Real-time monitoring of high-intensity focused ultrasound treatment using axial strain and axial-shear strain elastograms

Agarose as a Tissue Mimic for the Porcine Heart, Kidney, and Liver: Measurements and a Springpot Model

Agarose gels are often used as a tissue mimic. The goal of this work was to determine the appropriate agarose concentrations that result in mechanical properties that match three different porcine organs. Strain tests were carried out with an amplitude varying from 0.01% to 10% at a frequency of 1 Hz on a range of agarose concentrations and porcine organs. Frequency sweep tests were performed from 0.1 Hz to a maximum of 9.5 Hz at a shear strain amplitude of 0.1% for agarose and porcine organs. In agarose samples, the effect of pre-compression of the samples up to 10% axial strain was considered during frequency sweep tests. The experimental measurements from agarose samples were fit to a fractional order viscoelastic (springpot) model. The model was then used to predict stress relaxation in response to a step strain of 0.1%. The prediction was compared to experimental relaxation data, and the results agreed within 12%. The agarose concentrations (by mass) that gave the best fit were 0.25% for the liver, 0.3% for the kidney, and 0.4% for the heart. At a frequency of 0.1 Hz and a shear strain of 0.1%, the agarose concentrations that best matched the shear storage modulus of the porcine organs were 0.4% agarose for the heart, 0.3% agarose for the kidney, and 0.25% agarose for the liver.

Hyperspectral image-based analysis of thermal damage for ex-vivo bovine liver utilizing radiofrequency ablation

BACKGROUND & OBJECTIVE

Thermal ablation is the predominant methodology to treat liver tumors for segregating patients who are not permitted to have surgical intervention. However, noticing or predicting the size of the thermal strategies is a challenging endeavor. We aim to analyze the effects of ablation district volume following radiofrequency ablation (RFA) of ex-vivo liver exploiting a custom Hyperspectral Imaging (HSI) system.

MATERIALS AND METHODS

RFA was conducted on the ex-vivo bovine liver at focal and peripheral blood vessel sites and observed by Custom HSI system, which has been designed to assess the exactness and proficiency using visible and near-infrared wavelengths region for tissue thermal effect. The experiment comprised up to ten trials with RFA. The experiment was carried out in two stages to assess the percentage of the thermal effect on the investigated sample superficially and for the side penetration effect. Measuring the diffuse reflectance (Ŗ_d) of the sample to identify the spectral reflectance shift which could differentiate between normal and ablated tissue exploiting the designed cross-correlation algorithm for monitoring of thermal ablation.

RESULTS

Determination of the diffuse reflection (Ŗ_d) spectral signature responses from normal, thermal effected, and thermal ablation regions of the investigated liver sample. Where the ideal wavelength range at (600-640 nm) could discriminate between these different regions. Then, exploited the converted RGB image of the HS liver tissue after RFA for more validations which shows that the optimum wavelength for differentiation at (530-560 nm and 600-640 nm). Finally, applying statistical analysis to validate our results presenting that wavelength 600 nm had the highest standard deviation (δ) to differentiate between various thermally affected regions regarding the normal tissue and wavelength 640 nm shows the highest (δ) to differentiate between the ablated and normal regions.

CONCLUSION

The designed and implemented medical imaging system incorporated the hyperspectral camera capabilities with the associate cross-correlation algorithm that could successfully distinguish between the ablated and thermally affected regions to assist the surgery during the tumor therapy.

Displacement Imaging During Focused Ultrasound Median Nerve Modulation: A Preliminary Study in Human Pain Sensation Mitigation

Focused ultrasound (FUS)-based viscoelastic imaging techniques using high frame rate (HFR) ultrasound to track tissue displacement can be used for mechanistic monitoring of FUS neuromodulation. However, a majority of techniques avoid imaging during the active push transmit (interleaved or postpush acquisitions) to mitigate ultrasound interference, which leads to missing temporal information of ultrasound effects when FUS is being applied. Furthermore, critical for clinical translation, use of both axial steering and real-time (<1 s) capabilities for optimizing acoustic parameters for tissue engagement are largely missing. In this study, we describe a method of noninterleaved, single Vantage imaging displacement within an active FUS push with simultaneous axial steering and real-time capabilities using a single ultrasound acquisition machine. Results show that the pulse sequence can track micron-sized displacements using frame rates determined by the calculated time-of-flight (TOF), without interleaving the FUS pulses and imaging acquisition. Decimation by 3-7 frames increases signal-to-noise ratio (SNR) by 15.09±7.03 dB. Benchmarking tests of CUDA-optimized code show increase in processing speed of 35- and 300-fold in comparison with MATLAB parallel processing GPU and CPU functions, respectively, and we can estimate displacement from steered push beams ±10 mm from the geometric focus. Preliminary validation of displacement imaging in humans shows that the same driving pressures led to variable nerve engagement, demonstrating important feedback to improve transducer coupling, FUS incident angle, and targeting. Regarding the use of our technique for neuromodulation, we found that FUS altered thermal perception of thermal pain by 0.9643 units of pain ratings in a single trial. Additionally, 5 [Formula: see text] of nerve displacement was shown in on-target versus off-target sonications. The initial feasibility in healthy volunteers warrants further study for potential clinical translation of FUS for pain suppression.

Development of temperature controller-integrated portable HIFU driver for thermal coagulation

BACKGROUND

Temperature monitoring during high-intensity focused ultrasound (HIFU) therapy on tissue is essential to regulate the degree of thermal coagulation and to achieve the desired treatment outcomes eventually. The aim of the current study was to design and investigate the feasibility of a proportional-integral-derivative (PID) temperature controller-integrated portable HIFU driver for thermal coagulation.

METHODS

A portable HIFU driver was designed and operated at a maximum output voltage of 50 V with pulse-width modulation signals at 2 MHz. The temperature of ex vivo bovine liver tissue was monitored using a K-type thermocouple during the 2-MHz HIFU exposure.

RESULTS

The tissue temperature was maintained at 60 °C using a PID controller-integrated HIFU driver that modulated the output voltage during the 300-s HIFU exposure. The ex vivo testing demonstrated that the tissue temperature at the focal point approached the chosen temperature, i.e., 60 °C, within 70 s. The temperature was maintained with a deviation of less than 4 °C until the HIFU driver voltage was turned off at 300 s.

CONCLUSIONS

The designed PID controller-integrated HIFU driver can be used as a small portable tool to regulate the tissue temperature in real time and achieve thermal coagulation via HIFU sonication.

Performance of Shear Wave Elastography in Delineating the Radiofrequency Ablation Boundary: An in Vivo experiment

This study was aimed at exploring the cutoff value of Young's modulus of ablated tissue and the optimal scale at which shear wave elastography (SWE) can delineate the ablation boundary. The livers of 30 rabbits were radiofrequency (RF) ablated, and ultrasonic imaging, including SWE and contrast-enhanced ultrasound (CEUS), was performed. The ablation boundary in the SWE image was located using CEUS, and the SWE parameters of the boundary were measured to calculate the cutoff value of Young's modulus. The cutoff value of the ablated tissue was 48-50 kPa 2 h to 28 d post-ablation. The regions of increased stiffness in SWE images at a scale of 0-50 kPa overlapped well with the non-enhanced regions of CEUS images in 88% of specimens. Therefore, elasticity values differed significantly between ablated and non-ablated tissues, and the cutoff value for Young's modulus differentiated these tissues. SWE delineated the ablation boundary well at the optimal SWE scale with respect to the cutoff value.

Validation of hybrid angular spectrum acoustic and thermal modelling in phantoms

In focused ultrasound (FUS) thermal ablation of diseased tissue, acoustic beam and thermal simulations enable treatment planning and optimization. In this study, a treatment-planning methodology that uses the hybrid angular spectrum (HAS) method and the Pennes' bioheat equation (PBHE) is experimentally validated in homogeneous tissue-mimicking phantoms. Simulated three-dimensional temperature profiles are compared to volumetric MR thermometry imaging (MRTI) of FUS sonications in the phantoms, whose acoustic and thermal properties are independently measured. Additionally, Monte Carlo (MC) uncertainty analysis is performed to quantify the effect of tissue property uncertainties on simulation results. The mean error between simulated and experimental spatiotemporal peak temperature rise was +0.33°C (+6.9%). Despite this error, the experimental temperature rise fell within the expected uncertainty of the simulation, as determined by the MC analysis. The average errors of the simulated transverse and longitudinal full width half maximum (FWHM) of the profiles were -1.9% and 7.5%, respectively. A linear regression and local sensitivity analysis revealed that simulated temperature amplitude is more sensitive to uncertainties in simulation inputs than in the profile width and shape. Acoustic power, acoustic attenuation and thermal conductivity had the greatest impact on peak temperature rise uncertainty; thermal conductivity and volumetric heat capacity had the greatest impact on FWHM uncertainty. This study validates that using the HAS and PBHE method can adequately predict temperature profiles from single sonications in homogeneous media. Further, it informs the need to accurately measure or predict patient-specific properties for improved treatment planning of ablative FUS surgeries.

Detection of gaps between high-intensity focused ultrasound (HIFU)-induced lesions using transient axial shear strain elastograms

PURPOSE

High-intensity focused ultrasound (HIFU) is becoming an effective and noninvasive treatment modality for cancer and solid tumors. In order to avoid the cancer relapse and guarantee the success of ablation, there should be no gaps left among all HIFU-generated lesions. However, there are few imaging approaches available for detecting the HIFU lesion gaps in real time during ablation.

METHODS

Transient axial shear strain elastograms (ASSEs) were proposed and evaluated both numerically and experimentally to detect the lesion gaps immediately after the cessation of therapeutic HIFU exposure. Acoustic intensity and subsequent acoustic radiation force were first calculated by solving the nonlinear Khokhlov-Zabolotskaya-Kuznetzov (KZK) equation. Motion of being- and already-treated lesions during and after HIFU exposure was simulated using the transient dynamic analysis module of finite element method (FEM). The corresponding B-mode sonography of tissue-mimicking phantom with two HIFU lesions inside was simulated by FIELD II, and then axial strain elastograms (ASEs) under static compression and transient ASSEs were reconstructed. An ultrasound imaging probe was integrated with the HIFU transducer and used to obtain radio frequency (RF) echo signals at high frame rate using plane wave imaging (PWI). The resulting strains were mapped using the correlation-based method and block search strategy.

RESULTS

Acoustic radiation force from the therapeutic HIFU burst is sufficiently strong to produce significant displacement. As a result, large and highly localized axial shear strain appears in the gap zone between two HIFU-generated lesions and then disappears after sufficient HIFU ablation (no gap between them). Such capability of detecting the lesion gap is validated at the varied acoustic radiation force density, gap width, and the size of the lesion. In contrast, conventional ASEs using the static compression cannot distinguish whether a gap exists between lesions. Static ASEs and transient ASSEs reconstructed using both high-speed photography and sonography in the gel phantom show the same conclusion as that in the simulation. Ex vivo tissue experiments further confirmed that the presence of large axial shear strain in the gap zone. The ratios of axial shear strain in the porcine kidney and liver samples had statistical differences for two HIFU-generated lesions without and with a gap (P < 0.05).

CONCLUSIONS

Large axial shear strain induced by the acoustic radiation force from therapeutic HIFU burst only appears between two HIFU-generated lesions with a gap between them. Transient ASSEs reconstructed immediately after the cession of HIFU exposure can easily, reliably, and sensitively detect the gap between produced lesions, which would provide real-time feedback to enhance the success of HIFU ablation.

Non-invasive peripheral nerve stimulation via focused ultrasound in vivo

Focused ultrasound (FUS) has been employed on a wide range of clinical applications to safely and non-invasively achieve desired effects that have previously required invasive and lengthy procedures with conventional methods. Conventional electrical neuromodulation therapies that are applied to the peripheral nervous system (PNS) are invasive and/or non-specific. Recently, focused ultrasound has demonstrated the ability to modulate the central nervous system and ex vivo peripheral neurons. Here, for the first time, noninvasive stimulation of the sciatic nerve eliciting a physiological response in vivo is demonstrated with FUS. FUS was applied on the sciatic nerve in mice with simultaneous electromyography (EMG) on the tibialis anterior muscle. EMG signals were detected during or directly after ultrasound stimulation along with observable muscle contraction of the hind limb. Transecting the sciatic nerve downstream of FUS stimulation eliminated EMG activity during FUS stimulation. Peak-to-peak EMG response amplitudes and latency were found to be comparable to conventional electrical stimulation methods. Histology along with behavioral and thermal testing did not indicate damage to the nerve or surrounding regions. The findings presented herein demonstrate that FUS can serve as a targeted, safe and non-invasive alternative to conventional peripheral nervous system stimulation to treat peripheral neuropathic diseases in the clinic.

Tumor characterization and treatment monitoring of postsurgical human breast specimens using harmonic motion imaging (HMI)

BACKGROUND

High-intensity focused ultrasound (HIFU) is a noninvasive technique used in the treatment of early-stage breast cancer and benign tumors. To facilitate its translation to the clinic, there is a need for a simple, cost-effective device that can reliably monitor HIFU treatment. We have developed harmonic motion imaging (HMI), which can be used seamlessly in conjunction with HIFU for tumor ablation monitoring, namely harmonic motion imaging for focused ultrasound (HMIFU). The overall objective of this study was to develop an all ultrasound-based system for real-time imaging and ablation monitoring in the human breast in vivo.

METHODS

HMI was performed in 36 specimens (19 normal, 15 invasive ductal carcinomas, and 2 fibroadenomas) immediately after surgical removal. The specimens were securely embedded in a tissue-mimicking agar gel matrix and submerged in degassed phosphate-buffered saline to mimic in vivo environment. The HMI setup consisted of a HIFU transducer confocally aligned with an imaging transducer to induce an oscillatory radiation force and estimate the resulting displacement.

RESULTS

3D HMI displacement maps were reconstructed to represent the relative tissue stiffness in 3D. The average peak-to-peak displacement was found to be significantly different (p = 0.003) between normal breast tissue and invasive ductal carcinoma. There were also significant differences before and after HMIFU ablation in both the normal (53.84 % decrease) and invasive ductal carcinoma (44.69 % decrease) specimens.

CONCLUSIONS

HMI can be used to map and differentiate relative stiffness in postsurgical normal and pathological breast tissues. HMIFU can also successfully monitor thermal ablations in normal and pathological human breast specimens. This HMI technique may lead to a new clinical tool for breast tumor imaging and HIFU treatment monitoring.

Harmonic motion imaging for abdominal tumor detection and high-intensity focused ultrasound ablation monitoring: an in vivo feasibility study in a transgenic mouse model of pancreatic cancer

Harmonic motion imaging (HMI) is a radiationforce- based elasticity imaging technique that tracks oscillatory tissue displacements induced by sinusoidal ultrasonic radiation force to assess the resulting oscillatory displacement denoting the underlying tissue stiffness. The objective of this study was to evaluate the feasibility of HMI in pancreatic tumor detection and high-intensity focused ultrasound (HIFU) treatment monitoring. The HMI system consisted of a focused ultrasound transducer, which generated sinusoidal radiation force to induce oscillatory tissue motion at 50 Hz, and a diagnostic ultrasound transducer, which detected the axial tissue displacements based on acquired radio-frequency signals using a 1-D cross-correlation algorithm. For pancreatic tumor detection, HMI images were generated for pancreatic tumors in transgenic mice and normal pancreases in wild-type mice. The obtained HMI images showed a high contrast between normal and malignant pancreases with an average peak-to-peak HMI displacement ratio of 3.2. Histological analysis showed that no tissue damage was associated with HMI when it was used for the sole purpose of elasticity imaging. For pancreatic tumor ablation monitoring, the focused ultrasound transducer was operated at a higher acoustic power and longer pulse length than that used in tumor detection to simultaneously induce HIFU thermal ablation and oscillatory tissue displacements, allowing HMI monitoring without interrupting tumor ablation. HMI monitoring of HIFU ablation found significant decreases in the peak-to-peak HMI displacements before and after HIFU ablation with a reduction rate ranging from 15.8% to 57.0%. The formation of thermal lesions after HIFU exposure was confirmed by histological analysis. This study demonstrated the feasibility of HMI in abdominal tumor detection and HIFU ablation monitoring.

High-intensity focused ultrasound monitoring using harmonic motion imaging for focused ultrasound (HMIFU) under boiling or slow denaturation conditions

Harmonic motion imaging for focused ultrasound (HMIFU) is a recently developed high-intensity focused ultrasound (HIFU) treatment monitoring method that utilizes an amplitude-modulated therapeutic ultrasound beam to induce an oscillatory radiation force at the HIFU focus and estimates the focal tissue displacement to monitor the HIFU thermal treatment. In this study, the performance of HMIFU under acoustic, thermal, and mechanical effects was investigated. The performance of HMIFU was assessed in ex vivo canine liver specimens (n = 13) under slow denaturation or boiling regimes. A passive cavitation detector (PCD) was used to assess the acoustic cavitation activity, and a bare-wire thermocouple was used to monitor the focal temperature change. During lesioning with slow denaturation, high quality displacements (correlation coefficient above 0.97) were observed under minimum cavitation noise, indicating the tissue initial-softening-then- stiffening property change. During HIFU with boiling, HMIFU monitored a consistent change in lesion-to-background displacement contrast (0.46 ± 0.37) despite the presence of strong cavitation noise due to boiling during lesion formation. Therefore, HMIFU effectively monitored softening-then-stiffening during lesioning under slow denaturation, and detected lesioning under boiling with a distinct change in displacement contrast under boiling in the presence of cavitation. In conclusion, HMIFU was shown under both boiling and slow denaturation regimes to be effective in HIFU monitoring and lesioning identification without being significantly affected by cavitation noise.

Thermometry and ablation monitoring with ultrasound

In this review we present the current status of ultrasound thermometry and ablation monitoring, with emphasis on the diverse approaches published in the literature and with an eye on which methods are closest to clinical reality. It is hoped that this review will serve as a guide to the expansion of sonographic methods for treatment monitoring and thermometry since the last brief review in 2007.