We previously developed a 2D locally regularized strain estimation technique that was already validated with ex vivo
tissues. In this study, our technique is assessed with in vivo data, by examining breast abnormalities in clinical
conditions. Method reliability is analyzed as well as tissue strain fields according to the benign or malignant character of
the lesion. Ultrasound RF data were acquired in two centers on ten lesions, five being classified as fibroadenomas, the
other five being classified as malignant tumors, mainly ductal carcinomas from grades I to III. The estimation procedure
we developed involves maximizing a similarity criterion (the normalized correlation coefficient or NCC) between pre- and
post-compression images, the deformation effects being considered. The probability of correct strain estimation is
higher if this coefficient is closer to 1. Results demonstrated the ability of our technique to provide good-quality strain
images with clinical data. For all lesions, movies of tissue strain during compression were obtained, with strains that can
reach 15%. The NCC averaged over each movie was computed, leading for the ten cases to a mean value of 0.93, a
minimum value of 0.87 and a maximum value of 0.98. These high NCC values confirm the reliability of the strain
estimation. Moreover, lesions were clearly identified for the ten cases investigated. Finally, we have observed with
malignant lesions that compared to ultrasound data, strain images can put in relief a more important lesion size, and can
help in evaluating the lesion invasive character.
Strain imaging is useful for visualizing information related to tissue stiffness. However, strain is a parameter that depends on the boundary conditions, tissue connectivity and geometry. As a result, tissue hardness cannot be quantitatively evaluated from the strain distribution. Therefore, reconstruction of the elastic modulus (Young's Modulus) distribution has been investigated for quantitative evaluation of tissue hardness. A method has been recently proposed [NITT 00] to calculate locally the Young's modulus of tissues from the estimations of 3D displacement field within the medium. This approach requires a specific annular ultrasonic probe. The aim of our work, based on Nitta's approach, is to build a Young modulus mapping using clinical ultrasonic equipment. Results from finite-element simulations and a physical phantom are presented.
Segmentation of deformable structures remains a challenging task in ultrasound imaging especially in low signal-to-noise ratio applications. In this paper a fully automatic method, dedicated to the luminal contour segmentation in intracoronary ultrasound imaging is introduced. The method is based on an active contour with a priori properties that evolves according to the statistics of the ultrasound texture brightness, determined as being mainly Rayleigh distributed. However, contrary to classical snake-based algorithms, the presented technique neither requires from the user the pre-selection of a region of interest tight around the boundary, nor parameter tuning. This fully automatic character is achieved by an initial contour that is not set, but estimated and thus adapted to each image. Its estimation combines two statistical criteria extracted from the a posteriori probability, function of the contour position. These criteria are the location of the function maximum (or maximum a posteriori estimator) and the first zero-crossing of the function derivative. Then starting form the initial contour, a region of interest is automatically selected and the process iterated until the contour evolution can be ignored. In vivo coronary images from 15 patients, acquired with a 20 MHz central frequency Jomed Invision ultrasound scanner were segmented with the developed method. Automatic contours were compared to those manually drawn by two physician in terms of mean absolute difference. Results demonstrate that the error between automatic contours and the average of manual ones (0.099±0.032mm) and the inter-expert error (0.097±0.027mm) are similar and of small amplitude.
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