Real-time visualization of high-intensity focused ultrasound treatment using ultrasound imaging

Utility of a tumor-mimic model for the evaluation of the accuracy of HIFU treatments. results of in vitro experiments in the liver

Presented in this article is a tumor-mimic model that allows the evaluation, before clinical trials, of the targeting accuracy of a high intensity focused ultrasound (HIFU) device for the treatment of the liver. The tumor-mimic models are made by injecting a warm solution that polymerizes in hepatic tissue and forms a 1 cm discrete lesion that is detectable by ultrasound imaging and gross pathology. First, the acoustical characteristics of the tumor-mimics model were measured in order to determine if this model could be used as a target for the evaluation of the accuracy of HIFU treatments without modifying HIFU lesions in terms of size, shape and homogeneity. On average (n = 10), the attenuation was 0.39 +/- 0.05 dB.cm(-1) at 1 MHz, the ultrasound propagation velocity was 1523 +/- 1 m.s(-1) and the acoustic impedance was 1.84 +/- 0.00 MRayls. Next, the tumor-mimic models were used in vitro in order to verify, at a preclinical stage, that lesions created by HIFU devices guided by ultrasound imaging are properly positioned in tissues. The HIFU device used in this study is a 256-element phased-array toroid transducer working at a frequency of 3 MHz with an integrated ultrasound imaging probe working at a frequency of 7.5 MHz. An initial series of in vitro experiments has shown that there is no significant difference in the dimensions of the HIFU lesions created in the liver with or without tumor-mimic models (p = 0.3049 and p = 0.8796 for the diameter and depth, respectively). A second in vitro study showed that HIFU treatments performed on five tumor-mimics with safety margins of at least 1 mm were properly positioned. The margins obtained were on average 9.3 +/- 2.7 mm (min. 3.0 - max. 20.0 mm). This article presents in vitro evidence that these tumor-mimics are identifiable by ultrasound imaging, they do not modify the geometry of HIFU lesions and, thus, they constitute a viable model of tumor-mimics indicated for HIFU therapy.

Development of a portable therapeutic and high intensity ultrasound system for military, medical, and research use

We have developed a portable high power ultrasound system with a very low output impedance amplifier circuit (less than 0.3 Omega) that can transfer more than 90% of the energy from a battery supply to the ultrasound transducer. The system can deliver therapeutic acoustical energy waves at lower voltages than those in conventional ultrasound systems because energy losses owing to a mismatched impedance are eliminated. The system can produce acoustic power outputs over the therapeutic range (greater then 50 W) from a PZT-4, 1.54 MHz, and 0.75 in diameter piezoelectric ceramic. It is lightweight, portable, and powered by a rechargeable battery. The portable therapeutic ultrasound unit has the potential to replace "plug-in" medical systems and rf amplifiers used in research. The system is capable of field service on its internal battery, making it especially useful for military, ambulatory, and remote medical applications.

New integrated imaging high intensity focused ultrasound probe for transrectal prostate cancer treatment

The present study proposes a new integrated imaging (II) high-intensity focused ultrasound (HIFU) probe intended as an improvement to the Ablatherm prostate cancer treatment. Because of a perforation in the center of the II probe, the expected lesion differs from the one obtained for the original Ablatherm probe. In this paper, the new geometry and the strategy followed to establish the treatment parameters are presented. The original probe has a 40-mm focal length, a 50-mm aperture and is truncated at 31 mm. The II probe has a 45-mm focal length, a 61-mm aperture, a central perforation of 25 mm and is truncated at 31 mm. Both probes operate at 3 MHz. A mathematical model for lesion prediction was used for setting the treatment parameters for the II probe. These parameters should ensure equivalence between the lesions obtained with the original and II probes. Simulation-obtained parameters were validated by in-vitro and in-vivo (on liver of 70 New Zealand rabbits) experiments. The new II probe was used clinically to treat 30 patients. The mean age was 70.9 +/- 5.3 years (SD), the mean prostate volume 26.9 +/- 7.7 mL and the mean serum prostate specific antigen (PSA) concentration before treatment was 9.2 +/- 5.5 ng/mL. Simulations showed that for the II probe acoustical power and duration when the transducer is inactive should be reduced of 14% and 1s. In-vitro and in-vivo experiments confirmed the equivalence between the lesions obtained with the two probes. The lesion volume obtained under in-vitro conditions (for a traversed tissue depth of 16 mm to the focus) was 5 +/- 0.4 cm(3) and 5.1 +/- 0.5 cm(3) for the original and II probes, respectively. Under in-vivo conditions, the lesion volume (for a traversed tissue depth of 18 mm) was 5.3 +/- 1.1 cm(3) and 5.1 +/- 1.1 cm(3) for the original and II probes, respectively. During the clinical trial, a correction of + 1s in the exposure time was required to recreate the same degree of efficacy observed with the original probe (p = 0.97): 66.7 % of negative biopsies and 75% of patients with PSA at 3 mo < or =1 ng/mL. The morbidity observed was minimal and identical to that observed with the original probe.

Image-guided focused ultrasound ablation of breast cancer: current status, challenges, and future directions

Image-guided focussed ultrasound (FUS) ablation is a non-invasive procedure that has been used for treatment of benign or malignant breast tumours. Image-guidance during ablation is achieved either by using real-time ultrasound (US) or magnetic resonance imaging (MRI). The past decade phase I studies have proven MRI-guided and US-guided FUS ablation of breast cancer to be technically feasible and safe. We provide an overview of studies assessing the efficacy of FUS for breast tumour ablation as measured by percentages of complete tumour necrosis. Successful ablation ranged from 20% to 100%, depending on FUS system type, imaging technique, ablation protocol, and patient selection. Specific issues related to FUS ablation of breast cancer, such as increased treatment time for larger tumours, size of ablation margins, methods used for margin assessment and residual tumour detection after FUS ablation, and impact of FUS ablation on sentinel node procedure are presented. Finally, potential future applications of FUS for breast cancer treatment such as FUS-induced anti-tumour immune response, FUS-mediated gene transfer, and enhanced drug delivery are discussed. Currently, breast-conserving surgery remains the gold standard for breast cancer treatment.

Effects of high‐intensity focused ultrasound on nerve conduction

The effects of various exposures (intensity, duration) of high‐intensity focused ultrasound (HIFU) on sciatic nerve conduction were investigated in vivo in rats. The objective was to identify HIFU exposures that produce biological effects ranging from partial to complete conduction block, indicating potential use of HIFU as an alternative to current clinical methods of inducing nerve conduction block. In the study, 26 nerves were exposed and treated with 5‐s applications of 5.7‐MHZ HIFU with acoustic intensities of 390, 2,255, 3,310, and 7,890 W/cm2 (spatial peak, temporal peak). Compound muscle action potentials (CMAPs), in response to electrical stimulation of the nerve proximal to the HIFU site, were recorded from the plantar foot muscles immediately before and after HIFU treatment and 2 and 4 h after treatment. Furthermore, a preliminary long‐term investigation was performed on 27 nerves with the same four sets of HIFU parameters. CMAPs were measured at the survival endpoint for each animal (7 or 28 days after treatment). For nerves treated with the three lower exposures, CMAPs decreased initially within 4 h or 7 days after HIFU treatment and then recovered to their baseline level at 28 days after treatment. For the highest exposure, however, CMAPs remained absent even 28 days after treatment. These exposure‐dependent effects of HIFU on nerve function suggest its future potential as a novel treatment for severe spasticity and pain. Muscle Nerve, 2007

Real-time 3D image-guided HIFU therapy

Real-time three-dimensional ultrasound imaging (4D US) was utilized to monitor the treatment site during high-intensity focused ultrasound (HIFU) treatment. To obtain real-time monitoring during HIFU sonication, a 4D US imaging system and HIFU were synchronized and interference on the US image adjusted so that the region of interest was visible during treatment. The system was tested using tissue mimicking phantom gels and chicken breast tissue. The 4D US showed hyperechoic spots at the focal region of the HIFU transducer which then slowly faded after HIFU treatment. The hyperechoic regions were used as an indication of coagulative necrosis which occurs at temperatures higher than 60 degrees C. Different intensities of HIFU were applied to observe the difference in lesion formation and to determine the threshold intensity that produced hyperechoic regions due to the thermal and mechanical effects of focused ultrasound waves. The position, orientation, and shape of various lesions were examined in the three dimensional ultrasound images, and the volume of the lesions was measured. These volumes were compared to the volume measurements obtained from dissection of the tissue and phantom gels.

High-intensity focused ultrasound: current potential and oncologic applications

OBJECTIVE

The objective of this article is to introduce the reader to the principles and applications of high-intensity focused ultrasound (HIFU).

CONCLUSION

Although a great deal about HIFU physics is understood, its clinical applications are currently limited, and multiple trials are underway worldwide to determine its efficacy.

Using the acoustic interference pattern to locate the focus of a high-intensity focused ultrasound (HIFU) transducer

One of the main problems encountered when using conventional B-mode ultrasound (US) for targeting and monitoring purposes during ablation therapies employing high-intensity focused US (HIFU) is the appearance of strong interference in the obtained diagnostic US images. In this study, instead of avoiding the interference noise, we demonstrate how we used it to locate the focus of the HIFU transducer in both in vitro tissue-mimicking phantoms and an ex vivo tissue block. We found that when the B-mode image plane coincided with the HIFU focal plane, the interference noise was maximally converged and enhanced compared with the off-focus situations. Stronger interference noise was recorded when the angle (alpha) between the US image plane and the HIFU axis was less than or equal to 90 degrees. By intentionally creating a target (group of bubbles) at the 3.5-MHz HIFU focus (7.1 mm in length and 0.7 mm in diameter), the position of the maximal noise convergence coincided well with the target. The difference between the predicted focus and the actual one (bubbles) on x and z axes (axes perpendicular to the HIFU central axis, Fig. 1) were both about 0.9 mm. For y axis (HIFU central axis), the precision was within 1.0 mm. For tissue block ablation, the interference noise concentrated at the position of maximal heating of the HIFU-induced lesions. The proposed method can also be used to predict the position of the HIFU focus by using a low intensity output scheme before permanent changes in the target tissue were made.

A novel 100 watt high intensity focused ultrasound driving system

Ultrasound B-Mode imaging systems are often used to image tissues to which High Intensity Focused Ultrasound (HIFU) is being applied. Since HIFU operates at frequencies in the same range as a B-mode imager, the two technologies interfere when they are simultaneously applied to the patient. This causes the imaging window to 'whiteout' and obscure the target region, thus defeating the purpose of using the ultrasonic imager in the first place. The system in this paper controls the interference window by pulsing the HIFU at a frame rate synchronized to the ultrasonic imager. The frame rate is either approximated by using an on-board oscillator, or it is captured from stray electromagnetic waves received from the ultrasound transducer. The system is capable of delivering 100 W of peak electrical output power to a HIFU transducer. It is controlled by a custom software application, and synchronizes with B-Mode imaging systems with frame rates from 10-100 Hz. An on-board Direct Digital Synthesizer generates HIFU RF from 1-5 MHz. The complete system provides precision positioning of the interference window in a small and portable desktop package.

Pulsatile flow phantom for ultrasound image-guided HIFU treatment of vascular injuries

A pulsatile flow phantom was developed for studies of ultrasound image-guided high intensity focused ultrasound (HIFU) application in transcutaneous hemostasis of injured blood vessels. The flow phantom consisted of a pulsatile pump system with instrumented excised porcine carotid artery, which was imbedded in a transparent agarose gel to model structural configuration of in vivo tissues. Heparinized porcine blood was circulated through the phantom. The artery was injured using an 18-gauge needle to model a penetrating injury in human peripheral vasculature. A HIFU transducer with the diameter of 7 cm, focal length of 6.3 cm and frequency of 3.4 MHz was used to seal the puncture. Ultrasound imaging was used to localize and target the puncture site and to monitor the HIFU treatment. Triphasic blood flows present in the human arteries were reproduced, with flow rates of 50 to 500 mL/min, pulse rates of 62 to 138 beats/min and peak pressures of 100 to 250 mm Hg. The penetrating injury of an artery was mimicked successfully in the flow phantom setting and was easily visualized both optically through the transparent gel and with power Doppler ultrasound imaging. Hemostasis was achieved in 55 +/- 31 s (n = 9) of HIFU application. Histologic observations showed that a HIFU-sealed puncture was filled with clotted blood and covered with a fibrin cap. The pulsatile flow phantom provides a controlled and repeatable environment for studies of transcutaneous image-guided HIFU application in hemostasis of a variety of blood vessel injuries.

Role of acoustic cavitation in the delivery and monitoring of cancer treatment by high-intensity focused ultrasound (HIFU)

Acoustic cavitation has been shown to play a key role in a wide array of novel therapeutic ultrasound applications. This paper presents a brief discussion of the physics of thermally relevant acoustic cavitation in the context of high-intensity focussed ultrasound (HIFU). Models for how different types of cavitation activity can serve to accelerate tissue heating are presented, and results suggest that the bulk of the enhanced heating effect can be attributed to the absorption of broadband acoustic emissions generated by inertial cavitation. Such emissions can be readily monitored using a passive cavitation detection (PCD) scheme and could provide a means for real-time treatment monitoring. It is also shown that the appearance of hyperechoic regions (or bright-ups) on B-mode ultrasound images constitutes neither a necessary nor a sufficient condition for inertial cavitation activity to have occurred during HIFU exposure. Once instigated at relatively large HIFU excitation amplitudes, bubble activity tends to grow unstable and to migrate toward the source transducer, causing potentially undesirable pre-focal damage. Potential means of controlling inertial cavitation activity using pulsed excitation so as to confine it to the focal region are presented, with the intention of harnessing cavitation-enhanced heating for optimal HIFU treatment delivery. The role of temperature elevation in mitigating bubble-enhanced heating effects is also discussed, along with other bubble-field effects such as multiple scattering and shielding.

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.

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.

Blood flow occlusion via ultrasound image-guided high-intensity focused ultrasound and its effect on tissue perfusion

This study investigated the induction of tissue necrosis by arterial blood flow occlusion using ultrasound image-guided high-intensity focused ultrasound (HIFU). We constructed a prototype HIFU transducer in combination with an imaging probe that provided color Doppler imaging and ultrasound contrast imaging. The HIFU beam was aimed into a branch of the renal artery in vivo. The renal artery branches of eight rabbits were occluded by HIFU at an intensity of 4 kW/cm(2) (from 2 to 10 times of each sonication for 5 s). When the HIFU exposure was successful, complete cessation of blood flow was observed by color Doppler imaging with success rate of 100% (8/8). Furthermore, lack of perfusion was observed in the renal cortex with a contrast-enhanced image. Postmortem histologic evaluation showed a wedge-shaped area of infarction in six of seven cases, corresponding to the lack of the contrast medium in the ultrasound image. These results demonstrated that ultrasound image-guided HIFU can be used to induce arterial occlusion, thus producing infarction and necrosis of the perfused tissue.

Spatial variability in acoustic backscatter as an indicator of tissue homogenate production in pulsed cavitational ultrasound therapy

Spatial variability in acoustic backscatter is investigated as a potential feedback metric for assessment of lesion morphology during cavitation-mediated mechanical tissue disruption ("histotripsy"). A 750-kHz annular array was aligned confocally with a 4.5 MHz passive backscatter receiver during ex vivo insonation of porcine myocardium. Various exposure conditions were used to elicit a range of damage morphologies and backscatter characteristics [pulse duration = 14 micros, pulse repetition frequency (PRF) = 0.07-3.1 kHz, average I(SPPA) = 22-44 kW/cm2]. Variability in backscatter spatial localization was quantified by tracking the lag required to achieve peak correlation between sequential RF A-lines received. Mean spatial variability was observed to be significantly higher when damage morphology consisted of mechanically disrupted tissue homogenate versus mechanically intact coagulation necrosis (2.35 +/- 1.59 mm versus 0.067 +/- 0.054 mm, p < 0.025). Statistics from these variability distributions were used as the basis for selecting a threshold variability level to identify the onset of homogenate formation via an abrupt, sustained increase in spatially dynamic backscatter activity. Specific indices indicative of the state of the homogenization process were quantified as a function of acoustic input conditions. The prevalence of backscatter spatial variability was observed to scale with the amount of homogenate produced for various PRFs and acoustic intensities.

Hemorrhage control using high intensity focused ultrasound

Hemorrhage control is a high priority task in advanced trauma care, because hemorrhagic shock can result in less than a minute in cases of severe injuries. Hemorrhage was found to be solely responsible for 40-50% of traumatic civilian and battlefield deaths in recent years. The majority of these deaths were due to abdominal and pelvic injuries with hidden and inaccessible bleeding of solid organs such as liver, spleen, and kidneys, as well as major blood vessels. High intensity focused ultrasound (HIFU) offers a promising method for hemorrhage control. An important advantage of HIFU is that it can deliver energy to deep regions of tissue where hemorrhage is occurring, allowing cauterization at depth of parenchymal tissues, or in difficult-to-access anatomical regions, while causing no or minimal biological effects in the intervening and surrounding tissues. Moreover, HIFU can cause both thermal and mechanical effects that are shown to work synergistically for rapid hemorrhage control. The major challenges of this method are in development of bleeding detection techniques for accurate localization of the injury sites, delivery of large HIFU doses for profuse bleeding cases, and ensuring safety when critical structures are in the vicinity of the injury. Future developments of acoustic hemostasis technology are anticipated to be for applications in peripheral vascular injuries where an acoustic window is usually available, and for applications in the operating room on exposed organs.

A real-time measure of cavitation induced tissue disruption by ultrasound imaging backscatter reduction

A feedback method for obtaining real-time information on the mechanical disruption of tissue through ultrasound cavitation is presented. This method is based on a substantial reduction in ultrasound imaging backscatter from the target volume as the tissue structure is broken down. Ex-vivo samples of porcine liver were exposed to successive high-intensity ultrasound pulses at a low duty cycle to induce mechanical disruption of tissue parenchyma through cavitation (referred to as histotripsy). At the conclusion of treatment, B-scan imaging backscatter was observed to have decreased by 22.4 +/- 2.3 dB in the target location. Treated samples of tissue were found to contain disrupted tissue corresponding to the imaged hypoechoic volume with no remaining discernable structure and a sharp boundary. The observed, substantial backscatter reduction may be an effective feedback mechanism for assessing treatment efficacy in ultrasound surgery using pulsed ultrasound to create cavitation.

Three-dimensional spatial and temporal temperature imaging in gel phantoms using backscattered ultrasound

Thermal therapies such as radio frequency, heated saline, and high-intensity focused ultrasound ablations are often performed suboptimally due to the inability to monitor the spatial and temporal distribution of delivered heat and the extent of tissue necrosis. Ultrasound-based temperature imaging recently was proposed as a means to measure noninvasively the deposition of heat by tracking the echo arrival time shifts in the ultrasound backscatter caused by changes in speed of sound and tissue thermal expansion. However, the clinical applicability of these techniques has been hampered by the two-dimensional (2-D) nature of traditional ultrasound imaging, and the complexity of the temperature dependence of sound speed for biological tissues. In this paper, we present methodology, results, and validation of a 3-D spatial and temporal ultrasound temperature estimation technique in an alginate-based gel phantom to track the evolution of heat deposition over a treatment volume. The technique was experimentally validated for temperature rises up to approximately 10 degrees C by comparison with measurements from thermocouples that were embedded in the gel. Good agreement (rms difference = 0.12 degrees C, maximum difference = 0.24 degrees C) was observed between the noninvasive ultrasound temperature estimates and thermocouple measurements. Based on the results obtained for the temperature range studied in this paper, the technique demonstrates potential for applicability in image guidance of thermal therapy for determining the location of the therapeutic focal spot and assessing the extent of the heated region at subablative intensities.

Monitoring imaging of lesions induced by high intensity focused ultrasound based on differential ultrasonic attenuation and integrated backscatter estimation

We investigated the feasibility of two monitoring imaging methods to visualize and evaluate the high intensity focused ultrasound (HIFU) induced lesions in vitro during and after their formation, which were based on differential ultrasonic parameter estimation. Firstly, ultrasonic attenuation slope of tissue sample was estimated based on the spectral analysis of ultrasound RF backscattered signals. The differential attenuation slope maps were acquired, which were interpreted as the differences between the pretreatment image and those obtained in different stages during HIFU therapy. Secondly, ultrasonic integrated backscatter (IBS), defined as the frequency average of the backscatter transfer function over the useful bandwidth, was proposed quantitatively to evaluate the extent of lesions with the same RF signals as the first method. Differential IBS maps were also acquired to visualize temporal evolution of lesion formation. It was found in pig liver in vitro that more precise definition of the treated area was obtained from the differential IBS images than from differential attenuation slope images. Dramatic increase in both attenuation and IBS value was observed during the therapy, which may be related to dramatic enhancement of cavitation due to boiling and accompanying tissue damage. Two methods to obtain one differential image were compared and the cumulative differential image was found to be able to eliminate noises and artifacts to some extent, which was the cumulation of a series of differential images acquired from the differences between the temporally adjacent RF data frames. Moreover, we presented a bidirectional color code for identification of the artifacts due to tissue movements caused by HIFU radiation force. We conclude that cumulative differential IBS images have the potential to monitor the formation of HIFU-induced lesions.

Image-guided acoustic hemostasis for hemorrhage in the posterior liver

We investigated the use of ultrasound image-guided high intensity focused ultrasound (HIFU) to stop bleeding from injuries in the posterior liver. A HIFU transducer with focal length of 3.5 cm and frequency of 3.2 MHz was integrated with an intraoperative high-resolution ultrasound-imaging probe. Wedge tissue extractions, 30-mm long, 5-mm wide and 8-mm deep, were made in the posterior liver surface of five pigs to induce bleeding. The device was positioned on the anterior surface of the liver and HIFU was applied using ultrasound image-guidance. Hemostasis was achieved in 66 +/- 18 s (mean +/- standard deviation) for 17 HIFU treatments. During 7 min of sham HIFU treatment, none of the control incisions (n = 7) became hemostatic. Ultrasound image-guided HIFU offers a promising method for hemostasis in surgical settings in which the hemorrhage site is hidden and/or not accessible.

Design and development of a prototype endocavitary probe for high-intensity focused ultrasound delivery with integrated magnetic resonance imaging

PURPOSE

To integrate a high intensity focused ultrasound (HIFU) transducer with an MR receiver coil for endocavitary MR-guided thermal ablation of localized pelvic lesions.

MATERIALS AND METHODS

A hollow semicylindrical probe (diameter 3.2 cm) with a rectangular upper surface (7.2 cm x 3.2 cm) was designed to house a HIFU transducer and enable acoustic contact with an intraluminal wall. The probe was distally rounded to ease endocavitary insertion and was proximally tapered to a 1.5-cm diameter cylindrical handle through which the irrigation tubes (for transducer cooling) and electrical connections were passed. MR compatibility of piezoceramic and piezocomposite transducers was assessed using gradient-echo (GRE) sequences. The radiofrequency (RF) tuning of identical 6.5 cm x 2.5 cm rectangular receiver coils on the upper surface of the probe was adjusted to compensate for the presence of the conductive components of the HIFU transducers. A T1-weighted (T1-W) sliding window dual-echo GRE sequence monitored phase changes in the focal zone of each transducer. High-intensity (2400 W/cm(-2)), short duration (<1.5 seconds) exposures produced subtherapeutic temperature rises.

RESULTS

For T1-W images, signal-to-noise ratio (SNR) improved by 40% as a result of quartering the conductive surface of the piezoceramic transducer. A piezocomposite transducer showed a further 28% improvement. SNRs for an endocavitary coil in the focal plane of the HIFU trans-ducer (4 cm from its face) were three times greater than from a phased body array coil. Local shimming improved uniformity of phase images. Phase changes were detected at subtherapeutic exposures.

CONCLUSION

We combined a HIFU transducer with an MR receiver coil in an endocavitary probe. SNRs were improved by quartering the conductive surface of the piezoceramic. Further improvement was achieved with a piezocomposite transducer. A phase change was seen on MR images during both subtherapeutic and therapeutic HIFU exposures.

Differential ultrasonic imaging for the characterization of lesions induced by high intensity focused ultrasound

High intensity focused ultrasound (HIFU) is an effective technique for noninvasive local creating coagulative necrotic lesions in deep target volumes without damage to the overlaying or surrounding tissues. It is very important to detect and evaluate lesions generated by HIFU during treatment procedures. This study describes the development of several differential ultrasonic imaging techniques to characterize lesions based on estimation of relative changes in tissue properties derived from backscattered RF data. A single, spherical HIFU transducer was used to produce lesions in soft tissues. The RF signals were recorded as outputs from a modified diagnostic ultrasound system. After some preprocessing, the integrated backscatter values, which can be used as an indicator of the microstructure and backscattering property of tissues, were calculated before and after HIFU treatment. The differential integrated backscatter values were subsequently used to form images revealing the lesion areas. The differential attenuation imaging with the same RF data was also performed, which has been proposed by a few researchers. The results of the differential integrated backscatter imaging were compared with that of the differential attenuation imaging and the former method offers some advantages over the latter method. The two methods above are both based on spectrum analysis and would spend much computational time. Therefore, some simple digital differential imaging methods, including absolute difference (AD), sum absolute differences (SAD), and sum squared differences (SSD) algorithms, were also proposed to detect HIFU-induced lesions. However, these methods cannot provide the information of the degree of tissue damage. Experiments in vitro bovine muscle and liver validated the method of differential integrated backscatter imaging for the characterization of HIFU-induced lesions. And the AD, SAD, and SSD algorithms can be implemented in real-time during HIFU therapy to visualize the lesions.

Feasibility of noncontact intracardiac ultrasound ablation and imaging catheter for treatment of atrial fibrillation

Atrial fibrillation (AF) affects 1% of the population and results in a cost of 2.8 billion dollars from hospitalizations alone. Treatments that electrically isolate portions of the atria are clinically effective in curing AF. However, such minimally invasive catheter treatments face difficulties in mechanically positioning the catheter tip and visualizing the anatomy of the region. We propose a noncontact, intracardiac transducer that can ablate tissue and provide rudimentary imaging to guide therapy. Our design consists of a high-power, 20 mm by 2 mm, 128-element, transducer array placed on the side of 7-French catheter. The transducer will be used in imaging mode to locate the atrial wall; then, by focusing at that location, a lesion can be formed. Imaging of previously formed lesions could potentially guide placement of subsequent lesions. Successive rotations of the catheter will potentially enable a contiguous circular lesion to be created around the pulmonary vein. The challenge of intracardiac-sized transducers is achieving high intensities (300-5000 W/cm2) needed to raise the temperature of the tissue above 43 degrees C. In this paper, we demonstrate the feasibility of an intracardiac-sized transducer for treatment of atrial fibrillation. In simulations and proof-of-concept experiments, we show a 37 degrees C temperature rise in the lesion location and demonstrate the possibility of lesion imaging.

Biological and physical mechanisms of HIFU-induced hyperecho in ultrasound images

Guidance and monitoring of high intensity focused ultrasound (HIFU) therapy, using ultrasound imaging, has primarily utilized formation of a hyperechoic region at the HIFU focus. We investigated biologic and physical mechanisms of a hyperecho, as well as safety of this phenomenon, using thermal, acoustic and light microscopy observations. Single, short-duration HIFU pulses (30-60 ms) were able to produce a hyperechoic region at the HIFU focus, 2 cm deep in a rabbit thigh muscle. When hyperechoic regions appeared, inertial cavitation was detected in vivo using a custom-made passive cavitation detection system. Light micrographs showed a large number of cavities (approximately 100/mm3), 1-10 microm in diameter, in a cytoplasm of cells located at the HIFU focus. Blood congestion was observed around a focal region, indicating an injury of microvasculature. Cellular necrosis was observed at 2 d after the treatment, while healing, scar tissue formation and regeneration were observed at 7 d and 14 d. The results indicate that a possibility of adverse tissue effects has to be taken into consideration when the hyperecho formation, induced by very-short HIFU pulses, is used for pretreatment targeting.

Improved visualization of high-intensity focused ultrasound lesions

Spectral parameter imaging in both the fundamental and harmonic of backscattered radio-frequency (RF) data were used for immediate visualization of high-intensity focused ultrasound (HIFU) lesion sites. A focused 5-MHz HIFU transducer with a coaxial 9-MHz focused single-element diagnostic transducer was used to create and scan lesions in chicken breast and freshly excised rabbit liver. B-mode images derived from the backscattered RF signal envelope were compared with midband fit (MBF) spectral parameter images in the fundamental (9-MHz) and harmonic (18-MHz) bands of the diagnostic probe. Images of HIFU-induced lesions derived from the MBF to the calibrated spectrum showed improved contrast (approximately 3 dB) of tumor margins versus surround compared with images produced from the conventional signal envelope. MBF parameter images produced from the harmonic band showed higher contrast in attenuated structures (core, shadow) compared with either the conventional envelope (3.3 dB core; 11.6 dB shadow) or MBF images of the fundamental band (4.4 dB core; 7.4 dB shadow). The gradient between the lesion and surround was 3.4 dB/mm, 6.9 dB/mm and 17.2 dB/mm for B-mode, MBF-fundamental mode and MBF-harmonic mode, respectively. Images of threshold and "popcorn" lesions produced in freshly excised rabbit liver were most easily visualized and boundaries best-defined using MBF-harmonic mode.

Image-Guided High-Intensity Focused Ultrasound for Conduction Block of Peripheral Nerves

The objective of our work has been to investigate the use of ultrasound image-guided high-intensity focused ultrasound (HIFU) to non-invasively produce conduction block in rabbit sciatic nerves in vivo, a technique that could become a treatment of spasticity and pain. The work reported here involved the investigation of the duration of such conduction blocks after HIFU treatment and whether they resulted in axon degeneration. The right sciatic nerves of 12 rabbits were treated, under guidance of ultrasound imaging, with repeated 5-s applications of 3.2 MHz HIFU with in situ intensity of 1930 W/cm(2) (spatial-average, temporal-average) until conduction block was achieved. Survival endpoints were 0, 7, or 14 days after HIFU treatment, at which point the nerve conduction was assessed. Qualitative and quantitative histological analysis of nerve sections proximal and distal to the HIFU site was performed. Conduction block of all 12 nerves was achieved with average HIFU treatment time of 10.5+/-4.9 s (mean+/-SD). The volume of necrosis of adjacent muscle was measured to be 1.59+/-1.1 cm(3) (mean+/-SD). For all nerves, conduction block remained at the survival endpoint and the block resulted in degeneration of axons distal to the HIFU site, as confirmed by electrophysiological and histological methods. Potential clinical applications include treatment of spasticity in patients with spinal cord injury or pain in cancer patients.

Combination of HIFU therapy with contrast-enhanced sonography for quantitative assessment of therapeutic efficiency on tumor grafted mice

The objective was to evaluate treatment efficiency of a new high-intensity focused ultrasound (HIFU) prototype combining a therapeutic transducer with a sonographic probe. The optimal HIFU sequence was defined on ex vivo samples before in vivo evaluation of tumor ablation was performed by perfusion quantification after contrast agent injection. The original feature of this prototype is a 9-MHz sonographic probe in a HIFU device and connected to an Aplio (Toshiba) sonograph. Acoustical power and treatment time were determined on ex vivo livers to generate 1-cm-long lesions. Lesion reproducibility was assessed for the power and treatment time selected. The gap between lesions and HIFU displacement shot procedures were optimized to ablate a 1-cm3 volume. The optimized protocol was applied to five murine tumors in vivo. Tumor ablation was quantified according to (1) contrast uptake (CU) after HIFU using perfusion software (Toshiba) in "vascular recognition imaging" mode and Sonovue (Bracco) contrast agent, and (2) the percentage of necrosis quantified on histologic slides. Ex vivo results: optimized settings, at 442 W/cm2 applied during three cycles (3 s on/5 s off) generated 10 identical elementary lesions measuring 9.78 (+/-0.66) * 2.11 (+/-0.33) mm2. A 4-mm gap between adjacent lesions and a 2-min pause between shot lines were found optimal. In vivo results: 60 % (+/-22) mean reduction in CU after HIFU and tumor necrosis histologically estimated at 58 % (+/-5.7) were quantified for the five animals. The therapeutic potential of this HIFU prototype was demonstrated in vivo through objective quantification of tumor ablation based on CU.

Differences in the lesion formation process between focused ultrasound and microwave ablations

The objective is to understand the differences in the lesion formation processes between microwave and high-intensity focused ultrasound (HIFU) ablation. The lesions formed by microwaves and HIFU were real-time monitored and compared using transparent tissue-mimicking phantoms at 60 and 70 W of driving electrical power. Microwaves and HIFU produced lesions different in shape, size, and developing processes. For HIFU ablations, the hyperechoic region appeared bigger in ultrasonic images, as compared with the protein denatured region observed optically at the end of 100 s ablations. On the contrary, the hyperechoic signal was only limited to a small region along the antenna of a microwave ablator. Careful monitoring and controlling the lesion formation process is essential for successful microwaves and HIFU thermal ablations.

Effects of nonlinear propagation, cavitation, and boiling in lesion formation by high intensity focused ultrasound in a gel phantom

The importance of nonlinear acoustic wave propagation and ultrasound-induced cavitation in the acceleration of thermal lesion production by high intensity focused ultrasound was investigated experimentally and theoretically in a transparent protein-containing gel. A numerical model that accounted for nonlinear acoustic propagation was used to simulate experimental conditions. Various exposure regimes with equal total ultrasound energy but variable peak acoustic pressure were studied for single lesions and lesion stripes obtained by moving the transducer. Static overpressure was applied to suppress cavitation. Strong enhancement of lesion production was observed for high amplitude waves and was supported by modeling. Through overpressure experiments it was shown that both nonlinear propagation and cavitation mechanisms participate in accelerating lesion inception and growth. Using B-mode ultrasound, cavitation was observed at normal ambient pressure as weakly enhanced echogenicity in the focal region, but was not detected with overpressure. Formation of tadpole-shaped lesions, shifted toward the transducer, was always observed to be due to boiling. Boiling bubbles were visible in the gel and were evident as strongly echogenic regions in B-mode images. These experiments indicate that nonlinear propagation and cavitation accelerate heating, but no lesion displacement or distortion was observed in the absence of boiling.

A method to synchronize high-intensity, focused ultrasound with an arbitrary ultrasound imager

Ultrasound imaging is useful for monitoring high-intensity, focused ultrasound (HIFU) therapy; however, interference on the ultrasound image, caused by HIFU excitation, must be avoided. A method to synchronize HIFU excitation with ultrasound imaging is described here. Synchronization was tested with two unmodified, commercial imagers and two tissue phantoms.

The use of a micro-bubble agent to enhance rabbit liver destruction using high intensity focused ultrasound

Liver tissues in New Zealand rabbits were ablated using high intensity focused ultrasound (HIFU, 14300 W/cm(2), 1.0 MHz). The animals were intravenously administered 0.2 ml of micro-bubble agent in the experimental (n=20) group and an isovolumetric normal saline solution in the control (n=27) group before HIFU treatment which was performed as a linear scan. In both groups, the preselected tissue volumes were destroyed without harming the overlying tissues. Necrosis rate (NR, cm(3)/s) was used to reflect the ablation efficiency, which was the tissue volume of occurring coagulative necrosis per 1s HIFU exposure. NR was improved in the experimental group (0.0570+/-0.0433 vs 0.0120+/-0.0122, P=0.0002). Pathological studies confirmed that there were no residual intact targets within the exposed volume. These findings suggested that the introduction of the micro-bubble agent enhanced HIFU liver destruction.

Annular phased-array high-intensity focused ultrasound device for image-guided therapy of uterine fibroids

An ultrasound (US), image-guided high-intensity focused ultrasound (HIFU) device was developed for noninvasive ablation of uterine fibroids. The HIFU device was an annular phased array, with a focal depth range of 30-60 mm, a natural focus of 50 mm, and a resonant frequency of 3 MHz. The in-house control software was developed to operate the HIFU electronics drive system for inducing tissue coagulation at different distances from the array. A novel imaging algorithm was developed to minimize the HIFU-induced noise in the US images. The device was able to produce lesions in bovine serum albumin-embedded polyacrylamide gels and excised pig liver. The lesions could be seen on the US images as hyperechoic regions. Depths ranging from 30 to 60 mm were sonicated at acoustic intensities of 4100 and 6100 W/cm2 for 15 s each, with the latter producing average lesion volumes at least 63% larger than the former. Tissue sonication patterns that began distal to the transducer produced longer lesions than those that began proximally. The variation in lesion dimensions indicates the possible development of HIFU protocols that increase HIFU throughput and shorten tumor treatment times.

Hemorrhage control in arteries using high-intensity focused ultrasound: a survival study

High-intensity focused ultrasound (HIFU) has been shown to provide an effective method for hemorrhage control of blood vessels in acute animal studies. The objective of the current study was to investigate the long-term effects of HIFU-induced hemostasis in punctured arteries. The femoral arteries ( approximately 2mm in diameter) of 25 adult anesthetized rabbits were surgically exposed, and either punctured and treated with HIFU (n=15), served as control (no puncture and no HIFU application: n=7), or were punctured and left untreated (n=3). Treated animals were allowed to recover, and examined and/or sacrificed on days 0, 1, 3, 7, 14, 28, and 60 after treatment to obtain ultrasound images and samples of blood and tissue. Hemostasis (arrest of bleeding) was achieved in all 15 of the HIFU-treated arteries. Eleven of the arteries were patent after HIFU treatment, and four arteries were occluded, as determined by Doppler ultrasound. The median HIFU application time to achieve hemostasis was 20s (range 7-55 s) for the patent arteries and 110 s (range 50-134 s) for the occluded arteries. In untreated animals, bleeding had not stopped after 120 s. One of the occluded arteries had reopened by day 14. No immediate or delayed re-bleeding was observed after HIFU treatment. Maximal blood flow velocities were similar in HIFU-treated patent vessels and control vessels. No significant difference in hematocrits was found between HIFU-treated and control groups at different time points after the procedure. Light microscopy observations of the HIFU-treated arteries showed disorganization of adventitia, and coagulation and thinning of the tunica media. The general organization of the adventitia and tunica media recovered to normal appearance within 28 days, with some thinning of the tunica media observed up to day 60. Neointimal hyperplasia was observed on days 14 and 28. The results show that HIFU can produce effective and long-term (up to 60 days) hemostasis of punctured femoral arteries while preserving normal blood flow and vessel wall structure in the majority of vessels.

Gel phantom for use in high-intensity focused ultrasound dosimetry

An optically transparent phantom was developed for use in high-intensity focused ultrasound (US), or HIFU, dosimetry studies. The phantom is composed of polyacrylamide hydrogel, embedded with bovine serum albumin (BSA) that becomes optically opaque when denatured. Acoustic and optical properties of the phantom were characterized as a function of BSA concentration and temperature. The speed of sound (1544 m/s) and acoustic impedance (1.6 MRayls) were similar to the values in soft tissue. The attenuation coefficient was approximately 8 times lower than that of soft tissues (0.02 Np/cm/MHz for 9% BSA). The nonlinear (B/A) coefficient was similar to the value in water. HIFU lesions were readily seen during formation in the phantom. In US B-mode images, the HIFU lesions were observed as hyperechoic regions only if the cavitation activity was present. The phantom can be used for fast characterization and calibration of US-image guided HIFU devices before animal or clinical studies.

Cavitation detection during shock-wave lithotripsy

A system was built to detect cavitation in pig kidney during shock-wave lithotripsy (SWL) with a Dornier HM3 lithotripter. Active detection using echo on B-mode ultrasound, and passive cavitation detection using coincident signals on confocal orthogonal receivers, were used to interrogate the renal collecting system (urine) and the kidney parenchyma (tissue). Cavitation was detected in urine immediately upon shock-wave (SW) administration in urine or urine plus X-ray contrast agent but, in native tissue, cavitation required hundreds of SWs to initiate. Localization of cavitation was confirmed by fluoroscopy, sonography and by thermally marking the kidney using the passive cavitation detection receivers as high-intensity focused ultrasound sources. Cavitation collapse times in tissue and native urine were about the same, but less than in urine after injection of X-ray contrast agent. The finding that cavitation occurs in kidney tissue is a critical step toward determining the mechanisms of tissue injury in SWL.

Extracorporeal application of high-intensity focused ultrasound for prostatic tissue ablation

OBJECTIVE

To investigate the efficacy and safety of extracorporeal prostatic tissue ablation using high-intensity focused ultrasound (HIFU) in vivo in animals, and in a clinical feasibility study in men, as this is an investigational minimally invasive treatment alternative for locally confined prostatic carcinoma, but may have significant side-effects.

PATIENTS, MATERIALS AND METHODS

Ultrasound (1.04 MHz excitation frequency) was generated by an extracorporeal cylindrical piezo-ceramic element and focused by a paraboloidal reflector to a focal size of 32 x 4 mm. The focal distance and aperture diameter were both 100 mm. HIFU was applied extracorporeally at different intensities and pulse duration (up to 6 s) to 11 dog prostates in vivo (median intensity 1192 W/cm2) and eight patients (median intensity 3278 W/cm2, range 2384-3576) under general anaesthesia. The lesions were assessed macroscopically and histologically after HIFU and any side-effects evaluated.

RESULTS

Thermoablation was feasible in vivo and in all patients. Macroscopic analysis and histology showed sharply demarcated coagulative necrosis. Side-effects, including skin and rectal burns, occurred only after transvesical application in the in vivo study. There were no side-effects in patients after perineal application.

CONCLUSION

Extracorporeal HIFU is technically feasible and induces sharply demarcated tissue damage in the prostate. From the early results of this phase 1 study, the perineal approach seems to be safe.

Evaluation of ultrasound tissue damage based on changes in image echogenicity in canine kidney

Sufficiently high intensity ultrasound can create hyperechoic regions in an ultrasound image due to local bubble generation. We explore the link between the temporal extent of these hyperechoic regions and tissue damage caused by ultrasound therapy. The decay rate of increased echogenicity from the focal zone in insonated live exteriorized canine kidney was quantified and correlated to the spatial extent of tissue damage. The decay half-time, t(half), defined as the time for echogenicity enhancement to decay by a factor of 2, was observed in all cases to be greater than 41 s in spatial zones in which extensive histological damage was observed. In cases in which the measured thalf was less than 11 s, the damage was limited to minor hemorrhage, or it was not detected. These t(half) discrimination boundaries of 41 and 11 s were not statistically different for cases in which contrast agent was used to enhance therapeutic efficiency. This was true even though contrast agent infusion significantly reduced the therapy pulse duration threshold for damage production.

Hyperecho in ultrasound images of HIFU therapy: involvement of cavitation

High-intensity focused ultrasound (US), or HIFU, treatment of soft tissues has been shown to result in a hyperechoic region in B-mode US images. We report on detecting cavitation in vivo in correlation with the appearance of a hyperechoic region. The US system consisted of a HIFU transducer (3.3 MHz), a broadband A-mode transducer for active and passive cavitation detection and an US-imaging probe that were all confocal and synchronized. HIFU, at in situ intensities of 220 to 1710 W/cm(2), was applied for 10 s to pig muscles in vivo. Active and passive cavitation detection results showed a strong correlation between the onset of cavitation and the appearance of a hyperechoic region. Passive cavitation detection results showed that inertial cavitation typically occurred prior (within 0.5 s) to the appearance of a hyperechoic region. The observed cavitation activity confirms that bubbles are present during the formation of a hyperechoic region at the HIFU focus.

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.

A comparison of HIFU-induced lesion size measurement based on gross histological examination and image of bovine thigh in vitro

The aim of this study was to investigate the agreement in HIFU-induced lesion sizes between measurements based on gross histological examination and those from images. Experiments were conducted in an experimental arrangement with a three-way multiscan ultrasonic inspection system and imaging was done by B-mode ultrasound (US). Bovine thigh muscle with and without fascia lata was treated with an in situ spatially averaged focal intensity ranging between 750 W cm(-2) and 1565 W cm(-2) and sonication-time was variable from 8 to 20s. Assessment of the two measurement methods showed a rather weak correlation in the samples without fascia lata. For clarity and convenience in the discussion, sample muscles with and without fascia lata were labeled F and M, respectively. It was difficult to measure the lesion size from ultrasonographic images of F samples, so there was disagreement in the samples with fascia lata. This investigation showed that the presence of fascia lata in bovine thigh muscle has the likely effect of affecting the ultrasound image and makes it difficult to distinguish coagulated tissue from surrounding healthy tissue. There was no significant correlation between ultrasonography and the gross histological findings in M samples. Data supported that at for an in situ spatially averaged focal intensity ranging between 750 W cm(-2) and 1565 W cm(-2) and relatively shorter exposures (sonication-time variable from 1 to 8s) higher correlation between image and gross histology measurement was found in excised bovine muscle specimens.

Bubble dynamics and size distributions during focused ultrasound insonation

The deposition of ultrasonic energy in tissue can cause tissue damage due to local heating. For pressures above a critical threshold, cavitation will occur, inducing a much larger thermal energy deposition in a local region. The present work develops a nonlinear bubble dynamics model to numerically investigate bubble oscillations and bubble-enhanced heating during focused ultrasound (HIFU) insonation. The model is applied to calculate two threshold-dependent phenomena occurring for nonlinearly oscillating bubbles: Shape instability and growth by rectified diffusion. These instabilities in turn are shown to place physical boundaries on the time-dependent bubble size distribution, and thus the thermal energy deposition.

In vivo feasibility of image-guided transvaginal focused ultrasound therapy for the treatment of intracavitary fibroids

OBJECTIVE

To determine the feasibility of uterine tissue ablation in vivo using a transvaginal focused ultrasound applicator guided by ultrasound imaging.

DESIGN

Randomized in vivo animal study.

SETTING

Academic research environment.

ANIMAL(S)

Healthy anesthetized sheep.

INTERVENTION(S)

Uterine treatment location was determined using a computerized targeting system. Five sonications 10 seconds in duration and averaging 2,000 W/cm(2) of focal ultrasound intensity were applied in each animal's uterus. Animals were euthanized either immediately or 2, 7, or 30 days post-treatment.

MAIN OUTCOME MEASURE(S)

Gross and microscopic analysis of the dissected uterus was used to quantitatively and qualitatively determine the ablated region and treatment side effects.

RESULT(S)

Treatments resulted in coagulative necrosis. Histopathological analysis showed that over 7 days, inflammatory cells appeared and smooth muscle bundles regenerated. By day 30, treated tissues healed and scar tissue formed. None of the animals showed abnormal behavior or medical problems. Complications in three animals were damage to the vaginal wall and colon, possibly due to inadequate applicator cooling and an empty bladder during treatment.

CONCLUSION(S)

Transvaginal image-guided high-intensity focused ultrasound has potential for treating uterine fibroids. Further safety testing of this treatment will prepare it for human use.

Image-guided HIFU neurolysis of peripheral nerves to treat spasticity and pain

Spasticity, a major complication of central nervous system disorders, signified by uncontrollable muscle contractions, is very difficult to treat effectively. We report on the use of ultrasound (US) image-guided high-intensity focused US (HIFU) to target and suppress the function of the sciatic nerve complex of rabbits in vivo, as a possible treatment of spasticity. The image-guided HIFU device included a 3.2-MHz spherically curved transducer and an intraoperative imaging probe. A focal acoustic intensity of 1480 to 1850 W/cm(2), applied using a scanning method, was effective in achieving complete conduction block in 100% of 22 nerve complexes with HIFU treatment times of 36 +/- 14 s (mean +/- SD). Gross examination showed blanching of the nerve at the HIFU treatment site and lesion volumes of 2.8 +/- 1.4 cm(3) encompassing the nerve complex. Histologic examination indicated axonal demyelination and necrosis of Schwann cells as probable mechanisms of nerve block. With accurate localization and targeting of peripheral nerves using US imaging, HIFU could become a promising tool for the suppression of spasticity.

Characterization of cardiovascular liver motion for the eventual application of elasticity imaging to the liverin vivo

Elastography, which uses ultrasound to image the tissue strain that results from an applied displacement, can display tumours and heat-ablated tissue with high contrast. However, its application to liver in vivo may be problematic due to the presence of respiratory and cardiovascular sources of displacement. The aim of this study was to measure the cardiovascular-induced component of natural liver motion for the purpose of planning future work that will either use the motion to produce elasticity images or will compensate for it when employing an external source of displacement. A total of 36 sequences of 7 s real-time radio frequency (RF) echo images of the liver were acquired from six healthy volunteers during breath-hold using a stationary 3.5 MHz transducer. For each image sequence, the axial and lateral components of displacement were measured for each pair of consecutive RF images using 2D-echo tracking. The spatio-temporal character of these displacements was then analysed using a novel approach, employing proper orthogonal decomposition, whereby the dominant motion patterns are described by eigenvectors with the highest eigenvalues. The motion patterns of different liver segments were complex, but they were also found to be cyclic, highly repeatable and capable of producing measurable displacements in the liver. These observations provide good evidence to suggest that it may be possible to correct for natural liver motion when using an externally applied displacement for elasticity imaging. It was also found that about 65%-70% of all liver motion could be described using the first eigenvector. Use of only this component of the motion will greatly simplify the design of a mechanical system to be used in an objective study of elasticity imaging of phantoms and excised tissues in the presence of simulated cardiovascular-induced liver motion.

Perspectives in clinical uses of high-intensity focused ultrasound

Focused ultrasound holds promise in a large number of therapeutic applications. It has long been known that high intensity focused ultrasound can kill tissue through coagulative necrosis. However, it is only in recent years that practical clinical applications are becoming possible, with the development of high power ultrasound phased arrays and noninvasive monitoring methods. These technologies, combined with more sophisticated treatment planning methods allow noninvasive focusing in areas such as the brain, that were once thought to be unreachable. Meanwhile, exciting investigations are underway in microbubble-enhanced heating which could significantly reduce treatment times. These developments have promoted an increase in the number of potential applications by providing valuable new tools for medical research. This paper provides an overview of the scientific and engineering advances that are allowing the growth in clinical focused ultrasound applications. It also discusses some of these prospective applications, including the treatment of brain disorders and targeted drug delivery.

Analysis of clinical effect of high-intensity focused ultrasound on liver cancer

AIM: To evaluate the clinical effect of high-intensity focused ultrasound (HIFU) in the treatment of patients with liver cancer.

METHODS: HIFU treatment was performed in 100 patients with liver cancer under general anesthesia and by a targeted ultrasound. Evaluation of efficacy was made on the basis of clinical symptoms, liver function tests, AFP, MRI or CT before and after the treatment.

RESULTS: After HIFU treatment, clinical symptoms were relieved in 86.6%(71/82) of patients. The ascites disappeared in 6 patients. ALT (95 ± 44) U/L and AST (114 ± 58) U/L before HIFU treatment were reduced to normal in 83.3%(30/36) and 72.9%(35/48) patients, respectively, after the treatment. AFP was lowered by more than 50% in 65.3%(32/49) patients. After HIFU treatment, MRI or CT findings indicated coagulation necrosis and blood supply reduction or disappearance of tumor in the target region.

CONCLUSION: HIFU can efficiently treat the patients with liver cancer. It will offer a significant noninvasive therapy for local treatment of liver tumor.