An autocorrelation-based method for improvement of sub-pixel displacement estimation in ultrasound strain imaging

An Optimization Approach for Creating Application-specific Ultrasound Speckle Tracking Algorithms

OBJECTIVE

Ultrasound speckle tracking enables in vivo measurement of soft tissue deformation or strain, providing a non-invasive diagnostic tool to quantify tissue health. However, adoption into new fields is challenging since algorithms need to be tuned with gold-standard reference data that are expensive or impractical to acquire. Here, we present a novel optimization approach that only requires repeated measurements, which can be acquired for new applications where reference data might not be readily available or difficult to get hold of.

METHODS

Soft tissue motion was captured using ultrasound for the medial collateral ligament (MCL) of three quasi-statically loaded porcine stifle joints, and medial ligamentous structures of a dynamically loaded human cadaveric knee joint. Using a training subset, custom speckle tracking algorithms were created for the porcine and human ligaments using surrogate optimization, which aimed to maximize repeatability by minimizing the normalized standard deviation of calculated strain maps for repeat measurements. An unseen test subset was then used to validate the tuned algorithms by comparing the ultrasound strains to digital image correlation (DIC) surface strains (porcine specimens) and length change values of the optically tracked ligament attachments (human specimens).

RESULTS

After 1500 iterations, the optimization routine based on the porcine and human training data converged to similar values of normalized standard deviations of repeat strain maps (porcine: 0.19, human: 0.26). Ultrasound strains calculated for the independent test sets using the tuned algorithms closely matched the DIC measurements for the porcine quasi-static measurements (R > 0.99, RMSE < 0.59%) and the length change between the tracked ligament attachments for the dynamic human dataset (RMSE < 6.28%). Furthermore, strains in the medial ligamentous structures of the human specimen during flexion showed a strong correlation with anterior/posterior position on the ligaments (R > 0.91).

CONCLUSION

Adjusting ultrasound speckle tracking algorithms using an optimization routine based on repeatability led to robust and reliable results with low RMSE for the medial ligamentous structures of the knee. This tool may be equally beneficial in other soft-tissue displacement or strain measurement applications and can assist in the development of novel ultrasonic diagnostic tools to assess soft tissue biomechanics.

3-D Strain Imaging of the Carotid Bifurcation: Methods and in-Human Feasibility

Atherosclerotic plaque development in the carotid artery bifurcation elevates the risk for stroke, which is often initiated by plaque rupture. The risk-to-rupture of a plaque is related to its composition. Two-dimensional non-invasive carotid elastography studies have found a correlation between wall strain and plaque composition. This study introduces a technique to perform non-invasive volumetric elastography in vivo. Three-dimensional ultrasound data of carotid artery bifurcations were acquired in four asymptomatic individuals using an electrocardiogram-triggered multislice acquisition device that scanned over a length of 35 mm (350 slices) using a linear transducer (L11-3, f_c = 9 MHz). For each slice, three-angle ultrasound plane wave data were acquired and beamformed. A correction for breathing-induced motion was applied to spatially align the slices, enabling 3-D cross-correlation-based compound displacement, distensibility and strain estimation. Distensibility values matched with previously published values, while the corresponding volumetric principal strain maps revealed locally elevated compressive and tensile strains. This study presents for the first time 3-D elastography of carotid arteries in vivo.

The challenges of measuring in vivo knee collateral ligament strains using ultrasound

Ultrasound-based methods have shown promise in their ability to characterize non-uniform deformations in large energy-storing tendons such as the Achilles and patellar tendons, yet applications to other areas of the body have been largely unexplored. The noninvasive quantification of collateral ligament strain could provide an important clinical metric of knee frontal plane stability, which is relevant in ligament injury and for measuring outcomes following total knee arthroplasty. In this pilot cadaveric experiment, we investigated the possibility of measuring collateral ligament strain with our previously validated speckle-tracking approach, but encountered a number of challenges during both data acquisition and processing. Given the clinical interest in this type of tool, and the fact that this is a developing area of research, the goal of this article is to transparently describe this pilot study, both in terms of methods and results, while also identifying specific challenges to this work and areas for future study. Some challenges faced relate generally to speckle-tracking of soft tissues (e.g. the limitations of using a 2D imaging modality to characterize 3D motion), while others are specific to this application (e.g. the small size and complex anatomy of the collateral ligaments). This work illustrates a clear need for additional studies, particularly relating to the collection of ground-truth data and more thorough validation work. These steps will be critical prior to the translation of ultrasound-based measures of collateral ligament strains into the clinic.

Recovering vector displacement estimates in quasistatic elastography using sparse relaxation of the momentum equation

We consider the problem of estimating the 2D vector displacement field in a heterogeneous elastic solid deforming under plane stress conditions. The problem is motivated by applications in quasistatic elastography. From precise and accurate measurements of one component of the 2D vector displacement field and very limited information of the second component, the method reconstructs the second component quite accurately. No a priori knowledge of the heterogeneous distribution of material properties is required. This method relies on using a special form of the momentum equations to filter ultrasound displacement measurements to produce more precise estimates. We verify the method with applications to simulated displacement data. We validate the method with applications to displacement data measured from a tissue mimicking phantom, and in-vivo data; significant improvements are noticed in the filtered displacements recovered from all the tests. In verification studies, error in lateral displacement estimates decreased from about 50% to about 2%, and strain error decreased from more than 250% to below 2%.

An insight into elasticity analysis of common carotid artery using ultrasonography

Elastance is a distinguished marker in diagnosing various arterial diseases as studies have reported carotid artery–related diseases linked with stiffness index (β) values greater than 5. This study was to estimate elasticity of common carotid artery by measuring the diameter during systolic and diastolic phases using pixel tracing of successive frames and blood pressure. The B-mode ultrasonography video containing arterial wall motion was captured and fragmented into image frames. Each pixel on the greyscale image was converted into RGB intensity values. The diameter of the artery as well as the thickness of the wall was measured by tracing the pixel displacements from successive frames during arterial pulsation. The study was conducted on 19 subjects aged 25–40 years. The systolic and diastolic carotid artery lumen diameters and carotid intima-media thickness were calculated as 7.1 ± 0.7, 6.3 ± 0.6 and 0.5 ± 0.05 mm (mean ± standard deviation), respectively. The mean stiffness index (β), Peterson’s modulus and Young’s modulus of elasticity were 5.2 ± 1.1, 69 ± 15 kPa and 453 ± 99 kPa, respectively. The pixel displacements in tunica intima, tunica media and tunica adventitia were not homogeneous, due to varied macro-constituents such as endothelial tissues, smooth muscle cells, elastin lamina, fibrous tissue and micro-constituents such as collagen, fibroblast and elastin. We found that women have smaller arteries, and the stiffness increased during the systolic phase.

Strain at the internal cervical os assessed with quasi-static elastography is associated with the risk of spontaneous preterm delivery at ≤34 weeks of gestation

AIM

To evaluate the association between cervical strain assessed with quasi-static elastography and spontaneous preterm delivery.

METHODS

Quasi-static elastography was used to estimate cervical strain in 545 pregnant women with singleton pregnancies from 11 weeks to 28 weeks of gestation. Cervical strain was evaluated in one sagittal plane and in the cross-sectional planes of the internal cervical os and external cervical os. The distribution of strain values was categorized into quartiles for each studied region and their association with spontaneous preterm delivery at ≤34 weeks and at <37 weeks of gestation was evaluated using logistic regression.

RESULTS

The prevalence of spontaneous preterm delivery at <37 weeks of gestation was 8.2% (n=45), and that at ≤34 weeks of gestation was 3.8% (n=21). Strain in the internal cervical os was the only elastography value associated with spontaneous preterm delivery. Women with strain values in the 3rd and 4th quartiles had a significantly higher risk of spontaneous preterm delivery at ≤34 weeks and at <37 weeks of gestation when compared to women with strain values in the lowest quartile. When adjusting for a short cervix (<25 mm) and gestational age at examination, women with strain values in the 3rd quartile maintained a significant association with spontaneous preterm delivery at ≤34 weeks (OR 9.0; 95% CI, 1.1-74.0; P=0.02), whereas women with strain values in the highest quartile were marginally more likely than women with lowest quartile strain values to deliver spontaneously at ≤37 weeks of gestation (OR 95% CI: 2.8; [0.9-9.0]; P=0.08).

CONCLUSION

Increased strain in the internal cervical os is associated with higher risk of spontaneous preterm delivery both at ≤34 and <37 weeks of gestation.

Quantitative contrast-enhanced ultrasound measurement of cerebrospinal fluid flow for the diagnosis of ventricular shunt malfunction

OBJECT

Cerebral shunt malfunction is common but often difficult to effectively diagnose. Current methods are invasive, involve ionizing radiation, and can be costly. The authors of this study investigated the feasibility of quantitatively measuring CSF flow in a shunt catheter using contrast-enhanced ultrasound.

METHODS

A syringe pump was used to push a solution of gas-filled microbubbles at specific flow rates through a shunt catheter while a high-frequency ultrasound imaging system was used to collect ultrasound images for offline processing. Displacement maps and velocity profiles were generated using a speckle-tracking method based on a cross-correlation algorithm. An additional correction factor, to account for a predictable underestimation and to adjust the measured flow rates, was calculated based on the geometry of the ultrasound imaging plane and assuming a simple model of laminar flow.

RESULTS

The developed method was able to differentiate between physiologically relevant flow rates, including no flow and 0.006 to 0.09 ml/min, with reasonable certainty. The quantitative measurement of flow rates through the catheter using this method was determined to be in good agreement with the expected flow rate.

CONCLUSIONS

This study demonstrated that contrast-enhanced ultrasound has the potential to be used as a minimally invasive and cost-effective alternative method for outpatient shunt malfunction diagnosis.

Evaluation of cervical stiffness during pregnancy using semiquantitative ultrasound elastography

OBJECTIVE

To evaluate cervical stiffness during pregnancy using ultrasound-derived elastography, a method used to estimate the average tissue displacement (strain) within a defined region of interest when oscillatory compression is applied.

METHODS

Strain was calculated in two regions of interest, the endocervical canal and the entire cervix, from three anatomical planes of the cervix: mid-sagittal in the plane used for cervical length measurement and in cross-sectional planes located at the internal and external cervical os. Associations between strain values, method of ascertainment and patient characteristics were assessed using linear mixed models to account for within-subject correlation. Inter-rater agreement in defining the degree of cervical stiffness was evaluated in 120 regions of interest acquired by two operators in 20 patients.

RESULTS

A total of 1557 strain estimations were performed in 262 patients at 8-40 weeks of gestation. Adjusting for other sources of variation, (1) cervical tissue strain estimates obtained in the endocervical canal were on average 33% greater than those obtained in the entire cervix; (2) measurements obtained in the cross-sectional plane of the external cervical os and sagittal plane were 45% and 13% greater than those measured in the cross-sectional plane of the internal cervical os, respectively; (3) mean strain rates were 14% and 5% greater among parous women with and without a history of preterm delivery compared with those of nulliparous women, respectively, and were on average 13% greater among women with a cervical length of between 25 and 30 mm compared to those with a cervical length of > 30 mm; and (4) cervical tissue strain was more strongly associated with cervical length than with gestational age.

CONCLUSION

Semiquantitative elastography can be employed to evaluate changes in cervical stiffness during pregnancy.