We used to measure it for every neurovascular patient, but then reduced our routine to judging intensity of atherosclerosis in the common carotid artery as a vague hint of predicting coronary or general atherosclerosis.
Reasons for this are manifold:
- The interrater reliability is bad – even if the location (say 1 cm below the bifurcation) or the insonation angle is fixed (which is hard to standardize – thing of thick necks)
- The reliability is bad from systole to diastole, from hour to hour and due to circadian changes in vessel diameter
- The prognostic validity is bad. It used to be better when studies were done monocentrically, often with one ultrasound specialist. But nowadays 20.000 pts are included in therapeutic studies… More recent metanalyses such as this and this lead one to doubt the method.
- Insonate any CCA longitudinally where it runs absolutely parallel to the skin.
- Choose the medial wall where the IMT seems largest without being elevated to a plaque
- Identify the angle where the vessel diameter is biggest, trying not to hit the carotid tangentially
- Stop the picture during diastole (the least diameter of the vessel, the biggest IMT)
- Zoom in until the two lines fill the screen at least to one third
- Mark the exact lines where the grey changes to white and measure the distance
Don’t bother with measuring multiple times or from multiple angles or at varying locations – the validity of the measurement is not good enough to justify the effort.
If you want to know whether someone has atherosclerosis, look for plaques and stenoses. If you find nothing and the patient has few risk factors then the IMT makes sense – this is primary care and prophylaxis more than hospitalists’ problems.