IMT predicts cardiovascular risk
Used by pharmaceutical research groups for evaluating the regression and/or progression of atherosclerotic cardiovascular disease.
IMT is strogly associated to atherosclerosis.
It is worth noting that not all the processes of thickening of the intima-media are due to atherosclerosis. Intimal thickening is in fact a complex process, depending on a variety of factors, not necessarily related to atherosclerosis. Local hemodynamics plays an important role, higher blood pressure and changes in shear stress being potential causes of intimal thickening, even differences in cholesterol homeostasis.
Often, the measurement of the IMT is measured in three tracts: in the common carotid, at one or two cms from the flow divider, at the bifurcation and in the internal carotid artery.
IMT measurements of the deep wall, by ultrasound, are generally more reliable than measurements performed on the outer wall.
Measuring the intima-media thickness in the left CCA. Gray-scale US image shows the cursors placed perpendicular to the long axis of the CCA to include only the intima and media in the thickness measurement. In this case, the distance between the cursors is 2.8 mm.
The intima-media thickness of the CCA is thought to be associated with risk factors for stroke. The bifurcation intima-media thickness and the presence of plaque are more directly associated with risk factors for ischemic heart disease (12). Intima-media thickness measurements must be obtained from a gray-scale image, not from a color Doppler image. We recommend use of higher-frequency linear transducers (>7 MHz) with compound and harmonic imaging to reduce the near field artifacts (13). Intima-media thickness measurements may be obtained at the near or far wall of the CCA, bulb, and ICA. Only the intima (echogenic layer) and the media (echo-poor layer) are included in the measurement. Increased intima-media thickness has also been reported as a physiologic effect of aging (14).
An intima-media thickness of less than 1 mm is normal.
Automated edge detection:
Automated edge detection programmes have been suggested as a better approach to reduce variability in the measurement of CIMT. Most automated edge detection programmes have been designed for the measurement of far wall CCA, whereas the near wall is not considered, nor are measurements of the CB or the ICA. The main potential advantage of automated edge detection programmes is that they may reduce variability in CIMT readings as a result of reduction in differences between readers and by the elimination of change in reading behaviour over time (reader drift). The beneficial effect of automated edge detection in reduction of measurement variability depends on the contribution of sonographers and readers on the total variability in the CIMT measurement. A number of studies have indicated that differences between sonographers have a much larger effect on CIMT measurements than differences between readers. Furthermore, a formal quantification of the extent of reduction in measurement error using either approach has not been done. In general, when ultrasound images show clear interfaces automated edge detection programmes work well, whereas when the interfaces on the ultrasound images are less clear, the automated edge detection programme needs to be manually overridden, thereby eliminating the advantages of the use of automated edge detection.
Sources:
http://en.wikipedia.org/wiki/Intima-media_thickness
http://radiographics.rsna.org/content/25/6/1561.full?sid=f466f5a0-462f-4a47-b771-12c80e7de335
http://radiographics.rsna.org/content/25/5/1141.full?sid=f466f5a0-462f-4a47-b771-12c80e7de335
http://www.cardiovascularultrasound.com/content/9/1/32
