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Spectroscopic imaging (CSI : Chemical Shift Imaging)

Metabolic imaging (CSI) consists in recording the spectroscopic data for a group of voxels, in slice(s) (2D) or by volume (3D). It is based on a repetition of STEAM or PRESS type sequences to which is added spatial phase encoding (figure 15.7). The number and direction of phase encodings depend on the number of dimensions explored (1D, 2D or 3D), adding on to acquisition time. The duration of the sequence is equal to TR ∙ Nph1D ∙ Nph2D ∙ Nph3D ∙ NSA (NphxD number of phase encoding steps in direction x).

To reduce acquisition time, variants of the basic sequence have been proposed:

• Multiple Slice CSI accelerates the acquisition of several slices compared to 2D CSI
• Turbo CSI (several echoes received by rewinding the phase encoding gradient before each additional echo, derived from the repetition of the last spin echo pulse and the gradient)
• Fast CSI provides an important speed gain compared to 2D CSI, by performing spatial encoding in one direction during signal acquisition, by means of oscillating gradients, similar to spatial encoding in an echo planar sequence. These techniques are less sensitive than the classic CSI sequence. The speed gain is particularly useful in 3D spectroscopic imaging or in the case of motion artifacts.
• CSI with parallel acquisition (SENSE CSI).

Signal processing calls on Fourier transforms (1 for each phase-encoded dimension phase + 1 for spectral analysis) and requires data correction (correction of the baseline, phase, smoothing truncation artifacts…).

The results appear in the form of parametric images («metabolic maps ») or a matrix of the spectra of the regions to be studied (figure 15.8).