Characteristics of sonographic modalities used for imaging cerebrovascular circulation

Study	Location	Frequency	Common Usage
Transcranial Doppler	Extracranial (extravascular)	Multiples of 2 MHz	Measures velocity of blood through intracranial vessels
Carotid duplex ultrasonography	Extracranial (extravascular)	5–10 MHz	Duplex procedure: brightness mode (B-mode) allows imaging and Doppler imaging enables color-flow volume and directional information; spectral analysis measures blood flow velocity; measures intimal medial thickness and evaluates stenosis
Intravascular ultrasound	Intracranial, extracranial (intravascular)	20–40 MHz	Determination of morphology and composition of atherosclerotic plaque within the extracranial and intracranial circulation; allows evaluation of correct stent diameter and the amount of balloon inflation pressure needed during angioplasty; enables visualization of stent apposition and expansion, mural thrombus, plaque ulceration, and aneurysm and vessel dissection