Long acquisition times, low resolution, and voxel contamination are major difficulties in the application of magnetic resonance spectroscopic imaging (MRSI). To overcome these difficulties, an online-optimized acquisition of k-space, termed sequential forward array selection (SFAS), was developed to reduce acquisition time without sacrificing spatial resolution. A 2D proton MRSI region of interest (ROI) was defined from a scout image and used to create a region of support (ROS) image. The ROS was then used to optimize and obtain a subset of k-space (i.e., a subset of nonuniform phase encodings) and hence reduce the acquisition time for MRSI. Reconstruction and processing software was developed in-house to process and reconstruct MRSI using the projections onto convex sets method. Phantom and in vivo studies showed that good-quality MRS images are obtainable with an approximately 80% reduction of data acquisition time. The reduction of the acquisition time depends on the area ratio of ROS to FOV (i.e., the smaller the ratio, the greater the time reduction). It is also possible to obtain higher-resolution MRS images within a reasonable time using this approach. Key words: fast spectroscopic imaging; sequential forward array selection; projections onto convex sets reconstruction; partial k-space sampling; region of support Magnetic resonance spectroscopy (MRS) was first introduced in the early 1970s to measure in vivo tissue metabolism in intact biological structures (1,2). Since then, MRS has been utilized to measure the metabolic status of almost every organ system in the body, and in particular is an established tool for studying neurochemical and metabolic abnormalities in the human brain. However, because they require relatively long acquisition times and have a low sensitivity (particularly on low-field MRI systems), MRS studies are frequently limited to single-voxel acquisitions, which may not capture information from the most important pathologic regions. MRS imaging (MRSI), also known as chemical shift imaging (CSI), is a combination of MRS and MR imaging (MRI). It is a completely noninvasive, multivoxel technique that can acquire information that is representative of both anatomy and regional metabolic states. In addition, MRSI often provides higher-spatialresolution in vivo biochemical information than the single-voxel approach. MRSI has been used for basic physiological research and clinical imaging of metabolites (3). Proton ( 1 H) MRSI studies have identified both focal and global neuronal metabolic changes in a variety of diseases, including brain tumor (4), subacute and acute cerebral infarction (5), multiple sclerosis (6), AIDS dementia (7), Alzheimer's disease (8), degenerative ataxia (9), epilepsy (10), and psychiatric disorders (11). Most of these diseases present challenges to neuronal viability, which particularly relate to a reduction in the N-acetyl-L-aspartic acid (NAA) concentration. Proton MRSI is also capable of revealing the accumulation of lipids (12) and lactate (13) in ischemic myoca...