Direct strain estimation in elastography using spectral cross-correlation

Spectral estimation of tissue strain has been performed previously by using the centroid shift of the power spectrum or by estimating the variation in the mean scatterer spacing in the spectral domain. The centroid shift method illustrates the robustness of the direct, incoherent strain estimator. In this paper, we present a strain estimator that uses spectral cross-correlation of the pre- and postcompression power spectrum. The centroid shift estimator estimates strain from the mean center frequency shift, while the spectral cross-correlation estimates the shift over the entire spectrum. Spectral cross-correlation is shown to be more sensitive to small shifts in the power spectrum and, thus, provides better estimation for smaller strains when compared to the spectral centroid shift. Spectral cross-correlation shares all the advantages gained using the spectral centroid shift, in addition to providing accurate and precise strain estimation for small strains. The variance and noise properties of the spectral strain estimators quantified by their respective strain filters are also presented.     

Segmentation of elastographic images using a coarse-to-fine active contour model

Delineation of radiofrequency-ablation-induced coagulation (thermal lesion) boundaries is an important clinical problem that is not well addressed by conventional imaging modalities. Elastography, which produces images of the local strain after small, externally applied compressions, can be used for visualization of thermal coagulations. This paper presents an automated segmentation approach for thermal coagulations on 3-D elastographic data to obtain both area and volume information rapidly. The approach consists of a coarse-to-fine method for active contour initialization and a gradient vector flow, active contour model for deformable contour optimization with the help of prior knowledge of the geometry of general thermal coagulations. The performance of the algorithm has been shown to be comparable to manual delineation of coagulations on elastograms by medical physicists (r = 0.99 for volumes of 36 radiofrequency-induced coagulations). Furthermore, the automatic algorithm applied to elastograms yielded results that agreed with manual delineation of coagulations on pathology images (r = 0.96 for the same 36 lesions). This algorithm has also been successfully applied on in vivo elastograms.     

Normal and shear strain estimation using beam steering on linear-array transducers

In current ultrasound elastography, only the axial component of the displacement vector is estimated and used to produce strain images. A method was recently proposed by our group to estimate both the axial and lateral components of a displacement vector following a uniaxial compression. Previous work evaluated the technique using both simulations and a mechanically translated phased array transducer. In this paper, we present initial results using beam steering on a linear array transducer attached to a commercial scanner to acquire echo signals for estimating 2-D displacement vectors. Single-inclusion and anthropomorphic breast phantoms with different boundary properties between the inclusion and background material are imaged by acquiring echo data along beam lines ranging from -15 degrees to 15 degrees relative to the compression direction. 1-D cross-correlation is used to calculate "angular displacements" in each acquisition direction, yielding axial and lateral components of the displacement vector. Strain tensor components are estimated from these displacements. Features on shear strain images generated for the inclusion phantom agree with those predicted using FEA analysis. Experimental results demonstrate the utility of this technique on clinical scanners. Shear strain tensors obtained using this method may provide useful information for the differentiation of benign from malignant tumors. For the linear array transducer used in this study, the optimum angular increment is around 3 degrees. However, more work is required for the selection of an appropriate value for the maximum beam angle for optimal performance of this technique.     

Experimental corroboration of the nonstationary strain estimation errors in elastography

The nonstationary variation in the noise performance of the cross-correlation-based strain estimator due to frequency-dependent attenuation and lateral and elevational signal decorrelation have been addressed theoretically in recent papers using the strain-filter approach. In this paper, we present the experimental verification and corroboration of the nonstationary effects on the strain estimation results. The accuracy and precision of the strain estimate deteriorates with lateral position in the elastogram, due to the lateral motion of tissue scatterers, and with depth, due to frequency-dependent attenuation. The results illustrate that the best strain-estimation noise performance is obtained in the focal zone of the transducer and around the axis of symmetry of the phantom. (E-mail: tvarghese@facstaff.wisc.edu)     

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.     

超声弹性成像诊断多囊卵巢综合征的价值探讨

目的探讨超声弹性成像对多囊卵巢综合征(PCOS)的诊断价值。方法选取42例PCOS患者(PCOS组)和42例健康女性(对照组),于月经周期第7天测量卵巢体积、卵泡数、弹性成像模式及应变比值,比较两组间的差异。分析应变比值与卵巢体积和卵泡数间的相关性;绘制受试者工作特征(ROC)曲线分析卵泡数、卵巢体积及应变比值对PCOS的诊断效能。结果 PCOS组患者平均卵巢体积显著大于对照组[(12.9±3.8)cm^3vs.(5.6±1.7)cm^3];卵泡数显著多于对照组[(18.6±5.2)个vs.(8.5±2.6)个],差异均有统计学意义(均P<0.001)。PCOS组弹性成像模式以1型为主,对照组以3型为主,差异有统计学意义(P<0.001);PCOS组应变比值为8.4±1.6,对照组为4.5±1.2,差异有统计学意义(P<0.001)。以卵泡数≥12个诊断PCOS的敏感性85.4%、特异性89.5%、阳性预测值89.1%、阴性预测值86.0%,曲线下面积0.959;以卵巢体积≥10 cm3诊断PCOS的敏感性100%、特异性70.8%、阳性预测值77.4%、阴性预测值100%,曲线下面积0.962;以应变比值5.7诊断PCOS的敏感性88.1%、特异性83.3%,阳性预测值84.1%、阴性预测值87.5%、曲线下面积0.883。应变比值与卵巢体积和卵泡数均呈正相关(r=0.565、0.653,均P<0.001)。结论超声弹性成像在PCOS诊断中具有较好的临床应用价值。     

On the differences between two-dimensional and three-dimensional simulations for assessing elastographic image quality: a simulation study

In this work, we introduced an elastographic simulation framework, which estimates upper bounds on elastographic image quality by accounting for three-dimensional (3D) tissue motion and the 3D nature of the ultrasound beam. For the boundary conditions and the range of applied strains considered in this study, it was observed that for applied strains smaller than 0.7%, fast two-dimensional (2D) simulations and 3D simulations predicted similar upper bounds on elastographic signal-to-noise (SNR(e)) and contrast-to-noise ratios (CNR(e)); however, for applied strains greater than 0.7%, the predictions by 2D simulations grossly overestimated the achievable results when compared with upper bound results from 3D simulations. It was also found that linear increments in the elevational-to-lateral beamwidth ratio (beam ratio) resulted in nonlinear degradation in the achievable upper bounds on elastographic signal-to-noise ratio. For the modulus contrast ratio of ten between the target and the background, the peak difference in the prediction of contrast-to-noise by 2D and 3D simulations was approximately 10 dB, whereas, for modulus contrast ratio of 1.5, the peak difference increased to approximately 30 dB. No significant difference was observed between the spatial resolution predicted by 2D and 3D simulations; however, increase in beam ratio resulted in decrease in target detectability, especially at lower modulus contrast ratios.     

Three-dimensional electrode displacement elastography using the Siemens C7F2 fourSight four-dimensional ultrasound transducer

Because ablation therapy alters the elastic modulus of tissues, emerging strain imaging methods may enable clinicians for the first time to have readily available, cost-effective, real-time guidance to identify the location and boundaries of thermal lesions. Electrode displacement elastography is a method of strain imaging tailored specifically to ultrasound-guided electrode-based ablative therapies (e.g., radio-frequency ablation). Here tissue deformation is achieved by applying minute perturbations to the unconstrained end of the treatment electrode, resulting in localized motion around the end of the electrode embedded in tissue. In this article, we present a method for three-dimensional (3D) elastographic reconstruction from volumetric data acquired using the C7F2 fourSight four-dimensional ultrasound transducer, provided by Siemens Medical Solutions USA, Inc. (Issaquah, WA, USA). Lesion reconstruction is demonstrated for a spherical inclusion centered in a tissue-mimicking phantom, which simulates a thermal lesion embedded in a normal tissue background. Elastographic reconstruction is also performed for a thermal lesion created in vitro in canine liver using radio-frequency ablation. Postprocessing is done on the acquired raw radio-frequency data to form surface-rendered 3D elastograms of the inclusion. Elastographic volume estimates of the inclusion compare reasonably well with the actual known inclusion volume, with 3D electrode displacement elastography slightly underestimating the true inclusion volume.     

Estimating tissue strain from signal decorrelation using the correlation coefficient

A simple relationship between the correlation coefficient and the applied strain, applicable only at low strains, is presented in this article. This relationship is derived for a Gaussian modulated cosine point spread function. The performance of the strain estimator is analyzed using a theoretical expression for the correlation coefficient along with simulation and experimental results. Both the theoretical and simulation results diverge from the ideal relationship between the strain and the correlation coefficient as the applied strain is increased. Simulation results illustrate that the strain estimate obtained using the correlation coefficient is a biased estimate with a large variability. Experimental results, however, illustrate that strain estimation using the 1-D correlation coefficient estimate is applicable only at high signal-to-noise ratios in the radiofrequency signal and in the absence of lateral and elevational signal decorrelation.     

Reduction of stress nonuniformities by apodization of compressor displacement in elastography

Elastography is a method for imaging the elastic properties of compliant tissues that produces gray-scale strain or elasticity images called elastograms. The method is based on external tissue compression, with ultrasonic detection of local target displacements and subsequent computation of strain profiles along the compression axis. The internal strain variations are a result of the tissue elasticity variations and the applied deformation or compression. A number of mechanical artifacts that appear in elastograms have been identified. One such artifact appears as the result of a nonuniform stress distribution under the compressors used, including darkening (low stress) of the central region and brightening (high stress) of the peripheral regions under the compressor. On an elastogram, these areas may be misinterpreted as being respectively harder and softer than the rest of the target. In this article, a displacement apodization method for the minimization of this artifact is discussed, and its effects are studied using finite element simulations. When the isometric compression of standard elastography was replaced by an apodized displacement profile calculated from reciprocity conditions, a significant improvement in stress uniformity under the compressor was achieved.     

超声弹性成像比值法与评分法在乳腺肿物诊断中的比较

目的 比较弹性应变率比值法以及Itoh评分法、改良5分法对乳腺良恶性肿瘤的诊断价值.方法 对手术病理证实的269例患者共377个病灶(良性278个,恶性99个)进行了超声弹性成像检查,获得弹性成像图后,以Itoh评分法、改良5分法分别对病灶进行评分;测量病灶与周围组织的弹性应变率比值,以3.08为界点判断病灶良恶性.以病理诊断为金标准,构建受试者应用曲线,比较3种方法对乳腺肿物的诊断效率.结果 构建ROC曲线后,3种方法的曲线下面积(AUC)分别为:比值法0.965,改良5分法0.951,Itoh5分法0.907.比值法与改良5分法对乳腺肿物的诊断价值相似(P≥0.05),优于Itoh5分法(P=0.003).对于改良5分法评分为3分和4分的病灶,比值法的准确性要高于改良评分法(P=0.021).结论 弹性应变率比值测量为弹性成像检查提供了更为客观、有价值的诊断标准.     

Magnetic resonance elastography: Non-invasive mapping of tissue elasticity

Magnetic resonance elastography (MRE) is a phase-contrast-based MRI imaging technique that can directly visualize and quantitatively measure propagating acoustic strain waves in tissue-like materials subjected to harmonic mechanical excitation. The data acquired allows the calculation of local quantitative values of shear modulus and the generation of images that depict tissue elasticity or stiffness. This is significant because palpation, a physical examination that assesses the stiffness of tissue, can be an effective method of detecting tumors, but is restricted to parts of the body that are accessible to the physician's hand. MRE shows promise as a potential technique for 'palpation by imaging', with possible applications in tumor detection (particularly in breast, liver, kidney and prostate), characterization of disease, and assessment of rehabilitation (particularly in muscle). We describe MRE in the context of other recent techniques for imaging elasticity, discuss the processing algorithms for elasticity reconstruction and the issues and assumptions they involve, and present recent ex vivo and in vivo results.     

超声弹性成像半定量评价甲状腺占位性病变的价值探讨

目的探讨弹性应变率比值对甲状腺占位性病变良恶性的鉴别诊断价值。方法对98例患者的129个病灶行甲状腺超声弹性成像检查,测量病灶与周围同层正常甲状腺组织的弹性应变率比值;以病理诊断为金标准,采用受试者工作特征（ROC）曲线确定弹性应变率比值的最佳临界值。结果恶性组平均应变率比值为8．08,良性组为2．14,两组比较差异有统计学意义（P〈0．001）。通过ROC曲线确定的弹性应变率比值诊断甲状腺占位性病变良恶性的临界值为4．095;其诊断的敏感性为81．0％,特异性91．7％,准确性89．9％。结论超声弹性应变率比值可以用来初步评估甲状腺占位性病变的良恶性,为甲状腺占位性病变的初步判断提供了一项新的半定量诊断指标。     

经直肠超声弹性成像在前列腺癌诊断中的应用

目的 探讨经直肠超声弹性成像(TRE)在前列腺癌诊断中的应用价值。方法 118例疑诊前列腺癌患者行经直肠超声成像(TRUS)、TRE及超声引导下经直肠前列腺穿刺活检,以穿刺活检结果为金标准,分析TRE对前列腺癌的诊断价值。结果 TRE与TRUS对于诊断前列腺周围区癌灶的敏感性分别为73.6％、52.8％,特异性分别为72.7％、71.9％,二者敏感性差异有统计学意义;TRE与TRUS对于诊断位于前列腺中央区及移行区癌灶的敏感性分别为57.1％、57.1 ％,特异性分别为71.3％、70.7％,其差异均无统计学意义。癌组织应变比值(strain ratio,SR)平均值为27.35±35.20,良性病灶SR平均值为2.91±0.55,二者差异有统计学意义。由ROC曲线可知,当SR切点值为3.6时曲线下面积为0.80,其预测前列腺癌的敏感性为85.7％,特异性为67.7％。结论TRE的应用可以提高前列腺癌的诊断率;TRE对于诊断位于前列腺周围区的病灶具有较高敏感性;以SR 3.6作为切点值,对于诊断前列腺癌具有较高的敏感性及特异性。     

Clinical Application of Real-Time Sonoelastography for Evaluation of Medial Epicondylitis: A Pilot Study

The aim of this study was to evaluate the diagnostic potential of real-time sonoelastography (RSE) in medial epicondylitis by comparing clinically diagnosed patients and patients without medial elbow pain. From July 2016 to December 2017, gray-scale sonographic findings (swelling, cortical irregularity, hypo-echogenicity, calcification and tear), color Doppler findings (hyperemia) and sonoelastographic findings (elastographic grade on a 3-point visual scale and strain ratio from two regions of interest) for 63 elbows of 56 patients were compared. Twenty-four patients with 29 imaged elbows were clinically diagnosed with medial epicondylitis, and 32 patients with 34 imaged elbows had no medial elbow pain. Cortical irregularity, hypo-echogenicity, calcification, elastographic grade and strain ratio revealed significant differences (p < 0.05). Among these, strain ratio had the highest diagnostic performance (area under the curve: 0.985). Real-time sonoelastography, which can obtain both elastographic grade and strain ratio, is valuable as a supplementary tool in the diagnosis of medial epicondylitis.     

超声弹性成像应变率比值法在乳腺良恶性肿块中的诊断价值

目的探讨超声弹性成像应变率比值对乳腺肿块良恶性的诊断准确性。方法对162例患者的173个乳腺肿块分别进行常规超声和超声弹性成像超声检查,测量肿块周围正常乳腺组织与肿块组织间的超声弹性成像应变率比值(E2/E1),并将结果与术后病理结果进行比较。结果超声弹性成像应变率比值法诊断乳腺肿块的ROC曲线下面积为0.950(95%CI,0.916～0.983),取约登指数最大值(0.779)时的截断点,确定E2/E1=0.355为诊断良恶性肿块的最佳临界点。良性肿块的E2/E1为0.57±0.15,恶性肿块的E2/E1为0.29±0.08,二者比较差异有统计学意义(P<0.05)。超声弹性成像应变率比值法与常规超声对乳腺恶性肿块的敏感性差异无统计学意义,但联合应用二者较单独应用超声弹性成像应变率比值法敏感性有显著提高(P<0.05)。结论超声弹性成像应变率比值作为一项相对客观诊断乳腺良恶性病变的半定量指标,具有较高的诊断准确性,与常规超声联合应用可提高乳腺恶性肿块的检出率。     

弹性成像5分评分法与应变率比值法鉴别诊断肝脏良恶性病变

目的 比较弹性成像5分评分法与应变率比值法对肝脏良恶性病变的鉴别诊断价值。 方法 对165例患者(共183个肝脏局灶性病变)进行肝脏弹性成像检查,分别对病灶进行弹性评分及应变率比值测定。以病理结果作为金标准,构建ROC曲线并计算曲线下面积,分别计算弹性评分法和应变率比值用于鉴别诊断肝脏良恶性肿瘤的敏感度、特异度、准确率、阳性预测值和阴性预测值;比较两种方法对肝脏局灶性病变的诊断效能。 结果 采用应变率比值鉴别诊断肝脏良恶性病灶时,曲线下面积为0.85;采用弹性评分时,曲线下面积为0.82,二者差异无统计学意义(P=0.14)。应变率比值诊断肝脏良恶性肿瘤的敏感度、特异度、准确率、阳性预测值及阴性预测值分别为71.26%(62/87)、91.67%(88/96)、81.97%(150/183)、88.57%(62/70)、77.88%(88/113);弹性评分诊断肝脏恶性肿瘤的敏感度、特异度、准确率、阳性预测值及阴性预测值分别为88.51%(77/87)、60.42%(58/96)、73.77%(135/183)、66.96%(77/115)、85.29%(58/88)。应变率比值法的特异度高于弹性评分法(P<0.05),但其敏感度较低(P<0.05),二者准确率差异无统计学意义(P>0.05),二者诊断能力的差异主要体现在弹性评分为3分的病灶中。 结论 应用弹性成像鉴别诊断肝脏局灶性病变的良恶性时,应变率比值法及弹性评分法各有优缺点,在临床应用中应结合具体情况选择合理方法。     

Transient elastography using impulsive ultrasound radiation force: a preliminary comparison with surface palpation elastography

The use of impulsive acoustic radiation force for transient strain imaging was investigated and compared with conventional elastography. A series of experiments were performed to evaluate the performances of the technique on gelatine phantoms containing inclusions and to determine a range of applications where radiation force elastography may be useful compared with static elastography. Slip boundaries and cylindrical inclusions of varying elastic modulus were placed in background materials. A focused ultrasound transducer was used to apply localised radiation force to a small volume of tissue mimic (100 mm3) for durations of 8 ms. A conventional real-time ultrasound imaging probe was used to obtain radio- frequency echo signals. The resulting strains were mapped using ultrasound correlation-based methods. The instantaneous strain immediately following cessation of the radiation force was observed at depth within homogeneous gels and within stiff inclusions. The highly localised and transient strain that is produced at depth permits the sensing of variations in tissue elastic properties that are difficult to detect with conventional elastography, due to greater independence from boundary conditions. In particular, radiation force elastograms were more homogeneous in the background and within the inclusions and displayed a superior contrast-transfer-efficiency, particularly for regions that had negative modulus contrast or that were disconnected from the background or the anterior medium by a low friction boundary.     

Preliminary validation of angle-independent myocardial elastography using MR tagging in a clinical setting

Myocardial elastography (ME), a radio-frequency (RF) based speckle tracking technique, was employed in order to image the entire two-dimensional (2D) transmural deformation field in full echocardiographic views and was validated against tagged magnetic resonance imaging (tMRI) in normal as well as reperfused (i.e., treated myocardial infarction [MI]) human left ventricles. RF ultrasound and tMRI frames were acquired at the papillary muscle level in 2D short-axis (SA) views at the frame rates of 136 (fps; real-time) and 33 fps (electrocardiogram [ECG]-gated), respectively. In ME, in-plane, 2D (lateral and axial) incremental displacements were iteratively estimated using one-dimensional (1D) cross-correlation and recorrelation techniques in a 2D search with a 1D matching kernel. In tMRI, cardiac motion was estimated by a template-matching algorithm on a 2D grid-shaped mesh. In both ME and tMRI, cumulative 2D displacements were obtained and then used to estimate 2D Lagrangian finite systolic strains, from which polar (i.e., radial and circumferential) strains, namely angle-independent measures, were further obtained through coordinate transformation. Principal strains, which are angle-independent and less centroid-dependent than polar strains, were also computed and imaged based on the 2D finite strains using methodology previously established. Both qualitatively and quantitatively, angle-independent ME is shown to be capable of (1) estimating myocardial deformation in good agreement with tMRI estimates in a clinical setting and of (2) differentiating abnormal from normal myocardium in a full left-ventricular view. The principal strains were concluded to be a potential diagnostic measure for detection of cardiac disease with reduced centroid dependence.     

弹性应变率比值在乳腺实性肿物良恶性鉴别诊断中的价值初探

目的 探讨弹性应变率比值对乳腺良恶性肿瘤的诊断价值.方法 对手术病理证实269例患者共377个病灶(良性278个,恶性99个)进行了超声弹性成像检查.获得弹性成像图后,测量病灶与周围组织的弹性应变率比值,与病理诊断结果相对照,采用受试者工作特征(ROC)曲线来确定弹性应变率的临界值.结果 良恶性病灶弹性应变率的差异有统计学意义(P<0.05);通过ROC曲线确定的弹性应变率的诊断临界值是3.08,约登指数0.88.敏感性97.38%.特异性91.33%.结论 弹性应变率比值叮用于乳腺良恶性肿瘤的诊断,为超声弹性成像检查提供了一项新的诊断指标.