The contribution of tissue removal to lumen improvement after directional coronary atherectomy remains controversial. The purpose of this study was to validate the intravascular ultrasound measurement of plaque volume and use it to study the contribution of tissue removal to lumen improvement after directional coronary atherectomy. With use of intravascular ultrasound, 12 human coronary vessels were imaged in vitro. With use of computer-assisted planimetry, the external elastic membrane and lumen cross-sectional areas were manually traced and the plaque+media area was calculated at 1 mm axial intervals. Then, plaque+media volume was calculated by Simpson's rule. After imaging, ultrasound measurements of plaque+media volume were compared with histologic measurements. Similarly, volumetric intravascular ultrasound imaging was performed before and after directional atherectomy in 47 patients. In vitro, the mean plaque+media volume measured by intravascular ultrasound was 134.0 +/- 94.8 mm3 and compared well with that derived by histology (187.4 +/- 128.8 mm3, r = 0.96, p < 0.001). In vivo, the lumen volume increased from 27.2 +/- 12.3 to 58.7 +/- 30.3 mm3, and the mean plaque+media volume decreased from 122.0 +/- 74.0 to 97.5 +/- 63.5 mm3. The mean intravascular ultrasound atherectomy index was 76 +/- 23%. In 11 of the 47 patients (23.4%), tissue removal alone accounted for lumen improvement. Volumetric intravascular ultrasound image analysis indicates that the mechanism of directional coronary atherectomy primarily is tissue removal. As a result, the contribution of arterial remodeling (expansion and dissection) probably is less important.