When it is done/advised ?
There are two types of carotid artery stenosis:

• Symptomatic carotid artery stenosis:
  This refers to narrowing of the carotid artery that is brought to medical attention by occurrence of transient ischemic attacks or previous strokes.
  
• Asymptomatic stenosis:
  This refers to narrowing of the carotid artery that is discovered incidentally without any obvious symptoms. For e.g. it is found during a routine physical examination, when the physician hears a sound with the stethoscope placed on the neck vessels, which is caused by blood flowing past the narrowing.