Fast Acquisition Techniques

Fast acquisition techniques are usually restricted to techniques for multidimensional spectroscopic imaging (CSI). While the acquisition time in a CSI experiment is mainly defined by the number of phased-encoding steps used for encoding the spatial information, the time for a single-voxel experiment is proportional to the number b a c

Fig. 6.9. Spectra obtained from grey matter in a healthy subject using PRESS (a) and TE-averaged PRESS (b) sequences. In conventional spectroscopy sequences (a), the resonances between 2 and 2.6 ppm are assigned to a mixture of glutamate (Glu) and glutamine (Gln), designated Glx. At 3 T, a TE-averaged PRESS data acquisition gives an unobstructed single line response for glutamate at 2.38 ppm (b)

Fig. 6.9. Spectra obtained from grey matter in a healthy subject using PRESS (a) and TE-averaged PRESS (b) sequences. In conventional spectroscopy sequences (a), the resonances between 2 and 2.6 ppm are assigned to a mixture of glutamate (Glu) and glutamine (Gln), designated Glx. At 3 T, a TE-averaged PRESS data acquisition gives an unobstructed single line response for glutamate at 2.38 ppm (b)

of signal averages and the repetition time TR. None of the techniques used to decrease the time needed to cover the fc-space for spectroscopic imaging can be directly applied to single-voxel spectroscopy.

Different fast CSI acquisition techniques make use of increased gradient performance and specific fc-space trajectories to acquire the full spatial and spectral information in a reduced amount of time. These techniques utilize approaches initially developed for conventional fast imaging methods and include echoplanar spectroscopic imaging (EPSI, PEPSI) [55,56], spiral acquisitions [57] or recently developed flyback techniques [58].

Some of the techniques described above are very hardware-demanding, and an alternative approach, with very little or no extra hardware requirements, uses a method commonly known as parallel imaging. These techniques work with dedicated coil arrays, using the known B1 field distribution of every coil element to reduce the number of required phased-encoding steps for full encoding of the required spatial information. Parallel imaging techniques have successfully been applied to spectroscopic imaging, even though post-processing is very demanding, and the complex nature of the spec-troscopic data makes the resulting spectra prone to artefacts or quality losses.

Even though all the techniques enable a significant reduction in acquisition time, down to less than 15 min for a full brain metabolite map, the resulting spectra always suffer from reduced SNR compared for instance to conventional PRESS chemical shift imaging. To regain this SNR, multiple averaging would be required, preventing the acceptance of these approaches for clinical applications. Greater field strengths with higher intrinsic SNR may in the future open new prospects for the use of these methods.

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