A quantitative comparison of modulus images obtained using nanoindentation with strain elastograms

Tissue stiffness is generally known to be associated with pathologic changes. Ultrasound (US) elastography, on the other hand, is capable of imaging tissue strain, which may or may not be well-correlated with tissue stiffness. Hence, a quantitative comparison between the elastographic tissue strain images and the corresponding tissue modulus images needed to be performed to evaluate the usefulness of elastography in imaging tissue stiffnesss properties. Simulations were performed to demonstrate and quantify the similarities between modulus images and strain elastograms. This was followed by comparing nanoindenter-based modulus images with strain elastograms of thin slices of tissue-mimicking phantoms. Finally, some beef slices, canine prostates, ovine kidneys and breast cancers grown in mice were used to demonstrate the qualitative correspondence between modulus images and strain elastograms. The simulations and the experiments indicated that it is feasible to perform quantitative comparisons between strain images (using elastography) and modulus images on certain tissue structures and geometries. A good quantitative correspondence (correlation values of greater than 0.8) between structures in the modulus and strain images could be obtained at scales equal to or larger than 20 Qlambda (where Q is the quality factor defined as the ratio of the center frequency over the band width and lambda is the wavelength of the US system) modulus contrasts larger than 5, applied strains between 0.5% and 3% and window lengths for computing strain elastograms between 3 Qlambda and 5 Qlambda. The gelatin-phantom experiments showed lower values of correlation (values around 0.5) than with theory and simulations. The decrease in correlation was attributed to the presence of measurement noise in both strain elastography and modulus imaging, an increase of dimensionality of the problem (from 2-D to 3-D), local anisotropy, heterogeneity and nonstationarity. Experiments on real tissue slices showed further decrease in the correlation to around 0.3, possibly due to additional confounding factors such as time-dependent mechanical properties and geometrical distortions in the tissue during imaging. The work presented in this paper demonstrates that there is an intrinsic relationship between strain elastograms and the actual distribution of soft tissue elastic moduli, and bodes well for continued work in the area of elastography.     

Estimating axial and lateral strain using a synthetic aperture elastographic imaging system

Model-based elastography is an emerging technique with clinical applications in imaging vascular tissues, guiding minimally invasive therapies and diagnosing breast and prostate cancers. Its usage is limited because ultrasound can measure only the axial component of displacement with high precision. The goal of this study was to assess the effect of lateral sampling frequency, lateral beam-width and the number of active transmission elements on the quality of axial and lateral strain elastograms. Elastographic imaging was performed on gelatin-based phantoms with a modified commercial ultrasound scanner. Three groups of radio-frequency (RF) echo frames were reconstructed from fully synthetic aperture data. In the first group, all 128 transmission elements (corresponding to a lateral beamwidth of 0.22 mm at the center of the field of view) were used to reconstruct RF echo frames with A-line densities that varied from 6.4 lines/mm to 51.2 lines/mm. In the second group, the size of the aperture was varied to produce RF echo frames with lateral beamwidths ranging from 0.22 mm to 0.43 mm and a fixed A-line density of 25.6 lines/mm. In the third group, sparse arrays with varying number of active transmission elements (from 2 to 128) were used to reconstruct RF echo frames, whose A-line density and lateral beamwidth were fixed to 25.6 lines/mm and 0.22 mm, respectively. Applying a two-dimensional (2-D) displacement estimator to the pre- and post-deformed RF echo frames produced displacement elastograms. Axial and lateral strain elastograms were computed from displacement elastograms with a least squares strain estimator. The quality of axial and lateral strain elastograms improved with increasing applied strain and A-line density but decreased with increasing lateral beamwidth and deteriorated as the number of active transmission elements in the sparse arrays were reduced. This work demonstrated that the variance incurred when estimating the lateral component of displacement was reduced considerably when elastography was performed with a synthetic aperture ultrasound imaging system. Satisfactory axial and lateral strain elastograms were produced using a sparse array with as few as 16 active transmission elements.     

In vitro uterine strain imaging: preliminary results.

OBJECTIVE: Uterine abnormalities, such as leiomyomas, endometrial polyps, and adenomyosis, are often clinically associated with irregular uterine bleeding. These abnormalities can have similar B-mode characteristics but require different treatment. The objective of this study was to develop diagnostic techniques based on ultrasound strain imaging that would allow in vivo visualization and characterization of endometrial and myometrial uterine abnormalities, enabling physicians to improve diagnosis and treatment. METHODS: Ultrasound strain imaging was performed on 29 uteri removed via elective hysterectomy. An ultrasound system with a linear array transducer was used to obtain radio frequency echo data during manual freehand compressions of the tissue. Radio frequency data were post-processed with a 2-dimensional block-matching algorithm to generate strain images. RESULTS: In the uteri involved in this study, there were 19 leiomyomas, 1 case of adenomyosis, and 3 endometrial polyps observed on strain imaging. Leiomyomas appeared stiffer than the surrounding normal myometrium in strain images and were characterized by a slipping artifact at their boundary. Endometrial polyps appeared softer than the normal surrounding myometrium. The average strain contrast in small leiomyomas (<1.5 cm) compared to the myometrium was 1.75 +/- 1.14; the strain contrast was 2.50 +/- 1.15 in large leiomyomas and 0.40 +/- 0.05 in endometrial polyps. Leiomyoma strain contrast results were consistent with modulus contrast values from mechanical testing results. CONCLUSIONS: Ultrasound strain imaging can differentiate between endometrial polyps and leiomyomas. More data are necessary to validate these results and to ascertain whether other uterine abnormalities can also be differentiated.     

Lesion detection in simulated elastographic and echographic images: A psychophysical study

The image quality of two ultrasonic imaging techniques was studied: conventional echography and the recently introduced elastography. The image quality was assessed by estimating the detectability of disc-shaped lesions of various sizes and contrast levels. The study was designed to verify the hypothesis that elastograms could show lesions at a higher subjective and objective level of detectability than echograms of the same object contrast. This hypothesis was adopted because homogeneous elastograms can present a higher point signal-to-noise ratio than uniform echograms. Both elastograms and echograms were generated by two-dimensional (2D) simulations. The subjective assessment was performed by psychophysical experiments using the staircase up-down method. The threshold contrast of detection for both modalities was determined at different diameters of the disc-shaped lesion. These values were used to construct the contrastdetail curves for both techniques. For identical object contrasts, elastography was found to have significantly higher detectability at all lesion diameters considered. The contrast thresholds were also used for an objective evaluation with the lesion signal-to-noise ratio. The objective measure evaluated at the subjective threshold of detection for both modalities was not found to be identical, nor constant over the range of lesion diameters as expected.     

Axial strain imaging of intravascular data: results on polyvinyl alcohol cryogel phantoms and carotid artery

Mapping the local elastic properties of an atherosclerotic artery is of major interest for predicting the disease evolution or an intervention outcome. These properties can be investigated by elastography, which estimates the strain distribution within a medium in response to a stress. But because diseased arteries are highly heterogeneous, a small global deformation may result in high local strains in the softest regions. For those reasons, we use in this paper the strain estimation method we recently developed to compute elastograms of original vessel-mimicking cryogel phantoms and a fresh excised human carotid artery. This adaptive method has been effectively proved to be accurate in a wider range of strains (0–7%) than commonly used gradient-based methods, and very adapted for investigating highly heterogeneous tissues. Resulting elastograms cover a wider range of strains (0–3.5%) than all previously reported intravascular elastograms, improving the discrimination between healthy and diseased regions. (E-mail: elisabeth.brusseau@creatis.insa-lyon.fr)     

Spatially variant regularization for tissue strain measurement and shear modulus reconstruction

Purpose Regarding the regularization of the displacement vector measurement and shear modulus reconstruction, we propose to properly set the regularization parameters, i.e., to use spatially variant regularization parameters at each point in the region of interest, because the measurement accuracies of the displacements and strains vary spatially. Method As the measurement accuracies of the strains can be evaluated using the correlation coefficient when using the cross-spectrum phase gradient method, preliminarily the regularization parameters were set proportional to the reciprocal of a power of the correlation coefficient. Results and conclusion Such a regularization scheme realizes the spatially uniform stabilities of the strain measurement and shear modulus reconstruction. The effectiveness of this method was verified by showing the regularized results of the axial strain measurement and of one-dimensional (1-D) shear modulus reconstruction obtained in vivo from a human liver carcinoma (that was treated by interstitial microwave coagulation therapy) as well as by a 1-D shear modulus reconstruction obtained using an agar phantom.     

Ultrasonic technique for imaging tissue vibrations: preliminary results

We propose an ultrasound (US)-based technique for imaging vibrations in the blood vessel walls and surrounding tissue caused by eddies produced during flow through narrowed or punctured arteries. Our approach is to utilize the clutter signal, normally suppressed in conventional color flow imaging, to detect and characterize local tissue vibrations. We demonstrate the feasibility of visualizing the origin and extent of vibrations relative to the underlying anatomy and blood flow in real-time and their quantitative assessment, including measurements of the amplitude, frequency and spatial distribution. We present two signal-processing algorithms, one based on phase decomposition and the other based on spectral estimation using eigen decomposition for isolating vibrations from clutter, blood flow and noise using an ensemble of US echoes. In simulation studies, the computationally efficient phase-decomposition method achieved 96% sensitivity and 98% specificity for vibration detection and was robust to broadband vibrations. Somewhat higher sensitivity (98%) and specificity (99%) could be achieved using the more computationally intensive eigen decomposition-based algorithm. Vibration amplitudes as low as 1 mum were measured accurately in phantom experiments. Real-time tissue vibration imaging at typical color-flow frame rates was implemented on a software-programmable US system. Vibrations were studied in vivo in a stenosed femoral bypass vein graft in a human subject and in a punctured femoral artery and incised spleen in an animal model.     

Noninvasive two-dimensional strain imaging of arteries: validation in phantoms and preliminary experience in carotid arteries in vivo

Cardiac disease and stroke are the major causes of death in the Western World. Atherosclerosis of the carotid artery is the most important predictor of stroke. Elastography is a technique to assess the composition and vulnerability of an atherosclerotic plaque. Contrary to intravascular applications, the ultrasound beam and radial strain are not aligned in noninvasive acquisitions. In this study, 2D displacement and strain images were determined and used to calculate the radial and circumferential strain. Rf-data were acquired using a Philips SONOS 7500 live 3D ultrasound system, equipped with an 11_3L (3 to 11 MHz) linear array transducer and rf-interface. A homogeneous, hollow cylinder phantom [20% gelatin, 1% SiC scatterers (10 microM)] was measured in a water tank at different intraluminal pressures. In addition, measurements in patients (n = 12) were made to evaluate the in vivo applicability of the technique. Longitudinal and cross-sectional recordings were made, both in phantoms and patients. Strain along the ultrasound beam (axial strain) was determined using cross-correlation analysis for signal-windows from both the pre- and post-compression data. For lateral strain, new ultrasound lines were generated between the acquired lines using interpolation. A cross-correlation based search algorithm was applied to determine lateral displacement and strain. Longitudinal axial strain images in the phantom showed a decreasing strain from the lumen- vessel wall interface to the outer region that can be described by a 1 over r(2) relationship. The lateral strain image showed no strain in this direction indicating a plane strain situation. In the cross-sectional view, compression of the material in regions at 12 and 6 o'clock was observed, whereas expansion was observed in regions at 3 and 9 o'clock. This pattern is in accordance with theory, but can only be partly corrected for: in the transition regions, zero axial strain was measured. The lateral strain image showed a complementary pattern. In patients, low strain was observed in nonatherosclerotic artery walls. High and low strain regions were found in atherosclerotic plaques. High quality elastograms were generated both in longitudinal and cross-sectional views. In conclusion, 2D noninvasive elastography of atherosclerotic carotid plaques is feasible. Phantom studies revealed elastograms in accordance with theory. Additional in vivo validation is needed to assess the value of this technique for identifying plaque vulnerability and composition.     

Optical coherence tomography detection of shear wave propagation in inhomogeneous tissue equivalent phantoms and ex-vivo carotid artery samples

In this work, we explored the potential of measuring shear wave propagation using optical coherence elastography (OCE) in an inhomogeneous phantom and carotid artery samples based on a swept-source optical coherence tomography (OCT) system. Shear waves were generated using a piezoelectric transducer transmitting sine-wave bursts of 400 μs duration, applying acoustic radiation force (ARF) to inhomogeneous phantoms and carotid artery samples, synchronized with a swept-source OCT (SS-OCT) imaging system. The phantoms were composed of gelatin and titanium dioxide whereas the carotid artery samples were embedded in gel. Differential OCT phase maps, measured with and without the ARF, detected the microscopic displacement generated by shear wave propagation in these phantoms and samples of different stiffness. We present the technique for calculating tissue mechanical properties by propagating shear waves in inhomogeneous tissue equivalent phantoms and carotid artery samples using the ARF of an ultrasound transducer, and measuring the shear wave speed and its associated properties in the different layers with OCT phase maps. This method lays the foundation for future in-vitro and in-vivo studies of mechanical property measurements of biological tissues such as vascular tissues, where normal and pathological structures may exhibit significant contrast in the shear modulus.OCIS codes: (170.4500) Optical coherence tomography, (170.6935) Tissue characterization     

Colon polyp detection using smoothed shape operators: preliminary results

Computer-aided detection (CAD) algorithms identify locations in computed tomographic (CT) images of the colon that are most likely to contain polyps. Existing CAD methods treat the CT data as a voxelized, volume image. They estimate a curvature-based feature at the mucosal surface voxels. However, curvature is a smooth notion, while our data are discrete and noisy. As a second order differential quantity, curvature amplifies noise. In this paper, we present the smoothed shape operators method (SSO), which uses a geometry processing approach. We extract a triangle mesh representation of the colon surface, and estimate curvature on this surface using the shape operator. We then smooth the shape operators on the surface iteratively. Throughout, we use techniques explicitly designed for discrete geometry. All our computation occurs on the surface, rather than in the voxel grid. We evaluate our algorithm on patient data and provide free-response receiver-operating characteristic performance analysis over all size ranges of polyps. We also provide confidence intervals for our performance estimates. We compare our performance with the surface normal overlap (SNO) method for the same data. A preliminary evaluation of our method on 35 patients yielded the following results (polyp diameter range; sensitivity; false positives/case): (10mm; 100%; 17.5), (5-10 mm; 89.7%, 21.23), (<5 mm; 59.1%; 23.9) and (overall; 80.3%; 23.9). The evaluation of the SNO method yielded: (10 mm; 75%; 17.5), (5-10 mm; 43.1%; 21.23), (<5 mm; 15.9%; 23.9) and (overall; 38.5%; 23.9).     

Functional analysis of gallbladder using three-dimensional ultrasound: preliminary results

In evaluating gallbladder function, the clinical feasibility of three-dimensional (3-D) ultrasound (US) using volumetric acquisition was assessed in 30 patients: 18 patients with gallstones and 12 healthy volunteers. 3-D ultrasonography was performed in each patient before and after a fat-meal test. The ejection fraction of the gallbladder from two volumes measured by computer-aided analysis was calculated. There were significant differences in the gallbladder ejection fractions among the diseased and control groups (p < 0.0001). Compared with the results of oral cholecystograms, the ejection fraction of the gallbladder using 3-D volumetry and the grade of oral cholecystography showed good correlation in predicting the gallbladder dysfunction (gamma = 0.71, p = 0.002). Even in cases failing oral cholecystography, the gallbladder ejection fraction using 3-D US was calculated without any difficulty. Conclusively, volumetric acquisition of 3-D US enables us to evaluate gallbladder function and provides a reliable diagnostic yield superior to oral cholecystography.     

Hyperspectral images classification with convolutional neural network and textural feature using limited training samples

In this letter, a new deep learning framework, which integrates textural features of gray level co-occurrence matrix (GLCM) into convolutional neural networks (CNNs) is proposed for hyperspectral images (HSIs) classification using limited number of labeled samples. The proposed method can be implemented in three steps. Firstly, the GLCM textural features are extracted from the first principal component after the principal components analysis (PCA) transformation. Secondly, a CNN is built to extract the deep spectral features from the original HSIs, and the features are concatenated with the textural features obtained in the first step in a concat layer of CNN. Finally, softmax is employed to generate classification maps at the end of the framework. In this way, the CNN focuses on the learning of spectral features only, and the generated textural features are used directly as one set of features before softmax. These lead to the reduction of the requirements for the size of training samples and the improvement of computing efficiency. The experimental results are presented for three HSIs and compared with several advanced deep learning and spectral-spatial classification techniques. The competitive classification accuracy can be obtained, especially when only a limited number of training samples are available.     

Measurement of attenuation and scatterer spacing in human liver tissue: preliminary results.

A study measuring two quantitative parameters of human liver in vivo was performed to assess the reliability of measurement of the two parameters, and to evaluate their potential for diagnosing and grading diffuse fibrotic liver disease. The parameters measured were attenuation and "mean scatterer spacing," a measure of tissue structure. Components of variance analysis demonstrated that variation in the measured parameters was a function of the subject being examined, with significant variation noted between data acquisition sessions performed the same day. There was no significant additional variation of the measurements from week to week over a one-month period. A good correlation of the parameters with the severity of liver disease indicates that the technique may be useful in the clinical evaluation of diffuse liver disease.     

Mega-HAART: preliminary results and correlation with baseline resistance

This is a brief report of preliminary results of the assessment of the correlation between baseline viral drug susceptibility and virological response in patients receiving mega-HAART. A total of 37 patients received > or = 6 drugs with a median follow-up of 8 months. There was evidence of extensive loss of viral drug susceptibility at baseline among the 24 patients analysed. Results showed that the magnitude and duration of virological response was associated with phenotypic viral drug resistance, although resistance did not lead to virological failure in all cases, and susceptibility did not lead to response in all cases.     

Cardiac motion and strain detection using 4D CT images: comparison with tagged MRI,and echocardiography

Ischemic heart disease is a leading cause of death in the modern world. Coronary obstruction is the basis for ischemic heart disease and leads to decreased cardiac supply and decreased contractility of the myocardium. Recently, high quality 4D computed tomography (CT) has become available for cardiac imaging and provides the clinician with high quality anatomical images. In this article, a new method is proposed to detect 3D motion and strain from 4D cardiac CT images by constraining intensity constancy, myocardial volume changes and motion smoothness assumptions. The proposed method is validated by using manual tracking of the cardiac CT landmarks. The average error for the manual tracking, by an expert, was 2.9 ± 0.9 mm. As an additional validation, the cardiac CT strain values were compared to the cardiac tagged magnetic resonance imaging (MRI) and 2D B-mode echocardiography strain values of the same patients. The correlation ratio was significantly high for CT and tagged MRI radial strain values (r = 0.76, 95 % confidence interval, P < 0.001). The correlation ratio was meaningful for CT and echocardiography radial strain values as well (r = 0.67, 95 % confidence interval, P < 0.001). The correlation ratio for CT and tagged MRI circumferential strain values was acceptable (r = 0.73, 95 % confidence interval, P < 0.001), while the correlation ratio for CT and echocardiography circumferential strain values was good as well (r = 0.61, 95 % confidence interval, P < 0.001). In general, motion and strain values computed from cardiac CT images agree with motion and strain values computed from tagged MRI and echocardiography images.     

Ultrasonic vibration dectection with wavelets: preliminary results

Several arterial disorders are known to cause systolic audio vibrations in tissue: they include stenoses, vasospasm, aneurysms, bleeds and arteriovenous fistulas. High-amplitude vibrations can be discovered with conventional Doppler ultrasound (US) instruments; however, differentiating brief, low-amplitude vibrations from other nonstationary echo sources is difficult. Further, characterizing the frequency and amplitude of vibrations is not feasible with conventional Doppler US. The automated detection and estimation of both the frequency and amplitude of vibrations with durations less than 100 ms and amplitudes of a micrometer or less have remained a signal-processing challenge. These vibrations may be associated with both nonstationary colored noise and strong low-frequency clutter. The normalized continuous Morlet wavelet power-spectrum analysis of quadrature Doppler echoes, followed by a binary hypothesis test for noise, results in simulated detection rates above 99.9%, with 0.1% false alarms for signal-on signal-to-noise ratios (SNRs) as low as one. Two clinical examples are included.     

Ultrasonic interrogation of tissue vibrations in arterial and organ injuries: preliminary in vivo results

Soft tissues surrounding vascular injuries are known to vibrate at audible and palpable frequencies, producing bruits and thrills. We report the results of a feasibility study where Doppler ultrasound (US) was used to quantitatively estimate the tissue vibrations after induced trauma in an animal model. A software-programmable US system was used to acquire quadrature-demodulated ensembles of received US echoes bypassing clutter filtering and other conventional Doppler processing stages. The waveforms of tissue velocity surrounding the injury site were then estimated from the clutter data using autocorrelation and analyzed to determine vibration characteristics. Six New Zealand white rabbits and two juvenile pigs were used for the study. The femoral artery of the anesthetized animal was punctured with an 18-gauge needle to model a peripheral arterial trauma, and the liver was surgically exposed and incised to model organ trauma. Two types of oscillatory tissue motion were observed: "vibrations" with high frequency (>50 Hz) and low peak-peak amplitude (<1 microm) and "flutter" with low frequency (<50 Hz) and high peak-peak amplitude (>1 microm). Active bleeding in femoral artery punctures produced tissue vibrations at the frequency of 323 +/- 214 Hz (mean +/- standard deviation, pooled for both rabbits and pigs) and the amplitude of 0.24 +/- 0.15 microm. Active bleeding in liver incisions produced vibrations at the frequency of 120 +/- 47 Hz and the amplitude of 0.33 +/- 0.25 microm. Flutter was observed in punctured arteries at the frequency of 28 +/- 13 Hz the amplitude of 2.92 +/- 1.75 microm, and in incised livers at the frequency of 26 +/- 6 Hz and the amplitude of 1.53 +/- 0.76 microm. In a punctured artery, the vibration frequency and phase of tissue surrounding the artery were highly correlated between neighboring locations in tissue (correlation coefficient = 0.98), and with the flow oscillations in the lumen (correlation coefficient = 0.96). This preliminary study indicates that tissue vibrations could provide additional physiologic information for detecting, localizing and monitoring internal bleeding using US.