Intravascular ultrasound image subtraction: a contrast enhancing technique to facilitate automatic three-dimensional visualization of the arterial lumen.

At 30 MHz, the intravascular ultrasound backscatter of blood confounds the discrimination of the lumen from the arterial wall. This study validates a subtraction method which creates a still-frame image with a sharp demarcation of the lumen. The method involves subtraction of consecutive images and 2D ensemble averaging of the absolute pixel values. Subtraction exploits the dynamic properties of flowing red blood cells. Three phantom arteries were used, with erythrocytes in their lumens and wall. For this reason, it was not possible, in one single original image, to discriminate the blood in the lumen from the phantom wall. Based on 26 consecutive original images, in the mean subtraction image contrast between lumen and phantom wall grey values increased eightfold from 10.9 (5.3-19.2) (mean and range) in the original image to 87.7 (73.6-107.0) (P < 0.001). A sufficiently large contrast increase to allow automatic segmentation was obtained by using five original images (0.3-s acquisition time) for any single mean subtraction image. Low blood flow velocities (down to 0.5 cm/s) did not alter this result. Automatic segmentation of the lumen allowed fast 3D reconstruction of the lumen in all three phantom arteries. In phantom arteries, the intravascular ultrasound image subtraction technique improved contrast between lumen and wall which enabled automated lumen segmentation and fast 3D visualization of both the lumen and defects in the wall.