IVUS catheter motion: Implications for ultrasonic speckle and correction in IVUS palpography

Intravascular Ultrasound (IVUS) palpography is a techniques that depicts the distribution of the mechanical strain over the luminal surface of coronary arteries. It utilizes conventional radiofrequency (RF) signals acquired at two different levels of a compressional load. The signals are cross-correlated to obtain the microscopic tissue displacements, which can be directly translated into local strain of the vessel wall. However, (apparent) tissue motion and nonuniform deformation of the vessel wall due to catheter jolting and rotation reduce signal correlation and result in void strain estimates. Implications of probe motion were studied on the tissue-mimicking phantom. The measured circumferential tissue displacement and level of the speckle decorrelation amounted to 12° and 0.58 for the catheter displacement of 800 μm, respectively. To compensate for the motion artifacts in IVUS palpography, a novel method, based on the feature-based scale-space Optical Flow (OF) was employed. The computed OF vector field quantifies the amount of the local tissue misalignment in consecutive frames. Subsequently, the extracted motion pattern is used to realign the signals prior to the cross-correlation analysis, reducing signal decorrelation and increasing the number of valid strain estimates. The advantage of applying the motion compensation algorithms was demonstrated in a mid-scale validation study on 14 in-vivo pullbacks. Both methods substantially increase the number of valid strain estimates in the partial and compounded palpograms. A mean relative improvement amounts to 28% and 14%, respectively. Implementation of motion compensation method increase the diagnostic value of IVUS palpography.