Optimal imaging of the carotid artery for the purpose of IMT measurement are obtained in longitudinal plane, with the patient in the supine position and the head turned slightly to the contralateral side (Fig. 10.2).

High frequency ultrasound probes of 10–12 MHz or greater are required to produce images of sufficient resolution to allow for accurate and reproducible quantification of IMT.

The distal common carotid artery is regarded to be most suitable for accurate and reproducible measurement of carotid intima-media thickness [7].

Other segments of the carotid artery to include the internal carotid artery and bulb segment have been also studied. These sites are more commonly affected by focal plaque, but are more influenced by measurement variability due to arterial angulation.

The double-line sign refers to a relatively echodense intimal-luminal interface which overlies a more echolucent medial-adventitial interface, which together comprise the ultrasonographic appearance of the carotid intima-media thickness (Fig. 10.2).

Digital images are recorded in a cine-loop manner and transferred to a workstation where commercially available edge-detection software is used for CIMT quantification (Fig. 10.3):

Measurement of the far-wall of the common carotid artery (furthest from the ultrasound probe) is preferred over near-wall measurement, which has been shown to be subject to more variation in gain settings, signal loss from acoustic shadowing from the relatively echo-bright adventitia, and limitations involving axial resolution. Far-wall CIMT determination by B-mode ultrasound has been shown to more accurately correlate with true arterial wall thickness in several studies [8–10].

Sonographer experience and technical aspects can significantly affect reproducibility and accurateness of data collection. Figures 10.4, 10.5 and 10.6 illustrate some commonly encountered pitfalls and technical obstacles in image acquisition.

Methods to quantitate the degree of ultrasound “brightness” are available either through ­gray-scale quantification or through the quantification of the ultrasonic back-scatter characteristics. The use of back-scatter is less dependent on overall image gain and insonation angle but is more technically demanding. Histologic data suggest that echo-bright carotid atherosclerosis contains increased fibrous elements, whereas echolucent carotid atherosclerosis has increased lipid content.

3-dimensional, tomographic techniques are under development for volumetric quantification of carotid atherosclerosis.

CIMT values range within a healthy population according to age, gender, and race, and normal values for intima-media thickness have been defined based on this distribution [1, 11].

Multiple large population studies have demonstrated that intima-media thickness:

Normal CIMT is often considered to be the ≤75th percentile of CIMT distribution, a value which generally corresponds to an absolute value of CIMT of <1.0 mm (Fig. 10.3).

Focal plaque has been defined as a focal increase in CIMT  >  1.5 times that of surrounding CIMT, or alternatively as areas of CIMT  >  1.2 mm (Fig. 10.7).