Motion-moderated transfer function for volume rendering 4D CMR data

Cardiovascular Magnetic Resonance (CMR) produces time-varying volume data by combining conventional MRI techniques with ECG gating. It allows physicians to inspect the dynamics of a beating heart, such as myocardium motion and blood flows. Because the material intensity changes over time in a typical CMR scan, this poses a challenging problem in specifying an effective transfer function for depicting the geometry of a beating heart or other moving objects. In this paper, we propose to moderate the traditional transfer function based on intensity and intensity gradient. This enables us to depict the exterior boundary of a beating heart in a temporally consistent manner. We examine several different ways of moderating an intensity-based transfer function, and evaluate these designs in conjunction with practical CMR data. We present a ray-casting pipeline which includes optional flow estimation and a mechanism to assist temporal coherence in animation.