Spatiotemporal alignment of in utero BOLD‐MRI series

Abstract

Purpose To present a method for spatio-temporal alignment of in-utero magnetic resonance imaging (MRI) time series acquired during maternal hyperoxia for enabling improved quantitative tracking of blood-oxygen-level dependent (BOLD) signal changes that characterize oxygen transport through the placenta to fetal organs. Methods The proposed pipeline for spatio-temporal alignment of images acquired with a single shot gradient echo echo-planar imaging includes (i) signal non-uniformity correction, (ii) intra-volume motion correction based on non-rigid registration, (iii) correction of motion and non-rigid deformations across volumes, and (iv) detection of the outlier volumes to be discarded from subsequent analysis. BOLD MRI time series collected from ten pregnant women during 3T scans were analyzed using this pipeline. To assess pipeline performance, signal fluctuations between consecutive time points were examined. In addition, volume overlap and distance between manual ROI delineations in a subset of frames and the delineations obtained through propagation of the ROIs from the reference frame were used to quantify alignment accuracy. A previously demonstrated rigid registration approach was used for comparison. Results The proposed pipeline improved anatomical alignment of placenta and fetal organs over the state-of-the-art rigid motion correction methods. In particular, unexpected temporal signal fluctuations during the first normoxia period were significantly decreased (p<0.01) and volume overlap and distance between region boundaries measures were significantly improved (p<0.01). Conclusion The proposed approach to align MRI time series enables more accurate quantitative studies of placental function by improving spatio-temporal alignment across placenta and fetal organs.

Placenta microstructure and microcirculation imaging with diffusion MRI

“…This undoubtedly affects the visual quality of DWI parameter maps , limiting our ability to image small‐scale structures, such as spiral arteries. Motion correction in the placenta is a difficult and little studied problem, and requires consideration of non‐rigid motion (both inter‐slice and inter‐volume) . In the future, we aim to develop algorithms for motion correction in conjunction with protocols interspersing high and low b ‐value slices, as used for four subjects here, since these improve registration between diffusion weighted volumes.…”

“…Development of non‐invasive, in vivo techniques for measuring blood flow (beyond Doppler ultrasound) and oxygenation is an active field of research. For example, maternal blood oxygenation and maternal blood flow have been quantified with blood‐oxygen‐level dependent MRI and dynamic contrast‐enhanced MRI , respectively. Diffusion‐weighted MRI (DWI) is also emerging as a promising technique for quantifying placental function , but previous studies are limited to standard simple diffusion models, such as apparent diffusion coefficient (ADC) mapping, diffusion tensor imaging (DTI) , and intravoxel incoherent motion (IVIM) .…”

T2*‐weighted placental MRI: basic research tool or emerging clinical test for placental dysfunction?

“…Most human studies investigated the changes in placental signal intensity during maternal hyperoxia (3-10 min of breathing 100% oxygen) [15][16][17][18][19][20][21] , and one study investigated the effect of uterine contractions 12 . In addition, three animal studies described changes in placental signal intensity during maternal hypoxia 22 , normoxic hypercapnia 23 and injection of a vasoconstrictor 24 .…”

“…In addition, three animal studies described changes in placental signal intensity during maternal hypoxia, normoxic hypercapnia and injection of a vasoconstrictor. Human studies were performed at both 1.5 T and 3 T using single‐shot GE‐EPI sequences. However, the specific echo‐time (TE) and repetition time (TR) varied between studies (Table S1).…”

Multi‐modal functional MRI to explore placental function over gestation

“…Quantitative features have been obtained previously using placental relaxometry, 16,[18][19][20]22,33,34 BOLD MRI, 35 dynamic contrast-enhanced MRI 24,33 or diffusion MRI. [28][29][30] Anatomical imaging of the placenta has been performed by numerous groups and several attempts have been done to objectively assess placental appearance, 36 but to our knowledge no signature was so far able to quantify changes across gestation nor attempted quantitative assessment.…”