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 (SO) 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 and corresponding putative SO of the SSS were quantified from the generated susceptibility maps. Systematic error in the measured SO due to the modified pipeline was also studied through simulations.

Results: Simulations showed that the systematic error in SO 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 in the SSS quantified across all gestations was 0.42 ± 0.03 ppm. Based on ∆χ, the average putative SO 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 and SO.

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.