Differential Intensity Projection for Visualisation and Quantification of Plaque Neovascularisation in Contrast-Enhanced Ultrasound Images of Carotid Arteries

Studies have reported that intraplaque neovascularisation (IPN) is closely correlated with plaque vulnerability. In this study, a new image processing approach, differential intensity projection (DIP), was developed to visualise and quantify IPN in contrast-enhanced non-linear ultrasound image sequences of carotid arteries. DIP used the difference between the local temporal maximum and the local temporal average signals to identify bubbles against tissue non-linear artefact and noise. The total absolute and relative areas occupied by bubbles within each plaque were calculated to quantify IPN. In vitro measurements on a laboratory phantom were made, followed by in vivo measurements in which 24 contrast-enhanced non-linear ultrasound image sequences of carotid arteries from 48 patients were selected and motion corrected. The results using DIP were compared with those obtained by maximum intensity projection (MIP) and visual assessment. The results indicated that DIP can significantly reduce non-linear propagation tissue artefacts and is much more specific in detecting bubble signals than MIP, being able to reveal microbubble signals that are buried in tissue artefacts in the corresponding MIP image. A good correlation was found between microvascular area (MVA) (r = 0.83, p < 0.001)/microvascular density (r = 0.77, p < 0.001) obtained using DIP and the corresponding expert visual grades, comparing favourably to r = 0.26 and 0.23 obtained using MIP on the same data. In conclusion, the proposed method exhibits great potential in quantification of IPN in contrast-enhanced ultrasound images of carotid arteries.