Dual-Imaging Modality Approach to Evaluate Cerebral Hemodynamics in Growth-Restricted Fetuses: Oxygenation and Perfusion.

OBJECTIVE: To evaluate a dual-imaging modality approach to obtain a combined estimation of venous blood oxygenation (SνO2) using susceptibility-weighted magnetic resonance imaging (SWI-MRI), and blood perfusion using power Dopp-ler ultrasound (PDU) and fractional moving blood volume (FMBV) in the brain of normal growth and growth-restricted fetuses. METHODS: Normal growth (n = 33) and growth-restricted fetuses (n = 10) from singleton pregnancies between 20 and 40 weeks of gestation were evaluated. MRI was performed and SνO2 was calculated using SWI-MRI data obtained in the straight section of the superior sagittal sinus. Blood perfusion was estimated using PDU and FMBV from the frontal lobe in a mid-sagittal plane of the fetal brain. The association between fetal brain SνO2 and FMBV, and the distribution of SνO2 and FMBV values across gestation were calculated for both groups. RESULTS: In growth-restricted fetuses, the brain SνO2 values were similar, and the FMBV values were higher across gestation as compared to normal growth fetuses. There was a significantly positive association between SνO2 and FMBV values (slope = 0.38 ± 0.12; r = 0.7; p = 0.02) in growth-restricted fetuses. In normal growth fetuses, SνO2 showed a mild decreasing trend (slope = -0.7 ± 0.4; p = 0.1), whereas FMBV showed a mild increasing trend (slope = 0.2 ± 0.2; p = 0.2) with advancing gestation, and a mild but significant negative association (slope = -0.78 ± 0.3; r = -0.4; p = 0.04) between these two estimates. CONCLUSION: Combined MRI (SWI) and ultrasound (FMBV) techniques showed a significant association between cerebral blood oxygenation and blood perfusion in normal growth and growth-restricted fetuses. This dual-imaging approach could contribute to the early detection of fetal "brain sparing" and brain oxygen saturation changes in high-risk pregnancies.

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 (SvO2) 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, ∆χv and corresponding putative SvO2 of the SSS were quantified from the generated susceptibility maps. Systematic error in the measured SvO2 due to the modified pipeline was also studied through simulations. RESULTS: Simulations showed that the systematic error in SvO2 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 ∆χv in the SSS quantified across all gestations was 0.42 ± 0.03 ppm. Based on ∆χv, the average putative SvO2 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 ∆χv and SvO2. 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.

Imaging putative foetal cerebral blood oxygenation using susceptibility weighted imaging (SWI).

OBJECTIVE: To evaluate the magnetic susceptibility, ∆χ v , as a surrogate marker of venous blood oxygen saturation, S v O 2, in second- and third-trimester normal human foetuses. METHODS: Thirty-six pregnant women, having a mean gestational age (GA) of 31 2/7 weeks, underwent magnetic resonance imaging (MRI). Susceptibility-weighted imaging (SWI) data from the foetal brain were acquired. ∆χ v of the superior sagittal sinus (SSS) was quantified using MR susceptometry from the intra-vascular phase measurements. Assuming the magnetic property of foetal blood, ∆χ do , is the same as that of adult blood, S v O 2 was derived from the measured Δχ v . The variation of ∆χ v and S v O 2, as a function of GA, was statistically evaluated. RESULTS: The mean ∆χ v in the SSS in the second-trimester (n = 8) and third-trimester foetuses (n = 28) was found to be 0.34± 0.06 ppm and 0.49 ±0.05 ppm, respectively. Correspondingly, the derived S v O 2 values were 69.4% ±3.27% and 62.6% ±3.25%. Although not statistically significant, an increasing trend (p = 0.08) in Δχ v and a decreasing trend (p = 0.22) in S v O 2 with respect to advancing gestation was observed. CONCLUSION: We report cerebral venous blood magnetic susceptibility and putative oxygen saturation in healthy human foetuses. Cerebral oxygen saturation in healthy human foetuses, despite a slight decreasing trend, does not change significantly with advancing gestation. KEY POINTS: • Cerebral venous magnetic susceptibility and oxygenation in human foetuses can be quantified. • Cerebral venous oxygenation was not different between second- and third-trimester foetuses. • Foetal cerebral venous oxygenation does not change significantly with advancing gestation.