Hypothesis Validation of Far-Wall Brightness in Carotid-Artery Ultrasound for Feature-Based IMT Measurement Using a Combination of Level-Set Segmentation and Registration

Intima-media thickness (IMT) is now being considered as an indicator of atherosclerosis. Our group has developed several feature-based IMT measurement algorithms such as the Completely Automated Layer EXtraction (CALEX) (which is a class of patented AtheroEdge Systems from Global Biomedical Technologies, Inc., CA, USA). These methods are based on the hypothesis that the highest pixel intensities are in the far wall of the common carotid artery (CCA) or the internal carotid artery (ICA). In this paper, we verify that this hypothesis holds true for B-mode longitudinal ultrasound (US) images of the carotid wall. This patented methodology consists of generating the composite image (the arithmetic sum of images) from the database by first registering the carotid image frames with respect to a nearly straight carotid-artery frame from the same database using: (1) B-spline-based nonrigid registration and (2) affine registration. Prior to registration, we segment the carotid-artery lumen using a level-set-based algorithm followed by morphological image processing. The binary lumen images are registered, and the transformations are applied to the original grayscale CCA images. We evaluated our technique using a database of 200 common carotid images of normal and pathologic carotids. The composite image presented the highest intensity distribution in the far wall of the CCA/ICA, validating our hypothesis. We have also demonstrated the accuracy and improvement in the IMT segmentation result with our CALEX 3.0 system. The CALEX system, when run on newly acquired US images, shows the IMT error of about 30 μm. Thus, we have shown that the CALEX algorithm is able to exploit the far-wall brightness for accurate IMT measurements.

I. Introduction

Atherosclerosis is the thickening and narrowing of the arteries due to formation of plaque on the walls of the artery. It is one of the leading causes of stroke and is the first clinical manifestation of cardiovascular disease. Recent research has been focused on determining early indicators of atherosclerosis. Intima-media thickness (IMT) is an early indicator of atherosclerosis [1] and precedes luminal narrowing due to plaque formation. Since plaque formation starts in the walls of the artery, IMT could be a better indicator than lumen area or blood velocity. Population studies have shown a strong correlation between carotid IMT and several cardiovascular risk factors [2], and IMT has also been found to be associated with the extent of atherosclerosis and end organ damage of high-risk patients [3]. B-mode ultrasound (US) is a noninvasive method to measure IMT, particularly in easily accessible arteries such as the carotid. IMT measurements using ultrasonography correlate well with histopathology and are reproducible [4].