Ultrasound tissue displacement imaging with application to breast cancer

Contribution of Sonoelastographic Scoring to B-Mode Sonography in the Evaluation of Breast Masses

Objectives:

The study aims were to evaluate the contribution of sonoelastography to Breast Imaging Reporting, and Data System (BI-RADS) scoring of breast images.

Materials and Methods:

Two observers evaluated the BI-RADS category, Tsukuba score, and the strain index of 83 lesions of 73 consecutive patients. A new scoring system was established to evaluate the lesions by using the BI-RADS score, Tsukuba score, and strain index ratio.

Results:

There was a statistically significant difference between the strain index value of benign (3.08 ± 2.71) and malignant group (4.62 ± 2.70) ( P < .05). The sensitivity and specificity were 59.1% and 65.1% for the 3.12 cut-off value for the strain index. In the receiver operating characteristic (ROC) analysis, the area under the curve (AUC) of the only BI-RADS score was 0.834, both BT (BI-RADS + revised Tsukuba score) score and the total score (BI-RADS + revised Tsukuba score + strain index score) was 0.843. The interclass correlation coefficient for the two observers’ measurements of the strain index was weak, with .266 ( P < .05).

Conclusion:

The potential contribution of sonoelastography on lesion characterization is still controversial. In this study, the agreement among the observers was inadequate, and the contribution of sonoelastography on BI-RADS classification was limited. In addition, in the daily practice of sonoelastograpic evaluation, the Tsukuba score, was easier to apply and should be used rather than strain index measurements.

Harmonic motion imaging of human breast masses: an in vivo clinical feasibility

Non-invasive diagnosis of breast cancer is still challenging due to the low specificity of the imaging modalities that calls for unnecessary biopsies. The diagnostic accuracy can be improved by assessing the breast tissue mechanical properties associated with pathological changes. Harmonic motion imaging (HMI) is an elasticity imaging technique that uses acoustic radiation force to evaluate the localized mechanical properties of the underlying tissue. Herein, we studied the in vivo feasibility of a clinical HMI system to differentiate breast tumors based on their relative HMI displacements, in human subjects. We performed HMI scans in 10 female subjects with breast masses: five benign and five malignant masses. Results revealed that both benign and malignant masses were stiffer than the surrounding tissues. However, malignant tumors underwent lower mean HMI displacement (1.1 ± 0.5 µm) compared to benign tumors (3.6 ± 1.5 µm) and the adjacent non-cancerous tissue (6.4 ± 2.5 µm), which allowed to differentiate between tumor types. Additionally, the excised breast specimens of the same patients (n = 5) were imaged post-surgically, where there was an excellent agreement between the in vivo and ex vivo findings, confirmed with histology. Higher displacement contrast between cancerous and non-cancerous tissue was found ex vivo, potentially due to the lower nonlinearity in the elastic properties of ex vivo tissue. This preliminary study lays the foundation for the potential complementary application of HMI in clinical practice in conjunction with the B-mode to classify suspicious breast masses.

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.

Diverging beam with synthetic aperture technique for rotation elastography: preliminary experimental results

Rotation Elastogram (RE) is a 2D spatial distribution map of the estimated local rigid-body rotation undergone by a target when subjected to an external compression, which is one of the recent variants in elastographic imaging. A recent study has shown that inclusion-contrast in RE is independent of inclusion-background modulus contrast and thus may be helpful in distinguishing between barely-stiff benign and malignant lesions. However, estimation of quality RE requires not only precise axial displacement estimates but also lateral displacement estimates. The widely used conventional focused beamforming technique using linear array (CFB-LA) provides better lateral resolution only over the depth of focus, which still results in poorer quality lateral displacement estimates compared to the axial displacement estimates. As an alternative to overcome this depth-dependent lateral resolution and obtain an improved lateral resolution, synthetic aperture-based approaches have been proposed in literature. Recently, we developed a synthetic aperture-based method, diverging beam with synthetic aperture technique (DB-SAT) that was aimed to not only reduce the ultrasound system complexity, but also provide improved lateral resolution throughout the depth of imaging and at higher frame-rate than that is possible in CFB-LA. In this paper, we report the preliminary experimental findings on the use of DB-SAT on RE and compare the resultant image quality against that obtained using often-employed CFB-LA and the synthetic transmit aperture (STA) technique. The investigation was done on tissue-mimicking phantoms and using contrast-to-noise ratio (CNR) as the metric for performance evaluation. The estimated CNR values from the REs obtained using CFB-LA, STA, and DB-SAT were 2.69  ±  0.81, 1.35  ±  0.22, and 14.71  ±  9.83, respectively, for inclusion present at 55 mm depth. The obtained results clearly demonstrated that the quality of RE can be improved significantly, especially at larger depth, using DB-SAT compared to that obtained using CFB-LA and STA technique.

Understanding the Contrast Mechanism in Rotation Elastogram: A Parametric Study

Ultrasound elastography has been found to be useful in different clinical applications. For example, in breast imaging, axial strain elastography provides information related to tissue stiffness, which is used to characterize breast lesions as either benign or malignant. In addition, these lesions also differ in their bonding properties. Benign breast lesions are loosely bonded and malignant breast lesions are firmly bonded to the surrounding tissues. Therefore, only benign breast lesions will rotate/slip on the application of deformation. This rotation of lesions can be visualized with rotation elastography, which utilizes axial and lateral shear strain components. The contrast obtained in rotation elastography depends on various mechanical as well as ultrasound elastography parameters. However, there is no reported work that provides an understanding of the influence of these parameters on the visualized rotation contrast. In this work, the authors studied the rotation contrast by varying the mechanical parameters such as the inclusion b/a ratio, relative inclusion-background Young's modulus, amount of applied deformation and orientation of the inclusion. First, the authors performed finite-element analysis to understand the fundamental rotation contrast of the inclusion. Next, rotation elastograms obtained from ultrasound simulations in Field II and experiments on tissue-mimicking phantoms were investigated. Mean contrast was used as a metric to evaluate the quality of rotation elastograms in finite-element analysis, and contrast-to-noise ratio was used in Field II simulations and phantom experiments. The results indicate that rotation contrast was observed only in the case of loosely bonded inclusions. Further, the rotation contrast was found to depend on the inclusion asymmetry and its orientation with respect to the axis of deformation. Interestingly, it was found that a loosely bonded inclusion contrasts with surrounding tissue in rotation elastography, even in the absence of any inclusion-background modulus contrast.

Further characterization of changes in axial strain elastograms due to the presence of slippery tumor boundaries

Elastography measures tissue strain, which can be interpreted under certain simplifying assumptions to be representative of the underlying stiffness distribution. This is useful in cancer diagnosis where tumors tend to have a different stiffness to healthy tissue and has also shown potential to provide indication of the degree of bonding at tumor-tissue boundaries, which is clinically useful because of its dependence on tumor pathology. We consider the changes in axial strain for the case of a symmetrical model undergoing uniaxial compression, studied by characterizing changes in tumor contrast transfer efficiency (CTE), inclusion to background strain contrast and strain contrast generated by slip motion, as a function of Young's modulus contrast and applied strain. We present results from a finite element simulation and an evaluation of these results using tissue-mimicking phantoms. The simulation results show that a discontinuity in displacement data at the tumor boundary, caused by the surrounding tissue slipping past the tumor, creates a halo of "pseudostrain" across the tumor boundary. Mobile tumors also appear stiffer on elastograms than adhered tumors, to the extent that tumors that have the same Young's modulus as the background may in fact be visible as low-strain regions, or those that are softer than the background may appear to be stiffer than the background. Tumor mobility also causes characteristic strain heterogeneity within the tumor, which exhibits low strain close to the slippery boundary and increasing strain toward the center of the tumor. These results were reproduced in phantom experiments. In addition, phantom experiments demonstrated that when fluid lubrication is present at the boundary, these effects become applied strain-dependent as well as modulus-dependent, in a systematic and characteristic manner. The knowledge generated by this study is expected to aid interpretation of clinical strain elastograms by helping to avoid misinterpretation as well as provide additional diagnostic criteria stated in the paper and stimulate further research into the application of elastography to tumor mobility assessment.

Spatial Compounding Technique to Obtain Rotation Elastogram: A Feasibility Study

The perception of stiffness and slipperiness of a breast mass on palpation is used by physicians to assess the level of suspicion of a lesion as being malignant or benign. However, most current ultrasound elastography imaging methods provide only stiffness-related information. There is no existing approach that provides information about the local rigid body rotation undergone by only a loosely bonded, asymmetrically oriented lesion subjected to a small quasi-static compression. The inherent poor lateral resolution in ultrasound imaging poses a limitation in estimating the local rigid body rotation. Several techniques have been reported in the literature to improve the lateral resolution in ultrasound imaging, and among them is spatial compounding. In this study, we explore the feasibility of obtaining better-quality rotation elastograms with spatial compounding through simulations using Field II and experiments on tissue-mimicking phantoms. The phantom was subjected to axial compression (∼1%-2%) from the top, and the angular axial and lateral displacement estimates were obtained using a multilevel 2-D displacement tracking algorithm at different insonification angles. A rotation elastogram (RE) was obtained by taking half of the difference between the lateral gradient of the axial displacement estimates and the axial gradient of the lateral displacement estimates. Contrast-to-noise ratio was used to quantify the improvements in quality of RE. Contrast-to-noise ratio values were calculated by varying the maximum steering angle and the incremental angle, and its effects on RE quality were evaluated. Both simulation and experimental results corroborated and indicated a significant improvement in the quality of RE using compounding technique.

Rotation Elastogram Estimation Using Synthetic Transmit-aperture Technique: A Feasibility Study

It is well-documented in literature that benign breast lesions, such as fibroadenomas, are loosely bonded to their surrounding tissue and tend to slip under a small quasi-static compression, whereas malignant lesions being firmly bonded to their surrounding tissue do not slip. Recent developments in quasi-static ultrasound elastography have shown that an image of the axial-shear strain distribution can provide information about the bonding condition at the lesion-surrounding tissue boundary. Further studies analyzing the axial-shear strain elastograms revealed that nonzero axial-shear strain values appear inside the lesion, referred to as fill-in, only when a lesion is loosely bonded and asymmetrically oriented to the axis of compression. It was argued that the fill-in observed in axial-shear strain elastogram is a surrogate of the actual rigid-body rotation undergone by such a benign lesion due to slip boundary condition. However, it may be useful and perhaps easy to interpret, if the actual rigid-body rotation of the lesion can itself be visualized directly. To estimate this rotation tensor and its spatial distribution map (called a Rotation Elastogram [RE]), it would be necessary to improve the quality of lateral displacement estimates. Recently, it has been shown in the context of Non-Invasive Vascular Elastography (NIVE) that the Synthetic Transmit Aperture (STA) technique can be adapted for elastography to improve the lateral displacement estimates. Therefore, the focus of this work was to investigate the feasibility of employing the STA technique to improve the lateral displacement estimation and assess the resulting improvement in the RE quality. This investigation was done using both simulation and experimental studies. The image quality metric of contrast-to-noise ratio (CNR) was used to evaluate the quality of rotation elastograms. The results demonstrate that the contrast appeared in RE only in the case of loosely bonded inclusion, and the quality of RE improved considerably by employing the STA technique.

Review of ultrasound image guidance in external beam radiotherapy part II: intra-fraction motion management and novel applications

Imaging has become an essential tool in modern radiotherapy (RT), being used to plan dose delivery prior to treatment and verify target position before and during treatment. Ultrasound (US) imaging is cost-effective in providing excellent contrast at high resolution for depicting soft tissue targets apart from those shielded by the lungs or cranium. As a result, it is increasingly used in RT setup verification for the measurement of inter-fraction motion, the subject of Part I of this review (Fontanarosa et al 2015 Phys. Med. Biol. 60 R77-114). The combination of rapid imaging and zero ionising radiation dose makes US highly suitable for estimating intra-fraction motion. The current paper (Part II of the review) covers this topic. The basic technology for US motion estimation, and its current clinical application to the prostate, is described here, along with recent developments in robust motion-estimation algorithms, and three dimensional (3D) imaging. Together, these are likely to drive an increase in the number of future clinical studies and the range of cancer sites in which US motion management is applied. Also reviewed are selections of existing and proposed novel applications of US imaging to RT. These are driven by exciting developments in structural, functional and molecular US imaging and analytical techniques such as backscatter tissue analysis, elastography, photoacoustography, contrast-specific imaging, dynamic contrast analysis, microvascular and super-resolution imaging, and targeted microbubbles. Such techniques show promise for predicting and measuring the outcome of RT, quantifying normal tissue toxicity, improving tumour definition and defining a biological target volume that describes radiation sensitive regions of the tumour. US offers easy, low cost and efficient integration of these techniques into the RT workflow. US contrast technology also has potential to be used actively to assist RT by manipulating the tumour cell environment and by improving the delivery of radiosensitising agents. Finally, US imaging offers various ways to measure dose in 3D. If technical problems can be overcome, these hold potential for wide-dissemination of cost-effective pre-treatment dose verification and in vivo dose monitoring methods. It is concluded that US imaging could eventually contribute to all aspects of the RT workflow.

Pitavastatin calcium improves endothelial function and delays the progress of atherosclerosis in patients with hypercholesterolemia

BACKGROUND

Statins have proven efficacy in inhibiting the onset and progress of atherosclerosis. The effectiveness of pitavastatin in reversing carotid atherosclerosis associated with hypercholesterolemia (HC) is unknown.

OBJECTIVES

To explore the simultaneous effects of pitavastatin calcium on brachial arterial flow-mediated vasodilatation (FMD), carotid intima-media thickness (IMT), and arterial stiffness (β), three surrogate markers of atherosclerosis were studied in HC patients.

METHODS

A randomized, double-blind trial was performed with 40 HC subjects who fulfilled the inclusion/exclusion criteria. Patients were given pitavastatin calcium 1 mg/d (Group 1) or 2 mg/d (Group 2) for 8 weeks. There were 20 patients in each group, and 30 gender- and age-matched healthy subjects as controls were recruited. FMD of the brachial artery, carotid IMT, and arterial stiffness indicated by β were measured at baseline and at 8 weeks after starting pitavastatin calcium therapy using ultrasound techniques. Biochemical tests were also made on all subjects.

RESULTS

At baseline, higher total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C), reduced FMD, and increased β and IMT were observed in HC patients (P<0.001 for all) compared with controls. After 8 weeks, TC was decreased by 20.59%/27.56% and LDL-C 30.92%/35.64%, respectively, in comparison to baseline groups; the HC groups had reduced β and improved endothelial function over the 8-week follow-up (P<0.05-0.001); nonetheless, no significant alterations of IMT were found (P>0.05). Significant negative interactions between TC/LDL and FMD (P<0.05-0.001), positive interactions between TC and IMT (P=0.003) and between TC/LDL and β (P<0.001-0.000) were found.

CONCLUSIONS

Treatment with pitavastatin calcium exerted favorable effects on endothelial function and arterial stiffness. It also improved carotid atherosclerosis in patients with HC.

Efficacy of Sonoelastography in Distinguishing Benign from Malignant Breast Masses

OBJECTIVE

The study aimed to evaluate the influence of sonoelastographic strain ratio in distinguishing benign from malignant breast masses.

MATERIALS AND METHODS

Patients who were referred for diagnostic biopsy of a breast mass were examined by ultrasound and sonoelastography prior to percutaneous biopsy. Sonoelastography was performed twice by the same observer in the same session. The strain ratios (SR) were calculated for both measurements as well as the mean strain ratio. Results were compared with histopathologic findings. For each strain ratio, a threshold value was determined using a ROC analysis for the differentiation of benign and malignant masses.

RESULTS

After histopathological examination of 135 mass lesions in 132 female patients (mean age 48±12 years), 65 masses were diagnosed as benign and 70 as malignant. According to the Tsukuba classification with 5 scores; 44 of 65 benign masses had scores of either 1 or 2 while 56 of 70 malignant lesions had scores of either 4 or 5. No benign lesion was classified as score 5, and no malignant lesion as score 1. The mean cut-off in the two ROC measurements in distinguishing benign from malignant lesions was calculated as 4.52. When a threshold value of 4.52 was used for the mean strain ratio: the sensitivity, specificity, PPV, NPV, and accuracy rates were determined as 85.5%, 84.8%, 85.5%, 84.8% and 85.2%, respectively.

CONCLUSION

The threshold value for strain ratio in the differentiation of benign and malignant masses was detected as 4.52, and a significant intra-observer difference was not observed in this study. The diagnostic value of sonoelastograghy in distinguishing benign from malignant breast masses was higher in comparison to conventional ultrasound.

Confocal acoustic radiation force optical coherence elastography using a ring ultrasonic transducer

We designed and developed a confocal acoustic radiation force optical coherence elastography system. A ring ultrasound transducer was used to achieve reflection mode excitation and generate an oscillating acoustic radiation force in order to generate displacements within the tissue, which were detected using the phase-resolved optical coherence elastography method. Both phantom and human tissue tests indicate that this system is able to sense the stiffness difference of samples and quantitatively map the elastic property of materials. Our confocal setup promises a great potential for point by point elastic imaging in vivo and differentiation of diseased tissues from normal tissue.

The diagnostic importance of evaluation of solid breast masses by sonoelastography

OBJECTIVE

The aim of this study was to determine whether the use of a scoring method by sonoelastography in solid breast masses is helpful in differentiating benign and malignant lesions.

MATERIAL AND METHODS

One hundred and eighty solid breast masses in 155 patients (147 benign, 33 malignant) were prospectively evaluated in a two-year period. For each lesion, B-mode sonography and sonoelastography images were obtained. Elasticity scores of the lesions were determined with a 5-point scoring method by sonoelastography. The findings were compared with histopathology. The diagnostic performances of the sonoelastographic scoring and B-mode sonography methods were determined.

RESULTS

The mean scores on sonoelastography were 2.61±0.62 for benign lesions and 3.73±0.69 for malignant lesions. When a cutoff point between scores 3 and 4 was used, accuracy, sensitivity, specificity, positive and negative predictive values for B-mode sonography were found as 81%, 89%, 79%, 46% and 97%, respectively; these were 87%, 73%, 91%, 69% and 92% for the sonoelastographic scoring method.

CONCLUSION

After B-mode sonography analysis, the evaluation with the 5-point scoring method by sonoelastography might be a complementary method that increases specificity when differentiating between benign and malignant solid breast masses.

Sonoelastographic qualitative analysis for management of salivary gland masses

OBJECTIVES

Our aim was to investigate whether the use of a qualitative elasticity scoring method by sonoelastography is beneficial for management of salivary gland masses.

METHODS

Thirty-six patients with salivary gland masses (30 parotid and 6 sub-mandibular) were prospectively included in this study. For each lesion, B-mode sonographic and sonoelastographic images were obtained. Elasticity scores were determined by a 4-point scoring method. Differences among scores for benign and malignant masses were assessed by the Mann-Whitney U test. Qualitative variables were compared by the Pearson χ² test. The findings were compared with histopathologic diagnoses.

RESULTS

The score values of 28 benign masses ranged from 1 to 4, whereas the values of 8 malignant masses ranged from 2 to 4. The mean scores ± SD were 2.25 ± 0.92 for benign lesions and 3.0 ± 0.75 for malignant lesions (P < .05). When we considered scores 1 and 2 as benign and scores 3 and 4 as malignant, 10 false-positive results were determined by the 4-point scoring method, and 64.2% of benign masses were diagnosed.

CONCLUSIONS

Sonoelastography might be regarded as another sonographic parameter for management of salivary gland masses in terms of detecting benign masses.

Potential use of ultrasound speckle tracking for motion management during radiotherapy: preliminary report

We prospectively evaluated real-time ultrasound speckle tracking for monitoring soft tissue motion for image-guided radiotherapy. Two human volunteers and 1 patient with a proven hepatocellular carcinoma, who was being prepared for radiation therapy treatment, were scanned using a clinical ultrasound scanner modified to acquire and store radiofrequency signals. Scans were performed of the liver in the volunteers and the patient. In the patient, the speckle-tracking results were compared to those measured on a treatment-planning 4-dimensional computed tomogram with tumors contoured manually in each phase and with estimates made by hand on gray scale ultrasound images. The surface of the right lung and the prostate were scanned in a volunteer. The liver and lung surface were scanned during respiration. To simulate prostate motion, the ultrasound probe was rocked in an anterior-posterior direction. The correlation coefficients of all motion measurements were significantly correlated at all sites (P < .00001 for all sites) with 0 time delays. Ultrasound speckle-tracking motion estimates of tumor motion were within 2 mm of estimates made by hand tracking on gray scale ultrasound images and the 4-dimensional computed tomogram. The total tumor motion was greater than 20 mm. The angular displacement of the prostate was within 0.02 radians (1.1°) with displacements measured by hand. Speckle tracking could be used to monitor organ motion during radiotherapy.

Qualitative and semiquantitative evaluations of solid breast lesions by sonoelastography

OBJECTIVES

Our purpose was to determine whether the combination of a qualitative elasticity scoring method and a semiquantitative strain index method by sonoelastography is useful for differentiating between benign and malignant breast masses.

METHODS

Seventy-eight lesions in 71 consecutive patients with solid breast masses (62 benign and 16 malignant) were prospectively included in this study. For each lesion, B-mode sonographic and sonoelastographic images were obtained. After elasticity scores had been determined with a 5-point scoring method, strain indices of the lesions were calculated using a same-level and normal-appearing breast region as an internal reference by means of strain ratio measurement. The findings were compared with histopathologic findings. With the use of receiver operating characteristic curves, the diagnostic performances of the elasticity scoring and strain index methods were determined.

RESULTS

The mean scores ± SD on sonoelastography were 2.69 ± 0.59 for benign lesions and 3.75 ± 0.68 for malignant lesions. The mean stiffness index values were 2.03 ± 2.67 for benign lesions and 5.97 ± 4.45 for malignant lesions. The areas under the curves were 0.864 for 5-point scoring and 0.840 for the strain index. Sensitivity and specificity were 80% and 95%, respectively, for 5-point scoring, 87.5% and 72.6% for B-mode sonography, and 80% and 93% for the strain index when a cutoff point of 3.52 was used. A semiquantitative evaluation using the strain index did not contribute to the qualitative scoring evaluation.

CONCLUSIONS

After 5-point scoring with sonoelastography, additional measurement with the strain index is not mandatory for differentiating between benign and malignant breast masses.

Three-dimensional canine heart model for cardiac elastography

PURPOSE

A three-dimensional finite element analysis based canine heart model is introduced that would enable the assessment of cardiac function.

METHODS

The three-dimensional canine heart model is based on the cardiac deformation and motion model obtained from the Cardiac Mechanics Research Group at UCSD. The canine heart model is incorporated into ultrasound simulation programs previously developed in the laboratory, enabling the generation of simulated ultrasound radiofrequency data to evaluate algorithms for cardiac elastography. The authors utilize a two-dimensional multilevel hybrid method to estimate local displacements and strain from the simulated cardiac radiofrequency data.

RESULTS

Tissue displacements and strains estimated along both the axial and lateral directions (with respect to the ultrasound scan plane) are compared to the actual scatterer movement obtained using the canine heart model. Simulation and strain estimation algorithms combined with the three-dimensional canine heart model provide high resolution displacement and strain curves for improved analysis of cardiac function. The use of principal component analysis along parasternal cardiac short axis views is also presented.

CONCLUSIONS

A 3D cardiac deformation model is proposed for evaluating displacement tracking and strain estimation algorithms for cardiac strain imaging. Validation of the model is shown using ultrasound simulations to generate axial and lateral displacement and strain curves that are similar to the actual axial and lateral displacement and strain curves.

Axial-shear strain imaging for differentiating benign and malignant breast masses

Axial strain imaging has been utilized for the characterization of breast masses for over a decade; however, another important feature namely the shear strain distribution around breast masses has only recently been used. In this article, we examine the feasibility of utilizing in vivo axial-shear strain imaging for differentiating benign from malignant breast masses. Radio-frequency data was acquired using a VFX 13-5 linear array transducer on 41 patients using a Siemens SONOLINE Antares real-time clinical scanner at the University of Wisconsin Breast Cancer Center. Free-hand palpation using deformations of up to 10% was utilized to generate axial strain and axial-shear strain images using a two-dimensional cross-correlation algorithm from the radio-frequency data loops. Axial-shear strain areas normalized to the lesion size, applied strain and lesion strain contrast was utilized as a feature for differentiating benign from malignant masses. The normalized axial-shear strain area feature estimated on eight patients with malignant tumors and 33 patients with fibroadenomas was utilized to demonstrate its potential for lesion differentiation. Biopsy results were considered the diagnostic standard for comparison. Our results indicate that the normalized axial-shear strain area is significantly larger for malignant tumors compared with benign masses such as fibroadenomas. Axial-shear strain pixel values greater than a specified threshold, including only those with correlation coefficient values greater than 0.75, were overlaid on the corresponding B-mode image to aid in diagnosis. A scatter plot of the normalized area feature demonstrates the feasibility of developing a linear classifier to differentiate benign from malignant masses. The area under the receiver operator characteristic curve utilizing the normalized axial-shear strain area feature was 0.996, demonstrating the potential of this feature to noninvasively differentiate between benign and malignant breast masses.

DYNAMIC OPTICAL COHERENCE ELASTOGRAPHY: A REVIEW

With the development of optical coherence tomography, the application optical coherence elastography (OCE) has gained more and more attention in biomechanics for its unique features including micron-scale resolution, real-time processing, and non-invasive imaging. In this review, one group of OCE techniques, namely dynamic OCE, are introduced and discussed including external dynamic OCE mapping and imaging of ex vivo breast tumor, external dynamic OCE measurement of in vivo human skin, and internal dynamic OCE including acoustomotive OCE and magnetomotive OCE. These techniques overcame some of the major drawbacks of traditional static OCE, and broadened the OCE application fields. Driven by scientific needs to engineer new quantitative methods that utilize the high micron-scale resolution achievable with optics, results of biomechanical properties were obtained from biological tissues. The results suggest potential diagnostic and therapeutic clinical applications. Results from these studies also help our understanding of the relationship between biomechanical variations and functional tissue changes in biological systems.

Multilevel hybrid 2D strain imaging algorithm for ultrasound sector/phased arrays

Two-dimensional (2D) cross-correlation algorithms are necessary to estimate local displacement vector information for strain imaging. However, most of the current two-dimensional cross-correlation algorithms were developed for linear array transducers. Although sector and phased array transducers are routinely used for clinical imaging of abdominal and cardiac applications, strain imaging for these applications has been performed using one-dimensional (1D) cross-correlation analysis. However, one-dimensional cross-correlation algorithms are unable to provide accurate and precise strain estimation along all the angular insonification directions which can range from -45 degrees to 45 degrees with sector and phased array transducers. In addition, since sector and phased array based images have larger separations between beam lines as the pulse propagates deeper into tissue, signal decorrelation artifacts with deformation or tissue motion are more pronounced. In this article, the authors propose a multilevel two-dimensional hybrid algorithm for ultrasound sector and phased array data that demonstrate improved tracking and estimation performance when compared to the traditional 1D cross-correlation or 2D cross-correlation based methods. Experimental results demonstrate that the signal-to-noise and contrast-to-noise ratio estimates improve significantly for smaller window lengths for the hybrid method when compared to the currently used one-dimensional or two-dimensional cross-correlation algorithms. Strain imaging results on ex vivo thermal lesions created in liver tissue and in vivo on cardiac short-axis views demonstrate the improved image quality obtained with this method.

Quasi-Static Ultrasound Elastography

Elastography is a new imaging modality where elastic tissue parameters related to the structural organization of normal and pathological tissues are imaged. Basic principles underlying the quasi-static elastography concept and principles are addressed. The rationale for elastographic imaging is reinforced using data on elastic properties of normal and abnormal soft tissues. The several orders of magnitude difference between the elastic modulus of normal and abnormal tissues which is the primary contrast mechanism in elastographic imaging underlines the probability of success with this imaging modality. Recent advances enabling the clinical practice of elastographic imaging in real-time on clinical ultrasound systems is also discussed.In quasi-static elastography, radiofrequency echo signals acquired before and after a small (about 1%) of applied deformation are correlated to estimate tissue displacements. Local tissue displacement vector estimates between small segments of the pre- and post-deformation signals are estimated and the corresponding strain distribution imaged. Elastographic imaging techniques are based on the hypothesis that soft tissues deform more than stiffer tissue, and these differences can be quantified in images of the tissue strain tensor or the Young's modulus.Clinical applications of quasi-static elastography have mushroomed over the last decade, with the most commonly imaged areas being the breast, prostate, thyroid, cardiac, treatment monitoring of ablation procedures and vascular imaging applications.

Principal component analysis of shear strain effects

Shear stresses are always present during quasi-static strain imaging, since tissue slippage occurs along the lateral and elevational directions during an axial deformation. Shear stress components along the axial deformation axes add to the axial deformation while perpendicular components introduce both lateral and elevational rigid motion and deformation artifacts into the estimated axial and lateral strain tensor images. A clear understanding of these artifacts introduced into the normal and shear strain tensor images with shear deformations is essential. In addition, signal processing techniques for improved depiction of the strain distribution is required. In this paper, we evaluate the impact of artifacts introduced due to lateral shear deformations on the normal strain tensors estimated by varying the lateral shear angle during an axial deformation. Shear strains are quantified using the lateral shear angle during the applied deformation. Simulation and experimental validation using uniformly elastic and single inclusion phantoms were performed. Variations in the elastographic signal-to-noise and contrast-to-noise ratios for axial deformations ranging from 0% to 5%, and for lateral deformations ranging from 0 to 5 degrees were evaluated. Our results demonstrate that the first and second principal component strain images provide higher signal-to-noise ratios of 20 dB with simulations and 10 dB under experimental conditions and contrast-to-noise ratio levels that are at least 20 dB higher when compared to the axial and lateral strain tensor images, when only lateral shear deformations are applied. For small axial deformations, the lateral shear deformations significantly reduces strain image quality, however the first principal component provides about a 1-2dB improvement over the axial strain tensor image. Lateral shear deformations also significantly increase the noise level in the axial and lateral strain tensor images with larger axial deformations. Improved elastographic signal and contrast-to-noise ratios in the first principal component strain image are always obtained for both simulation and experimental data when compared to the corresponding axial strain tensor images in the presence of both axial and lateral shear deformations.

Effect of lesion boundary conditions on axial strain elastograms: a parametric study

Ultrasound elastography produces strain images of compliant tissues under quasi-static compression. When a material is compressed, there are several parameters that affect the stress-distribution and, hence, the strain distribution in the material. The state of bonding of an inclusion to the background material is a critical parameter. Heretofore, in the field of elastography, the inclusion was considered to be firmly bonded to the background material and analytical solutions were derived for the elasticity problem involving simple geometries like circular inclusion (for two dimensional [2D]) and spherical inclusion (three dimensional [3D]). Under these conditions, simple analytical expressions relating the strain contrast to the modulus contrast were derived. However, it is known that the state of bonding of some tumors to their surrounding tissues depends on the type of the lesion. For example, benign lesions of the breast are known to be loosely bonded to the surrounding tissue, while malignant breast lesions are firmly bonded. In this study, we perform a parametric study using finite element modeling (FEM) to investigate the validity of the analytical expression relating the strain contrast to the modulus contrast, when the state of bonding at the inclusion/background interface spans a large dynamic range. The results suggest that estimated modulus contrast using the analytical expression is sensitive to the region-of-interest within the inclusion that is considered in the computation of the strain contrast. By considering the inclusion region lying along the axis of lateral symmetry instead of whole region of the inclusion, the estimated modulus contrast (obtained using the analytical expression present in the literature) can be computed to within a systematic error of 10% of the actual modulus contrast. Additional estimation errors are expected to accrue in experimental and in vivo conditions.

Two-dimensional strain imaging: a new echocardiographic advance with research and clinical applications

Over the past two decades the quest for quantitative evaluation of left ventricular function and regional wall motion has escalated, allowing several aspects of myocardial contractile patterns to be quantified, both during stress echocardiography and in the assessment of dyssynchrony. Most of the literature to date has used Tissue Doppler Imaging (TDI) techniques to assess essentially long-axis function due to the angle dependency of Doppler based techniques. This brief review introduces the early development, validation and potential clinical applications of a new technique of quantifying two-dimensional (radial and circumferential) strains and strain rates through tracking myocardial "speckles". In-vivo and in-vitro validation of this 2D-strain imaging technique has been undertaken and reached a point where it is considered ready for more widespread investigations into clinical utility. One important advantage over TDI techniques is that it is not limited by dependency on the angle of insonation. Several recent studies looking at ventricular function in specific groups of patients have reported practical ability to distinguish the abnormally from the normally contracting regions of ventricular walls. It provides new and complementary quantitative information about ventricular dyssynchrony and regional wall motion abnormalities. More research studies are needed to determine the sensitivity and specificity of the measurements obtained using this technique and define its strengths and limitations. In particular, whether the measured values correlate well with clinical outcomes will need to be established in longitudinal interventional studies. The clinical utilities of this technique over the coming years are likely to expand rapidly.

Visualization of bonding at an inclusion boundary using axial-shear strain elastography: a feasibility study

Ultrasound elastography produces strain images of compliant tissues under quasi-static compression. In axial-shear strain elastography, the local axial-shear strain resulting from application of quasi-static axial compression to an inhomogeneous material is imaged. The overall hypothesis of this work is that the pattern of axial-shear strain distribution around the inclusion/background interface is completely determined by the bonding at the interface after normalization for inclusion size and applied strain levels, and that it is feasible to extract certain features from the axial-shear strain elastograms to quantify this pattern. The mechanical model used in this study consisted of a single stiff circular inclusion embedded in a homogeneous softer background. First, we performed a parametric study using finite-element analysis (FEA) (no ultrasound involved) to identify possible features that quantify the pattern of axial-shear strain distribution around an inclusion/background interface. Next, the ability to extract these features from axial-shear strain elastograms, estimated from simulated pre- and post-compression noisy RF data, was investigated. Further, the feasibility of extracting these features from in vivo breast data of benign and malignant tumors was also investigated. It is shown using the FEA study that the pattern of axial-shear strain distribution is determined by the degree of bonding at the inclusion/background interface. The results suggest the feasibility of using normalized features that capture the region of positive and negative axial-shear strain area to quantify the pattern of the axial-shear strain distribution. The simulation results showed that it was feasible to extract the features, as identified in the FEA study, from axial-shear strain elastograms. However, an effort must be made to obtain axial-shear strain elastograms with the highest signal-to-noise ratio (SNR(asse)) possible, without compromising the resolution. The in vivo results demonstrated the feasibility of producing and extracting features from the axial-shear strain elastograms from breast data. Furthermore, the in vivo axial-shear strain elastograms suggest an additional feature not identified in the simulations that may potentially be used for distinguishing benign from malignant tumors-the proximity of the axial-shear strain regions to the inclusion/background interface identified in the sonogram.

Improvement of elastographic displacement estimation using a two-step cross-correlation method

The cross-correlation algorithm used to compute the local strain components for elastographic imaging requires a minimum radio-frequency data segment length of around 10 wavelengths to obtain accurate and precise strain estimates with a reasonable signal-to-noise ratio. Shorter radio-frequency data segments generally introduce increased estimation errors as the information content in the data segment reduces. However, shorter data segments and increased overlaps are essential to improve the axial resolution in the strain image. In this paper, we propose a two-step cross-correlation technique that enables the use of window lengths on the order of a single wavelength to provide displacement and strain estimates with similar noise properties as those obtained with a 10 wavelength window. The first processing step utilizes a window length on the order of 10 wavelengths to obtain coarse displacement estimates between the pre- and post-compression radio frequency data frames. This coarse displacement is then interpolated and utilized as the initial guess-estimate for the second cross-correlation processing step using the smaller window. This step utilizes a single wavelength window to improve the axial resolution in strain estimation, without significantly compromising the noise properties of the image. Simulation and experimental results show that the signal-to-noise and contrast-to-noise ratio estimates improve significantly at the smaller window lengths with the two-step processing when compared with the use of a similar sized window in the currently utilized single window method.

Signal-to-noise ratio, contrast-to-noise ratio and their trade-offs with resolution in axial-shear strain elastography

In axial-shear strain elastography, the local axial-shear strain resulting from the application of quasi-static axial compression to an inhomogeneous material is imaged. In this paper, we investigated the image quality of the axial-shear strain estimates in terms of the signal-to-noise ratio (SNR(asse)) and contrast-to-noise ratio (CNR(asse)) using simulations and experiments. Specifically, we investigated the influence of the system parameters (beamwidth, transducer element pitch and bandwidth), signal processing parameters (correlation window length and axial window shift) and mechanical parameters (Young's modulus contrast, applied axial strain) on the SNR(asse) and CNR(asse). The results of the study show that the CNR(asse) (SNR(asse)) is maximum for axial-shear strain values in the range of 0.005-0.03. For the inclusion/background modulus contrast range considered in this study (<10), the CNR(asse) (SNR(asse)) is maximum for applied axial compressive strain values in the range of 0.005%-0.03%. This suggests that the RF data acquired during axial elastography can be used to obtain axial-shear strain elastograms, since this range is typically used in axial elastography as well. The CNR(asse) (SNR(asse)) remains almost constant with an increase in the beamwidth while it increases as the pitch increases. As expected, the axial shift had only a weak influence on the CNR(asse) (SNR(asse)) of the axial-shear strain estimates. We observed that the differential estimates of the axial-shear strain involve a trade-off between the CNR(asse) (SNR(asse)) and the spatial resolution only with respect to pitch and not with respect to signal processing parameters. Simulation studies were performed to confirm such an observation. The results demonstrate a trade-off between CNR(asse) and the resolution with respect to pitch.

Noise performance and signal-to-noise ratio of shear strain elastograms

In this paper, we develop a theoretical expression for the signal-to-noise ratio (SNR) of shear strain elastograms. The previously-developed ideas for the axial strain filter (ASF) and lateral strain filter (LSF) are extended to define the concept of the shear strain filter (SSF). Some of our theoretical results are verified using simulations and phantom experiments. The results indicate that the signal-to-noise ratio of shear-strain elastograms (SNRsse) improves with increasing shear strain and with improvements in system parameters such as the sonographic signal-to-noise ratio (SNRs) beamwidth, center frequency and fractional bandwidth. The results also indicate that the amount of axial strain present along with the shear strain is an important parameter that determines the upper bound on SNRsse. The SNRsse will be higher in the absence of additional deformation due to axial strain.

Improvements in elastographic contrast-to-noise ratio using spatial-angular compounding

Spatial-angular compounding is a new technique developed for improving the signal-to-noise ratio (SNR) in elastography. Under this method, elastograms of a region-of-interest (ROI) are obtained from a spatially weighted average of local strain estimated along different insonification angles. In this article, we investigate the improvements in the strain contrast and contrast-to-noise ratio (CNR) of the spatially compounded elastograms. Spatial angular compounding is also applied and evaluated in conjunction with global temporal stretching. Quantitative experimental results obtained using a single-inclusion tissue-mimicking phantom demonstrate that the strain contrast reduces slightly but the CNR improves by around 8 to 13 dB. We also present experimental spatial angular compounding results obtained from an in vitro thermal lesion in canine liver tissue embedded in a gelatin phantom that demonstrate the improved visual characteristics (due to the improved CNR) of the compound elastogram. The experimental results provide guidelines for the practical range of maximum insonification angles and estimates of the optimum angular increment.

Impact of gas bubbles generated during interstitial ablation on elastographic depiction of in vitro thermal lesions

OBJECTIVE

Artifacts from gas bubble formation during radio frequency ablation along with the poor intrinsic contrast between normal and treated regions (zone of necrosis) are considerable problems for the visualization of the necrotic region on conventional sonography. Sonographic elastography is very effective for visualizing the zone of necrosis, but it uses the same echo signals to estimate strain as those used to form gray scale images. Thus, the impact of gas bubbles on strain images or elastograms must be investigated.

METHODS

Radio frequency ablation was performed in vitro on liver tissue samples, approximately 40 x 40 x 20 mm, encased in 80-mm cubed gelatin phantoms. Elastograms generated at different instants during the ablation procedures were obtained on a real-time scanner with a 5-MHz linear array. Sequences of elastograms illustrate the growth of the thermal lesion.

RESULTS

Degradation of the distal boundary of the thermal lesion was observed. The degradation was confined to the lower-fifth quadrant of the thermal lesion. However, accurate estimates of lesion areas could still be obtained by extrapolation of the thermal lesion boundary.

CONCLUSIONS

Elastograms of thermal lesions in vitro can be obtained during radio frequency ablation. Some loss of thermal lesion boundary information on strain images was observed in regions where attenuation due to gas bubbles reduced the signal-noise ratio of the echo signals.

Comparing elastographic strain images with modulus images obtained using nanoindentation: preliminary results using phantoms and tissue samples

Conventional elastography involves quasistatic mechanical compression (external or internal) of the tissue under ultrasonic insonification to obtain radiofrequency (RF) A-lines before and after compression. Cross-correlation of the pre- and postcompression A-lines results in displacement images with axial gradients that produce the strain images (strain elastograms). Though the strain elastograms show structural similarities to the modulus images, they are not related in a simple way to the modulus images because the strains depend on both modulus and geometry of the materials being deformed. Therefore, a quantification of the similarities between the strain and modulus images may enhance the interpretation confidence of strain elastograms in depicting tissue structure. To demonstrate similarities between modulus images and strain elastograms, a feasibility study of using nanoindentation to obtain modulus images of thin slices of tissue and tissue-mimicking phantoms (agar-gelatin mixtures) was performed first, with encouraging results. This was followed by a comparison of modulus images and strain elastograms obtained from the same sample slices. The experimental results indicated that, under certain experimental conditions, it is feasible to perform quantitative comparisons between strain images (using elastography) and modulus images. A good visual, as well as quantitative, correspondence between structures in the modulus and strain images could be obtained at a 3-mm scale.

Detection of lesions using differential rates of speckle decorrelation

Breast lesions that may be cancerous generally possess a higher elastic modulus than the surrounding tissue. Thus, they are sensed to move differently from the surrounding tissue during finger-based palpation. Since the palpation is subjective in nature, a method using ultrasound for detection of the differential motion associated with hard lesions is proposed. It is intended that regions of detected 'lump' be highlighted on top of a conventional B-mode image so that the user can positively associate palpated lumps with specific regions on the anatomical B-mode image. We detect the differential motion associated with a lesion by detecting a local abnormality in a map of image-to-image correlations as the transducer is swept in an elevational direction. Experiments were performed with three different tissue mimicking phantoms. The results indicate that the location of the mobile lesion can be successfully estimated whereas immobile lesions are not very detectable. This approach is simple and requires only a short period of time for each scan. It has practically no theoretical incremental cost.

A finite-element approach for Young's modulus reconstruction

Modulus imaging has great potential in soft-tissue characterization since it reveals intrinsic mechanical properties. A novel Young's modulus reconstruction algorithm that is based on finite-element analysis is reported here. This new method overcomes some limitations in other Young's modulus reconstruction methods. Specifically, it relaxes the force boundary condition requirements so that only the force distribution at the compression surface is necessary, thus making the new method more practical. The validity of the new method is demonstrated and the performance of the algorithm with noise in the input data is tested using numerical simulations. Details of how to apply this method under clinical conditions is also discussed.

Application of nonlinear phenomena induced by focused ultrasound to bone imaging

A tissue deformability image is obtained with the vibroacoustography imaging method using mechanical low-frequency (LF) excitation. This ultrasonic excitation is created locally by means of a focused annular array emitting two primary beams at two close frequencies, f(1) and f(2) (f(2) = f(1) + f(LF)). The LF acoustic emission resulting from the vibration of the medium is detected by a sensitive hydrophone and then used to form the image. This noninvasive imaging method was demonstrated in this study to be suitable for bone imaging, with x and y transverse resolutions less than 300 micro m. Two bone sites susceptible to demineralization were tested: the calcaneus and the neck of the femur. The vibroacoustic method provides valuable ultrasonic images regarding the structure and the elastic properties of bone tissue. Correlation was made between vibroacoustic bone images, performed in vitro, and images acquired by other imaging methods (i.e., bone ultrasound attenuation and x-ray computerized tomography (CT)). Moreover, the amplitudes of vibroacoustic signals radiating from phosphocalcic ceramic samples (bone substitute) of different porosity were evaluated. The good correlation between these results and the description of our images and the quality of vibroacoustic images indicate that bone decalcification could be detected using vibroacoustography.

A freehand elastographic imaging approach for clinical breast imaging: system development and performance evaluation

A prototype freehand elastographic imaging system has been developed for clinical breast imaging. The system consists of a fast data acquisition system, which is able to capture sequences of intermediate frequency echo frames at full frame rate from a commercial ultrasound scanner whilst the breast is deformed using hand-induced transducer motion. Two-dimensional echo tracking was used in combination with global distortion compensation and multi-compression averaging to minimise decorrelation noise incurred when stress is applied using hand-induced transducer motion. Experiments were conducted on gelatine phantoms to evaluate the quality of elastograms produced using the prototype system relative to those produced using mechanically induced transducer motion. The strain sensitivity and contrast-to-noise ratio of freehand elastograms compared favourably with elastograms produced using mechanically induced transducer motion. However, better dynamic range and signal-to-noise ratio was achieved when elastograms were created using mechanically induced transducer motion. Despite the loss in performance incurred when stress is applied using hand-induced transducer motion, it was concluded that the prototype system performed sufficiently well to warrant clinical evaluation.

Maximum-likelihood approach to strain imaging using ultrasound

A maximum-likelihood (ML) strategy for strain estimation is presented as a framework for designing and evaluating bioelasticity imaging systems. Concepts from continuum mechanics, signal analysis, and acoustic scattering are combined to develop a mathematical model of the ultrasonic waveforms used to form strain images. The model includes three-dimensional (3-D) object motion described by affine transformations, Rayleigh scattering from random media, and 3-D system response functions. The likelihood function for these waveforms is derived to express the Fisher information matrix and variance bounds for displacement and strain estimation. The ML estimator is a generalized cross correlator for pre- and post-compression echo waveforms that is realized by waveform warping and filtering prior to cross correlation and peak detection. Experiments involving soft tissuelike media show the ML estimator approaches the Cramer-Rao error bound for small scaling deformations: at 5 MHz and 1.2% compression, the predicted lower bound for displacement errors is 4.4 microns and the measured standard deviation is 5.7 microns.

Strain imaging with a deformable mesh

Ultrasonic strain imaging has drawn much attention recently because of its ability to noninvasively provide information on spatial variation of the elastic properties of soft tissues. Traditionally, local strain is estimated by scaling and cross correlating pre- and postcompression ultrasound echo fields. However, when the motion field generated by compression is more complex, scaling and cross correlation can no longer provide precise displacement estimates because of signal decorrelation. We introduce a new algorithm based on the deformable mesh method. This algorithm can accommodate more general forms of motion, namely, the motion that can be described by bilinear transformations. We applied the new algorithm to three sets of data in order to evaluate its performance. In the first set of data, primitive motions such as shearing and rotation are simulated. The second set of data is collected by compressing a tissue-mimicking phantom with three hard inclusions. The third experiment involves an ex vivo pig kidney embedded in a block of gelatin. The results from all three experiments show improvements with the new algorithm over other methods.

Elastographic imaging of thermal lesions in soft tissue: a preliminary study in vitro

The use of elastography for the visualization of thermal lesions in biological soft tissue in vitro was investigated. Thermal lesions were created in samples of postmortem ovine kidney using a surgical neodymium: YAG (Nd:YAG) laser. The kidney samples were cast in gel, and elastographic images of the lesions were constructed using sonographic information and external markers to locate the region of interest. Gross pathology of the kidney samples confirmed the dimensions of the lesions. Good correlation between the lesion length along the laser fiber axis and maximum diameter measured off of the fiber axis determined from elastographic images and gross pathology photographs was found.

Modeling of the correlation of analytic ultrasound radiofrequency signals for angle-independent motion detection

Conventional pulsed ultrasound systems are able to assess motion of scatterers in the direction of the ultrasound beam, i.e., axial motion, by determining the lag at which the maximum correlation occurs between consecutively-received radiofrequency (rf) signals. The accuracy, resolution, and processing time of this technique is improved by making use of a model for the correlation of rf signals. All previously-described correlation models only include axial motion, but it is common knowledge that lateral motion, i.e., motion in the plane perpendicular to the beam axis, reduces the correlation of rf signals in time. In the present paper, a model for the correlation of analytic rf signals in depth and time is derived and verified. It also includes, aside of some signal and transducer parameters, both axial and lateral motion. The influence of lateral motion on the correlation of (analytic) rf signals is strongly related to local phase and amplitude characteristics of the ultrasound beam. It is shown how the correlation model, making use of an ultrasound transducer with a circular beam shape, can be applied to estimate, independent of angle, the magnitude of the actual motion. Furthermore, it is shown that the model can be applied to estimate the local signal-to-noise ratio and rf bandwidth.

Characterization of elastographic noise using the envelope of echo signals

A theoretical formulation characterizing the noise performance of strain estimation using envelope signals is presented for the cross-correlation based strain estimator in elastography, using a modified strain filter approach. The strain filter describes the relationship among the elastographic signal-to-noise ratio (SNRe), sensitivity, contrast-to-noise ratio and dynamic range for a given resolution in the elastogram, as determined by the cross-correlation window length and window overlap. Theoretical results indicate that the envelope strain filter noise performance (SNRe level) is about half that obtained in the ratio frequency (RF) case (fo = 7.5 MHz). Simulation results corroborate the trend predicted using the strain filter. Experimental SNRe vs. strain plots presented in this article illustrate the same trend as the theoretical results. These plots allow a quantitative comparison of the elastograms obtained with RF and envelope signal processing. For small strains, the performance obtained using RF signals is superior to that obtained for envelope signals (since jitter errors are smaller due to the utilization of phase information in RF signals). However, for large tissue strains, envelope analysis provides an accurate estimate of the tissue strain (since envelope signal decorrelation is smaller than RF signal decorrelation at large strains). An algorithm that combines the low-noise characteristics of the cross-correlation analysis using RF signals at small strains and envelope signals for estimation of large tissue strains is proposed to improve the dynamic range in the elastogram.

2-D companding for noise reduction in strain imaging

Companding is a signal preprocessing technique for improving the precision of correlation-based time delay measurements. In strain imaging, companding is applied to warp 2-D or 3-D ultrasonic echo fields to improve coherence between data acquired before and after compression. It minimizes decorrelation errors, which are the dominant source of strain image noise. The word refers to a spatially variable signal scaling that compresses and expands waveforms acquired in an ultrasonic scan plane or volume. Temporal stretching by the applied strain is a single-scale (global), 1-D companding process that has been used successfully to reduce strain noise. This paper describes a two-scale (global and local), 2-D companding technique that is based on a sum-absolute-difference (SAD) algorithm for blood velocity estimation. Several experiments are presented that demonstrate improvements in target visibility for strain imaging. The results show that, if tissue motion can be confined to the scan plane of a linear array transducer, displacement variance can be reduced two orders of magnitude using 2-D local companding relative to temporal stretching.

Ultrasonic measurement of differential displacement and strain in a vascular model

The potential in intravascular ultrasound imaging for characterizing regional arterial elasticity was examined in an experimental tissue-equivalent vessel model. Differential intrawall displacement measurement, the first step in regional elasticity determination, was investigated using a crosscorrelation tracking algorithm. Calibration studies showed that tracking accuracy varied significantly with tracking direction (axial versus lateral) and position in the field of the transducer. Midfield geometric error in the axial direction for a nominal displacement of 100 microns was 5.5 microns whereas the corresponding error in the lateral direction was 31.7 microns. Displacement was tracked in serial intravascular images of vessel phantoms acquired during stepwise pressurization experiments from 0-250 mmHg. Two-dimensional grey scale maps of axial, lateral and net intrawall displacement components over the full pressurization range were generated. Displacement profiles demonstrated successful detection of differential radial displacement and good correlation with theoretical profiles (root mean square difference 3%). The corresponding experimental strain profiles were significantly noisier (root mean square difference 76%) due to small fluctuations in the displacement data. This work demonstrates that, with further refinement, regional strain mapping in vessel walls with intravascular ultrasound imaging is feasible. Mechanical characterization of arteries may provide a new tool to aid and treating atherosclerotic lesions.