Last modified: 2017-05-13
Abstract
Patient-specific vessel material properties are in general lacking in image-based computational models, limiting the accuracy of their stress/strain calculations. A noninvasive approach of combining in vivo 3D multi-contrast and Cine magnetic resonance imaging (MRI) and computational modeling was introduced to quantify patient-specific carotid plaque material properties for potential plaque model improvements. Carotid plaque stress and strain conditions with in vivo and old material model were investigated. A computational plaque stress index (CPVI) was proposed to combine mechanical analysis, plaque morphology and compositions for more complete carotid plaque vulnerability assessment.
In vivo 3D multi-contrast and Cine MRI carotid plaque data were acquired from 8 patients with follow-up (18 months) with written informed consent obtained. 3D thin-layer model and an established iterative procedure were used to determine parameter values in the Mooney-Rivlin models for the 81slices from 16 plaque samples. Effective Young’s Modulus (YM) values were calculated for comparison and analysis.
The average YM, circumferential shrink (C-Shrink) and lumen circumference variation measure by Cine MRI of the 81 slices was 411 kPa, 5.62%, and 8.91%, respectively. Average YM values by vessel varied from 109 kPa (softest) to 922 kPa (stiffest), a 746% difference. The average absolute variation of average stress values from 16 carotid plaques were 16.42% between in vivo material model and old material model, where 8 cases had in vivo material model stress greater than old material model stress and 8 cases had old material model stress greater than in vivo material model stress. The range of absolute variation values was [0.29%, 30.98%]. The average absolute variation of average strain values from 16 carotid plaques were 71.99% between in vivo material model and old material model, where 9 cases had in vivo material model strain greater than old material model strain and 7 cases had old material model strain greater than in vivo material model strain. The range of absolute variation values was [2.82%, 377.34%]. YM values showed positive correlation with Max stress (r=0.3531, p=0.1797) and critical stress (r=0.5733, p=0.0202). YM values also showed strong negative correlation with Max strain (r=-0.8246, p<0.0001) and critical strain (r=-0.7376, p=0.0011). The five intervals (unit: kPa) [0, 46.8), [46.8, 80), [80, 92), [92, 103), and [103, +∞) from in vivo material models were used for CPVI values of 0, 1, 2, 3 and 4, respectively. The optimized agreement rate was 85.19%.
In vivo carotid vessel material properties have large variations from patient to patient. The use of patient-specific material properties in plaque models could potentially improve the accuracy of model stress/strain calculations. Large-scale studies are needed to further demonstrate that CPVI has the potential to improve the current image-based screening and plaque vulnerability assessment schemes.