2-D MR spectroscopy combined with 2-D/3-D spatial encoding

In addition to detecting water and lipids in human tissues using magnetic resonance imaging (MRI), a number of metabolite resonances have been recorded noninvasively using one (chemical shift)-dimensional (1-D) proton (1H) magnetic resonance (MR) spectroscopy (MRS) on whole-body MRI scanners (1.5 and 3 T). However, severe overlap of resonances in 1-D MRS limits the unambiguous identification of many metabolites. Different versions of spectral editing sequences allow detection and quantification of selected metabolites. This approach, too, is limited in that it detects only one metabolite per acquisition, and many metabolites still cannot be detected owing to severe overlap. Adding another spectral dimension can overcome this limitation by providing resolution of metabolite resonances along the second dimension, thereby reducing the ambiguity, especially for quantifying J-coupled metabolites. In this article, we review progress with two-dimensional (2-D) MRS such as localized J-resolved spectroscopy (JPRESS) and localized correlated spectroscopy (L-COSY and their multidimensional versions, namely echo-planar-correlated spectroscopic imaging EP-COSI) and echo-planar J-resolved spectroscopic imaging (EP-JRESI), where 2-D spectral encoding is combined with two-or three-dimensional spatial encoding. These '4-D' or '5-D' spectroscopic imaging sequences can be extremely time consuming. However, acquisition using nonuniform undersampling (NUS) strategies and compressed sensing (CS) accelerates their acquisition times.