Quantification of Arterial Wall with MRI and Image Processing

Abstract: Quantification of atherosclerosis in vivo in animal models is of great importance when treatments against atherosclerosis are evaluated. In the studies presented in this thesis a non-invasive method combining MRI and image processing is evaluated. In the first study cholesterol-fed rabbits were examined with a clinical MRI scanner with 1,5T in field strength. By thresholding two different MRI sequences, segmentation of arterial lumen and tissues surrounding the aortic vessel wall was performed. The cholesterol-fed rabbits had significantly larger vessel wall percentage area than the healthy controls. In the second study spontaneously atherosclerotic (Watanabe) rabbits were examined with equal MRI sequences as in the first study. The images were processed with a Gradient Vector Flow snake algorithm. By applying the snake algorithm, the inner and outer boundaries of the arterial wall were identified and vessel wall area and thickness were calculated. Close agreement was obtained between MRI-snakes and histopathological measurements. Reproducibility, evaluated by comparing results from three different observers, was high. In the third study cholesterol-fed rabbits were examined after 12 weeks of diet and compared to healthy controls. MRI-snakes results were compared to histopathological area measurements of intima and media layers of the arterial wall. No significant vessel wall changes were established during this period. There was a significant correlation between the two methods when the results were compared segment-wise for each individual animal. In the fourth study 8 rabbits underwent a double injury, cholesterol diet and desendothelialization of thoracic and abdominal aorta followed by angioplasty of a 30 mm long segment in the distal part of the abdominal aorta. The abdominal aorta was examined with MRI-snakes on three different occasions: baseline, 6 weeks and 10 weeks post-PTA. A significant increase of the wall thickness in the lesioned segment compared to a proximal vessel segment was found at 6 weeks post-PTA.