Malignancy diagnosis of liver lesion in contrast enhanced ultrasound using an end-to-end method based on deep learning.

BACKGROUND: Contrast-enhanced ultrasound (CEUS) is considered as an efficient tool for focal liver lesion characterization, given it allows real-time scanning and provides dynamic tissue perfusion information. An accurate diagnosis of liver lesions with CEUS requires a precise interpretation of CEUS images. However,it is a highly experience dependent task which requires amount of training and practice. To help improve the constrains, this study aims to develop an end-to-end method based on deep learning to make malignancy diagnosis of liver lesions using CEUS. METHODS: A total of 420 focal liver lesions with 136 benign cases and 284 malignant cases were included. A deep learning model based on a two-dimensional convolution neural network, a long short-term memory (LSTM), and a linear classifier (with sigmoid) was developed to analyze the CEUS loops from different contrast imaging phases. For comparison, a 3D-CNN based method and a machine-learning (ML)-based time-intensity curve (TIC) method were also implemented for performance evaluation. RESULTS: Results of the 4-fold validation demonstrate that the mean AUC is 0.91, 0.88, and 0.78 for the proposed method, the 3D-CNN based method, and the ML-based TIC method, respectively. CONCLUSIONS: The proposed CNN-LSTM method is promising in making malignancy diagnosis of liver lesions in CEUS without any additional manual features selection.

Deep Multi-Objective Learning from Low-Dose CT for Automatic Lung-RADS Report Generation.

Radiology report generation through chest radiography interpretation is a time-consuming task that involves the interpretation of images by expert radiologists. It is common for fatigue-induced diagnostic error to occur, and especially difficult in areas of the world where radiologists are not available or lack diagnostic expertise. In this research, we proposed a multi-objective deep learning model called CT2Rep (Computed Tomography to Report) for generating lung radiology reports by extracting semantic features from lung CT scans. A total of 458 CT scans were used in this research, from which 107 radiomics features and 6 slices of segmentation related nodule features were extracted for the input of our model. The CT2Rep can simultaneously predict position, margin, and texture, which are three important indicators of lung cancer, and achieves remarkable performance with an F1-score of 87.29%. We conducted a satisfaction survey for estimating the practicality of CT2Rep, and the results show that 95% of the reports received satisfactory ratings. The results demonstrate the great potential in this model for the production of robust and reliable quantitative lung diagnosis reports. Medical personnel can obtain important indicators simply by providing the lung CT scan to the system, which can bring about the widespread application of the proposed framework.

High Frequency Ultrasound Elastography for Estimating the Viscoelastic Properties of the Cornea Using Lamb Wave Model.

OBJECTIVE: Estimating the elasticity distribution in the cornea is important because corneal elasticity is usually influenced by corneal pathologies and surgical treatments, especially for early corneal sclerosis. Because the thickness of the cornea is typically less than 1 mm, high-resolution ultrasound elastography as well as the Lamb wave model is required for viscoelastic property estimation. In the present study, an array high-frequency ultrasound (HFUS) elastography method based on ultrafast ultrasound imaging was proposed for estimating the viscoelastic properties of porcine cornea. METHODS: The elastic wave was generated by an external vibrator, after which the wave propagation image was obtained using a 40-MHz array transducer. Viscoelasticity estimation was performed by fitting the phase velocity curve using the Lamb wave model. The performance of the proposed HFUS elastography system was verified using 2-mm-thick thin-layer gelatin phantoms with gelatin concentrations of 7% and 12%. Ex vivo experiments were carried out using fresh porcine cornea with artificial sclerosing. RESULTS: Experimental results showed that the estimated elasticity was close to the standard value obtained in the phantom study when the Lamb wave model was used for elasticity measurement. However, the error between the standard elasticity values and the elasticity values estimated using group shear wave velocity was large. In the ex vivo eyeball experiments, the estimated elasticities and viscosities were respectively 9.1 ± 1.3 kPa and 0.5 ± 0.10 Pa·s for a healthy cornea and respectively 15.9 ± 2.1 kPa and 1.1 ± 0.12 Pa·s for a cornea with artificial sclerosis. A 3D HFUS elastography was also obtained for distinguishing the region of sclerosis in the cornea. CONCLUSION: The experimental results demonstrated that the proposed HFUS elastography method has high potential for the clinical diagnosis of corneal diseases compared with other HFUS single-element transducer elastography systems.

Characterization of Hand Tendons Through High-Frequency Ultrasound Elastography.

Tendon stiffness plays an important role in the tendon healing process, and many studies have indicated that measuring the shear wave velocity (SWV) on tendons relates to their stiffness. Because the thickness of hand tendons is a few millimeters, high-resolution imaging is required for visualizing hand tissues. However, the resolution of current ultrasound elastography systems is insufficient. In this study, a high-frequency (HF) ultrasound elastography system is proposed for measuring the SWVs of hand tendons. The HF ultrasound elastography system uses an external vibrator to create shear waves on hand tendons. Then, it uses a 40-MHz HF ultrasound array transducer with ultrafast ultrasound imaging technology to measure the SWV for characterizing hand tendons. A handheld device that combines a transducer and a vibrator allows the user to scan hand tissues. The biases of HF ultrasound elastography were measured in gelatin phantom experiments and were less than 6% compared to standard mechanical testing approach. Human experiments showed the ability to use HF ultrasound elastography to distinguish different SWVs of hand tendons. The SWVs were 0.73 ± 0.65 m/s and 1 ± 0.54 m/s for flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP), respectively, and 0.52 ± 0.14 m/s and 4.02 ± 0.77 m/s for extensor tendon under stretch and contraction conditions, respectively. The simplicity and convenience of the HF ultrasound elastography system for measuring hand tendon stiffness make it a promising tool for evaluating the severity of hand injuries and the performance of rehabilitation after hand injuries.

Ultrasonic Microelastography to Assess Biomechanical Properties of the Cornea.

OBJECTIVE: To both qualitatively and quantitatively investigate corneal biomechanical properties through an ultrasonic microelastography imaging system, which is potentially useful in the diagnosis of diseases, such as keratoconus, postrefractive keratectasia, and tracking treatment such as cross-linking surgery. METHODS: Our imaging system has a dual-frequency configuration, including a 4.5 MHz ring transducer to push the tissue and a confocally aligned 40 MHz needle transducer to track micron-level displacement. Two-dimensional/three-dimensional acoustic radiation force impulse (ARFI) imaging and Young's modulus in the region of interest were performed on ex vivo porcine corneas that were either cross-linked using formalin solution or preloaded with intraocular pressure (IOPs) from 5 to 30 mmHg. RESULTS: The increase of corneal stiffness and the change in cross-linked volume following formalin crosslinking could be precisely observed in the ARFI images and reflected by the reconstructed Young's modulus while the B-mode structural images remained almost unchanged. In addition, the relationship between the stiffness of the cornea and IOPs was investigated among 12 porcine corneas. The corneal stiffness is significantly different at various IOPs and has a tendency to become stiffer with increasing IOP. CONCLUSION: Our results demonstrate the principle of using ultrasonic microelastography techniques to image the biomechanical properties of the cornea. Integrating high-resolution ARFI imaging labeled with reconstructed Young's modulus and structural imaging of the cornea can potentially lead to a routinely performed imaging modality in the field of ophthalmology.

High-Resolution Shear Wave Imaging of the Human Cornea Using a Dual-Element Transducer.

Estimating the corneal elasticity can provide valuable information for corneal pathologies and treatments. Ophthalmologic pathologies will invariably cause changes to the elasticity of the cornea. For example, keratoconus and the phototoxic effects of ultraviolet radiation usually increase the corneal elasticity. This makes a quantitative estimation of the elasticity of the human cornea important for ophthalmic diagnoses. The present study investigated the use of a proposed high-resolution shear wave imaging (HR-SWI) method based on a dual-element transducer (comprising an 8-MHz element for pushing and a 32-MHz element for imaging) for measuring the group shear wave velocity (GSWV) of the human cornea. An empirical Young's modulus formula was used to accurately convert the GSWV to Young's modulus. Four quantitative parameters, bias, resolution, contrast, and contrast-to-noise ratio (CNR), were measured in gelatin phantoms with two different concentrations (3% and 7%) to evaluate the performance of HR-SWI. The biases of gelatin phantoms (3% and 7%) were 5.88% and 0.78%, respectively. The contrast and CNR were 0.76, 1.31 and 3.22, 2.43 for the two-side and two-layer phantoms, respectively. The measured image resolutions of HR-SWI in the lateral and axial directions were 72 and 140 µm, respectively. The calculated phase SWV (PSWV) and their corresponding Young's modulus from six human donors were 2.45 ± 0.48 m/s (1600 Hz) and 11.52 ± 7.81 kPa, respectively. All the experimental results validated the concept of HR-SWI and its ability for measuring the human corneal elasticity.

Quantitative Assessment of Thin-Layer Tissue Viscoelastic Properties Using Ultrasonic Micro-Elastography With Lamb Wave Model.

Characterizing the viscoelastic properties of thin-layer tissues with micro-level thickness has long remained challenging. Recently, several micro-elastography techniques have been developed to improve the spatial resolution. However, most of these techniques have not considered the medium boundary conditions when evaluating the viscoelastic properties of thin-layer tissues such as arteries and corneas; this might lead to estimation bias or errors. This paper aims to integrate the Lamb wave model with our previously developed ultrasonic micro-elastography imaging system for obtaining accurate viscoelastic properties in thin-layer tissues. A 4.5-MHz ring transducer was used to generate an acoustic radiation force for inducing tissue displacements to produce guided wave, and the wave propagation was detected using a confocally aligned 40-MHz needle transducer. The phase velocity and attenuation were obtained from k-space by both the impulse and the harmonic methods. The measured phase velocity was fit using the Lamb wave model with the Kelvin-Voigt model. Phantom experiments were conducted using 7% and 12% gelatin and 1.5% agar phantoms with different thicknesses (2, 3, and 4 mm). Biological experiments were performed on porcine cornea and rabbit carotid artery ex vivo. Thin-layer phantoms with different thicknesses were confirmed to have the same elasticity; this was consistent with the estimates of bulk phantoms from mechanical tests and the shear wave rheological model. The trend of the measured attenuations was also confirmed with the viscosity results obtained using the Lamb wave model. Through the impulse and harmonic methods, the shear viscoelasticity values were estimated to be 8.2 kPa for $0.9~\text {Pa}{\cdot} \text {s}$ and 9.6 kPa for $0.8~\text {Pa}{\cdot} \text {s}$ in the cornea and 27.9 kPa for $0.1~\text {Pa}\cdot \text {s}$ and 26.5 kPa for $0.1~\text {Pa}\cdot \text {s}$ in the artery.

Development of an intravascular ultrasound elastography based on a dual-element transducer.

The ability to measure the elastic properties of plaques and vessels would be useful in clinical diagnoses, particularly for detecting a vulnerable plaque. This study demonstrates the feasibility of the combination of intravascular ultrasound (IVUS) and acoustic radiation force elasticity imaging for detecting the distribution of stiffness within atherosclerotic arteries ex vivo. A dual-frequency IVUS transducer with two elements was used to induce the propagation of the shear wave (by the 8.5 MHz pushing element) which could be simultaneously monitored by the 31 MHz imaging element. The wave-amplitude image and the wave-velocity image were reconstructed by measuring the peak displacement and wave velocity of shear wave propagation, respectively. System performance was verified using gelatin phantoms. The phantom results demonstrate that the stiffness differences of shear modulus of 1.6 kPa can be distinguished through the wave-amplitude and wave-velocity images. The stiffness distributions of the atherosclerotic aorta from a rabbit were obtained, for which the values of peak displacement and the shear wave velocity were 3.7 ± 1.2 µm and 0.38 ± 0.19 m s-1 for the lipid-rich plaques, and 1.0 ± 0.2 µm and 3.45 ± 0.45 m s-1 for the arterial walls, respectively. These results indicate that IVUS elasticity imaging can be used to distinguish the elastic properties of plaques and vessels.

Freehand three-dimensional ultrasound imaging of carotid artery using motion tracking technology.

Ultrasound imaging has been extensively used for determining the severity of carotid atherosclerotic stenosis. In particular, the morphological characterization of carotid plaques can be performed for risk stratification of patients. However, using 2D ultrasound imaging for detecting morphological changes in plaques has several limitations. Due to the scan was performed on a single longitudinal cross-section, the selected 2D image is difficult to represent the entire morphology and volume of plaque and vessel lumen. In addition, the precise positions of 2D ultrasound images highly depend on the radiologists' experience, it makes the serial long-term exams of anti-atherosclerotic therapies are difficult to relocate the same corresponding planes by using 2D B-mode images. This has led to the recent development of three-dimensional (3D) ultrasound imaging, which offers improved visualization and quantification of complex morphologies of carotid plaques. In the present study, a freehand 3D ultrasound imaging technique based on optical motion tracking technology is proposed. Unlike other optical tracking systems, the marker is a small rigid body that is attached to the ultrasound probe and is tracked by eight high-performance digital cameras. The probe positions in 3D space coordinates are then calibrated at spatial and temporal resolutions of 10µm and 0.01s, respectively. The image segmentation procedure involves Otsu's and the active contour model algorithms and accurately detects the contours of the carotid arteries. The proposed imaging technique was verified using normal artery and atherosclerotic stenosis phantoms. Human experiments involving freehand scanning of the carotid artery of a volunteer were also performed. The results indicated that compared with manual segmentation, the lowest percentage errors of the proposed segmentation procedure were 7.8% and 9.1% for the external and internal carotid arteries, respectively. Finally, the effect of handshaking was calibrated using the optical tracking system for reconstructing a 3D image.

A Novel Adhesion Index for Verifying the Extent of Adhesion for the Extensor Digitorum Communis in Patients with Metacarpal Fractures.

This study aims to determine if the relative displacement between the extensor digitorum communis (EDC) tendon and its surrounding tissues can be used as an adhesion index (AI) for assessing adhesion in metacarpal fractures by comparing two clinical measures, namely single-digit-force and extensor lag (i.e., the difference between passive extension and full active extension). The Fisher-Tippett block-matching method and a Kalman-filter algorithm were used to determine the relative displacements in 39 healthy subjects and 8 patients with metacarpal fractures. A goniometer was used to measure the extensor lag, and a force sensor was used to measure the single-digit-force. Measurements were obtained twice for each patient to evaluate the performance of the AI in assessing the progress of rehabilitation. The Pearson correlation coefficient was calculated to quantify the various correlations between the AI, extensor lag, and single-digit-force. The results showed strong correlations between the AI and the extensor lag, the AI and the single-digit-force, and the extensor lag and the single-digit-force (r = 0.718, -0.849, and -0.741; P = 0.002, P < 0.001, and P = 0.001, respectively). The AI in the patients gradually decreased after continuous rehabilitation, but remained higher than that of healthy participants.

Evaluating the intensity of the acoustic radiation force impulse (ARFI) in intravascular ultrasound (IVUS) imaging: Preliminary in vitro results.

The ability to measure the elastic properties of plaques and vessels is significant in clinical diagnosis, particularly for detecting a vulnerable plaque. A novel concept of combining intravascular ultrasound (IVUS) imaging and acoustic radiation force impulse (ARFI) imaging has recently been proposed. This method has potential in elastography for distinguishing between the stiffness of plaques and arterial vessel walls. However, the intensity of the acoustic radiation force requires calibration as a standard for the further development of an ARFI-IVUS imaging device that could be used in clinical applications. In this study, a dual-frequency transducer with 11MHz and 48MHz was used to measure the association between the biological tissue displacement and the applied acoustic radiation force. The output intensity of the acoustic radiation force generated by the pushing element ranged from 1.8 to 57.9mW/cm(2), as measured using a calibrated hydrophone. The results reveal that all of the acoustic intensities produced by the transducer in the experiments were within the limits specified by FDA regulations and could still displace the biological tissues. Furthermore, blood clots with different hematocrits, which have elastic properties similar to the lipid pool of plaques, with stiffness ranging from 0.5 to 1.9kPa could be displaced from 1 to 4µm, whereas the porcine arteries with stiffness ranging from 120 to 291kPa were displaced from 0.4 to 1.3µm when an acoustic intensity of 57.9mW/cm(2) was used. The in vitro ARFI images of the artery with a blood clot and artificial arteriosclerosis showed a clear distinction of the stiffness distributions of the vessel wall. All the results reveal that ARFI-IVUS imaging has the potential to distinguish the elastic properties of plaques and vessels. Moreover, the acoustic intensity used in ARFI imaging has been experimentally quantified. Although the size of this two-element transducer is unsuitable for IVUS imaging, the experimental results reported herein can be applied in ARFI-IVUS imaging applications.

Application of a novel Kalman filter based block matching method to ultrasound images for hand tendon displacement estimation.

PURPOSE: Information about tendon displacement is important for allowing clinicians to not only quantify preoperative tendon injuries but also to identify any adhesive scaring between tendon and adjacent tissue. The Fisher-Tippett (FT) similarity measure has recently been shown to be more accurate than the Laplacian sum of absolute differences (SAD) and Gaussian sum of squared differences (SSD) similarity measures for tracking tendon displacement in ultrasound B-mode images. However, all of these similarity measures can easily be influenced by the quality of the ultrasound image, particularly its signal-to-noise ratio. Ultrasound images of injured hands are unfortunately often of poor quality due to the presence of adhesive scars. The present study investigated a novel Kalman-filter scheme for overcoming this problem. METHODS: Three state-of-the-art tracking methods (FT, SAD, and SSD) were used to track the displacements of phantom and cadaver tendons, while FT was used to track human tendons. These three tracking methods were combined individually with the proposed Kalman-filter (K1) scheme and another Kalman-filter scheme used in a previous study to optimize the displacement trajectories of the phantom and cadaver tendons. The motion of the human extensor digitorum communis tendon was measured in the present study using the FT-K1 scheme. RESULTS: The experimental results indicated that SSD exhibited better accuracy in the phantom experiments, whereas FT exhibited better performance for tracking real tendon motion in the cadaver experiments. All three tracking methods were influenced by the signal-to-noise ratio of the images. On the other hand, the K1 scheme was able to optimize the tracking trajectory of displacement in all experiments, even from a location with a poor image quality. The human experimental data indicated that the normal tendons were displaced more than the injured tendons, and that the motion ability of the injured tendon was restored after appropriate rehabilitation sessions. CONCLUSIONS: The obtained results show the potential for applying the proposed FT-K1 method in clinical applications for evaluating the tendon injury level after metacarpal fractures and assessing the recovery of an injured tendon during rehabilitation.

High-resolution tissue Doppler imaging of the zebrafish heart during its regeneration.

The human heart cannot regenerate after injury, whereas the adult zebrafish can fully regenerate its heart even after 20% of the ventricle is amputated. Many studies have begun to reveal the cellular and molecular mechanisms underlying this regenerative process, which have exciting implications for human cardiac diseases. However, the dynamic functions of the zebrafish heart during regeneration are not yet understood. This study established a high-resolution echocardiography for tissue Doppler imaging (TDI) of the zebrafish heart to explore the cardiac functions during different regeneration phases. Experiments were performed on AB-line adult zebrafish (n=40) in which 15% of the ventricle was surgically removed. An 80-MHz ultrasound TDI based on color M-mode imaging technology was employed. The cardiac flow velocities and patterns from both the ventricular chamber and myocardium were measured at different regeneration phases relative to the day of amputation. The peak velocities of early diastolic inflow, early diastolic myocardial motion, late diastolic myocardial motion, early diastolic deceleration slope, and heart rate were increased at 3 days after the myocardium amputation, but these parameters gradually returned to close to their baseline values for the normal heart at 7 days after amputation. The peak velocities of late diastolic inflow, ventricular systolic outflow, and systolic myocardial motion did not significantly differ during the heart regeneration.

Estimating the viscoelastic modulus of a thrombus using an ultrasonic shear-wave approach.

PURPOSE: Measurements of the viscoelastic properties of a thrombus can be used to assess whether blood clots are likely to become occlusive or to break apart and leak into the blood circulation and block smaller vessels. An accurate method for estimating both the shear elasticity and viscosity of a blood clot in vivo is still lacking, which prompted us to use a novel shear-wave approach to measure the viscoelastic modulus of blood clots. METHODS: The shear-wave dispersion ultrasound vibrometry was used to measure both the elasticity and viscosity of blood clots. The experimental system was verified by measuring the viscoelastic modulus of phantoms containing gelatin at different concentrations. Blood-clot experiments were carried out using porcine whole blood with hematocrits ranging from 3% to 40%. The measured values for both clots and gelatin phantoms were compared to those obtained using an embedded-sphere method in order to validate the accuracy of the viscoelastic modulus estimations. RESULTS: The shear elastic modulus increased from 406.9 ± 15.8 (mean ± SD) Pa for 3% gelatin to 1587.2 ± 28.9 Pa for 7% gelatin, while the viscosity increased from 0.12 ± 0.02 Pa s to 0.86 ± 0.05 Pa s, respectively. The shear modulus increased from 196.8 ± 58.4 Pa for 40%-hematocrit clots to 641.4 ± 76.3 Pa for 3%-hematocrit clots, while the viscosity increased from 0.29 ± 0.02 Pa s to 0.42 ± 0.01 Pa s, respectively. CONCLUSIONS: The results from the statistical analysis indicated that both the embedded-sphere and shear-wave approaches can provide accurate estimations of the shear elasticity for clots and gelatin phantoms. In contrast, the shear-wave approach as well as other methods of rheological measurements does not provide accurate viscosity estimations for blood clots. However, the measured viscosity range of 0.29-0.42 Pa s is reasonable for blood clots.

High-resolution acoustic-radiation-force-impulse imaging for assessing corneal sclerosis.

In ophthalmology, detecting the biomechanical properties of the cornea can provide valuable information about various corneal pathologies, including keratoconus and the phototoxic effects of ultraviolet radiation on the cornea. Also, the mechanical properties of the cornea can be used to evaluate the recovery from corneal refractive surgeries. Therefore, noninvasive and high-resolution estimation of the stiffness distribution in the cornea is important in ophthalmic diagnosis. The present study established a method for high-resolution acoustic-radiation-force-impulse (ARFI) imaging based on a dual-frequency confocal transducer in order to obtain a relative stiffness map, which was used to assess corneal sclerosis. An 11-MHz pushing element was used to induce localized displacements of tissue, which were monitored by a 48-MHz imaging element. Since the tissue displacements are directly correlated with the tissue elastic properties, the stiffness distribution in a tiny region of the cornea can be found by a mechanical B/D scan. The experimental system was verified using tissue-mimicking phantoms that included different geometric structures. Ex vivo cornea experiments were carried out using fresh porcine eyeballs. Corneas with localized sclerosis were created artificially by the injection of a formalin solution. The phantom experiments showed that the distributions of stiffness within different phantoms can be recognized clearly using ARFI imaging, and the measured lateral and axial resolutions of this imaging system were 177 and 153 µ m, respectively. The ex vivo experimental results from ARFI imaging showed that a tiny region of localized sclerosis in the cornea could be distinguished. All of the obtained results demonstrate that high-resolution ARFI imaging has considerable potential for the clinical diagnosis of corneal sclerosis.

The effect of flow acceleration on the cyclic variation of blood echogenicity under pulsatile flow.

It has been shown that the echogenicity of blood varies during a flow cycle under pulsatile flow both in vitro and in vivo. In general, the echogenicity of flowing whole blood increases during the early systole phase and then reduces to a minimum at late diastole. While it has been postulated that this cyclic variation is associated with the dynamics of erythrocyte aggregation, the mechanisms underlying this increasing echogenicity with flow velocity remain uncertain. The effect of flow acceleration has also been proposed as an explanation for this phenomenon, but no specific experiments have been conducted to test this hypothesis. In addition, the influence of ultrasonic attenuation on the cyclic variation of echogenicity requires clarification. In the present study, a Couette flow system was designed to simulate blood flowing with different acceleration patterns, and the flow velocity, attenuation, and backscattering coefficient were measured synchronously from 20%- and 40%-hematocrit porcine whole blood and erythrocyte suspensions using 35-MHz ultrasound transducers. The results showed ultrasonic attenuation exerted only minor effects on the echogenicity of blood under pulsatile flow conditions. Cyclic variations of echogenicity were clearly observed for whole blood with a hematocrit of 40%, but no variations were apparent for erythrocyte suspensions. The echogenicity did not appear to be enhanced when instantaneous acceleration was applied to flowing blood in any case. These findings show that flow acceleration does not promote erythrocyte aggregation, even when a higher peak velocity is applied to the blood. Comparison of the results obtained with different accelerations revealed that the cyclic variation in echogenicity observed during pulsatile blood flow may be jointly attributable to the effect of shear rate and the distribution of erythrocyte on aggregation.

Assessing the viscoelastic properties of thrombus using a solid-sphere-based instantaneous force approach.

The viscoelastic properties of thrombus play a significant role when the clot closes a leak in a vessel of the blood circulation. The common method used to measure the viscoelastic properties of a clot employs a rheometer but this might be unsuitable due to the clot fiber network being broken up by excessive deformation. This study assessed the feasibility of using a novel acoustic method to assess the viscoelastic properties of blood clots. This method is based on monitoring the motion of a solid sphere in a blood clot induced by an applied instantaneous force. Experiments were performed in which a solid sphere was displaced by a 1 MHz single-element focused transducer, with a 20 MHz single-element focused transducer used to track this displacement. The spatiotemporal behavior of the sphere displacement was used to determine the viscoelastic properties of the clot. The experimental system was calibrated by measuring the viscoelastic modulus of gelatin using different types of solid spheres embedded in the phantoms and, then, the shear modulus and viscosity of porcine blood clots with hematocrits of 0% (plasma), 20% and 40% were assessed. The viscoelastic modulus of each clot sample was also measured directly by a rheometer for comparison. The results showed that the shear modulus increased from 173 ± 52 (mean ± SD) Pa for 40%-hematocrit blood clots to 619.5 ± 80.5 Pa for plasma blood clots, while the viscosity decreased from 0.32 ± 0.07 Pa∙s to 0.16 ± 0.06 Pa∙s, respectively, which indicated that the concentration of red blood cells and the amount of fibrinogen are the main determinants of the clot viscoelastic properties.