Non-T₁-weighted ³¹P chemical shift imaging of the human liver

A ³¹P chemical shift imaging (CSI) protocol was developed for human liver studies. It is shown that at the commonly used repetition time (TR) of 1 s T₁-weighting reduces the integrated intensities of liver phosphate metabolite signals by 18 ± 15% (inorganic phosphate, P₁) to 46 ± 10% (phosphodiester, PDE), that is for an RF pulse angle of 60° (weighted average) in liver. The loss in signal-to-noise ratio ( S N) at TR = 20 s, sufficient to eliminate spectral distortions caused by saturation, compared with TR = 1 s (47-65%) can be overcome by using one-dimensional (1D)-phase encoding with a small number of phase-encode steps. The liver spectra obtained by 1D-CSI with 4-step phase-encoding (spatial resolution 10 cm) have the highest S N and, after multiplication of the PDE signal by a factor of 1.4, closely reflect the liver metabolite levels. It is concluded that clinical ³¹P MR studies of liver function can be performed without T₁-weighting and that the current practice to compromise the MRS quantification of lever metabolites with uncertainties caused by (differential changes in) T₁-weighting is not warranted.