Nonlinear processing in B-mode ultrasound affects carotid diameter assessment.

Noninvasive diameter assessment in the common carotid artery (CCA) by means of ultrasound is a useful technique for estimation of arterial mechanical and dynamic properties, clinical screening and treatment monitoring. Before presentation on screen, ultrasound images are subjected to nonlinear processing, e.g., logarithmic compression and noise-level thresholding, to improve visualization. In addition, signal saturation may occur, either in the received radiofrequency (RF) signals or in their envelopes. The objective of this study is to evaluate the effect of signal nonlinearities on CCA diameter measurements by means of noninvasive B-mode ultrasound, comparing the performance of two different edge detectors. In 14 healthy subjects, three repeated ultrasonic acquisitions (6 s) without saturation were performed. The acquired RF signals were subjected off-line to envelope detection, logarithmic compression and various degrees of saturation applied to the signals before or after envelope detection. For the purpose of CCA diameter estimation, artery walls were automatically outlined frame by frame. As automatic edge detectors, we considered the sustain attack filter (SAF), based on exponentially decaying reference functions, and a derivative approach (DER), relying on the positions of first derivative maxima. Both methods are applied within a region-of-interest located on the CCA. No regularization of the detected wall positions by means of pre- or postprocessing is presently applied to directly relate the outcome of the edge detectors to the applied nonlinear processing. Diameter values assessed with SAF are unaffected by logarithmic compression because of the possibility to integrate the compression characteristic of the ultrasound system into the method. The estimated diameters values obtained with DER instead show differences in the order of 10% because of compression. Saturation affects DER more than SAF; DER exhibits larger intrarecording and intrasubject variations in the estimated diameter values. Therefore, SAF gives more precise and robust CCA diameter estimates than DER, and is more suited for integration in algorithms meant for vascular ultrasound image segmentation. This study demonstrates the relevant effects of nonlinearities such as saturation and logarithmic compression on the quality of noninvasive US CCA diameter measurements.