Spectroscopic sampling of the left side of long-TE spin echoes: a free lunch?

Objective: Use of spectroscopically-acquired spin echoes typically involves Fourier transformation of the right side of the echo while largely neglecting the left side. For sufficiently long echo times, the left side may have enough spectral resolution to offer some utility. Since the acquisition of this side is "free", we deemed it worthy of attention and investigated the spectral properties and information content of this data.
Materials and Methods: Theoretical expressions for left- and right-side spectra were derived assuming Lorentzian frequency distributions. For left-side spectra, three regimes were identified based upon the relative magnitudes of reversible and irreversible transverse relaxation rates, R ₂ ' and R ₂ , respectively. Point-resolved spectroscopy (PRESS) data from muscle, fat deposit and bone marrow were acquired at 1.5 T to test aspects of the theoretical expressions.
Results: For muscle water or methylene marrow resonances, left-side signals were substantially or moderately larger than right-side signals but were similar in magnitude for muscle choline and creatine resonances. Left- versus right-side spectral-peak amplitude ratios depend sensitively on the relative values of R ₂ and R ₂ ' , which can be estimated given this ratio and a right-side linewidth measurement.
Conclusion: Left-side spectra can be used to augment signal-to-noise and to estimate spectral R ₂ and R ₂ ' values under some circumstances.