Abstract
The purpose of ultrasound elastography is to identify lesions by reconstructing the hardness characteristics of tissue reconstructed from ultrasound data. Conventional quasi-static ultrasound elastography is easily applied to obtain axial strain components along the compression direction, with the results inverted to represent the distribution of tissue hardness under the assumption of constant internal stresses. However, previous works of quasi-static ultrasound elastography have found it difficult to obtain the lateral and shear strain components, due to the poor lateral resolution of conventional ultrasound probes. The physical nature of the strain field is a continuous vector field, which should be fully described by the axial, lateral, and shear strain components, and the clinical value of lateral and shear strain components of deformed tissue is gradually being recognized by both engineers and clinicians. Therefore, a biomechanical-model-constrained filtering framework is proposed here for recovering a full displacement field at a high spatial resolution from the noisy ultrasound data. In our implementation, after the biomechanical model constraint is integrated into the state-space equation, both the axial and lateral displacement components can be recovered at a high spatial resolution from the noisy displacement measurements using a robust filter, which only requires knowledge of the worst-case noise levels in the measurements. All of the strain components can then be calculated by applying a gradient operator to the recovered displacement field. Numerical experiments on synthetic data demonstrated the robustness and effectiveness of our approach, and experiments on phantom data and in-vivo clinical data also produced satisfying results.
Highlights
The routine clinical practice of palpation represents a qualitative assessment of tissue stiffness based on the significant difference in elastic properties between normal and diseased tissues [1,2]
Among the various elastographic techniques [22,23,24], quasi-static ultrasound elastography is popular, whose basic steps are as follows: (1) a set of radio-frequency (RF) signals is collected from the specimen in its undeformed state; (2) the specimen is compressed by external loading, which can be assumed to be quasi-static, and another set of RF signals is recorded; (3) motion -tracking techniques, such as widely used cross-correlation techniques, are applied to estimate the displacement field between the two sets of RF signals recorded in the previous two steps; and (4) so called elastograms are reconstructed/computed from the displacement field
A modified phase-shift estimation (PSE) method that we proposed previous ly [42] is first used to compute the axial displacement from RF signals, and an H? filtering algorithm [51] is applied to generate statistically optimal estimates via the assimilation of measurements with a meaningful biomechanical model constraint
Summary
The routine clinical practice of palpation represents a qualitative assessment of tissue stiffness based on the significant difference in elastic properties between normal and diseased tissues [1,2]. Some proposed reconstruction algorithms [22,23,24,30] require knowledge of lateral displacements, while the robustness of many reconstruction methods could be affected by displacement measurements with a poor signal-to-noise ratio (SNR) [24,29,33] For both the calculation of strain images and the reconstruction of elastic parameters, accurate estimation of tissue displacements is the first important step that will critically affect the image quality. With the assumption of a constant Poisson’s ratio (0:49), i.e. based on the biomechanical constraint of tissue incompressibility [50], lateral displacements were recovered from axial-strain measurements using the least-square technique. The third section discusses the experimental results and the fourth section draws the conclusion s from this study
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