Quantitative susceptibility mapping in the human fetus to measure blood oxygenation in the superior sagittal sinus

Objectives: To present the feasibility of performing quantitative susceptibility mapping (QSM) in the human fetus to evaluate the oxygenation (S_vO₂) of cerebral venous blood in vivo.

Methods: Susceptibility weighted imaging (SWI) data were acquired from healthy pregnant subjects (n = 21, median = 31.3 weeks, interquartile range = 8.8 weeks). The susceptibility maps were generated from the SWI-phase images using a modified QSM processing pipeline, optimised for fetal applications. The processing pipeline is as follows: (1) mild high-pass filtering followed by quadratic fitting of the phase images to eliminate background phase variations; (2) manual creation of a fetal brain mask that includes the superior sagittal sinus (SSS); (3) inverse filtering of the resultant masked phase images using a truncated k-space approach with geometric constraint. Further, the magnetic susceptibility, ∆χ_v and corresponding putative S_vO₂ of the SSS were quantified from the generated susceptibility maps. Systematic error in the measured S_vO₂ due to the modified pipeline was also studied through simulations.

Results: Simulations showed that the systematic error in S_vO₂ when using a mask that includes a minimum of 5 voxels around the SSS and five slices remains < 3% for different orientations of the vessel relative to the main magnetic field. The average ∆χ_v in the SSS quantified across all gestations was 0.42 ± 0.03 ppm. Based on ∆χ_v, the average putative S_vO₂ in the SSS across all fetuses was 67% ± 7%, which is in good agreement with published studies.

Conclusions: This in vivo study demonstrates the feasibility of using QSM in the human fetal brain to estimate ∆χ_v and S_vO₂.

Key points: • A modified quantitative susceptibility mapping (QSM) processing pipeline is tested and presented for the human fetus. • QSM is feasible in the human fetus for measuring magnetic susceptibility and oxygenation of venous blood in vivo. • Blood magnetic susceptibility values from MR susceptometry and QSM agree with each other in the human fetus.