Investigating Placental Haemodynamics Using MRI

Hutchinson, George (2023) Investigating Placental Haemodynamics Using MRI. PhD thesis, University of Nottingham.

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Magnetic Resonance Imaging (MRI) has been proven to be a safe and powerful tool for investigating the small scale motion of blood as it moves throughout the human placenta in utero, using techniques such as Diffusion Weighted Imaging (DWI) which have been proven to be sensitive to placental haemodynamics and changes in placental insufficiency. However due to the complexity of the human placenta, and the sensitivity of DWI to flow effects interpreting the results of DWI in utero is challenging, as within a single voxel many haemodynamic features can combine, such as blood percolating across the intervillous space, turbulent blood supplied by the spiral arteries and the fetal circulation within the villous tree network. The direct effect of these processes combining is not obvious, and while DWI has been shown to be sensitive to placental dysfunction it is not always clear how changes in signal reflect changes in structure.

This thesis investigates the placental DWI response further, initially through the use of an Inverse Laplace Transform (ILT) based approach to fit the data and provide more understanding of the slow diffusive, and faster ’pseudodiffusive’ components, where I found multiple components, which were altered in placental dysfunction. I then fitted the same data using a more popular Intra-Voxel Incoherent Motion IVIM model, and made the first comparisons between data fit with an ILT based approach, and an IVIM model, to provide more context to what IVIM parameters are measuring within the placental DWI data. While I found that the ILT provided more information about the faster incoherent motion of blood, the IVIM model still provides similar information through easier to fit and interpret parameter maps.

This thesis outlines the first implementation of Phase Contrast Angiography (PCA) for measurement of the velocity of blood within the placenta, which was used to provide better understanding of placental haemodynamics. Importantly I found that the speed of blood in placentas with insufficiency (namely Pre Eclampsia (PE) and Fetal Growth Restriction (FGR)) was significantly faster than in those without. These measurements were also used to provide more context to DWI measures as the bulk flow of blood throughout the organ is also important for ensuring proper development and function of the placenta. This thesis also outlines the first measurements of the poorly understood venous drainage system using MRI, which has seen little attention but must also be important for sustaining the placental environment.

The presence of ’rebounds’ in placental Pulse Gradient Spin Echo (PGSE) data, where voxels which begin decaying (as in normal DWI) begin to regain a proportion of the initial signal intensity. Rebounds have theoretically been shown to be possible but have yet to be reported in experimental data, however I believe the rebounds presented here are not of the same form as those previously described, and instead were due to flow effects within the placenta. Here I introduced a method for identifying these voxels, and providing quantitative information about the underlying flow.

Finally, as all the results presented here are expected to depend on the exact properties of the PGSE sequence used, I investigated how the response of the placental DWI results presented here could vary with diffusion time (∆). This work could only be carried out in an imaging phantom due to the COVID-19 pandemic, and while it showed promise and outlined a method to combine diffusion encoded, and flow encoded measurements into a single set of imaging gradients to better investigate the placental environment, the work needs to be repeated in utero.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Gowland, Penny A
Keywords: MRI, Magnetic Resonance Imaging, Placenta, DWI, Diffusion Weighted Imaging, PCA, Phase Contrast Angiography, IVIM, Intra-Voxel Incoherent Motion, haemodynamics, hemodynamics
Subjects: Q Science > QP Physiology > QP1 Physiology (General) including influence of the environment
R Medicine > RC Internal medicine
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 72460
Depositing User: Hutchinson, George
Date Deposited: 26 Jul 2023 04:40
Last Modified: 26 Jul 2023 04:40

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