Development of finite element analysis of magnetic resonance elastography to investigate its potential use in abdominal aortic aneurysms
View/ Open
Hollis2016 Matlab codes.zip (8.344Kb)
Hollis2016 Python codes.zip (74.53Kb)
Date
02/07/2016Author
Hollis, Lyam Mark
Metadata
Abstract
Abdominal aortic aneurysm (AAA) is a form of cardiovascular disease whereby a change
in the material properties of the vessel wall results in a localised dilation of the abdominal
aorta. The primary risk of AAAs is rupture with mortality rates close to 90%. Whilst surgical
intervention can be performed to repair AAAs, such procedures are considered high
risk. As a result, surgery is only performed upon AAAs that are considered likely to rupture.
The current method of prediction is the diameter criterion, with surgical intervention
performed if the diameter of the AAA exceeds 5.5cm. Research has demonstrated that this
is a weak method of predicting rupture and as such other methodologies are sought. One
promising method is patient specific modelling (PSM) which involves the reconstruction
of individual patient AAA geometries from imaging datasets, and finite element analysis
(FEA) to calculate the stresses acting on the AAA wall, with the peak stress typically used
as the predictor. A weakness of this methodology is the lack of patient specific material
property values defined in the simulation. A potential technique to address this limitation
is magnetic resonance elastography (MRE), an MR-based technique which utilises a phase-contrast
sequence to characterise displacements caused by shear waves induced into the
tissue by an external mechanical driver. An inversion algorithm is used to calculate local
material property values of the tissue from these displacements. The aim of this thesis was
to investigate the capability of utilising MRE to obtain material property measurements
from AAAs that could be incorporated into PSM.
To achieve this an FE method of modelling MRE was developed. The influence of modelling
parameters upon the material property measurements made using the direct inversion (DI)
algorithm was investigated, with element type and boundary conditions shown to have an
effect. The modelling technique was then utilised to demonstrate the influence that the size
of an insert had upon shear modulus measurements of that insert using DI in both 2- and
3-dimensions, and the multi-frequency dual elasto-visco algorithm (MDEV), an extension
of DI combining information from multiple frequencies. Meanwhile a comparison of the
modelling technique against an MRE scan of a phantom showed that whilst measurements
made from the two techniques were different at low frequencies, they became similar as the
frequency increased. This suggested that such differences were attributable to increased
noise in the scanned data.
FEA of MRE performed on idealised AAA geometries demonstrated that AAA size, shear
viscosity of the thrombus and shear modulus of the AAA wall all influenced the accuracy of
MRE measurements in the thrombus. Meanwhile MRE scanning of a small cohort of AAA
patients had been undertaken and phase images investigated for signs of wave propagation
to investigate the capabilities of the current MRE setup. Phase images were dominated by
noise and there was no wave propagation visualised in any of the AAAs.
This thesis demonstrates that the current MRE setup is not capable of achieving accurate
measurements of material properties of AAA for PSM. Visualisation of wave propagation
in AAAs is technically demanding and requires further development. A more fundamental
concern however is the size dependence of the inversion algorithm used and the inability
to consistently make accurate measurements from AAA geometries.
The following license files are associated with this item: