Enhanced spectral resolution for correlated spectroscopic imaging using inner-product and covariance transform: a pilot analysis of metabolites and lipids in breast cancer in vivo.

Acquisition duration of correlated spectroscopy in vivo can be longer due to a large number of t increments along the indirect (F) dimension. Limited number of t increments on the other hand leads to poor spectral resolution along F. Covariance transformation (CT) instead of Fourier transform along t is an alternative way of increasing the resolution of the 2D COSY spectrum. Prospectively undersampled five-dimensional echo-planar correlated spectroscopic imaging (EP-COSI) data from ten malignant patients and ten healthy women were acquired and reconstructed using compressed sensing. The COSY spectrum at each voxel location was then generated using FFT, CT and a variant of CT called Inner Product (IP). Metabolite and lipid ratios were computed with respect to water from unsuppressed one-dimensional spectrum. The effects of t-ridging artifacts commonly seen with FFT were not observed with CT/IP. Statistically significant differences were observed in the fat cross peaks measured with CT/IP/FFT. Spectral resolution was increased ~ 8.5 times (~ 19.53 Hz in FFT, ~ 2.32 Hz in CT/IP) without affecting the spectral width along F was possible with CT/IP. CT and IP enabled substantially increased F resolution effectively with significant gain in scan time and reliable measure of unsaturation index as a biomarker for malignant breast cancer.