Intima-media thickness

Intima-media thickness (IMT), also called intimal medial thickness, is a measurement of the thickness of artery walls, usually by external ultrasound, occasionally by internal, invasive ultrasound catheters, see IVUS, to both detect the presence and to track the progression of atherosclerotic disease in humans.

IMT has increasingly been used in medical research since the mid 1990's to track changes in arterial walls and is occasionally in clinical medicine by more progressive clinicians. Historically, since the 1950's, focus was initially placed on detection and progression of the atherosclerotic process by its late affects on the lumens of arterial blood vessels, either narrowing or enlargement. This led to the still widely held beliefs that if the lumen looked OK, then little to no atherosclerotic disease was presumed to be present.

However, the atherosclerosis process occurs within the walls of blood vessels, not the lumen. Starting primarily in the 1980s, especially with improvements in both CAT scanner, see EBT, and ultrasound technology, see IVUS, plus better understanding of the atherosclerotic process from both basic science and clinical research efforts, attention started slowly and increasingly shifting to detecting and tracking arterial disease at earlier stages, well before changes to the lumen of the artery either occur or become detectable by any technology.

Ultrasound Methods
Since the 1990's, both small clinical and several larger scale pharmaceutical trials have used CIMT (carotid IMT) as a surrogate endpoint for evaluating the regression and/or progression of atherosclerotic cardiovascular disease

By ultrasound, IMT can be measured from either outside the body, in larger arteries which are relatively close to the skin (e.g. carotids, brachial, radial and/or femoral arteries), and/or internally by IVUS using special catheters which use ultrasound to look at blood vessels from inside out.

Key advantages of external ultrasound methods are: a. lower cost compared with most other methods b. relative comfort and convenience for the patient being examined c. lack of need for any IV’s of other body invasive methods (uaually) and d. 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 Intitutes 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 carefull 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 untrasound technicians remain unfamiliar with either perfoming 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 which 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.

However, one of the concerns with all CAT 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.