Model-based measurements of the diameter of the internal carotid artery in CT angiography images

Purpose: Computed tomography angiography (CTA) is often used to determine the degree of stenosis in patients that suffer from carotid artery occlusive disease. Accurate and precise measurements of the diameter of the stenosed internal carotid artery are required to make decisions on treatment of the patient. However, the inherent blurring of images hampers a straightforward measurement, especially for smaller vessels. The authors propose a model-based approach to perform diameter measurements in which explicit allowance is made for the blurring of structures in the images. Three features of the authors' approach are the use of prior knowledge in the fitting of the model at the site of the stenosis, the applicability to vessels both with circular and noncircular cross-section, and the ability to deal with additional structures close to the arteries such as calcifications. Methods: Noncircular cross-sections of vessels were modeled with elliptic Fourier descriptors. When calcifications or other high-intensity structures are adjacent to the lumen, both the lumen and the high-intensity structures were modeled in order to improve the diameter estimates of the vessel. Measurements were performed in CT scans of a phantom mimicking stenosed carotids and in CTA scans of two patients with an internal carotid stenosis. In an attempt to validate the measurements in CTA images, measurements were also performed in three-dimensional rotational angiography (3DRA) images of the same patients. Results: The validity of the approach for diameter measurements of cylindrical arteries in CTA images is evident from phantom measurements. When prior knowledge about the enhancement and the blurring parameter was used, accurate and precise diameter estimates were obtained down to a diameter of 0.4 mm. The potential of the presented approach, both with respect to the extension to noncircular cross-sections and the modeling of adjacent calcifications, appears from the patient data. The accuracy of the size estimates in the patient images could not be unambiguously established because no gold standard was available and the quality of the 3DRA images was often suboptimal. Conclusions: The authors have shown that the inclusion of a priori information results in accurate and precise diameter measurements of arteries with a small diameter. Furthermore, in patient data, the assumption of a circular cross-section often appears to be too simple. The extension to noncircular cross-sections and adjacent calcifications paves the way to realistic modeling of the carotid artery.

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