Intima-media thickness (IMT), also called intimal medial thickness, is a measurement of the thickness of tunica intima and tunica media, the innermost two layers of the wall of an artery. The measurement is usually made by external ultrasound and occasionally by internal, invasive ultrasound catheters; see Intravascular ultrasound. Measurements of the wall thickness of blood vessels can also be done using other imaging modalities.
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IMT is used to detect the presence of atherosclerotic disease in humans and, more contentiously, to track the regression, arrest or progression of atherosclerosis. Ultrasound IMT measurements were first proposed and validated in vitro by Paolo Pignoli in 1984 and later publicized in a 'most cited' article. The use of IMT as a non-invasive tool to track changes in arterial walls has increased substantially since the mid-1990s. Although IMT is predictive of future cardiovascular events, the usefulness of measuring change in IMT over time is disputed, as meta-analyses have found that change in IMT is not predictive of cardiovascular events. As such, the use of change in IMT as a surrogate endpoint measure of drug efficacy in clinical trials, or in clinical management of cardiovascular disease, is debated.
IMT is occasionally used in clinical practice, but its role is not clear. After systematically reviewing the evidence base for IMT, the United States Preventive Services Task Force found no support for its routine use in stratification of risk for people at intermediate cardiovascular risk. However, in 2003 the European Society of Hypertension–European Society of Cardiology guidelines for the management of arterial hypertension recommended the use of IMT measurements in high-risk patients to help identify target organ damage and in 2010 the American Heart Association and the American College of Cardiology advocated the use of IMT on intermediate risk patients if usual risk classification was not satisfactory.
IMT measurements in the carotid artery
Carotid IMT has been measured in numerous epidemiological and clinical studies. These have shown associations with numerous risk factors, including type 2 diabetes and impaired glucose tolerance, familial hypercholesterolemia, high-density lipoprotein cholesterol (HDL-C) and triglycerides, rheumatoid arthritis, non-alcoholic fatty liver disease, and air pollution. Since the 1990s, some small and larger scale clinical trials of lifestyle and pharmaceutical interventions have also used carotid artery IMT as a surrogate endpoint for evaluating the regression and/or progression of atherosclerotic cardiovascular disease. However the appropriateness of carotid IMT in this context is uncertain. Although carotid intima-media thickness is strongly associated with atherosclerosis, thickening of the intima-media may not always be due to atherosclerosis. Intima-medial thickening is a complex process, depending on a variety of factors, including local hemodynamics, blood pressure, shear stress and circumferential tensile stress. Changes in shear stress may adversely affect endothelial function and particle residence time, affecting the delivery and transport of potentially atherogenic particles into the arterial wall and consequent plaque formation. Blood pressure may affect IMT through blood vessel remodelling or wall hypertrophy in response to altered circumferential stress. Variations in IMT between different locations, such as the inflow side of branches, the inner curvature at bends and opposite the flow divider at bifurcations may reflect differences in local hemodynamic forces. However, an IMT greater than 0.9-1mm is highly likely to be indicative of atherosclerosis and increased risk of cardiovascular disease. Often, the measurement of the IMT is measured in three locations: in the common carotid (typically at one cm proximal to the flow divider), at the bifurcation, and in the internal carotid artery. IMT measurements of the far (deeper) wall, by ultrasound, are generally considered more reliable than measurements performed on the near (more superficial) wall; although measurement of both near and far wall IMT has also been advocated.
Ultrasound methods
By ultrasound, IMT can be measured from either outside the body, in larger arteries relatively close to the skin (e.g., carotids, brachial, radial and/or femoral arteries), and/or internally by IVUS using special catheters that use ultrasound to image blood vessels from inside out. The carotid artery is the usual site of measurement of IMT and the American Society of Echocardiography published a consensus statement on measurement of carotid IMT in 2007
Key advantages of external ultrasound methods are:
- lower cost compared with most other methods
- relative comfort and convenience for the patient being examined
- lack of need for any IV's of other body invasive methods (usually) and
- lack of any X-Ray radiation;
Ultrasound can be used repeatedly, over years, without compromising the patient's short or long term health status.
One 20 year National Institutes of Health ongoing study, called CARDIA, which began recruitment in 1985, is focusing on the efficacy of CIMT to identify subclinical cardiovascular disease at earlier, younger stages in over 5000 individuals.
Both the American Heart Association and the National Cholesterol Education Program, Third Adult Treatment Panel report, i.e. ATP III have encouraged the clinical use of CIMT, but caution that the procedure be done with attention to accuracy and reliability.
As of 2007, while IMT has increasingly become easier to measure using higher grade equipment and careful attention to image quality, most clinical carotid ultrasound software in widespread use in the United States is not designed to easily facilitate measurement of IMT and most clinical ultrasound technicians remain unfamiliar with either performing or the importance of IMT measurements. Instead, most carotid ultrasound examinations remain focused on the older concept of measuring blood velocities within the lumen as an indication of the anatomic changes that occur after disease has progressed to advanced stages of severity.
Radiographic methods
By radiographic, i.e. X-Ray, methods, after arteries have developed advanced calcified atherosclerotic plaque, IMT can also be semi-estimated by the distance between the outer edges of calcification (actually this leaves out most of the media) and the outer edges of an angiographic dye column within the artery lumen. This is a far more complex technique; it is invasive to the body due to the use of X-Ray radiation, catheters and angiographic contrast agents.
The radiographic approach can sometimes be done during angiography, however usually only when an artery segment happens to be visualized on end so that the calcification within the outer edges of plaques can be sufficiently seen.
Radiographic IMT is more often approximated using advanced CAT scanners due to the ability to use software to more slowly and carefully process the images (after the patient's scan has been completed) and then examine artery segments from whatever angle appears most appropriate. In particular some papers have recently produced that indicated that CT and US have an excellent agreement in the analysis of the IMT (in CT analysis is known as CAWT : carotid artery wall thickness)
However, one of the concerns with all CT scanners, both EBT and perhaps more so with the spiral scanners (which are more commonly used because they are less expensive to purchase), is the dose of X-Ray delivered to the patient’s body and concerns about the safety of repeated doses of X-Ray to track disease status over time.