High intensity focused ultrasound ablation of kidney guided by MRI

Quadratic versus linear models to estimate the mean scattering spacing as a function of temperature in ex-vivo tissue

Previous works have shown the feasibility of temperature estimation during ultrasonic therapy using pulse-echo diagnostic ultrasound. These methods are based on the measurement of thermally induced changes in backscattered RF echoes due to thermal expansion and changes in ultrasonic velocity. They assume a joint contribution of these two parameters and a linear dependence with temperature. In this work, the contributions of velocity changes and thermal expansion to the evolution of the mean scatterer spacing of ex vivo bovine skeletal muscle tissue samples were decoupled. This was achieved by employing an experimental setup which allows measuring the absolute velocity value, using the through-transmission technique in a direct transmission configuration. The mean-scatterer spacing was estimated from spectral analysis of the backscattered signals obtained in pulse-echo mode. We propose a quadratic model of the thermal expansion coefficient to fit the evolution of the mean-scatterer spacing with temperature. The temperature increase estimated by the linear model, in the range of 29.5-47 °C, presents a percentage error (mean square error) of 11 %, while for the quadratic model the error is 4.8 %.

Modeling for Quantitative Analysis of Nakagami Imaging in Accurate Detection and Monitoring of Therapeutic Lesions by High-Intensity Focused Ultrasound

OBJECTIVE

Nakagami imaging is an appealing monitoring and evaluation technique for high-intensity focused ultrasound treatment when bubbles are present in ultrasound images. This study aimed to investigate the accuracy of thermal lesion detection using Nakagami imaging.

METHODS

Simulations were conducted to explore and quantify the influence of the bubbles and the subresolvable effect at the boundary of the thermal lesion on thermal lesion detection. The thermal ablation experiments were conducted in phantom and porcine liver ex vivo.

RESULTS

In the simulation, the estimated lateral and axial size of the thermal lesion in the Nakagami image was 4.91 and 4.79 mm, close to the actual size (5 × 5 mm). The simulation results indicated that the subresolvable region in high-intensity focused ultrasound treatment thermal ablation mainly happened at the boundary between bubbles and the untreated region and does not affect the accuracy of thermal lesion detection. The accurate detection of the thermal lesion using Nakagami imaging mainly depends on bubbles and thermal lesion characterization. Our thermal ablation experiments confirmed that Nakagami imaging has the ability to accurately identify thermal lesions from bubbles.

CONCLUSION

The subresolvable effect is helpful for thermal lesion identification, and precision is related to the Nakagami values chosen for boundary division in Nakagami imaging. Therefore, Nakagami imaging is a promising method for accurately evaluating thermal lesions. Further studies in vivo and in clinical settings will be needed to explore its potential applications.

Image reconstruction method with compressed sensing for high-speed MR temperature measurement of abdominal organs. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2019;2019:2731-2735. [PMID: 31946459 DOI: 10.1109/embc.2019.8856422]

Magnetic Resonance guided High Intensity Focused Ultrasound (MRgHIFU) treatment is a low invasive tumor treatment using high energy from an ultrasound. The transducer generates sound wave and focuses a heat point within the body to eliminate the tumor. In heating, it is necessary to monitor the condition at the target area for safe and effective treatment. Magnetic Resonance Imaging(MRI) can monitor the target condition and temperature distribution during treatment. However, the acquisition time of MR data is long and has to be shortened to track the focal point. In this research, a rapid acquisition and reconstruction method using compressed sensing MRI is proposed. In order to reduce the number of phase encode times, k-space was divided into regions. Then, the value of the gradient was used to shorten the signal restoration time. In the computational experiments, image quality and temperature error were evaluated.

High-Intensity Focused Ultrasound Lesion Detection Using Adaptive Compressive Sensing Based on Empirical Mode Decomposition

Background:

The main goal of ultrasound therapy is to have clinical effects in the tissue without damage to the intervening and surrounding tissues. Treatments have been developed for both in vitro and in clinical applications. HIFU therapy is one of these. Non-invasive surgeries, such as HIFU, have been developed to treat tumors or to stop bleeding. In this approach, an adequate imaging method for monitoring and controlling the treatment is required.

Methods:

In this paper, an adaptive compressive sensing representation of ultrasound RF echo signals is presented based on empirical mode decomposition (EMD). According to the different numbers of intrinsic mode functions (IMFs) produced by the EMD, the ultrasound signals is adaptively compressive sampled in the source and then adaptively reconstructed in the receiver domains. In this paper, a new application of compressive sensing based on EMD (CS-EMD) in the monitoring of high-intensity focused ultrasound (HIFU) treatment is presented. Non-invasive surgeries such as HIFU have been developed for various therapeutic applications. In this technique, a suitable imaging method is necessary for monitoring of the treatment to achieve adequate treatment safety and efficacy. So far, several methods have been proposed, such as ultrasound radiofrequency (RF) signal processing techniques, and imaging methods such as X-ray, MRI, and ultrasound to monitor HIFU lesions.

Results:

In this paper, a CS-EMD method is used to detect the HIFU thermal lesion dimensions using different types of wavelet transform. The results of the processing on the real data demonstrate the potential for this technique in image-guided HIFU therapy.

Conclusions:

In this study, a new application of compressive sensing in the field of monitoring of the HIFU treatment is presented. To the best of our knowledge, so far no studies on compressive sensing have been carried out in the monitoring of the HIFU. Based on the results obtained, it was showed that the number of measurements and Intrinsic Mode Functions have the function of noise reduction. In addition, results were shown that the successful reconstruction of the compressive sensing signals can be gained using a threshold based algorithm. To this end, in this paper it was shown that by selecting an suitable number of measurements, the sparse transform, and a thresholding algorithm, we can achieve a more accurate detection of the HIFU thermal lesion size.

High-Intensity Focused Ultrasound Thermal Lesion Detection Using Entropy Imaging of Ultrasound Radio Frequency Signal Time Series

Background:

During the past few decades, high-intensity focused ultrasound (HIFU) modality has been gaining surging interest in various therapeutic applications such as non- or minimally-invasive cancer treatment. Among other attributes, robust and real-time HIFU treatment monitoring and lesion detection have become essential issues for successful clinical acceptance of the modality. More recently, ultrasound radio frequency (RF) time series imaging has been studied by a number of researchers.

Materials and Methods:

The objective of this study is to investigate the applicability of entropy parameter of RF time series of ultrasound backscattered signals, a. k. a. Entropy imaging, toward HIFU thermal lesion detection. To this end, five fresh ex vivo porcine muscle tissue samples were exposed to HIFU exposures with total acoustic powers ranging from 30 to 110 Watts. The contrast-to-speckle ratio (CSR) values of the entropy images and their corresponding B-mode images of pre-, during- and post-HIFU exposure for each acoustic power were calculated.

Results:

The novelty of this study is the use of Entropy parameter on ultrasound RF time series for the first time. Statistically significant differences were obtained between the CSR values for the B mode and entropy images at various acoustic powers. In case of 110 Watt, a CSR value 3.4 times higher than B-mode images was accomplished using the proposed method. Furthermore, the proposed method is compared with the scaling parameter of Nakagami imaging and same data which are used in this study.

Conclusion:

Entropy has the potential for using as an imaging parameter for differentiating lesions in HIFU surgery.

Passive acoustic mapping of cavitation using eigenspace-based robust Capon beamformer in ultrasound therapy

Pulse-echo imaging technique can only play a role when high intensity focused ultrasound (HIFU) is turned off due to the interference between the primary HIFU signal and the transmission pulse. Passive acoustic mapping (PAM) has been proposed as a tool for true real-time monitoring of HIFU therapy. However, the most-used PAM algorithm based on time exposure acoustic (TEA) limits the quality of cavitation image. Recently, robust Capon beamformer (RCB) has been used in PAM to provide improved resolution and reduced artifacts over TEA-based PAM, but the presented results have not been satisfactory. In the present study, we applied an eigenspace-based RCB (EISRCB) method to further improve the PAM image quality. The optimal weighting vector of the proposed method was found by projecting the RCB weighting vector onto the desired vector subspace constructed from the eigenstructure of the covariance matrix. The performance of the proposed PAM was validated by both simulations and in vitro histotripsy experiments. The results suggested that the proposed PAM significantly outperformed the conventionally used TEA and RCB-based PAM. The comparison results between pulse-echo images of the residual bubbles and cavitation images showed the potential of our proposed PAM in accurate localization of cavitation activity during HIFU therapy.

Toward high-intensity focused ultrasound lesion quantification using compressive sensing theory

Compressive sensing theory has in recent years been increasingly used in various pattern recognition applications. Compressive sensing theory makes it possible, under certain assumptions, to recover a signal or an image sampled below the Nyquist sampling limit. In this work, a new application of compressive sensing based on the threshold algorithm, in the area of controlling and monitoring of high-intensity focused ultrasound therapy, was investigated. In this work, a new method of high-intensity focused ultrasound lesion detection is presented based on a modified compressive sensing method in combination with the threshold algorithm and the wavelet transforms. In this study, analysis of the suggested method is performed using two sets of data: simulated and experimental ultrasound radio frequency data. The results of processing the data show that the proposed algorithm results in enhancement of the high-intensity focused ultrasound lesion contrast in comparison with the ultrasound B-mode and standard compressive sensing imaging methods. The results of the study show that the modified compressive sensing method could effectively detect thermal lesions in vitro. Comparing the estimated size of the thermal lesion (8.3 mm × 8.4 mm) using the proposed algorithm with the actual size of that from physical examination (10.1 mm × 9 mm) shows that we could detect high-intensity focused ultrasound thermal lesions with the difference of 0.8 mm × 0.5 mm.

Experimental study on ablation of leiomyoma by combination high-intensity focused ultrasound and iodized oil in vitro

AIM

The aim of the current study was to investigate whether iodized oil (IO) enhances high-intensity focused ultrasound (HIFU) ablation of uterine leiomyoma and to determine the features of hyperechoic changes in the target region.

METHODS

Forty samples of uterine leiomyoma were randomly divided into an experimental group and a control group. In the experimental group, the leiomyoma was ablated by HIFU 30 min after 1 mL of iodized oil had been injected into the center of the myoma. The hyperechoic values and areas in the target region were observed by B-modal ultrasound after HIFU ablation. The samples were cut successively into slices and stained by triphenyltetrazolium chloride (TTC) solution within 1 h after HIFU ablation. The diameters of TTC-non-stained areas were measured and tissues in the borderline of the TTC-stained and -non-stained areas were observed pathologically. All procedures in the control group were the same as those in the experimental group except IO was replaced by physiological saline.

RESULTS

The hyperechoic value in the target region in the experimental group was higher than that in the control group 4 min after HIFU ablation (P < 0.05). Hyperechoic areas in the target region as well as TTC-non-stained volumes in the experimental group were greater than those in the control group (P < 0.05). Routine pathologic observation showed that coagulation necrosis of leiomyoma occurred in the target region in both groups.

CONCLUSION

IO causes coagulation necrosis, enlarges tissue damage, and postpones the attenuation of hyperechoic changes in the target region when HIFU ablation is carried out for leiomyoma in vitro.

Thermal ablation of a confluent lesion in the porcine kidney with a clinically available MR-HIFU system

The incidence of small renal masses (SRMs) sized  <4 cm has increased over the decades (as co-findings/or due to introduction of cross sectional imaging). Currently, partial nephrectomy (PN) or watchful waiting is advised in these patients. Ultimately, 80-90% of these SRMs require surgical treatment and PN is associated with a 15% complication rate. In this aging population, with possible comorbidities and poor health condition, both PN and watchful waiting are non-ideal treatment options. This resulted in an increased need for early, non-invasive treatment strategies such as MR-guided high intensity focused ultrasound (MR-HIFU). (i) To investigate the feasibility of creating a confluent lesion in the kidney using respiratory-gated MR-HIFU under clinical conditions in a pre-clinical study and (ii) to evaluate the reproducibility of the MR-HIFU ablation strategy. Healthy pigs (n  =  10) under general anesthesia were positioned on a clinical MR-HIFU system with integrated cooling. A honeycomb pattern of seven overlapping ablation cells (4  ×  4  ×  10 mm³, 450 W, <30 s) was ablated successively in the cortex of the porcine kidney. Both MR thermometry and acoustic energy delivery were respiratory gated using a pencil beam navigator on the contralateral kidney. The non-perfused volume (NPV) was visualized after the last sonication by contrast-enhanced (CE) T ₁-weighted MR (T ₁ w) imaging. Cell viability staining was performed to visualize the extent of necrosis.

RESULTS

a median NPV of 0.62 ml was observed on CE-T ₁ w images (IQR 0.58-1.57 ml, range 0.33-2.75 ml). Cell viability staining showed a median damaged volume of 0.59 ml (IQR 0.24-1.35 ml, range 0-4.1 ml). Overlooking of the false rib, shivering of the pig, and too large depth combined with a large heat-sink effect resulted in insufficient heating in 4 cases. The NPV and necrosed volume were confluent in all cases in which an ablated volume could be observed. Our results demonstrated the feasibility of creating a confluent volume of ablated kidney cortical tissue in vivo with MR-HIFU on a clinically available system using respiratory gating and near-field cooling and showed its reproducibility.

Assessment of acute thermal damage volumes in muscle using magnetization-prepared 3D T2 -weighted imaging following MRI-guided high-intensity focused ultrasound therapy

PURPOSE

To evaluate magnetization-prepared 3D T₂ -weighted magnetic resonance imaging (MRI) measurements of acute tissue changes produced during ablative MR high-intensity focused ultrasound (MR-HIFU) exposures.

MATERIALS AND METHODS

A clinical MR-HIFU system (3T) was used to generate thermal lesions (n = 24) in the skeletal muscles of three pigs. T₁ -weighted, 2D T₂ -weighted, and magnetization-prepared 3D T₂ -weighted sequences were acquired before and after therapy to evaluate tissue changes following ablation. Tissues were harvested shortly after imaging, fixed in formalin, and gross-sectioned. Select lesions were processed into whole-mount sections. Lesion dimensions for each imaging sequence (length, width) and for gross sections (diameter of lesion core and rim) were assessed by three physicists. Contrast-to-background ratio between lesions and surrounding muscle was compared.

RESULTS

Lesion dimensions on T₁ and 2D T₂ -weighted imaging sequences were well correlated (R² ∼0.7). The contrast-to-background ratio between lesion and surrounding muscle was 7.4 ± 2.4 for the magnetization-prepared sequence versus 1.7 ± 0.5 for a conventional 2D T₂ -weighted acquisition, and 7.0 ± 2.9 for a contrast-enhanced T₁ -weighted sequence. Compared with diameter measured on gross pathology, all imaging sequences overestimated the lesion core by 22-33%, and underestimated the lesion rim by 6-13%.

CONCLUSION

After MR-HIFU exposures, measurements of the acute thermal damage patterns in muscle using a magnetization-prepared 3D T₂ -weighted imaging sequence correlate with 2D T₂ -weighted and contrast-enhanced T₁ -weighted imaging, and all agree well with histology. The magnetization-prepared sequence offers positive tissue contrast and does not require IV contrast agents, and may provide a noninvasive imaging evaluation of the region of acute thermal injury at multiple times during HIFU procedures.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. MAGN. RESON. IMAGING 2017;46:354-364.

HIFU for Palliative Treatment of Pancreatic Cancer

Pancreatic cancer is one of the deadliest malignancies, with only a 6 % 5-year survival rate and over 50 % of patients being diagnosed at the advanced stage. Current therapies are ineffective, and the treatment of patients with advanced disease is palliative. In the past decade, HIFU ablation has emerged as a modality for palliative treatment of pancreatic tumors. Multiple preclinical and non-randomized clinical trials have been performed to evaluate the safety and efficacy of this procedure. Substantial tumor-related pain reduction was achieved in most cases after HIFU treatment and few significant side effects were observed. In addition, some studies indicate that combination of HIFU ablation with chemotherapy may provide a survival benefit. This chapter summarizes the pre-clinical and clinical experience obtained to date in HIFU treatment of pancreatic tumors and discusses the challenges, limitations and new approaches in this modality.

Dependence of ablative ability of high-intensity focused ultrasound cavitation-based histotripsy on mechanical properties of agar

Cavitation-based histotripsy uses high-intensity focused ultrasound at low duty factor to create bubble clouds inside tissue to liquefy a region, and provides better fidelity to planned lesion coordinates and the ability to perform real-time monitoring. The goal of this study was to identify the most important mechanical properties for predicting lesion dimensions, among these three: Young's modulus, bending strength, and fracture toughness. Lesions were generated inside tissue-mimicking agar, and correlations were examined between the mechanical properties and the lesion dimensions, quantified by lesion volume and by the width and length of the equivalent bubble cluster. Histotripsy was applied to agar samples with varied properties. A cuboid of 4.5 mm width (lateral to focal plane) and 6 mm depth (along beam axis) was scanned in a raster pattern with respective step sizes of 0.75 and 3 mm. The exposure at each treatment location was either 15, 30, or 60 s. Results showed that only Young's modulus influenced histotripsy's ablative ability and was significantly correlated with lesion volume and bubble cluster dimensions. The other two properties had negligible effects on lesion formation. Also, exposure time differentially affected the width and depth of the bubble cluster volume.

Real-time monitoring of high-intensity focused ultrasound thermal therapy using the manifold learning method

High-intensity focused ultrasound (HIFU) induces thermal lesions by increasing the tissue temperature in a tight focal region. The main ultrasound imaging techniques currently used to monitor HIFU treatment are standard pulse-echo B-mode ultrasound imaging, ultrasound temperature estimation and elastography-based methods. The present study was carried out on ex vivo animal tissue samples, in which backscattered radiofrequency (RF) signals were acquired in real time at time instances before, during and after HIFU treatment. The manifold learning algorithm, a non-linear dimensionality reduction method, was applied to RF signals whichconstruct B-mode images to detect the HIFU-induced changes among the image frames obtained during HIFU treatment. In this approach, the embedded non-linear information in the region of interest of sequential images is represented in a 2-D manifold with the Isomap algorithm, and each image is depicted as a point on the reconstructed manifold. Four distinct regions are chosen in the manifold corresponding to the four phases of HIFU treatment (before HIFU treatment, during HIFU treatment, immediately after HIFU treatment and 10-min after HIFU treatment). It was found that disorganization of the points is achieved by increasing the acoustic power, and if the thermal lesion has been formed, the regions of points related to pre- and post-HIFU significantly differ. Moreover, the manifold embedding was repeated on 2-D moving windows in RF data envelopes related to pre- and post-HIFU exposure data frames. It was concluded that if mean values of the points related to pre- and post-exposure frames in the reconstructed manifold are estimated, and if the Euclidean distance between these two mean values is calculated and the sliding window is moved and this procedure is repeated for the whole image, a new image based on the Euclidean distance can be formed in which the HIFU thermal lesion is detectable.

Nakagami imaging for detecting thermal lesions induced by high-intensity focused ultrasound in tissue

High-intensity focused ultrasound induces focalized tissue coagulation by increasing the tissue temperature in a tight focal region. Several methods have been proposed to monitor high-intensity focused ultrasound-induced thermal lesions. Currently, ultrasound imaging techniques that are clinically used for monitoring high-intensity focused ultrasound treatment are standard pulse-echo B-mode ultrasound imaging, ultrasound temperature estimation, and elastography-based methods. On the contrary, the efficacy of two-dimensional Nakagami parametric imaging based on the distribution of the ultrasound backscattered signals to quantify properties of soft tissue has recently been evaluated. In this study, ultrasound radio frequency echo signals from ex vivo tissue samples were acquired before and after high-intensity focused ultrasound exposures and then their Nakagami parameter and scaling parameter of Nakagami distribution were estimated. These parameters were used to detect high-intensity focused ultrasound-induced thermal lesions. Also, the effects of changing the acoustic power of the high-intensity focused ultrasound transducer on the Nakagami parameters were studied. The results obtained suggest that the Nakagami distribution's scaling and Nakagami parameters can effectively be used to detect high-intensity focused ultrasound-induced thermal lesions in tissue ex vivo. These parameters can also be used to understand the degree of change in tissue caused by high-intensity focused ultrasound exposures, which could be interpreted as a measure of degree of variability in scatterer concentration in various parts of the high-intensity focused ultrasound lesion.

Precision control of lesions by high-intensity focused ultrasound cavitation-based histotripsy through varying pulse duration

The goal of this experimental study was to explore the feasibility of acquiring controllable precision through varying pulse duration for lesions generated by cavitation-based histotripsy. Histotripsy uses high-intensity focused ultrasound (HIFU) at low duty factor to create energetic bubble clouds inside tissue to liquefy a region. It uses cavitation-mediated mechanical effects while minimizing heating, and has the advantages of real-time monitoring and lesion fidelity to treatment planning. In our study, histotripsy was applied to three groups of tissue-mimicking agar samples of different stiffnesses (29.4 ± 5.3, 44.8 ± 5.9, and 66.4 ± 7.1 kPa). B-mode imaging was used first to quantify bubble cluster dimensions in both water and agar. Then, a 4.5-mm-wide square (lateral to the focal plane) was scanned in a raster pattern with a step size of 0.75 mm in agar histotripsy experiments to estimate equivalent bubble cluster dimensions based on the histotripsyinduced damage. The 15-s exposure at each treatment location comprised 5000 sine-wave tone bursts at a spatial-peak pulseaverage intensity of 41.1 kW/cm2, with peak compressional and rarefactional pressures of 102 and 17 MPa, respectively. The results showed that bubble cluster width and length increased with pulse duration and decreased with agar stiffness. Therefore, a significant improvement in histotripsy precision could be achieved by reducing the pulse duration.

Impact of preconditioning pulse on lesion formation during high-intensity focused ultrasound histotripsy

Therapeutic applications with high-intensity focused ultrasound (HIFU) fall into two classifications-one using thermal effect for coagulation or ablation while generally avoiding cavitation and the other using cavitation-mediated mechanical effects while suppressing heating. Representative of the latter, histotripsy uses HIFU at low duty factor to create energetic bubble clouds inside tissue to liquefy a region and has the advantages in real-time monitoring and lesion fidelity to treatment planning. We explored the impact of a preconditioning/heating pulse on histotripsy lesion formation in porcine muscle samples. During sonication, a targeted square region 9 mm wide (lateral to the focal plane) was scanned in a raster pattern with a step size of 0.75 mm. The 20-s exposure at each treatment location consisted of a 5-s duration preconditioning burst at spatial-peak intensities from 0-1386 W/cm² followed by 5000 tone bursts at high intensity (with spatial-peak pulse-average intensity of 47.34 kW/cm², spatial-peak temporal-average intensity of 284 W/cm², peak compressional pressure of 102 MPa and peak rarefactional pressure of 17 MPa). The temperature increase for all exposures was measured using a thermal imager immediately after each exposure. Lesion volume increased with increasing amplitude of the preconditioning pulse until coagulation was observed, but lesion width/area did not change significantly with the amplitude. In addition, the lesion dimensions became smaller when the global tissue temperature was raised before applying the histotripsy pulsing sequence. Therefore, the benefit of the preconditioning pulse was not caused by global heating.

Application of high-intensity focused ultrasound for fetal therapy: experimental study using an animal model of lower urinary tract obstruction

OBJECTIVE

The purpose of this study is to investigate whether high-intensity focused ultrasound (HIFU) exposure is able to produce a fistula between the bladder and abdominal wall of a fetus with lower urinary tract obstruction (LUTO).

MATERIALS AND METHODS

We constructed a prototype HIFU transducer in combination with an imaging probe. HIFU was applied to the lower abdomen of a rabbit neonate that was complicated by LUTO as an experimental model to produce a fistula; HIFU was applied in a tank filled with degassed water. Exposed lesions were assessed by histological analysis at necropsy.

RESULTS

When HIFU was applied at 5.5 kW/cm(2) of spatial-peak temporal average intensity (SPTA), a fistula was created between the lower abdominal wall and the urinary bladder; urine gushed out from the bladder through the fistula within 60 s after HIFU exposure.

CONCLUSION

The findings suggest that fetal diseases such as LUTO can be non-invasively treated using HIFU exposure from even outside the maternal body, though this study was performed in a water tank.

A fast tissue stiffness-dependent elastography for HIFU-induced lesions inspection

To monitor HIFU-induced lesion with elastography in quasi-real time, a fast correlation based elastographic algorithm using tissue stiffness-dependent displacement estimation (SdDE) is developed in this paper. The high time efficiency of the proposed method contributes to the reduction on both the number of the displacement points and the computational time of most of the points by utilizing local uniformity of the tissue under HIFU treatment. To obtain admirable comprehensive performance, the key algorithm parameter, a threshold to densify the displacement points, is optimized with simulation over a wedge-inclusion tissue model by compromising the axial resolution (AR) and the computational cost. With the optimum parameter, results from both simulations and phantom experiments show that the SdDE is faster in about one order of magnitude than the traditional correlation based algorithm. At the same time, other performance parameters, such as the signal-to-noise ratio (SNRe), the contrast-to-noise ratio (CNRe) and the axial resolution (AR), are superior to or comparable with that obtained from the traditional algorithm. In vitro experiments on bovine livers validate the improvement on the time efficiency under the circumstances of real tissue and real radio frequency (RF) signal. This preliminary work implies potential of the SdDE in dynamic or close real time guidance and monitoring of HIFU treatment.

An adaptive spectral estimation technique to detect cavitation in HIFU with high spatial resolution

In ultrasound-guided high-intensity focused ultrasound (HIFU) therapy, the changes observed on tissue are subtle during treatment; some ultrasound-guided HIFU protocols rely on the observation of significant brightness changes as the indicator of tissue lesions. The occurrence of a distinct hyperechogenic region ("bright-up") around the focus is often associated with acoustic cavitation resulting in microbubble formation, but it may indicate different physical events such as larger bubbles from boiling (known to alter acoustic impedance) or sometimes lesion formation. A reliable method to distinguish and spatially localize these causes within the tissue would assist the control of HIFU delivery, which is the subject of this paper. Spectral analysis of the radio frequency (RF) signal underlying the B-mode image provides more information on the physical cause, but the usual techniques that are methods on the Fourier transform require a long series for good spectral resolution and so they give poor spatial resolution. This paper introduces an active spectral cavitation detection method to attain high spatial resolution (0.15 × 0.15 mm per pixel) through a parametric statistical method (ARMA modeling) used on finite-length data sets, which enables local changes to be identified more easily. This technique uses the characteristics of the signal itself to optimize the model parameters and structure. Its performance is assessed using synthesized cavitation RF data, and it is then demonstrated in ex vivo bovine liver during and after HIFU exposure. The results suggest that good spatial and spectral resolution can be obtained by the design of suitable algorithms. In ultrasound-guided HIFU, the technique provides a useful supplement to B-mode analysis, with no additional time penalty in data acquisition.

Model-based ultrasound temperature visualization during and following HIFU exposure

This paper describes the application of signal processing techniques to improve the robustness of ultrasound feedback for displaying changes in temperature distribution in treatment using high-intensity focused ultrasound (HIFU), especially at the low signal-to-noise ratios that might be expected in in vivo abdominal treatment. Temperature estimation is based on the local displacements in ultrasound images taken during HIFU treatment, and a method to improve robustness to outliers is introduced. The main contribution of the paper is in the application of a Kalman filter, a statistical signal processing technique, which uses a simple analytical temperature model of heat dispersion to improve the temperature estimation from the ultrasound measurements during and after HIFU exposure. To reduce the sensitivity of the method to previous assumptions on the material homogeneity and signal-to-noise ratio, an adaptive form is introduced. The method is illustrated using data from HIFU exposure of ex vivo bovine liver. A particular advantage of the stability it introduces is that the temperature can be visualized not only in the intervals between HIFU exposure but also, for some configurations, during the exposure itself.

Dynamic changes of integrated backscatter, attenuation coefficient and bubble activities during high-intensity focused ultrasound (HIFU) treatment

This paper simultaneously investigated the transient characteristics of integrated backscatter (IBS), attenuation coefficient and bubble activities as time traces before, during and after HIFU treatment, with different HIFU parameters (acoustic power and duty cycle) in both transparent tissue-mimicking phantoms and freshly excised bovine livers. These dynamic changes of acoustic parameters and bubble activities were correlated with the visualization of lesion development selected from photos, conventional B-mode ultrasound images and differential IBS images over the whole procedure of HIFU treatment. Two-dimensional radiofrequency (RF) data were acquired by a modified diagnostic ultrasound scanner to estimate the changes of mean IBS and attenuation coefficient averaged in the lesion region, and to construct the differential IBS images and B-mode ultrasound images simultaneously. Bubble activities over the whole procedure of HIFU treatment were investigated by the passive cavitation detection (PCD) method and the changes in subharmonic and broadband noise were correlated with the transient characteristics of IBS and attenuation coefficient. When HIFU was switched on, IBS and attenuation coefficient increased with the appearance of bubble clouds in the B-mode and differential IBS image. At the same time, the level of subharmonic and broadband noise rose abruptly. Then, there was an initial decrease in the attenuation coefficient, followed by an increase when at lower HIFU power. As the lesion appeared, IBS and attenuation coefficient both increased rapidly to a value twice that of normal. Then the changes in IBS and attenuation coefficient showed more complex patterns, but still showed a slower trend of increases with lesion development. Violent bubble activities were visible in the gel and were evident as strongly echogenic regions in the differential IBS images and B-mode images simultaneously. This was detected by a dramatic high level of subharmonic and broadband noise at the same time. These bubble activities caused fluctuations in IBS and attenuation coefficient during HIFU treatment. After HIFU, IBS and attenuation coefficient decreased gradually accompanied by the fadeout of bright hyperechoic spot in the B-mode and differential IBS image, but were still higher than normal when they were stable. The increases of IBS and attenuation coefficient were greater when using higher acoustic power or a higher duty cycle of the therapeutic emission. These experiments indicated that the bubble activities had the dominant effects on the transient characteristics of IBS and attenuation. This should be taken into consideration when using the dynamic acoustic-property changes for the potentially real-time monitoring imaging of HIFU treatment.

Passive spatial mapping of inertial cavitation during HIFU exposure

A novel method for mapping inertial cavitation activity during high-intensity focused ultrasound (HIFU) exposure is presented. Inertial cavitation has been previously shown to result in increased heat deposition and to be associated with broadband noise emissions that can be readily monitored using a passive receiver without interference from the main HIFU signal. In the present study, the signals received passively by each of 64 elements on a standard diagnostic array placed coaxially with the HIFU transducer are combined using time exposure acoustics to generate maps of inertially cavitating regions during HIFU exposure of an agar-based tissue-mimicking material. The technique is shown to be effective in localizing single-bubble activity, as well as contiguous and disjoint cavitating regions instigated by creating regions of lower cavitation threshold within the tissue phantom. The cavitation maps obtained experimentally are also found to be in good agreement with computational simulations and theoretical predictions. Unlike B-mode imaging, which requires interleaving with the HIFU pulse, passive array-based mapping of cavitation activity is possible during HIFU exposure. If cavitating regions can be directly correlated to increased tissue damage, this novel cavitation mapping technique could enable real-time HIFU treatment monitoring.

In vitro and in vivo brain ablation created by high-intensity focused ultrasound and monitored by MRI

In this paper, magnetic resonance imaging (MRI) is investigated for monitoring small and large lesions created by high-intensity focused ultrasound (HIFU) in freshly excised lamb brain and in rabbit brain in vivo. A single-element spherically focused transducer of 5 cm diameter, focusing at 10 cm and operating at 1 MHz was used. A prototype MRI-compatible positioning device that is used to navigate the transducer is described. The effects of HIFU were investigated using T1-W and T2-W fast spin echo (FSE) and fluid-attenuated inversion recovery (FLAIR). T2-W FSE and FLAIR show better anatomical details within the brain than T1-W FSE, but with T1-W FSE, the contrast between lesion and brain is higher for both thermal and bubbly lesions. The best contrast between lesion and brain with T1-W FSE is obtained with TR above 500 ms, whereas with T2-W FSE, the best contrast is observed between 40 and 60 ms. The maximum contrast to noise ratio (CNR) measured with T1-W FSE was approximately 20. With T2-W FSE, the corresponding CNR was approximately 12. With this system, we were able to create large lesions (by producing overlapping lesions), and it was possible to monitor these lesions with MRI with excellent contrast. The length of the lesions in vivo brain was much higher than the length in vitro, indicating that the penetration in the in vitro brain is limited, possibly by reflection due to trapped bubbles in the blood vessels. This paper demonstrates that HIFU has the potential to treat brain tumors in humans. This could be done either using a single-element transducer with a frequency around 1 MHZ or using a multi-element transducer.

High-intensity focused ultrasound for the treatment of renal masses: current status and future potential

PURPOSE OF REVIEW

To outline the current status of high-intensity focused ultrasound (HIFU) for the treatment of renal tumors.

RECENT FINDINGS

Application of extracorporeal HIFU for renal tumors is well tolerated with no serious perioperative complications. However, the techniques available do not permit sufficient tumor destruction that can be considered as an alternative to surgical extirpation. Laparoscopic HIFU avoids problems with respiratory movement and interphases and may achieve a greater rate of tumor destruction.

SUMMARY

At the current time, HIFU of renal tumors has to be considered an experimental treatment approach. Prospective evaluation of laparoscopic HIFU is necessary to evaluate its oncologic efficacy.

Treatment monitoring and thermometry for therapeutic focused ultrasound

Therapeutic ultrasound is currently enjoying increasingly widespread clinical use especially for the treatment of cancer of the prostate, liver, kidney, breast, pancreas and bone, as well as for the treatment of uterine fibroids. The optimum method of treatment delivery varies between anatomical sites, but in all cases monitoring of the treatment is crucial if extensive clinical acceptance is to be achieved. Monitoring not only provides the operating clinician with information relating to the effectiveness of treatment, but can also provide an early alert to the onset of adverse effects in normal tissue. This paper reviews invasive and non-invasive monitoring methods that have been applied to assess the extent of treatment during the delivery of therapeutic ultrasound in the laboratory and clinic (follow-up after treatment is not reviewed in detail). The monitoring of temperature and, importantly, the way in which this measurement can be used to estimate the delivered thermal dose, is dealt with as a separate special case. Already therapeutic ultrasound has reached a stage of development where it is possible to attempt real-time feedback during exposure in order to optimize each and every delivery of ultrasound energy. To date, data from MR imaging have shown better agreement with the size of regions of damage than those from diagnostic ultrasound, but novel ultrasonic techniques may redress this balance. Whilst MR currently offers the best method for non-invasive temperature measurement, the ultrasound techniques under development, which could potentially offer more rapid visualisation of results, are discussed.

Application of high-intensity focused ultrasound for umbilical artery occlusion in a rabbit model

OBJECTIVES

To investigate the application of high-intensity focused ultrasound (HIFU) for fetal umbilical artery blood flow occlusion in a rabbit model.

METHODS

A prototype HIFU transducer in combination with an imaging probe with Doppler capability was constructed. Using this transducer, HIFU was applied at 1.4, 2.75 or 5.5 kW/cm(2) through the maternal abdominal skin to the fetal intra-abdominal umbilical arteries of four time-mated Japanese White rabbits (11 fetuses) on gestational day 25. Courses of 5-s HIFU exposure were performed until cessation of umbilical blood flow and cardiac arrest were confirmed by Doppler ultrasonography. Fetal necropsy was performed and exposed lesions were assessed by microscopic histological analysis.

RESULTS

The mean diameter of the fetal umbilical artery was 0.6 +/- 0.2 mm and the mean peak systolic velocity of arterial blood flow was 44.7 +/- 18.5 cm/s. When HIFU was applied at 5.5 kW/cm(2), blood flow was completely occluded within 15 courses. HIFU exposure brought about vacuolar degeneration and destruction of elastic fibers in the tunica media of the artery.

CONCLUSIONS

HIFU can be used to occlude umbilical artery blood flow in fetal rabbits.

Image-based control of the magnetic resonance imaging-guided focused ultrasound thermotherapy

Magnetic resonance imaging (MRI)-guided focused ultrasound surgery (FUS) is a full noninvasive approach for localized thermal ablation of deep tissues, coupling the following: (1) a versatile, nonionizing physical agent for therapy and (2) a state-of-the art diagnosis and on-line monitoring tool. A commercially available, Food and Drug Administration-approved device using the MRI-guided FUS exists since 2004 for the ablation of benign tumors (uterine fibroids); however, the ultimate goal of the technological, methodological, and medical research in this field is to provide a clinical-routine tool for fighting localized cancer. When addressing cancer applications, the accurate spatial control of the delivered thermal dose is mandatory. Contiguous destruction of the target volume must be achieved in a minimum time, whereas sparing as much as possible the neighboring healthy tissues and especially when some adjacent regions are critical. This paper reviews some significant developments reported in the literature related to the image-based control of the FUS therapy for kidney, breast, prostate, and brain, including the own experience of the authors on the active feedback control of the temperature during FUS ablation. In addition, preliminary results of an original study of MRI-guided FUS ablation of VX2 carcinoma in kidney, under active temperature control, are described here.

Oncologic outcomes for ablative therapy of kidney cancer

Needle ablative therapies for small incidental renal masses are emerging as alternatives to traditional extirpative surgery. Reasons include their associated decreased morbidity, shorter convalescence, and the ability to avert the higher risk of extirpative surgery in an aging patient population. Cryoablation (CA) and radiofrequency ablation are the two most thoroughly studied needle ablative methods used for renal cancer. High-intensity focused ultrasound has also been studied but with limited published human experience at this time. For both radiofrequency ablation and CA, in vitro experiments, animal studies, and (increasingly) human experience have been published, allowing us to define appropriate candidates for such therapies, their oncologic outcomes, and the potential pitfalls. While long-term data is being collected, the current literature suggests that CA and radiofrequency ablation can be safely performed and can effectively eradicate small renal cancers with cancer-specific survival rates similar to those of traditional surgical options.

Technology Insight: high-intensity focused ultrasound for urologic cancers

The growing interest in high-intensity focused ultrasound (HIFU) technology is mainly due to its many potential applications as a minimally invasive therapy. It has been introduced to urologic oncology as a treatment for prostate and kidney cancers. While its application in the kidney is still at the clinical feasibility phase, HIFU technology is currently used in daily practice in Europe for the treatment of prostate cancer. Literature describing the results of HIFU for prostate cancer is mainly based on several series of patients from clinical development teams. The latest published results suggest that HIFU treatment is a valuable option for well-differentiated and moderately-differentiated tumors, as well as for local recurrence after external-beam radiation therapy.

New Approaches to the Minimally Invasive Treatment of Kidney Tumors

The incidence of renal cortical neoplasms has dramatically increased with the widespread use of abdominal imaging over the past 20 years. Coincidentally, the proportion of tumors that are smaller and incidentally detected has risen as well, indicative of a stage migration. The widespread application of minimally invasive and laparoscopic techniques to other organ systems has spurred the development of minimally invasive approaches to the management of renal tumors. The available data regarding laparoscopic nephrectomy, laparoscopic partial nephrectomy, and tissue ablative techniques, such as renal cryoablation, radiofrequency ablation, and high-intensity focused ultrasound are reviewed.

MRI monitoring of the effect of tissue interfaces in the penetration of high intensity focused ultrasound in kidney in vivo

In this paper, we studied the effect of interfaces during the application of high intensity focused ultrasound (HIFU) ablation in rabbit kidney in vivo. In kidney ablation, mainly two types of interfaces are encountered: these are muscle-kidney and fat-kidney. It was observed that the intensity for which the probability of cavitation (POC) is one was decreased when HIFU penetrated through interfaces, meaning that an interface is a potential site of cavitation. We utilized the concept of scanning the area to be treated in two dimensions (rectangular grid) by applying low intensity ultrasound (diagnostic scan). When all the points of the grid show decrease of signal in T1-weighted fast spoiled gradient (FSPGR) which indicated heating, complete necrosis was observed in the targeted area during the application of HIFU (therapeutic scan). If ultrasound goes through an interface that includes air spaces, the diagnostic scan indicates spaces with poor ultrasound penetration and as a result, during the application of the therapeutic scan, some sites remain untreated. The muscle-kidney and fat-kidney interfaces cause reflection of ultrasound, which prevents the penetration of ultrasound. Microscopic bubbles in the interface may initiate cavitation, especially at high intensities. However, sometimes these types of interfaces do not include any bubbles and therefore the propagation of ultrasound is not inhibited.