IVUS Image Processing Methodologies

Background

Apart from the regions of interest (lumen, media-adventitia and stent border) and the plaque [which depending on its echogenisity can be classified as echolucent/soft (lipid rich) plaque, intermediate (fibrous) plaque, echo dense (calcific) plaque and mixed if it includes more than one acoustic subtypes] (Mintz et al. 2001) various artefacts are often seen in IVUS images which should be taken into consideration during the segmentation process (Figure 1). Some of these depend on the type of the catheter (mechanical or solid state) used, while others can be detected in all IVUS sequences. For example, the non-uniform rotational artefacts can be seen only in mechanical IVUS imaging and are due to asymmetric friction which leads the transducer to lag during one part of the rotation and whip through the other part of its 360^o rotation resulting in geometrical distortion of the image. On the other hand the ring down artefact, the shadowing artefact, the reverberations and the blood speckles are seen in both mechanical and solid state IVUS imaging. The ring down artefact is due to signal disorganization and appears as a bright halo of variable thickness around the catheter while the shadowing artefact occurs when a structure with a marked difference in acoustic impedance (e.g. calcific plaque) blocks the transmission of the ultrasound beyond that point resulting in a shadow behind the echoreflective structure, which follows the ultrasound signal. The reverberations are artefacts caused by secondary false echoes of the same structure and are more common in strong echoreflectors such as calcium, stent or the guidewire. Finally, the blood speckles are spots that are moving with a characteristic pattern and are due to the reflection of the signal by the red blood cells (Nissen et al. 1998). From the above it is obvious that there is a variety of artefacts which renders the automated segmentation of the IVUS images a challenging process.

Figure 1.

Structures and artefacts seen in IVUS images