Synopsis

Healthy placental function requires optimum percolation of blood throughout the intervillous space to provide adequate feto-maternal exchange. This depends on blood flow from the spiral arteries and permeability of the intervillous space - both being altered in conditions leading to fetal growth restriction. This study explores the use of diffusion based MRI to visualise and depict the blood flow within the placenta and explore the underlying micro-structure, including the repeatability of the measures.

Introduction

Healthy placental function requires optimum percolation of blood throughout the intervillous space to enable adequate transfer of gases, nutrients and waste products between the fetus and mother. This percolation depends on the flow of blood from the spiral arteries and the permeability of the intervillous space - both parameters that are altered in conditions leading to fetal growth restriction. Placental perfusion has previously been assessed using ASL¹, IVIM² and power Doppler ultrasound³. However we are not aware of any reported measures of net blood flow velocity within the placenta, which is a key measure for modelling blood flow through the intervillous space.

Aim

Methods

Scanning: 7 normal women (27-36 weeks gestational age [GA]) were recruited with local ethics committee approval. They were scanned on a 3T Philips Ingenia with a torso coil at SAR<2.0W/kg, in the left or right decubitus position⁴. Two subjects where discarded due to gross motion and low signal (posterior placentas).

IVIM measurements were acquired using single-shot, respiratory-gated, pulsed gradient spin echo (PGSE), EPI: TE/TR 61ms/1.8s, diffusion weighting b=0,1,3,9,18,32,54,88,147,200,350,400,500s/mm², 5 transaxial slices, resolution 2.5x2.5x6mm³, repeated twice. The signal from high b values (b=88-500s/mm²) was smoothed before fitting for diffusion coefficient (D). Next unsmoothed signals S(b) were fitted to the IVIM model:

$$ S(b) = S_{0} [(1 - f_{IVIM})e^{-bD} + f_{IVIM}e^{-bD*}] $$

where D was fixed at the value obtained from the fit of high b values, S₀ was the signal with no diffusion weighting, f_IVIM represents the moving blood volume fraction, D* was the pseudo diffusion coefficient.

Kurtosis⁵ (K) The unsmoothed data were also fitted to:

$$ S(b) = S_{0} [(1 - f_{IVIM})e^{-bD + \frac{1}{6}b^{2}D^{2}K} + f_{IVIM}e^{-bD*}] $$

Velocity was measured in three orthogonal directions using another PGSE sequence[6] repeated twice, and was calculated from:

$$ v_{x,y,z} =\dfrac{ \Delta\phi}{\gamma G_{x,y,z} T \Delta} $$

where ΔΦ was the net phase accumulated due to the diffusion gradients, G_x,y,z=21.6mT/m was gradient amplitude, $$$Δ$$$=15.9ms was lobe length and T=31.3ms was time between lobes.

Results

Fig.2 shows velocity, f_­IVIM and kurtosis for one subject. Flow into the placenta was observed on the maternal side and was in the opposite direction on the fetal side. The maximum net flow velocity was ~0.1cm/s. Fig.1 (overlay) indicates the relationship between regions of high v_flow, f_­IVIM and kurtosis. Fast flow (green, yellow) occured at the edge of the placenta, with high f_­IVIM regions overlapping (yellow) or occuring more centrally (green). High kurtosis occurred in areas of low f_­IVIM and slow v_flow (blue, little magenta or turquoise).

Fig.3 shows velocity histograms with a normal distribution fitted to the central peak. The box-plots show velocities above and below the 95th centile of the fitted peak. Forward and backward velocities were similar, but were higher in the basal plate than the placenta (P<0.0001 Mann Whitney test, each direction separately). Fig.4 shows example histograms and box-plots of f­_IVIM. f_­IVIM was significantly higher in the uterine wall and basal plate than the placenta (p=0.004 and 0.02 paired t-test).

Fig.5 indicates repeatability comparing data from two scans collected on each subject and example decay curves from two placental ROIs.

Discussion

This work measured coherent flow velocities within the placenta for the first time, and compared it to f_IVIM and kurtosis, interpreted as blood percolating through the villous tree with large or small mean free paths respectively. Net flow near the basal plate is consistent with flow from the spiral arteries, and net negative flow on the fetal side suggests that deoxygenated blood is pooling between the villous trees. We are not sensitive to fetal flow in the villous trees at this spatial resolution.

v_flow and f_IVIM were higher in the basal plate than the placenta in all subjects (except v_flow in Sub.1’s basal plate). The overlay maps indicate spatial correspondence between areas of high v_flow and high f_IVIM. Areas with lower v_flow or f_IVIM corresponded to areas of high kurtosis probably related to slow percolation away from the spiral arteries.

Sub.1 (earlier gestational age and experiencing uterine contractions) showed slow placental flow and low basal plate f­_IVIM (data sets acquired independently) but the placental flows and f_IVIM were unchanged. The data showed good agreement between repeated measurements.

Conclusion

Acknowledgements

References

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