Lagrangian carotid strain imaging indices normalized to blood pressure for vulnerable plaque

Abstract

Objective: Ultrasound Lagrangian carotid strain imaging (LCSI) utilizes physiological deformation caused by arterial pressure variations to generate strain tensor maps of the vessel walls and plaques. LCSI has been criticized for the lack of normalization of magnitude-based strain indices to physiological stimuli, namely blood pressure. We evaluated the impact of normalization of magnitude-based strain indices to blood pressure measured immediately after the acquisition of radiofrequency (RF) data loops for LCSI.

Materials and methods: A complete clinical ultrasound examination along with RF data loops for LCSI was performed on 50 patients (30 males and 20 females) who presented with >60% carotid stenosis and were scheduled for carotid endarterectomy. Cognition was assessed using the 60-minute neuropsychological test protocol.

Results: For axial strains correlation of maximum accumulated strain indices (MASI), cognition scores were -0.46 for non-normalized and -0.45, -0.49, -0.37, and -0.48 for systolic, diastolic, pulse pressure, and mean arterial pressure normalized data, respectively. The corresponding area under the curve (AUC) values for classifiers designed using maximum likelihood estimation of a binormal distribution with a median-split of the executive function cognition scores were 0.73, 0.70, 0.71, 0.70, and 0.71, respectively.

Conclusions: No significant differences in the AUC estimates were obtained between normalized and non-normalized magnitude-based strain indices.

Keywords: carotid strain imaging; cognition; elastography; strain indices; ultrasonography.

Figure 1:

Ultrasound B-mode (a) and corresponding accumulated axial (b), lateral (c) and shear (d) strain images. Strain images are overlaid on the corresponding ultrasound B-mode images.

Figure 2:

Box-and-Whisker plots of the maximum accumulated strain indices (MASI) for axial (A-MASI), lateral (L-MASI) and shear (S-MASI) over the patients in this study.

Figure 3:

Box-and-Whisker plots of the blood pressure measurements over the patients in this study.

Figure 4:

Box-and-Whisker plots of A-MASI (a), L-MASI (b) and S-MASI (c) normalized to the blood pressure measurements over the patients in this study.

Figure 5:

Histogram of the composite cognition z-scores on executive function. Z-scores below zero indicate poor-cognition while those above zero indicate improved cognition.

Figure 6:

ROC curves for MASI versus a median-split of the cognition scores for all patients in the study. MASI denotes the un-normalized results described previously, while MASI-S, MASI-D, MASI-PP and MASI-MAP denotes normalization with the systolic, diastolic, pulse pressure and mean arterial pressure (MAP) respectively. ROC results are shown for (a) strain indices derived from axial strain (A-MASI), (b) lateral strain (L-MASI) and c) shear strain (S-MASI).