Fast MR Spectroscopic Imaging

MR spectroscopic imaging (MRSI) has become widely available as a research tool for mapping tissue metabolic status. Clinical research applications have been limited by intrinsically low sensitivity and by time-consuming phase encoding techniques that are used with conventional MRSI. The long encoding times limit volume coverage, introduce motion sensitivity and tax patient tolerance. The increasing availability of high field scanners and sensitive surface coil arrays, and maturation of fast MRSI techniques now allows for shorter acquisition times, higher spatial resolution and whole organ coverage not feasible with conventional MRSI methods. Fast MRSI has shown considerable potential for applications in human brain where acceptable magnetic field homogeneity can be achieved to enable whole brain coverage. This lecture will describe the basic principles of the most commonly used fast MRSI methods, ranging from methods with moderate acceleration to high-speed encoding methods, and compare their performance (acceleration, sensitivity per unit time and unit volume, spectral width and spectral resolution) with that of conventional phase encoded MRSI. It will also discuss technical challenges for implementation of high-speed MRSI at high field strength. Many of these methods are based on high-speed MRI methods and largely fall into the following categories: (i) Echo planar spectroscopic imaging (EPSI), which uses interleaving of 1D, 2D and 3D echo-planar encoding into the spectroscopic acquisition, is one of the earliest high-speed MRSI approaches, which was originally developed by Mansfield (1) and later adapted for human applications (2-5). Advantages of EPSI include an adequate spectral width at field strengths up to 3 Tesla in case of 1D echo-planar encoding, a high spectral resolution, the feasibility of very short echo time, a high sensitivity comparable to conventional MRSI and the relative ease of data reconstruction. (ii) Spiral MRSI using interleaving of 2D and 3D spiral encoding into the spectroscopic acquisition offers faster acceleration than EPSI due to the efficiency of the spiral trajectory (6). Combination with echo time shifting is typically required to increase the spectral width. Spiral MRSI provides flexibility in shaping the point spread function and high sensitivity, but data reconstruction is more demanding than for EPSI and requires correction for stronger off-resonance effects. (iii) High-speed 2D imaging methods, such as EPI, RARE or spiral encoding, have been combined with echo time shifting to achieve chemical shift encoding (7). This approach is time efficient for applications that require higher spatial resolution than conventional MRSI and moderate spectral resolution. However, achieving high spectral resolution at large spectral bandwidth is time consuming and decreases SNR per unit time. (iv) Multiple spin echo acquisition with phase encoding of individual spin echoes has been developed to accelerate conventional MRSI (8). However, spectral resolution is reduced due to the shortened readout and signal modulation due to T2 relaxation needs to be corrected to avoid spatial blurring. (v) Partial parallel imaging using phased array coils has been applied to conventional (9) and high-speed MRSI (10,11) to achieve considerable acceleration of spatial encoding, including single-shot 2D encoding (12). Increases in encoding speed need to be balanced with increases in g-factor related noise that can compromise spatial localization and spectral quality. Recent developments using compressed sensing (13) and the combination of parallel imaging and compressed sensing (14) exploit the sparsity of spectral information to achieve even faster encoding speed.