Quantitative validation of two model-based methods for the correction of probe pressure deformation in ultrasound.

PURPOSE: The acquisition of good quality ultrasound (US) images requires good acoustic coupling between the ultrasound probe and the patient's skin. In practice, this good coupling is achieved by the operator applying a force to the skin through the probe. This force induces a deformation of the tissues underlying the probe. The distorted images deteriorate the quality of the reconstructed 3D US image. METHODS: In this work, we propose two methods to correct these deformations. These methods are based on the construction of a biomechanical model to predict the mechanical behavior of the imaged soft tissues. The originality of the methods is that they do not use external information (force or position value from sensors, or elasticity value from the literature). The model is parameterized thanks to the information contained in the image. This is allowed thanks to the optimization of two key parameters for the model which are the indentation d and the elasticity ratio α. RESULTS: The validation is performed on real images acquired on a gelatin-based phantom using an ultrasound probe inducing an increasing vertical indentation using a step motor. The results showed a good correction of the two methods for indentations less than 4 mm. For larger indentations, one of the two methods (guided by the similarity score) provides a better quality of correction, presenting a Euclidean distance between the contours of the reference image and the corrected image of 0.71 mm. CONCLUSION: The proposed methods ensured the correction of the deformed images induced by a linear probe pressure without using any information coming from sensors (force or position), or generic information about the mechanical parameters. The corrected images can be used to obtain a corrected 3D US image.

Predictive Model for Designing Soft-Tissue Mimicking Ultrasound Phantoms With Adjustable Elasticity.

The use of mechanically representative phantoms is important for experimental validation in ultrasound (US) imaging, elastography, and image registration. This article proposes a model to predict the elastic modulus of a soft tissue-mimicking phantom based on two very easily controllable parameters: gelatin concentration and refrigeration duration. The model has been validated on small- and large-scale phantoms; it provides a good prediction of the elastic modulus in both cases (error < 16.2%). The tissue-mimicking phantom is made following a low-cost and simple fabrication procedure using commercial household gelatin with psyllium hydrophilic mucilloid fiber to obtain echogenicity. A large range of elastic properties was obtained (15-100kPa) by adjusting the gelatin concentration between 5% and 20% (g/mL) and the refrigeration time of the sample between 2 and 168 h, allowing to mimic normal and pathological human soft tissues. The phantom's acoustic properties (velocity, attenuation, and acoustic impedance) are also assessed using the American Institute of Ultrasound in Medicine (AIUM) standard.