Quantification of HO by H-MR imaging at 3 T: a feasibility study.

Background: Indirect H-magnetic resonance (MR) imaging of O-labelled water allows imaging in vivo dynamic changes in water compartmentalisation. Our aim was to describe the feasibility of indirect H-MR methods to evaluate the effect of HO on the MR relaxation rates by using conventional a 3-T equipment and voxel-wise relaxation rates.

Methods: MR images were used to calculate the R1, R2, and R2* relaxation rates in phantoms (19 vials with different HO concentrations, ranging from 0.039 to 5.5%). Afterwards, an experimental animal pilot study (8 rats) was designed to evaluate the in vivo relative R2 brain dynamic changes related to the intravenous administration of O-labelled water in rats.

Results: There were no significant changes on the R1 and R2* values from phantoms. The R2 obtained with the turbo spin-echo T2-weighted sequence with 20-ms echo time interval had the higher statistical difference (0.67 s, interquartile range 0.34, p < 0.001) and Spearman correlation (rho 0.79). The R2 increase was adjusted to a linear fit between 0.25 and 5.5%, represented with equation R2 = 0.405 concentration + 0.3215. The highest significant differences were obtained for the higher concentrations (3.1-5.5%). The rat brain MR experiment showed a mean 10% change in the R2 value after the HO injection with progressive normalisation.

Conclusions: Indirect H-MR imaging method is able to measure HO concentration by using R2 values and conventional 3-T MR equipment. Normalised R2 relative dynamic changes after the intravenous injection of a HO saline solution provide a unique opportunity to map water pathophysiology in vivo, opening the analysis of aquaporins status and modifications by disease at clinically available 3-T proton MR scanners.