding gradient is used, PR has a shorter minimum TE, which has made PR particularly desirable for imaging very short T 2 nuclei (3-5). Another advantage of PR is its robustness to the motion artifacts from flow or respiration. One important example is the reduction of motion artifacts in a diffusionweighted MRI (6,7). Furthermore, the aliasing artifacts from radial under-sampling usually appear as streaks, which are visually less distracting than the wrap-around artifacts obtained with cartesian under-sampling.One of the disadvantages of PR is the increased scan time involved if the Nyquist sampling criterion needs to be satisfied. More specifically, the number of radial lines N s required to satisfy the Nyquist criterion is given by (3):where L is the field-of-view (FOV), and k max is the maximum k-space radius. Usually, the number of radial lines acquired by PR is about 57% larger than the number of k-space lines acquired on a cartesian grid, which results in the increased scan time (3). If streak aliasing artifacts can be tolerated in an application, the scan time can be reduced by using angular undersampling. One such undersampled PR application is contrast-enhanced vascular imaging (8).Because of the properties of PR, if the contrast enhanced vessels are located at the center of the FOV, the undersampling aliasing artifacts appear as streaks near the periphery of the FOV and usually do not interfere with vessels located at the center of FOV. Hence, this application of PR for angiography has been a success (8).Rather than tolerating the angular aliasing artifacts, however, the main goal of our research is to develop a novel reconstruction algorithm with minimal angular aliasing. The bases of such a novel algorithm are the following two observations: (a) most medical imaging is sparse in some sense, and (b) the under-sampled PR still provides a meaningful low resolution imag...

Purpose: To develop a fast, quantitative 3D magnetization transfer contrast (MTC) framework based on an unsupervised learning scheme, which will provide baseline reference signals for CEST and nuclear Overhauser enhancement imaging. Methods: Pseudo-randomized RF saturation parameters and relaxation delay times were applied in an MR fingerprinting framework to generate transient-state signal evolutions for different MTC parameters. Prospectively compressed sensingaccelerated (four-fold) MR fingerprinting images were acquired from 6 healthy volunteers at 3 T. A convolutional neural network framework in an unsupervised fashion was designed to solve an inverse problem of a two-pool MTC Bloch equation, and was compared with a conventional Bloch equation-based fitting approach. The MTC images synthesized by the convolutional neural network architecture were used for amide proton transfer and nuclear Overhauser enhancement imaging as a reference baseline image. Results: The fully unsupervised learning scheme incorporated with the two-pool exchange model learned a set of unique features that can describe the MTC-MR fingerprinting input, and allowed only small amounts of unlabeled data for training. The MTC parameter values estimated by the unsupervised learning method were in excellent agreement with values estimated by the conventional Bloch fitting approach, but dramatically reduced computation time by ~1000-fold. Conclusion: Given the considerable time efficiency compared to conventional Bloch fitting, unsupervised learning-based MTC-MR fingerprinting could be a powerful tool for quantitative MTC and CEST/nuclear Overhauser enhancement imaging.

The purpose of this study is to explore spatiotemporal brain activation patterns during perception of words from three different languages (Korean, English, Chinese) and pictures. Using 64 channel event-related potential (ERP) recording and source localization using distributed source model, we investigated, with high temporal resolution, whether similar or different spatiotemporal patterns of brain activation are involved in the perception of words of different languages and/or pictures. Experimental results seem to corroborate left hemispheric dominance in language processing, and temporal/spatial characteristics in word perception revealed by previous ERP and neuroimaging studies. Observed differences in spatial pattern of activation at specific time periods between English and Korean, and Korean and Chinese, could be explained in terms of required visual pattern analysis due to the orthographic characteristics of each language.

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The aim of this study was to identify the activated brain region that is involved with the affective dimension of thermal stimulation (not pain, but innocuous warming) using functional MR imaging. Twelve healthy, right-handed male subjects participated in the study. Thermal stimulation with two different temperatures of 41 degrees C and 34 degrees C was applied to the subjects using a fomentation pack, wrapped around the right lower leg of each subject. On the basis of the subjects' responses after the scanning sessions, the authors were able to observe that the subjects felt "warm" and "slightly pleasant and comfortable" under the 41 degrees C condition. The experimental results indicated that warm stimulation produced a significant increase of activation compared to thermal neutral stimulation in various regions such as contralateral insular, ipsilateral cerebellum, ipsilateral putamen, contralateral middle frontal gyrus, ipsilateral inferior frontal gyrus, contralateral postcentral gyrus, and contralateral paracentral lobule. The activated regions are known to be related to thermal sensory, affective/emotional awareness, cognitive functions, sensory-discrimination, and emotion/affective processing, and so on. These results suggest that an appropriate thermal stimulation can produce a positive emotion and activate emotion/affect related regions of the brain.

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