The ultrasound elastography inverse problem and the effective criteria

Actuator-assisted Subpitch Translation-capable Transducer for Elastography: Preliminary Performance Assessment

In conventional linear array (CLA)-based elastography tissue compression in one direction (e.g., axial) leads to an expansion in all other directions (lateral, elevation). Therefore, the estimation of the lateral displacements and strains may provide additional information on the tissue mechanical properties. However, these are not exploited fully due to the inherent limitation in lateral sampling. Recently, a method named actuator-assisted beam translation (ABT) was demonstrated to address this issue, wherein the focused beam was translated at subpitch locations using an external bench-top setup. However, because such bench-top setup may be impractical for routine clinical use, an ultrasound transducer was customized to have an internal actuator. The performance of the customized transducer was studied through experiments on phantoms for rotation elastography application, which requires precise lateral displacement estimation. Furthermore, the results obtained from ABT was compared against the currently practiced spatial displacement compounding (SDC) method, which is known to yield better quality lateral displacement estimates than conventional approaches. The results show that the ABT method yields a full-width half-maximum (FWHM) value, taken from the lateral profile across a point scatterer, which is 65% and 24% smaller than that obtained using CLA and SDC methods, respectively. Furthermore, the contrast-to-noise ratio (CNR) estimated from rotation elastogram obtained using ABT method is better by 300% and 35% compared with that obtained by using CLA and SDC methods, respectively. Furthermore, the results demonstrate an additional advantage of having larger field of view (FoV) for the ABT method compared with spatial compounding approach.

Diagnostic role of ultrasound elastography on lymph node metastases in patients with head and neck cancer

Introduction

Ultrasonography is the easiest non-invasive method to diagnose lymph node metastases in patients with head and neck cancer. However, since CT scans are often preferred in the evaluation of primary tumours of these patients, information about lymphatic metastases may also available in these patients. Therefore, ultrasound is not routinely employed in the evaluation of these patients. However, elastography technique, a recent development in ultrasound technology, could make use of ultrasonography in these patients even more widespread, even though it is still not widely used today.

Objectives

The aim of this study was to evaluate the role of sonographic elastography in the diagnosis of lymph node metastasis of head and neck cancer.

Methods

Twenty-three patients diagnosed with head and neck cancer and scheduled for surgical treatment including neck dissection were included in the study. All patients underwent neck examination by palpation, ultrasound elastography and computerized tomography with contrast. To compare the diagnostic performance of palpation, ultrasound elastography and computerized tomography, the findings of each examination method were compared with the histopathological examination results of neck specimens.

Results

15 (65.2%) patients had a primary tumour in the larynx; 7 (30.4%) in the oral cavity; and 1 (4.3%) in the parotid. 7 (30.4%) out of 23 patients underwent bilateral neck dissection. In total, 30 neck dissections were hereby taken into account during study. Ultrasound elastography showed higher accuracy (83.3%) and higher sensitivity (82.4%) than palpation and computerized tomography, but the specificity of ultrasound elastography was lower (84.6%) than palpation and computerized tomography.

Conclusions

Ultrasound elastography is helpful for the diagnosis of lymph node metastases in patients with head and neck cancer. Due to its non-invasive character, it can be used safety in combination with other radiological techniques to support or improve their diagnostic performance.

An Iterative Method for Estimating Nonlinear Elastic Constants of Tumor in Soft Tissue from Approximate Displacement Measurements

Objectives

Various elastography techniques have been proffered based on linear or nonlinear constitutive models with the aim of detecting and classifying pathologies in soft tissues accurately and noninvasively. Biological soft tissues demonstrate behaviors which conform to nonlinear constitutive models, in particular the hyperelastic ones. In this paper, we represent the results of our steps towards implementing ultrasound elastography to extract hyperelastic constants of a tumor inside soft tissue.

Methods

Hyperelastic parameters of the unknown tissue have been estimated by applying the iterative method founded on the relation between stress, strain, and the parameters of a hyperelastic model after (a) simulating the medium's response to a sinusoidal load and extracting the tissue displacement fields in some instants and (b) estimating the tissue displacement fields from the recorded/simulated ultrasound radio frequency signals and images using the cross correlation-based technique.

Results

Our results indicate that hyperelastic parameters of an unidentified tissue could be precisely estimated even in the conditions where there is no prior knowledge of the tissue, or the displacement fields have been approximately calculated using the data recorded by a clinical ultrasound system.

Conclusions

The accurate estimation of nonlinear elastic constants yields to the correct cognizance of pathologies in soft tissues.

Strain Ratio as a Quantification Tool in Strain Imaging

Ultrasound-based strain imaging is available in several ultrasound (US) scanners. Strain ratio (SR) can be used to quantify the strain recorded simultaneously in two different user-selected areas, ideally exposed to the same amount of stress. The aim of this study was to evaluate SR variability when assessed in an in-vitro setup with a tissue-mimicking phantom on resected tissue samples and in live tissue scanning with endoscopic applications. We performed an in vivo retrospective analysis of SR variability used for quantification of elastic contrasts in a tissue-mimicking phantom containing four homogenous inclusion in 38 resected bowel wall lesions and 48 focal pancreatic lesions. Median SR and the inter-quartile range (IQR) were calculated for all external and endoscopic ultrasound (EUS) applications. The IQR and median provide a measure of SR variability focusing on the two percentiles of the data closest to the median value. The overall SR variability was lowest in a tissue-mimicking phantom (mean QR/median SR: 0.07). In resected bowel wall lesions representing adenomas, adenocarcinomas, or Crohn lesions, the variability increased (mean IQR/Median: 0.62). During an in vivo endoscopic examination of focal pancreatic lesions, the variability increased further (mean IQR/Median: 2.04). SR variability increased when assessed for different targets with growing heterogeneity and biological variability from homogeneous media to live tissues and endoscopic application. This may indicate a limitation for the accuracy of SR evaluation in some clinical applications.

Strategies to Obtain Subpitch Precision in Lateral Motion Estimation in Ultrasound Elastography

In elastography, conventional linear array (CLA)-based RF data acquisition provides more accurate displacement measurements in the direction of beam propagation (axial direction) when compared to the perpendicular direction (lateral). Obtaining good quality lateral displacement estimates in ultrasound (US) elastography will lead to several benefits such as obtaining accurate inverse solutions, improving shear strain elastogram quality, getting good quality poroelastograms, and obtaining reliable rotation elastograms. For accomplishing high-precision lateral displacement estimation (LDE), one of the popular methods is by interpolating additional A-lines in between neighboring RF A-lines. We describe a method wherein true RF A-lines (not interpolated) are acquired and augmented at subpitch locations using CLA transducer, instead of interpolating the data, and using this new frame data for further image formation and/or processing to yield better lateral resolution and LDE. We demonstrate the proposed method by translating the US beam of CLA transducer in subpitch range by the following two approaches: 1) actuator-assisted beam translation and 2) electronic translation of subaperture of a CLA by activating odd and even number of consecutive elements sequentially, referred to as electronic beam translation. The performances of the different methods were studied through simulations and experiments on phantoms. The results demonstrate that these methods yield better quality LDE compared to those obtained from interpolation of RF A-lines. These methods may provide affordable ways to obtain subpitch precision LDE using CLA.

Adaptive mesh refinement for elastic modulus reconstruction in elastography

Meshes play a crucial role in determining the accuracy of the elastic modulus reconstruction in the elastography when the finite element method is employed. In this article, we propose an adaptive mesh refinement strategy which can ensure the coincidence of the meshes with the shape of the inclusions in the observed tissue. This strategy is based on the intensity distribution of the strain image where the variance of the strain distribution in each element of the meshes is used to measure the homogeneity of the element, that is, the larger the strain variance is the more inhomogeneous the element will be and hence more detailed information will be included in this element. For more accurate reconstruction of such detailed information, mesh refinement procedure is implemented in such elements. Besides, two refinement steps are employed for the reconstruction to improve the fitness of the reconstructed image and the observed tissue. Simulation results show that the two-stage adaptive mesh refinement algorithm performs well without needing any prior information about the internal geometric shape in tissue. Not only Young’s moduli of models but also shapes of the inclusions can be reconstructed perfectly and quickly with our proposed method.

A SENSITIVITY ANALYSIS FOR BREAST DISPLACEMENT BY VARYING MATERIAL PROPERTIES

In the quasi-static ultrasound breast elastography, the simulated displacement greatly influence the results. The breast modeling requires certain mechanical parameters for generating the simulated displacements. However, there are wide ranges of data in the existing literature for the breast mechanical parameters. Therefore, the sensitivity analysis of displacement to the changes of mechanical parameters worths a particular attention.

In this paper we construct a three-dimensional (3D) breast model using magnetic resonance images. The constitutive equations corresponding to both linear elastic and hyperelastic (nonlinear) mechanical behaviors are considered. Then, two series of finite element analysis are performed in order to investigate how the changes of mechanical parameters of the fat and fibroglandular tissues may influence the displacement. The obtained results demonstrate that the displacement are generally less sensitive in the hyperelastic modeling (to the changes of material properties) than they are in the linear elastic modeling. Furthermore, the breast deformation is more influenced by the changes of material properties of fat part than the fibroglandular part.