Mechanical evaluation of ovine brain tissue using rheometry and shear wave ultrasound elastography
Blackwell, James
Blackwell, James
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Publication Date
2023-08-24
Type
Thesis
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Abstract
The aim of this thesis is to measure the mechanical properties of brain matter to aid clinical decisions in tissue resection. Brain tissue has proven difficult to model in part due to its extreme softness, requiring specialised mechanical testing protocols and devices, and nonlinear constitutive modelling. First, a method for non-invasively monitoring the potential heating effects from transcranial shear wave ultrasound elastography (SWE) is proposed. This near real-time method utilised proton resonance frequency (PRF) shift thermometry and was found to be accurate to within 2.3% for energy depositions as low as 0.42 W/kg. Then, a first-of-its-kind torsion study of ex-vivo ovine brain tissue was conducted. A method to quantify the Young’s modulus of a material by measuring both the torque and the normal force is demonstrated and its limitations are discussed. A Young’s modulus of 0.7–0.9 kPa was found which agreed with previously published indentation tests. Finally, SWE of ex-vivo ovine brain tissue was conducted to measure the Young’s modulus. Results of 19–26 kPa were recorded, which are much larger than the rheometry results. This may demonstrate the strain-rate dependence of the brain response due to viscoelasticity. Existing clinical methods such as ultrasound and magnetic resonance imaging (MRI) were used for anatomical imaging. Changing the angle of the transducer appeared to change the value of the recorded stiffness value for ovine brain, but not for agar gel, which may suggest anisotropic mechanical effects, though it is noted that further tests are required to prove these results. In conclusion, this thesis demonstrates some of the different ways, both destructive and non-destructive, that brain tissue can be tested and highlights some of the shortcomings in current testing and modelling methods.
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NUI Galway