Real-Time Quantitative Elasticity Imaging of Deep Tissue Using Free-Hand Conventional Ultrasound

The Use of Noninvasive Velacur® for Discriminating between Volunteers and Patients with Chronic Liver Disease: A Feasibility Study

Background and Aims

Nonalcoholic fatty liver disease is the leading cause of chronic liver disease globally and can progress to cirrhosis, liver failure, and liver cancer. Current AASLD, AGA, and ADA guidelines recommend assessment for liver fibrosis in all patients with NAFLD. Serum biomarkers for fibrosis, while widely available, have notable limitations. Imaging-based noninvasive testing for liver fibrosis/cirrhosis is more accurate and is becoming more widespread.

Methods

We evaluated the feasibility of a novel shear wave absolute vibroelastography (S-WAVE) modality called Velacur® for assessing liver stiffness measurement (LSM) for fibrosis and attenuation coefficient estimation (ACE) in differentiating patients with chronic liver disease from normal healthy controls.

Results

Fifty-four healthy controls and 89 patients with NAFLD or cured HCV with a prior known LSM of >8 kPa were enrolled, and all subjects were evaluated with FibroScan® and Velacur®. Velacur® was able to discriminate patients with increased liver stiffness as determined by a FibroScan® score of >8 kPa from healthy controls with an AUC of 0.938 (0.88-0.96). For assessment of steatosis in NAFLD patients only, Velacur® could identify patients with steatosis from healthy controls with an AUC of 0.831 (0.777-0.880). The Velacur® scan quality assessment was superior in healthy controls, as compared to patients, and the scan quality, as assessed by the quality factor (QF) and interquartile range (IQR)/median, was affected by BMI. Velacur® was safe and well tolerated by patients, and there were no adverse events.

Conclusion

Velacur® assessment of liver stiffness measurement and liver attenuation is comparable to results obtained by FibroScan® and is an alternative technology for monitoring liver fibrosis progression in patients with chronic liver disease. This trial is registered with NCT03957070.

Multi-Frequency 3D Shear Wave Absolute Vibro-Elastography (S-WAVE) System for the Prostate

This article describes a novel system for quantitative and volumetric measurement of tissue elasticity in the prostate using simultaneous multi-frequency tissue excitation. Elasticity is computed by using a local frequency estimator to measure the three-dimensional local wavelengths of steady-state shear waves within the prostate gland. The shear wave is created using a mechanical voice coil shaker which transmits simultaneous multi-frequency vibrations transperineally. Radio frequency data is streamed directly from a BK Medical 8848 transrectal ultrasound transducer to an external computer where tissue displacement due to the excitation is measured using a speckle tracking algorithm. Bandpass sampling is used that eliminates the need for an ultra-fast frame rate to track the tissue motion and allows for accurate reconstruction at a sampling frequency that is below the Nyquist rate. A roll motor with computer control is used to rotate the transducer and obtain 3D data. Two commercially available phantoms were used to validate both the accuracy of the elasticity measurements as well as the functional feasibility of using the system for in vivo prostate imaging. The phantom measurements were compared with 3D Magnetic Resonance Elastography (MRE), where a high correlation of 96% was achieved. In addition, the system has been used in two separate clinical studies as a method for cancer identification. Qualitative and quantitative results of 11 patients from these clinical studies are presented here. Furthermore, an AUC of 0.87±0.12 was achieved for malignant vs. benign classification using a binary support vector machine classifier trained with data from the latest clinical study with leave one patient out cross-validation.

3-D Transducer Mounted Shear Wave Absolute Vibro-Elastography: Proof of Concept

Quantitative tissue stiffness characterization using ultrasound (US) has been shown to improve prostate cancer (PCa) detection in multiple studies. Shear wave absolute vibro-elastography (SWAVE) allows quantitative and volumetric assessment of tissue stiffness using external multifrequency excitation. This article presents a proof of concept of a first-of-a-kind 3-D hand-operated endorectal SWAVE system designed to be used during systematic prostate biopsy. The system is developed with a clinical US machine, requiring only an external exciter that can be mounted directly to the transducer. Subsector acquisition of radio frequency (RF) data allows imaging of shear waves with a high effective frame rate (up to 250 Hz). The system was characterized using eight different quality assurance phantoms. Due to the invasive nature of prostate imaging, at this early stage of development, validation of in vivo human tissue was instead carried out by intercostally scanning the livers of n = 7 healthy volunteers. The results are compared with 3-D magnetic resonance elastography (MRE) and an existing 3-D SWAVE system with a matrix array transducer (M-SWAVE). High correlations were found with MRE (99% in phantoms, 94% in liver data) and with M-SWAVE (99% in phantoms, 98% in liver data).

Three-Dimensional Multi-Frequency Shear Wave Absolute Vibro-Elastography (3D S-WAVE) With a Matrix Array Transducer: Implementation and Preliminary In Vivo Study of the Liver

Magnetic resonance elastography (MRE) is commonly regarded as the imaging-based gold-standard for liver fibrosis staging, comparable to biopsy. While ultrasound-based elastography methods for liver fibrosis staging have been developed, they are confined to a 1D or a 2D region of interest and to a limited depth. 3D Shear Wave Absolute Vibro-Elastography (S-WAVE) is a steady-state, external excitation, volumetric elastography technique that is similar to MRE, but has the additional advantage of multi-frequency excitation. We present a novel ultrasound matrix array implementation of S-WAVE that takes advantage of 3D imaging. We use a matrix array transducer to sample axial multi-frequency steady-state tissue motion over a volume, using a Color Power Angiography sequence. Tissue motion with the frequency components {40,50,60} and {45,55,65} Hz are acquired over a (90° lateral) × (40° elevational) × (16 cm depth) sector with an acquisition time of 12 seconds. We compute the elasticity map in 3D using local spatial frequency estimation. We characterize this new approach in tissue phantoms against measurements obtained with transient elastography and MRE. Six healthy volunteers and eight patients with chronic liver disease were imaged. Their MRE and S-WAVE volumes were aligned using T1 to B-mode registration for direct comparison in common regions of interest. S-WAVE and MRE results are correlated with R² = 0.92, while MRE and TE results are correlated with R² = 0.71. Our findings show that S-WAVE with matrix array has the potential to deliver a similar assessment of liver fibrosis as MRE in a more accessible, inexpensive way, to a broader set of patients.

Three-dimensional shear wave elastography on conventional ultrasound scanners with external vibration

Two-dimensional (2D) ultrasound shear wave elastography (SWE) has been widely used for soft tissue properties assessment. Given that shear waves propagate in three dimensions (3D), extending SWE from 2D to 3D is important for comprehensive and accurate stiffness measurement. However, implementation of 3D SWE on a conventional ultrasound scanner is challenging due to the low volume rate (tens of Hertz) associated with limited parallel receive capability of the scanner's hardware beamformer. Therefore, we developed an external mechanical vibration-based 3D SWE technique allowing robust 3D shear wave tracking and speed reconstruction for conventional scanners. The aliased shear wave signal detected with a sub-Nyquist sampling frequency was corrected by leveraging the cyclic nature of the sinusoidal shear wave generated by the external vibrator. Shear wave signals from different sub-volumes were aligned in temporal direction to correct time delays from sequential pulse-echo events, followed by 3D speed reconstruction using a 3D local frequency estimation algorithm. The technique was validated on liver fibrosis phantoms with different stiffness, showing good correlation (r = 0.99, p < 0.001) with values measured from a state-of-the-art SWE system (GE LOGIQ E9). The phantoms with different stiffnesses can be well-differentiated regardless of the external vibrator position, indicating the feasibility of the 3D SWE with regard to different shear wave propagation scenarios. Finally, shear wave speed calculated by the 3D method correlated well with magnetic resonance elastography performed on human liver (r = 0.93, p = 0.02), demonstrating the in vivo feasibility. The proposed technique relies on low volume rate imaging and can be implemented on the widely available clinical ultrasound scanners, facilitating its clinical translation to improve liver fibrosis evaluation.

Real-Time Crawling Wave Sonoelastography for Human Muscle Characterization: Initial Results

Imaging of musculoskeletal tissue dynamics is currently an exploratory field with the goal of aiding rehabilitation and performance evaluation of pathological or asymptomatic patients. In this pilot study, initial elasticity assessments of the biceps brachii were conducted in a novel crawling wave sonoelastography (CWS) system implemented on a research ultrasound instrument with graphical processing unit capabilities, displaying quantitative elasticity values at 4 frames per second. The CWS system computes the tissue stiffness with the generation of an interference pattern from external vibrators, which can overcome depth limitations of imaging systems with internal excitation sources. Validation on gelatin-based phantoms reported low bias of elasticity values (4.7%) at low excitation frequencies. Preliminary results on in vivo muscle characterization are in accordance with average elasticity values for relaxed and contracted tissues found in the literature, as well as for a range of weight loads.

Correlation between semiquantitative sonoelastography and immunohistochemistry in the evaluation of testicular focal lesions

BACKGROUND

Sonoelastography is a novel and promising imaging tool, which has been applied to breast, thyroid, and prostate tissues. The aim of this study was to evaluate focal lesions of the testes with diameters of <10 mm using sonoelastography, B-mode sonography (US), and colour Doppler ultrasonography (CDU).

METHODS

Thirty patients who were referred to our outpatient clinics for varicocoeles, scrotal pain, scrotal enlargements, epididymitis, palpable testicular nodules, or infertility, were prospectively enrolled into this study. Ultrasound evaluations had revealed that 27 subjects had focal testicular lesions with diameters of <10 mm and 3 subjects had 10-mm spherical non-homogeneous testicular nodules. All lesions were evaluated using semiquantitative sonoelastography, and the patients underwent orchifunicolectomies. The testicular lesions were examined histopathologically. The vascularization of the lesions and the surrounding testicular parenchyma was evaluated by analysing the immunohistochemical distribution of the cluster of differentiation 31 and by calculating the vascular indices (VI). Potential associations between the strain ratios (stiffness of the lesions) and the VI were tested.

RESULTS

Analyses of the strain fields obtained using semiquantitative sonoelastography yielded different values for the masses and the surrounding tissues, which led to significant increases in the strain ratios. Sonoelastography upheld all of the diagnoses that were suspected when the patients were physically examined, when the serum markers were analysed, and after the patients had undergone US and CDU. Histopathological examinations confirmed the neoplastic characteristics of these masses. A significant inverse correlation was determined between the sonoelastographic strain ratio and the VI (Pearson correlation coefficient, r, = - 0.93; P < 0.001).

CONCLUSION

Our investigation shows that semiquantitative sonoelastography may provide additional objective information to support the algorithm used to diagnose testicular lesions. This might be of crucial diagnostic importance for lesions with diameters of <10 mm, particularly if they are not palpable, are negative for serum tumour markers, and if the findings from ultrasonography and CDU are equivocal. The findings from semiquantitative sonoelastography might indicate the need for surgical exploration. Further investigations with larger numbers of patients are required to corroborate these data and to support the use of semiquantitative sonoelastography in the evaluation of testicular lesions.

Calibration for position tracking of swept motor 3-D ultrasound

Tracking the position and orientation of a 3-D ultrasound transducer has many clinical applications. Tracking requires calibration to find the transformation between the tracking sensor and the ultrasound coordinates. Typically the set of image slice data are scan converted to a Cartesian volume using assumed motor geometry and a single transformation to the sensor. We propose, instead, the calibration of individual slices using a 2-D calibration technique. A best fit to a subset of slices is performed to decrease data collection time compared with that for calibration of all slices, and to reduce the influence of random errors in individual calibrations. We compare our technique with four scan conversion-based techniques: 2-D N-wire on the center slice, N-wire using a 3-D volume, N-wire using a 3-D volume including the edge points and a new closed-form planar method using a 3-D volume. The proposed multi-slice technique produced the smallest point reconstruction error (0.82 mm using a tracked stylus).

Remote ultrasound palpation for robotic interventions using absolute elastography

Although robotic surgery has addressed many of the challenges presented by minimally invasive surgery, haptic feedback and the lack of knowledge of tissue stiffness is an unsolved problem. This paper presents a system for finding the absolute elastic properties of tissue using a freehand ultrasound scanning technique, which utilizes the da Vinci Surgical robot and a custom 2D ultrasound transducer for intraoperative use. An external exciter creates shear waves in the tissue, and a local frequency estimation method computes the shear modulus. Results are reported for both phantom and in vivo models. This system can be extended to any 6 degree-of-freedom tracking method and any 2D transducer to provide real-time absolute elastic properties of tissue.