Intravascular Ultrasound - Method

Method

To visualize an artery or vein, angiographic techniques are used and the physician positions the tip of a guidewire, usually 0.36 mm (0.014") diameter with a very soft and pliable tip and about 200 cm long. The physician steers the guidewire from outside the body, through angiography catheters and into the blood vessel branch to be imaged.

The ultrasound catheter tip is slid in over the guidewire and positioned, using angiography techniques so that the tip is at the farthest away position to be imaged. The sound waves are emitted from the catheter tip, are usually in the 20-40 MHz range, and the catheter also receives and conducts the return echo information out to the external computerized ultrasound equipment which constructs and displays a real time ultrasound image of a thin section of the blood vessel currently surrounding the catheter tip, usually displayed at 30 frames/second image.

The guide wire is kept stationary and the ultrasound catheter tip is slid backwards, usually under motorized control at a pullback speed of 0.5 mm/s. (The motorized pullback tends to be smoother than hand movement by the physician.)

The (a) blood vessel wall inner lining, (b) atheromatous disease within the wall and (c) connective tissues covering the outer surface of the blood vessel are echogenic, i.e. they return echoes making them visible on the ultrasound display.

By contrast, the blood itself and the healthy muscular tissue portion of the blood vessel wall is relatively echolucent, just black circular spaces, in the images.

Heavy calcium deposits in the blood vessel wall both heavily reflect sound, i.e. are very echogenic, but are also distinguishable by shadowing. Heavy calcification blocks sound transmission beyond and so, in the echo images, are seen as both very bright areas but with black shadows behind (from the vantage point of the catheter tip emitting the ultrasound waves).