Effect of pathological heterogeneity on shear wave elasticity imaging in the staging of deep venous thrombosis

Effect of rt-PA on Shear Wave Mechanical Assessment and Quantitative Ultrasound Properties of Blood Clot Kinetics In Vitro

OBJECTIVE

The consequences associated with blood clots are numerous and are responsible for many deaths worldwide. The assessment of treatment efficacy is necessary for patient follow-up and to detect treatment-resistant patients. The aim of this study was to characterize the effect of treatment on blood clots in vitro using quantitative ultrasound parameters.

METHODS

Blood from 10 pigs was collected to form three clots per pig in gelatin phantoms. Clots were subjected to 1) no treatment, 2) rt-PA (recombinant tissue plasminogen activator) treatment after 20 minutes of clotting, and 3) rt-PA treatment after 60 minutes of clotting. Clots were weighted before and after the experiment to assess the treatment effect by the mass loss. The clot kinetics was studied over 100 minutes using elastography (Young's modulus, shear wave dispersion, and shear wave attenuation). Homodyne K-distribution (HKD) parameters derived from speckle statistics were also studied during clot formation and dissolving (diffuse-to-total signal power ratio and intensity parameters).

RESULTS

Treated clots loosed significantly more mass than non-treated ones (P < .005). A significant increase in Young's modulus was observed over time (P < .001), and significant reductions were seen for treated clots at 20 or 60 minutes compared with untreated ones (P < .001). The shear wave dispersion differed for treated clots at 60 minutes versus no treatments (P < .001). The shear wave attenuation decreased over time (P < .001), and was different for clots treated at 20 minutes versus no treatments (P < .031). The HKD intensity parameter varied over time (P < .032), and was lower for clots treated at 20 and 60 minutes than those untreated (P < .001 and P < .02).

CONCLUSION

The effect of rt-PA treatment could be confirmed by a decrease in Young's modulus and HKD intensity parameter. The shear wave dispersion and shear wave attenuation were sensitive to late and early treatments, respectively. The Young's modulus, shear wave attenuation, and HKD intensity parameter varied over time despite treatment.

Miniaturized Stacked Transducer for Intravascular Sonothrombolysis With Internal-Illumination Photoacoustic Imaging Guidance and Clot Characterization

Thromboembolism in blood vessels can lead to stroke or heart attack and even sudden death unless brought under control. Sonothrombolysis enhanced by ultrasound contrast agents has shown promising outcome on effective treatment of thromboembolism. Intravascular sonothrombolysis was also reported recently with a potential for effective and safe treatment of deep thrombosis. Despite the promising treatment results, the treatment efficiency for clinical application may not be optimized due to the lack of imaging guidance and clot characterization during the thrombolysis procedure. In this paper, a miniaturized transducer was designed to have an 8-layer PZT-5A stacked with an aperture size of 1.4 × 1.4 mm2 and assembled in a customized two-lumen 10-Fr catheter for intravascular sonothrombolysis. The treatment process was monitored with internal-illumination photoacoustic tomography (II-PAT), a hybrid imaging modality that combines the rich contrast of optical absorption and the deep penetration of ultrasound detection. With intravascular light delivery using a thin optical fiber integrated with the intravascular catheter, II-PAT overcomes the penetration depth limited by strong optical attenuation of tissue. In-vitro PAT-guided sonothrombolysis experiments were carried out with synthetic blood clots embedded in tissue phantom. Clot position, shape, stiffness, and oxygenation level can be estimated by II-PAT at clinically relevant depth of ten centimeters. Our findings have demonstrated the feasibility of the proposed PAT-guided intravascular sonothrombolysis with real-time feedback during the treatment process.

Elastometry of clot phantoms via magnetomotive ultrasound-based resonant acoustic spectroscopy

Objective. An ultrasound-based system capable of both imaging thrombi against a dark field and performing quantitative elastometry could allow for fast and cost-effective thrombosis diagnosis, staging, and treatment monitoring. This study investigates a contrast-enhanced approach for measuring the Young’s moduli of thrombus-mimicking phantoms. Approach. Magnetomotive ultrasound (MMUS) has shown promise for lending specific contrast to thrombi by applying a temporally modulated force to magnetic nanoparticle (MNP) contrast agents and measuring resulting tissue displacements. However, quantitative elastometry has not yet been demonstrated in MMUS, largely due to difficulties inherent in measuring applied magnetic forces and MNP densities. To avoid these issues, in this work magnetomotive resonant acoustic spectroscopy (MRAS) is demonstrated for the first time in ultrasound. Main results. The resonance frequencies of gelatin thrombus-mimicking phantoms are shown to agree within one standard deviation with finite element simulations over a range of phantom sizes and Young’s moduli with less than 16% error. Then, in a proof-of-concept study, the Young’s moduli of three phantoms are measured using MRAS and are shown to agree with independent compression testing results. Significance. The MRAS results were sufficiently precise to differentiate between thrombus phantoms with clinically relevant Young’s moduli. These findings demonstrate that MRAS has potential for thrombus staging.

Effect of Thrombin and Incubation Time on Porcine Whole Blood Clot Elasticity and Recombinant Tissue Plasminogen Activator Susceptibility

Deep vein thrombosis is a major source of morbidity and mortality worldwide. Catheter-directed thrombolytics are the frontline approach for vessel recanalization, though fibrinolytic efficacy is limited for stiff, chronic thrombi. Although thrombin has been used in preclinical models to induce thrombosis, the effect on lytic susceptibility and clot stiffness is unknown. The goal of this study was to explore the effect of bovine thrombin concentration and incubation time on lytic susceptibility and stiffness of porcine whole blood clots in vitro. Porcine whole blood was allowed to coagulate at 37°C in glass pipets primed with 2.5 or 15 U/mL thrombin for 15 to 120 min. Lytic susceptibility to recombinant tissue plasminogen activator (rt-PA, alteplase) over a range of concentrations (3.15-107.00 µg/mL) was evaluated using percentage clot mass loss. The Young's moduli and degrees of retraction of the clots were estimated using ultrasound-based single-track-location shear wave elasticity and B-mode imaging, respectively. Percentage mass loss decreased and clot stiffness increased with the incubation period. Clots formed with 15 U/mL and incubated for 2 h exhibited properties similar to those of highly retracted clots: Young's modulus of 2.39 ± 0.36 kPa and percentage mass loss of 8.69 ± 2.72% when exposed to 3.15 µg/mL rt-PA. The histological differences between thrombin-induced porcine whole blood clots in vitro and thrombi in vivo are described.

Fibrous hydrogels under biaxial confinement

Confinement of fibrous hydrogels in narrow capillaries is of great importance in biological and biomedical systems. Stretching and uniaxial compression of fibrous hydrogels have been extensively studied; however, their response to biaxial confinement in capillaries remains unexplored. Here, we show experimentally and theoretically that due to the asymmetry in the mechanical properties of the constituent filaments that are soft upon compression and stiff upon extension, filamentous gels respond to confinement in a qualitatively different manner than flexible-strand gels. Under strong confinement, fibrous gels exhibit a weak elongation and an asymptotic decrease to zero of their biaxial Poisson’s ratio, which results in strong gel densification and a weak flux of liquid through the gel. These results shed light on the resistance of strained occlusive clots to lysis with therapeutic agents and stimulate the development of effective endovascular plugs from gels with fibrous structures for stopping vascular bleeding or suppressing blood supply to tumors.

Studies of the confinement of filamentous hydrogels such as fibrin, a major component of blood clots, can shed light on the resistance of strained occlusive clots to lysis with therapeutic agents. Here, the authors show the mechanism of the response of fibrous hydrogels to confinement.

Different precompression does not reduce the diagnostic value of virtual touch tissue imaging and quantification (VTIQ) in breast lesions, especially for the ratio of the shear wave velocity between lesions and surrounding tissues

OBJECTIVE

To investigate the accuracy of virtual touch tissue imaging and quantification (VTIQ) in the diagnosis of benign and malignant breast lesions under four different precompression levels. The shear wave velocity (SWV) ratios of lesion to surrounding tissue were also added for diagnosis.

METHODS

167 female patients with breast lesions were included in this single center prospective study. VTIQ was performed under four different precompression levels. The SWV of the lesion, surrounding fat, and gland tissue were measured at the same depth as much as feasible 7 times. The breast lesions studied were all histopathologically confirmed. The VTIQ parameters were compared between precompression levels. The area under the receiver operating characteristic curve (AUC) was calculated to evaluate the diagnostic performance of each parameter of the VTIQ.

RESULTS

The VTIQ parameters of the malignant lesions were significantly higher than those of benign lesions in all precompression levels (P < 0.001). SWV of the lesion, fat, and gland tissue increased significantly with increasing precompression. The VTIQ parameters had great diagnostic performance for breast lesions in all precompression levels (AUC = 0.765-0.911). There was no significant difference between the precompression levels of the lesion-to-fat SWV ratio and the lesion-to-gland SWV ratio in benign and malignant lesions, and the cut-off coefficients of variation were 7.42% and 8.55%, respectively.

CONCLUSIONS

Precompression can increase the stiffness of breast lesions, fat and gland tissues, but does not reduce diagnostic value of VTIQ parameters in the breast. Under different precompression levels, the diagnosis of breast lesions by the ratio of the SWV of the lesion to the surrounding tissues is more stable.

Deep thrombosis characterization using photoacoustic imaging with intravascular light delivery

Venous thromboembolism (VTE) is a condition in which blood clots form within the deep veins of the leg or pelvis to cause deep vein thrombosis. The optimal treatment of VTE is determined by thrombus properties such as the age, size, and chemical composition of the blood clots. The thrombus properties can be readily evaluated by using photoacoustic computed tomography (PACT), a hybrid imaging modality that combines the rich contrast of optical imaging and deep penetration of ultrasound imaging. With inherent sensitivity to endogenous chromophores such as hemoglobin, multispectral PACT can provide composition information and oxygenation level in the clots. However, conventional PACT of clots relies on external light illumination, which provides limited penetration depth due to strong optical scattering of intervening tissue. In our study, this depth limitation is overcome by using intravascular light delivery with a thin optical fiber. To demonstrate in vitro blood clot characterization, clots with different acuteness and oxygenation levels were placed underneath ten-centimeter-thick chicken breast tissue and imaged using multiple wavelengths. Acoustic frequency analysis was performed on the received PA channel signals, and oxygenation level was estimated using multispectral linear spectral unmixing. The results show that, with intravascular light delivery, clot oxygenation level can be accurately measured, and the clot age can thus be estimated. In addition, we found that retracted and unretracted clots had different acoustic frequency spectrum. While unretracted clots had stronger high frequency components, retracted clots had much higher low frequency components due to densely packed red blood cells. The PACT characterization of the clots was consistent with the histology results and mechanical tests.

Assessment of histological characteristics, imaging markers, and rt-PA susceptibility of ex vivo venous thrombi

Venous thromboembolism is a significant source of morbidity and mortality worldwide. Catheter-directed thrombolytics is the primary treatment used to relieve critical obstructions, though its efficacy varies based on the thrombus composition. Non-responsive portions of the specimen often remain in situ, which prohibits mechanistic investigation of lytic resistance or the development of diagnostic indicators for treatment outcomes. In this study, thrombus samples extracted from venous thromboembolism patients were analyzed ex vivo to determine their histological properties, susceptibility to lytic therapy, and imaging characteristics. A wide range of thrombus morphologies were observed, with a dependence on age and etymology of the specimen. Fibrinolytic inhibitors including PAI-1, alpha 2-antiplasmin, and TAFI were present in samples, which may contribute to the response venous thrombi to catheter-directed thrombolytics. Finally, a weak but significant correlation was observed between the response of the sample to lytic drug and its magnetic microstructure assessed with a quantitative MRI sequence. These findings highlight the myriad of changes in venous thrombi that may promote lytic resistance, and imaging metrics that correlate with treatment outcomes.

Diminished Sphenous Compartment Connective Tissue Elasticity has Little Impact on Low Grade Venous Insufficiency: An Ultrasound Shearwave Elastography Study

BACKGROUND

Greater Saphenous Vein (GSV) courses within saphenous compartment, an adipose-filled space bound by fasciae provides structural support. Ultrasound Shear-Wave Elastography (SWE) provides objective and quantitative data on tissue shear elasticity modulus.

OBJECTIVE

This study aims to analyze possible associations between early stage GSV insufficiency and saphenous intracompartmental SWE measurements.

METHODS

Two-hundred consecutive patients, ages 22 to 81 (mean=44.3) years, with venous insufficiency symptoms underwent Doppler and SWE examinations. Patients had no visible or palpable sign of venous disease or had telangiectasia and reticular veins only. Analyses regarding patient age, gender, presence of venous insufficiency of GSV proper and intracompartmental connective tissue elasticity were performed.

RESULTS

Ninety-six patients had Doppler evidence for either bilateral or unilateral insufficiency of GSV proper at mid-thigh level. Intracompartmental elasticity of patients with venous insufficiency (mean=4.36±2.24 kilopascals; range 1.55 to 10.44 kPa) did not differ significantly from those with normal veins (mean=4.82±2.61 kPa; range 2.20 to 12.65 kPa) (p=0.231). No threshold for predicting the presence of venous insufficiency could be determined. Neither were there any correlations between age, gender and intracompartmental elasticity. In patients with unilateral insufficiency, however, elastography values around insufficient veins were significantly lower compared to contralateral normal GSV (p<0.001).

CONCLUSION

Many intrinsic and patient factors affect intracompartmental connective tissue elastography measurements; thus, cut-off values obtained from specific populations have limited generalizability. Nevertheless, statistically significant intrapatient differences of intracompartmental elasticity among diseased and normal saphenous veins indicate that lack of elastic support from surrounding connective tissues contributes to venous insufficiency in early stages.

Cardiac thrombotic stability determined by contrast-enhanced echocardiography: investigative protocol and preliminary results

Objective

This study’s intent was to test a new system for scoring cardiac thrombotic stability, based on contrast-enhanced ultrasound (CEUS).

Methods

We used human whole blood for an in vitro thrombotic model involving 1-h (T_1h) and 7-day (T_7d) subsets. The T_1h group was monitored for 1 h continuously to observe for the formation of a new thrombus on the original thrombus base. Changes in thrombotic CEUS images, histologic features, and shear wave elastography were recorded over time. We also studied 28 patients diagnosed with cardiac thrombi, each examined by transthoracic echocardiography and CEUS.Thrombi were scored for substrate (T_s) and hardness (T_h) based on the visualized degree of contrast penetration into the thrombi. Statistical analyses of T_s and T_h reflected thrombolytic time and risk of embolism to other organs.

Results

Histologically, the loosely constructed ends of in vitro thrombi solidified over time. In addition, the average Young’s modulus of thrombi over time indicated a progressive increase in hardness. Contrast-enhancing agents were able to penetrate fresh, loose thrombi only, not chronic, stable thrombi. As T_s and T_h increased, prolonged thrombolytic time and greater risk of embolism to other organs were apparent.

Conclusions

Our data suggest that this new CEUS scoring system correlates well with cardiac thrombotic hardness and the quality of its underlying substrate, serving to quantify thrombotic stability.

Characterizing blood clots using acoustic radiation force optical coherence elastography and ultrasound shear wave elastography

Thromboembolism in a cerebral blood vessel is associated with high morbidity and mortality. Mechanical thrombectomy (MT) is one of the emergenc proceduresperformed to remove emboli. However, the interventional approaches such as aspiration catheters or stent retriever are empirically selected. An inappropriate selection of surgical devices can influence the success rate during embolectomy, which can lead to an increase in brain damage. There has been growing interest in the study of clot composition and using a priori knowledge of clot composition to provide guidance for an appropriate treatment strategy for interventional physicians. Developing imaging tools which can allow interventionalists to understand clot composition could affect management and device strategy. In this study, we investigated how clots of different compositions can be characterized by using acoustic radiation force optical coherence elastography (ARF-OCE) and compared with ultrasound shear wave elastography (SWE). Five different clots compositions using human blood were fabricated into cylindrical forms from fibrin-rich (21% red blood cells, RBCs) to RBC-rich (95% RBCs). Using the ARF-OCE and SWE, we characterized the wave velocities measured in the time-domain. In addition, the semi-analytical finite element model was used to explore the relationship between the phase velocities with various frequency ranges and diameters of the clots. The study demonstrated that the wave group velocities generally decrease as RBC content increases in ARF-OCE and SWE. The correlation of the group velocities from the OCE and SWE methods represented a good agreement as RBC composition is larger than 39%. Using the phase velocity dispersion analysis applied to ARF-OCE data, we estimated the shear wave velocities decoupling the effects of the geometry and material properties of the clots. The study demonstrated that the composition of the clots can be characterized by elastographic methods using ARF-OCE and SWE, and OCE demonstrated better ability to discriminate between clots of different RBC compositions, compared to the ultrasound-based approach, especially in clots with low RBC compositions.

Non-invasive imaging techniques for the differentiation of acute and chronic thrombosis

Thrombosis is the localized clotting of blood that can occur in both the arterial and venous circulation. It is a key factor in the pathogenesis of acute coronary syndrome, myocardial infarction and stroke and the primary cause of deep vein thrombosis and pulmonary embolism. Rapid and accurate diagnosis of thrombotic episodes is crucial in reducing the morbidity and potential mortality associated with arterial and venous thrombotic disorders by allowing early targeted therapeutic interventions. From a clinical perspective the ability to accurately assess the age and composition of thrombus is highly desirable given that anticoagulation and, in particular, fibrinolytic therapies are more effective in treating acute rather than chronic thrombosis. While there are no imaging tests used in routine clinical practice that can reliably determine the age of thrombus and differentiate between acute and chronic thrombosis there are several emerging non-invasive techniques that can provide an indication of the age of a thrombus depending on its location in the body. Examples of techniques developed for venous thrombosis include Doppler imaging with venous duplex ultrasonography, ultrasound B-mode imaging integrated with IER (intrinsic mode functions-based echogenicity ratio), elastography, scintigraphy imaging with ^99mTc-recombinant tissue plasminogen activator (^99mTc-rt-PA), and magnetic resonance direct thrombus imaging (MDRTI). Magnetic resonance imaging (MRI) has been used to noninvasively detect and differentiate acute and chronic arterial and venous thrombosis. These methods have limitations that need further investigation to enable cost-effective and clinically relevant treatment practices to be established in the future. This review will discuss the difference between acute and chronic thrombosis and the role of non-invasive imaging techniques in discriminating between the two.

Effect of Clot Stiffness on Recombinant Tissue Plasminogen Activator Lytic Susceptibility in Vitro

The lytic recombinant tissue plasminogen activator (rt-PA) is the only drug approved by the Food and Drug Administration for treating ischemic stroke. Less than 40% of patients with large vessel occlusions who are treated with rt-PA have improved blood flow. However, up to 6% of all patients receiving rt-PA develop intracerebral hemorrhage. Predicting the efficacy of rt-PA treatment a priori could help guide therapeutic decision making, such that rt-PA is administered only to those individuals who would benefit from this treatment. Clot composition and structure affect the lytic efficacy of rt-PA and have an impact on elasticity. However, the relationship between clot elasticity and rt-PA lytic susceptibility has not been adequately investigated. The goal of this study was to quantify the relationship between clot elasticity and rt-PA susceptibility in vitro. Human and porcine highly retracted and mildly retracted clots were fabricated in glass pipettes. The rt-PA lytic susceptibility was evaluated in vitro using the percent clot mass loss. The Young's moduli of the clots were estimated using ultrasound-based single-track-location shear wave elasticity imaging. The percent mass loss in mildly retracted porcine and human clots (28.9 ± 6.1% and 45.2 ± 7.1%, respectively) was significantly higher (p < 0.05) than in highly retracted porcine and human clots (10.9 ± 2.1% and 25.5 ± 10.0%, respectively). Furthermore, the Young's moduli of highly retracted porcine and human clots (5.33 ± 0.92 and 3.21 ± 1.97 kPa, respectively) were significantly higher (p < 0.05) than those of mildly retracted porcine and human clots (2.66 ± 0.55 and 0.79 ± 0.21 kPa, respectively). The results revealed an inverse relationship between the percent clot mass loss and Young's modulus. These findings motivate continued investigation of ultrasound-based methods to assess clot stiffness in order to predict rt-PA thrombolytic efficacy.

Ultrasound elastography is useful to distinguish acute and chronic deep vein thrombosis

Essentials Ultrasound elastography uses tissue deformation to assess the relative quantification of its elasticity. Compression and duplex ultrasonography may be unable to correctly determine the thrombus age. Ultrasound elastography may be useful to distinguish between acute and chronic deep vein thrombosis. The exact determination of the thrombus age could have both therapeutic and prognostic implications. BACKGROUND: Background Ultrasound elastography (UE) imaging is a novel sonographic technique that is commonly employed for relative quantification of tissue elasticity. Its applicability to venous thromboembolic events has not yet been fully established; in particular, it is unclear whether this technique may be useful in determining the age of deep vein thrombosis (DVT). Thus, the aim of this study was to assess the role of UE in distinguishing acute from chronic DVT. Methods Consecutive patients with a first unprovoked acute and chronic (3 months old) DVT of the lower limbs were analyzed. Patients with recurrent DVT or with a suspected recurrence were excluded. The mean elasticity index (EI) values of acute and chronic popliteal and femoral vein thrombosis were compared. The accuracy of the EI in distinguishing acute from chronic DVT was also assessed by measuring the sensitivity, specificity, positive and negative predictive values, and likelihood ratios. Results One-hundred and forty-nine patients (mean age 63.9 years, standard deviation 13.6; 73 males) with acute and chronic DVT were included. The mean EI of acute femoral DVT was higher than that of chronic femoral DVT (5.09 versus 2.46), and the mean EI of acute popliteal DVT was higher than that of chronic popliteal DVT (4.96 versus 2.48). An EI value of > 4 resulted in a sensitivity of 98.9% (95% confidence interval [CI] 93.3-99.9), a specificity of 99.1% (95% CI 94.8-99.9), a positive predictive value of 91.1% (95% CI 77.9-97.1), a negative predictive value of 98.6% (95% CI 91.3-99.9), a positive likelihood ratio of 13.23 (95% CI 93-653) and a negative likelihood ratio of 0.001 (95% CI 0.008-0.05) for acute DVT. Conclusions UE appears to be a promising technique for distinguishing between acute and chronic DVT. Larger prospective studies are warranted to confirm our preliminary findings.

Elastography techniques in the evaluation of deep vein thrombosis

Deep venous thrombosis (DVT) is a significant medical problem with an incidence of 1 in 1,000 adults and greatly reduces quality of life through post-thrombotic syndrome. Treatment choice for DVT can be influenced by the age of the clot. While new endovascular catheter techniques treat venous clots to potentially prevent post-thrombotic syndrome, they require improved imaging techniques to accurately determine clot age. This review investigates experimental and clinical evidence of elastography techniques for aging DVT. Strain elastography and shear wave elastography are the most common techniques to age thrombus. These elastography techniques can distinguish between acute and chronic clots by characterizing tissue stiffness. When clot age cannot be determined with ultrasound duplex analysis, elastography may offer a helpful adjunct. However, further investigation is required to validate accuracy and reproducibility for clinical implementation of this novel technique.