In vivo phosphorus magnetic resonance spectroscopic imaging of the whole human liver at 7 T using a phosphorus whole-body transmit coil and 16-channel receive array: Repeatability and effects of principal component analysis-based denoising

Abstract

Quantitative three-dimensional (3D) imaging of phosphorus (³¹ P) metabolites is potentially a promising technique with which to assess the progression of liver disease and monitor therapy response. However, ³¹ P magnetic resonance spectroscopy has a low sensitivity and commonly used ³¹ P surface coils do not provide full coverage of the liver. This study aimed to overcome these limitations by using a ³¹ P whole-body transmit coil in combination with a 16-channel ³¹ P receive array at 7 T. Using this setup, we determined the repeatability of whole-liver ³¹ P magnetic resonance spectroscopic imaging (³¹ P MRSI) in healthy subjects and assessed the effects of principal component analysis (PCA)-based denoising on the repeatability parameters. In addition, spatial variations of ³¹ P metabolites within the liver were analyzed. 3D ³¹ P MRSI data of the liver were acquired with a nominal voxel size of 20 mm isotropic in 10 healthy volunteers twice on the same day. Data were reconstructed without denoising, and with PCA-based denoising before or after channel combination. From the test-retest data, repeatability parameters for metabolite level quantification were determined for 12 ³¹ P metabolite signals. On average, ³¹ P MR spectra from 100 ± 25 voxels in the liver were analyzed. Only voxels with contamination from skeletal muscle or the gall bladder were excluded and no voxels were discarded based on (low) signal-to-noise ratio (SNR). Repeatability for most quantified ³¹ P metabolite levels in the liver was good to excellent, with an intrasubject variability below 10%. PCA-based denoising increased the SNR ~ 3-fold, but did not improve the repeatability for mean liver ³¹ P metabolite quantification with the fitting constraints used. Significant spatial heterogeneity of various ³¹ P metabolite levels within the liver was observed, with marked differences for the phosphomonoester and phosphodiester metabolites between the left and right lobe. In conclusion, using a ³¹ P whole-body transmit coil in combination with a 16-channel ³¹ P receive array at 7 T allowed ³¹ P MRSI acquisitions with full liver coverage and good to excellent repeatability.

Keywords: 7-T MRI; energy metabolism; liver; membrane turnover; phosphorus.