Quantitation Error in ¹H MRS Caused by B₁ Inhomogeneity and Chemical Shift Displacement

The quantitation accuracy in proton magnetic resonance spectroscopy (¹H MRS) improves at higher B₀ field. However, a larger chemical shift displacement (CSD) and stronger B₁ inhomogeneity exist. In this work, we evaluate the quantitation accuracy for the spectra of metabolite mixtures in phantom experiments at 4.7T. We demonstrate a position-dependent error in quantitation and propose a correction method by measuring water signals.

All experiments were conducted on a whole-body 4.7T MR system with a quadrature volume coil for transmission and reception. We arranged three bottles filled with metabolite solutions of N-acetyl aspartate (NAA) and creatine (Cr) in a vertical row inside a cylindrical phantom filled with water. Peak areas of three singlets of NAA and Cr were measured on three ¹H spectra at three volume of interests (VOIs) inside three bottles. We also measured a series of water spectra with a shifted carrier frequency and measured a reception sensitivity map.

The ratios of NAA and Cr at 3.92 ppm to Cr at 3.01 ppm differed amongst the three VOIs in peak area, which leads to a position-dependent error. The nature of slope depicting the relationship between peak areas and the shifted values of frequency was like that between the reception sensitivities and displacement at every VOI.

CSD and inhomogeneity of reception sensitivity cause amplitude modulation along the direction of chemical shift on the spectra, resulting in a quantitation error. This error may be more significant at higher B₀ field where CSD and B₁ inhomogeneity are more severe. This error may also occur in reception using a surface coil having inhomogeneous B₁. Since this type of error is around a few percent, the data should be analyzed with greater attention while discussing small differences in the studies of ¹H MRS.