Intravascular ultrasound (IVUS) provides direct depiction of coronary anatomy, including the degree and extent of coronary plaque, useful in quantitative research or clinical studies. These studies require fast and accurate analysis of coronary morphometry in volumetric IVUS images. Semi-automated interaction techniques are important for this task due to limitations of automated processing. We present B-spline-based surface fitting and manipulation methods that provide a foundation for such interaction techniques. They can be integrated easily with previously developed segmentation algorithms to provide a semi-automated segmentation system for identification of luminal and vessel borders in volumetric IVUS images.