Ultrasound imaging of thermal therapy in in vitro liver

Monitoring Microwave Ablation Using Ultrasound Echo Decorrelation Imaging: An ex vivo Study

In this study, a microwave-induced ablation zone (thermal lesion) monitoring method based on ultrasound echo decorrelation imaging was proposed. A total of 15 cases of ex vivo porcine liver microwave ablation (MWA) experiments were carried out. Ultrasound radiofrequency (RF) signals at different times during MWA were acquired using a commercial clinical ultrasound scanner with a 7.5-MHz linear-array transducer. Instantaneous and cumulative echo decorrelation images of two adjacent frames of RF data were calculated. Polynomial approximation images were obtained on the basis of the thresholded cumulative echo decorrelation images. Experimental results showed that the instantaneous echo decorrelation images outperformed conventional B-mode images in monitoring microwave-induced thermal lesions. Using gross pathology measurements as the reference standard, the estimation of thermal lesions using the polynomial approximation images yielded an average accuracy of 88.60%. We concluded that instantaneous ultrasound echo decorrelation imaging is capable of monitoring the change of thermal lesions during MWA, and cumulative ultrasound echo decorrelation imaging and polynomial approximation imaging are feasible for quantitatively depicting thermal lesions.

Advantages of cooled fiber for monitoring laser tissue ablation through temporal and spectral analysis of RF ultrasound signal: A case study

The promising minimally invasive laser thermal therapy technique may be improved if thermal lesions induced into the tissue can be carefully monitored in extension and morphology during the treatment. According to results obtained in several recent experimentations, solutions that avoid tissue carbonization during the treatment have been proposed, in order to allow deeper and longer lasting light penetration in treated tissue and to reduce failures of the applicator tip and fiber optic, dangerous for patients. In the work the advantages in using a cooled fiber are shown, in order not only to induce efficient lesions but also in performing an accurate monitoring by ultrasound. Indeed, one important limit of the ultrasound control is caused by the gas bubbles generation, which represent an acoustic barrier that invalidate the ultrasonic image representation of the treated tissue. Ultrasonic radiofrequency signals were acquired from the same bovine liver ex vivo sample by using both bare and cooled fiber and processed to produce B-mode and spectral parametric images by implementing TUV (Thermotherapy Ultrasonic View) algorithm. Radiofrequency signals, B-mode and TUV images were analysed and compared in order to evaluate the different tissue heating processes during ablation and the different lesion extensions induced into the tissue after the treatment. Cooled fiber avoided carbonization and strongly reduced gas bubbles generation inducing a larger lesion and allowing a more effective ultrasound monitoring. Moreover by correlating optical images of the lesions and the corresponding Integral TUV images, by using Dice and Jaccard coefficients, it was proven that TUV algorithm is able to characterize the tissue portions differently modified by ablation exhibiting better performances in the case of cooled fiber and revealing to be a potential tool capable to improve the laser delivery settings control.

Mean scatterer spacing estimation in normal and thermally coagulated ex vivo bovine liver

The liver has been hypothesized to have a unique arrangement of microvasculature that presents as an arrangement of quasiperiodic scatterers to an interrogating ultrasound pulse. The mean scatterer spacing (MSS) of these quasiperiodic scatterers has been proposed as a useful quantitative ultrasound biomarker for characterizing liver tissue. Thermal ablation is an increasingly popular method for treating hepatic tumors, and ultrasonic imaging approaches for delineating the extent of thermal ablation are in high demand. In this work, we examine the distribution of estimated MSS in thermally coagulated bovine liver and normal untreated bovine liver ex vivo. We estimate MSS by detecting local maxima in the spectral coherence function of radio frequency echoes from a clinical transducer, the Siemens VFX 9L4 transducer operating on an S2000 scanner. We find that normal untreated bovine liver was characterized by an MSS of approximately 1.3 mm. We examined regions of interest 12 mm wide laterally, and ranging from 12 mm to 18 mm axially, in 2 mm increments. Over these parameters, the mode of the MSS estimates was between 1.25 and 1.37 mm. On the other hand, estimation of MSS in thermally coagulated liver tissue yields a distribution of MSS estimates whose mode varied between 0.45 and 1.0 mm when examining regions of interest over the same sizes. We demonstrate that the estimated MSS in thermally coagulated liver favors small spacings because the randomly positioned scatterers in this tissue are better modeled as aperiodic scatterers. The submillimeter spacings result from the fact that this was the most probable spacing to be estimated if the discretely sampled spectral coherence function was a uniformly random two-dimensional function.

Ultrasonic assessment of thermal therapy in rat liver

One way to assess the efficacy of thermal therapy is to quantify changes in tissue properties through ultrasonic interrogation, which requires knowledge of the acoustic properties of thermally treated tissues. In this study, estimates of ultrasonic attenuation, speed of sound, backscatter coefficient (BSC), and scattering property estimates were generated from rat liver samples submersed for 10 minutes in a saline bath that was heated to one of seven temperature values over a range of 37-70°C. The attenuation coefficient increased monotonically with exposure temperature, with a maximum increase of 90%. Speed of sound changed by <1% for the different treatment conditions. The BSC had close agreement for all thermal doses over the frequency range of 8-15 MHz. Above this frequency range, samples heated ≥55°C demonstrated an increased BSC slope, and the effective scatterer diameter and effective acoustic concentration were able to distinguish treated from nontreated cases. The findings suggest that attenuation and either BSCs or scatterer property estimates above 15 MHz were sensitive to tissue changes in excised liver caused by thermal therapy.

An acoustic backscatter-based method for localization of lesions induced by high-intensity focused ultrasound

Ultrasound B-mode visualization of lesions produced in soft tissues using high-intensity focused ultrasound (HIFU) has been shown to be challenging when there is no cavitation activity and, therefore, no hyperechogenecity in the focal region. We investigated a method for the visualization and localization of HIFU-induced lesions after HIFU delivery was complete based on the change in backscattered radio-frequency (RF) signals. A HIFU transducer was used with focal dimension of 8 mm by 2 mm working at 5 MHz. HIFU was applied at different intensities to produce lesions in ex vivo chicken breast, with or without the generation of hyperecho in B-mode images. We compared lesion locations obtained from our RF-processing method, from measurement of physical lesions after exposure and from the B-mode images, if exposures had resulted in hyperecho. The results showed that the RF amplitude decreased as a function of time immediately after stopping the HIFU exposure. The lesions were clearly visualized in two-dimensional (2-D) images of the decay rate of RF amplitude, no matter with or without hyperecho. In experiments with hyperecho, when comparing to physical lesion locations, there was no statistically significant difference in the localization accuracy between the RF-based and the hyperecho-based method (p = 0.76). In cases without hyperecho, the distance between RF-based locations and measured lesion locations was 3.37 +/- 1.59 mm (mean +/- standard deviation). The axial and lateral difference were 2.00 +/- 2.31 mm and 0.85 +/- 2.15 mm, respectively, and no statistically significant difference was found between lesion coordinates (axial: p = 0.37 and lateral: p = 0.15). We demonstrated the feasibility of our proposed RF-based method for the localization of HIFU-induced lesions immediately after HIFU treatment. Using the decay rate in RF amplitude as the signature of lesion formation, our method can detect lesion locations even without the appearance of hyperecho.

Optimized noninvasive monitoring of thermal changes on digital B-mode renal sonography during revascularization therapy

OBJECTIVE

Noninvasive real-time thermal change monitoring of human internal organs can play a critical role in diagnosis and treatment of many disorders, including reperfusion of renal arteries during anticoagulation therapy.

METHODS

This article focuses on tissue temperature detection using ultrasound velocity changes in different structures and their related speckle shift from their primary locations on high-quality B-mode digital sonography. We evaluated different speckle-tracking techniques and optimized them using appropriate motion estimation methods to determine the best algorithm and parameters.

RESULTS

Performing thermal detection methods on simulated phantoms showed a good correlation between speckle shifts and the ground truth temperature. For the simulated images, average thermal error was 0.5 degrees C with an SD of 0.5 degrees C, where lower errors can be obtained in noiseless (motionless) data. The proposed technique was evaluated on real in vivo cases during surgical occlusion and reopening of the renal segmental artery and showed the potential of the algorithm for observation of internal organ changes using only digital ultrasound systems for diagnosis and therapy.

CONCLUSIONS

The adaptive Rood pattern search proved to be the best block-matching technique, whereas the multiresolution Horn-Schunck technique was the best gradient optical flow method. The extracted thermal change during in vivo revascularization therapy is promising. In addition, we present an evaluation of several block-matching and optical flow motion estimation techniques.

Acute increases in murine tumor echogenicity after antivascular ultrasound therapy: a pilot preclinical study

OBJECTIVE

This study was designed to determine whether the echogenicity of neoplastic tissues changed as a result of low-intensity insonation and whether such alterations were related to an anti-vascular effect.

METHODS

In 21 mice, implanted melanomas were insonated at either 1, 2, or 3 MHz using low-intensity ultrasound (spatial-average temporal-average intensity, 2.1 W/cm(2)). B-mode (mean gray scale) and contrast-enhanced power Doppler (percentage area of flow) measurements were made on each tumor before and after therapy.

RESULTS

There was an increase in the echogenicity of the tumors with the increase in the frequency of the therapy beam and an accompanying decrease in tumor vascularity.

CONCLUSIONS

Although the mechanisms responsible for the echogenicity change are not fully understood, it appears that an increase in the tumor mean gray scale was, at least in part, related to tissue inhomogeneities formed after disruption of the tumor neovasculature.

Subtraction elastography for the evaluation of ablation-induced lesions: a feasibility study

Different noninvasive or minimally invasive therapeutic ablation procedures can produce tissue necrosis associated with local-stiffness increase. Although elastography has been proved as a potential evaluation tool for many kinds of ablation-induced lesions, the application of subtraction technique in elastography to enhance the visualization of the ablation lesions has rarely been reported. In this paper, subtraction elastography is proposed to evaluate the ablation-induced lesions. Three models are constructed to simulate different kinds of ablated inclusions. The simulation results showed that subtraction elastography is superior to conventional elastography in detecting the ablation-induced lesions with higher signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). The artifacts induced by elastographic signal processing algorithms can be largely reduced in subtraction elastography. In addition, subtraction elastography is less influenced by the stiff background and can provide more reliable boundary information about the lesion than conventional elastography. Furthermore, the feasibility of subtraction elastography is validated by an in vitro experiment of ethanol-induced hepatic lesions. The preliminary results of this work suggest that subtraction elastography may be a good option for the evaluation of ablationinduced lesions.

In vivo guidance and assessment of liver radio-frequency ablation with acoustic radiation force elastography

The initial results from clinical trials investigating the utility of acoustic radiation force impulse (ARFI) imaging for use with radio-frequency ablation (RFA) procedures in the liver are presented. To date, data have been collected from 6 RFA procedures in 5 unique patients. Large displacement contrast was observed in ARFI images of both pre-ablation malignancies (mean 7.5 dB, range 5.7-11.9 dB) and post-ablation thermal lesions (mean 6.2 dB, range 5.1-7.5 dB). In general, ARFI images provided superior boundary definition of structures relative to the use of conventional sonography alone. Although further investigations are required, initial results are encouraging and demonstrate the clinical promise of the ARFI method for use in many stages of RFA procedures.

Dual-mode ultrasound transducer for image-guided interstitial thermal therapy

Deep-seated tumors can be treated by minimally invasive interstitial ultrasound thermal therapy. A miniature transducer emitting high-intensity acoustic waves is placed in contact with the targeted area to induce local thermal necrosis. Accurate positioning of the probe and treatment monitoring must be achieved for the technique to be effective. A piezocomposite technology was used for obtaining both high-quality imaging and effective treatment with the same transducer. Prototypes were designed and built to be compatible with an endoscopic approach for treating cholangiocarcinomas in the biliary ducts. The transducer had dimensions of 2.5 x 7.5 mm(2), it was cylindrically focused at 10 mm and it was operated at a center frequency of 11 MHz. Transducer efficiency was measured at 71%, and the impulse response corresponded to an axial resolution of 0.2 mm. In-vitro tests were conducted on samples of pig liver in which lesions up to 10 mm in depth were induced. B-mode images were obtained by mechanically rotating the transducer. Treatments were monitored in three ways: (i) classical M-mode images, (ii) images of local deformation of ultrasound lines during heating and (iii) comparison of the displacements induced in the tissue by radiation force, before and after treatments. The successful use of piezocomposite materials to manufacture dual-mode transducers opens new perspectives for interstitial ultrasound thermal therapy.

Ultrasound thermal change detection based on steerable filters

Growing tendency toward utilization of Laser and RF knives has opened a new port for thermal control applications in which ultrasound thermal detection is crucial. Ultrasound velocity is dependent on the thermal properties of the environment. In this paper we focus on tissue temperature detection using multiresolution steerable filter-based motion estimation. The proposed technique was evaluated on simulated and real in-vivo cases during surgical occlusion and reopening of renal segmental artery and demonstrated promising results for observation of internal organ temperature changes using only digital ultrasound systems for diagnosis and therapy. It is proved that being oriented in space and time, steerable filters can achieve more accurate results. Performing thermal detection methods on synthetic phantoms demonstrated good correlation between speckle shifts and the ground truth temperature. For the simulated images average thermal error was 0.68 degrees Celsius with a standard deviation of 0.79.

Focused ultrasound thermal therapy system with ultrasound image guidance and temperature measurement feedback

In this study, we developed a focused ultrasound (FUS) thermal therapy system with ultrasound image guidance and thermocouple temperature measurement feedback. Hydraulic position devices and computer-controlled servo motors were used to move the FUS transducer to the desired location with the measurement of actual movement by linear scale. The entire system integrated automatic position devices, FUS transducer, power amplifier, ultrasound image system, and thermocouple temperature measurement into a graphical user interface. For the treatment procedure, a thermocouple was implanted into a targeted treatment region in a tissue-mimicking phantom under ultrasound image guidance, and then the acoustic interference pattern formed by image ultrasound beam and low-power FUS beam was employed as image guidance to move the FUS transducer to have its focal zone coincident with the thermocouple tip. The thermocouple temperature rise was used to determine the sonication duration for a suitable thermal lesion as a high power was turned on and ultrasound image was used to capture the thermal lesion formation. For a multiple lesion formation, the FUS transducer was moved under the acoustic interference guidance to a new location and then it sonicated with the same power level and duration. This system was evaluated and the results showed that it could perform two-dimensional motion control to do a two-dimensional thermal therapy with a small localization error 0.5 mm. Through the user interface, the FUS transducer could be moved to heat the target region with the guidance of ultrasound image and acoustic interference pattern. The preliminary phantom experimental results demonstrated that the system could achieve the desired treatment plan satisfactorily.

Ultrasound monitoring of temperature change in liver tissue during laser thermotherapy: 10 degrees C intervals

In thermal tissue ablation, it is very important to control the increase in the temperature for having an efficient ablation therapy. We conducted this study to determine the efficacy of measuring pixel shift of ultrasound B-mode images as a function of change in tissue temperature. By fixing some micro thermocouples in liver tissues, temperature at different points was monitored invasively in vitro during laser-induced thermotherapy. According to our results optimum power and exposure time were determined for ultrasound temperature monitoring. Simultaneously, noninvasive temperature monitoring was performed with ultrasound B-mode images. These images were saved on computer from 25 degrees C to 95 degrees C with 10 degrees C steps. The speed of sound changes with each 10 degrees C temperature change that produce virtual shifts in the scatter positions. Using an image processing method, the pixel shift due to 10 degrees C temperature change was extracted by motion detection. The cubic regression function between the mean pixel shifts on ultrasound B-mode images caused by the change in speed of sound which in turn was a function of the mean change in temperature was evaluated. When temperature increased, pixel shift occurs in ultrasound images. The maximum pixel shift was observed between 60 to 70 degrees C. After 70 degrees C, the local pixel shift due to change in the speed of sound in liver tissue had an irregular decreasing. Pearson correlation coefficient between invasive and non-invasive measurements for 10 degrees C temperature changes was 0.93 and the non-linear function was suitable for monitoring of temperature. Monitoring of changes in temperature based on pixel shifts observed in ultrasound B-mode images in interstitial laser thermotherapy of liver seems a good modality.

Assessment of pixel shift in ultrasound images due to local temperature changes during the laser interstitial thermotherapy of liver: in vitro study

Laser interstitial thermotherapy (LITT) is an internal ablation therapy consisting of percutaneous or intraoperative insertion of laser fibers directly into the liver tumor with maximum diameter of 5 cm. It is very important to control the temperature increase, because tissue carbonization occurs with high temperatures, which can damage normal tissues. In this research, pixel shift changes on ultrasound B-mode images with temperature changes were measured. LITT in vitro was performed on 12 freshly excised sheep liver tissues using a Nd:YAG laser with a bare-tip optical fiber. The 1 W power setting was used for 700 s exposure time (2477 J/mm2). Invasive temperature monitoring was performed during the heating and cooling by attaching microthermocouples to the tissue. At the same time, ultrasound B-mode images were saved on the computer for each 5 degrees C temperature increase from 25 degrees C to 100 degrees C, for noninvasive temperature monitoring. These pixel shifts were measured by an echo-tracking algorithm. Linear and nonlinear regression analyses between the independent variable (temperature change) and the dependent variable (pixel shift on images) were performed. Regression functions and correlation coefficients were determined. It was shown that with a correlation coefficient of 0.998, the cubic function was suitable. Pixel shift increased for each 5 degrees C temperature increase and the maximum shift was observed during 60 to 70 degrees C. Beyond these temperatures, the pixel shift decreased. In this method, because of evaporation of tissue water and bubble formation and tissue carbonization, monitoring greater than 100 degrees C was difficult. It is possible to monitor temperature changes on the ultrasound B-mode images in interstitial laser thermotherapy of liver. Also, with the improvement of image processing, this method could be used for noninvasive temperature monitoring for a large number of patients during LITT.

Challenges and implementation of radiation-force imaging with an intracardiac ultrasound transducer

Intracardiac echocardiography (ICE) has been demonstrated to be an effective imaging modality for the guidance of several cardiac procedures, including radiofrequency ablation (RFA). However, assessing lesion size during the ablation with conventional ultrasound has been limited, as the associated changes within the B-mode images often are subtle. Acoustic radiation force impulse (ARFI) imaging is a promising modality to monitor RFAs as it is capable of visualizing variations in local stiffnesses within the myocardium. We demonstrate ARFI imaging with an intracardiac probe that creates higher quality images of the developing lesion. We evaluated the performance of an ICE probe with ARFI imaging in monitoring RFAs. The intracardiac probe was used to create high contrast, high resolution ARFI images of a tissue-mimicking phantom containing stiffer spherical inclusions. The probe also was used to examine an excised segment of an ovine right ventricle with a RFA-created surface lesion. Although the lesion was not visible in conventional B-mode images, the ARFI images were able to show the boundaries between the lesion and the surrounding tissue. ARFI imaging with an intracardiac probe then was used to monitor cardiac ablations in vivo. RFAs were performed within the right atrium of an ovine heart, and B-mode and ARFI imaging with the intracardiac probe was used to monitor the developing lesions. Although there was little indication of a developing lesion within the B-mode images, the corresponding ARFI images displayed regions around the ablation site that displaced less.

Noninvasive temperature estimation using sonographic digital images

OBJECTIVE

The purpose of this study was to develop and evaluate a speckle-tracking method for tissue temperature estimation due to heating fields using digital sonographic images.

METHODS

The temperature change estimation method is based on the thermal dependence of the ultrasound speed and the thermal expansion of the medium. Local changes in the speed of sound due to changes in the temperature produce apparent displacement of the scatterers, and the expansion introduces physical displacement. In our study, a new technique has been introduced in which the axial physical displacements were obtained from digital sonographic images. The axial speckle pattern displacement was determined with a cross-correlation algorithm. The displacement data were then used for computing the temperature changes. To monitor the temperature in real time, the computational time was decreased by restricting the search region in the cross-correlation algorithm and carrying out the cross-correlation function in the frequency domain via a fast Fourier transform algorithm.

RESULTS

Experiments were performed on tissue-mimicking phantoms. The imaging probe was a commercial linear array working at 10 MHz. In addition, the temperature changes during heating were measured invasively by negative temperature coefficient thermistors. There was good agreement between ultrasonic temperature estimations and invasive temperature measurements.

CONCLUSIONS

The proposed method verifies the capability of the speckle-tracking algorithm for determining both the magnitude and direction of displacement. The average error was 0.2 degrees C; the maximum error was 0.53 degrees C; and the SD was 0.19 degrees C. Therefore, the proposed algorithm is capable of extracting the temperature information from sonographic digital images.

Liver ablation guidance with acoustic radiation force impulse imaging: challenges and opportunities

Previous studies have established the feasibility of monitoring radiofrequency (RF) ablation procedures with acoustic radiation force impulse (ARFI) imaging. However, questions remained regarding the utility of the technique in clinically realistic scenarios and at scanning depths associated with abdominal imaging in adults. We address several of these issues and detail recent progress towards the clinical relevance of the ARFI technique. Results from in vitro bovine tissues and an in vivo ovine model are presented. Additional experiments were conducted with a tissue-mimicking phantom and parallel receive tracking techniques in order to further support the clinical feasibility of the method. Thermal lesions created during RF ablation are visualized with high contrast in both in vitro and in vivo hepatic tissues, and radial lesion growth can be monitored throughout the duration of the procedure. ARFI imaging is implemented on a diagnostic ultrasonic scanner, and thus may be a convenient option to guide RF ablation procedures, particularly when electrode insertion is also performed with sonographic guidance.

An investigation of the use of transmission ultrasound to measure acoustic attenuation changes in thermal therapy

The potential of using a commercial ultrasound transmission imaging system to quantitatively monitor tissue attenuation changes after thermal therapy was investigated. The ultrasound transmission imaging system used, the AcoustoCam (Imperium Inc., MD) allows ultrasonic images to be captured using principles similar to that of a CCD-type camera that collects light. Ultrasound energy is focused onto a piezoelectric array by an acoustic lens system, creating a gray scale acoustic image. In this work, the pixel values from the acoustic images were assigned acoustic attenuation values by imaging polyacrylamide phantoms of varying known attenuation. After the calibration procedure, data from heated polyacrylamide/bovine serum albumin (BSA) based tissue-mimicking (TM) phantoms and porcine livers were acquired. Samples were heated in water at temperatures of 35, 45, 55, 65, and 75 degrees C for 1 h. Regions of interest were chosen in the images and acoustic attenuation values before and after heating were compared. An increase in ultrasound attenuation was found in phantoms containing BSA and in porcine liver. In the presence of BSA, attenuation in the TM phantom increased by a factor of 1.5, while without BSA no significant changes were observed. The attenuation of the porcine liver increased by up to a factor of 2.4, consistent with previously reported studies. The study demonstrates the feasibility of using a quantitative ultrasound transmission imaging system for monitoring thermal therapy.

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.

Interstitial microwave thermal therapy and its application to the treatment of recurrent prostate cancer

Interstitial microwave thermal therapy may be an effective alternative to surgery for the treatment of some solid tumours. Arrays of helical antennae can produce complex heating patterns which when combined with active cooling of normal tissue structures can provide conformal heating for thermal coagulation of tumours. The development of a clinical protocol involving phantom and animal model studies, treatment planning, tissue property measurement and methods for on-line treatment monitoring is reviewed. The technology developed has been applied to the problem of recurrent prostate cancer following failed radiation treatment where available curative options are associated with high normal tissue morbidity. The purpose was to develop a treatment option for this group of patients with a very low side-effect profile that would not preclude further treatment if the disease progressed. Results of a Phase I/II trial demonstrate safety, promising efficacy and a low complication rate. As the technology for delivering this treatment matures, larger multi-institutional trials should be considered.

Acoustic radiation force impulse imaging of myocardial radiofrequency ablation: initial in vivo results

Acoustic radiation force impulse (ARFI) imaging techniques were used to monitor radiofrequency (RF) ablation of ovine cardiac tissue in vivo. Additionally, ARFI M-mode imaging methods were used to interrogate both healthy and ablated regions of myocardial tissue. Although induced cardiac lesions were not visualized well in conventional B-mode images, ARFI images of ablation procedures allowed determination of lesion location, shape, and relative size through time. The ARFI M-mode images were capable of distinguishing differences in behavior through the cardiac cycle between healthy and damaged tissue regions. As conventional sonography is often used to guide ablation catheters, ARFI imaging, which requires no additional equipment, may be a convenient modality for monitoring lesion formation in vivo.

Long-term Survival of Patients with Unresectable Colorectal Liver Metastases treated by Percutaneous Interstitial Laser Thermotherapy

In situ ablation of colorectal cancer (CRC) liver metastases is an accepted form of treatment for selected patients. It is associated with low morbidity and mortality and increases the number of patients who may benefit from therapy compared to resection alone. This study assesses the impact of interstitial laser thermotherapy (ILT) on local tumor control and long-term survival in patients with unresectable CRC liver metastases. Percutaneous ILT was performed in patients with unresectable CRC liver metastases between January 1992 and December 1999 using a bare-tip quartz fiber connected to an Nd:YAG laser source. This was prior to the routine use of a diffusing fiber for ablative therapy. Treatment was monitored with real-time ultrasonography. Tumors were considered unresectable based on their anatomic location or the extent of liver involvement. Patients with extrahepatic disease, more than five liver metastases, or tumors larger than 10 cm in diameter were excluded from this study. Local tumor control was assessed by dynamic computed tomography (CT) 6 months after therapy. Long-term follow-up was undertaken, and the impact of various factors on survival was analyzed. Eighty patients with a mean age of 63.8 years were suitable for ILT. In total, 168 liver tumors with a median diameter of 5 cm (range 1-10 cm) were so treated. There were no procedure-related deaths. The overall complication rate was 16%, with all cases managed conservatively. Bradycardia (n = 5), pneumothorax (n = 3), and persistent pyrexia (n = 3) were the most common complications. Complete tumor ablation was noted in 67% of patients assessed by CT 6 months following the initial therapy. Median follow-up was 35 months (range 4-96 months), with 10 patients alive at the end of this period. Altogether there were 67 deaths, which were related to hepatic disease in 55 cases and to extrahepatic disease in 9; they were unrelated to malignancy in 3 others. Three patients were excluded from follow-up after ILT down-staging of tumors that allowed complete surgical resection. The median disease-free survival of patients treated by ILT was 24.6 months, with a 5-year survival of 3.8%. Poor tumor differentiation and the presence of more than two hepatic metastases were associated with lower overall survival (p < 0.01). Fourteen patients treated by ILT for postoperative hepatic recurrences had the best outcome, with a median overall survival of 36.3 months and a 5-year survival of 17.2%. Percutaneous ILT is a minimally invasive, safe, effective technique that appears to improve overall survival in specific patients with unresectable CRC liver metastases, compared to the natural history of untreated disease reported in the literature.

Monitoring thermally-induced lesions with supersonic shear imaging

Thermally-induced lesions are generally stiffer than surrounding tissues. We propose here to use the supersonic shear imaging technique (SSI) for monitoring high-intensity focused ultrasound (HIFU) therapy. This new elasticity imaging technique is based on remotely creating shear sources using an acoustic radiation force at different locations in the medium. In these experiments, an HIFU probe is used to generate lesions in fresh tissue samples. A diagnostic transducer, controlled by our ultrafast scanner, is located in the therapeutic probe focal plane. It is used for both generating the shear waves and imaging the resulting propagation at frame rates reaching 5,000 images/s. Movies of the shear wave propagation can be computed off-line. The therapeutic and imaging sequences are interleaved and a set of wave propagation movies is performed during the heating process. From each movie, elasticity estimations have been performed using an inversion algorithm. It demonstrates the feasibility of detecting and quantifying the hardness of HIFU-induced lesions using SSI.

Acoustic radiation force impulse imaging of thermally- and chemically-induced lesions in soft tissues: preliminary ex vivo results

The ability of acoustic radiation force impulse (ARFI) imaging to visualize thermally- and chemically-induced lesions in soft tissues was investigated. Lesions were induced in freshly excised bovine liver samples. Chemical lesions were induced via the injection of formaldehyde and thermal lesions were created using a radiofrequency (RF) ablation system. Although conventional sonography was unable to visualize induced lesions, ARFI imaging was capable of monitoring lesion size and boundaries. Agreement was observed between lesion size in ARFI images and in results from pathology. The fact that ARFI imaging requires no additional equipment aside from that needed for conventional ultrasonic imaging makes it a promising modality for monitoring lesion development in situations where sonography is already involved as a guiding mechanism, such as in procedures requiring precise catheter placement.

Feasibility of a transurethral ultrasound applicator for coagulation in prostate

Transurethral resection of the prostate (TURP) is the surgical method routinely used in clinics to treat benign prostate hyperplasia (BPH). The purpose of this work is to demonstrate the feasibility of a transurethral ultrasound (US) applicator based on a miniature US flat transducer to coagulate prostatic tissues. Rabbit liver was found to comply well with human prostate. A significant fall in Doppler signal amplitude immediately after treatment demonstrated the applicator's ability to achieve haemostasis. The therapeutic depth extended from 6 to 10 mm, depending on conditions of exposure, and the coagulation rate ranged between 51% and 99%. The coagulated zone pinpointed on histological examination could be easily correlated to a permanent hypoechoic zone observed on B-scans of treated zones. This observation is most likely due to temperature-related changes in the acoustic attenuation of liver and, unfortunately, may not be visible in the prostate.

Interstitial laser thermotherapy for liver tumours

BACKGROUND

Primary hepatocellular carcinoma (HCC) and metastases from colorectal cancer are the most common malignant liver tumours. Surgical resection is the optimum treatment in suitable patients. Interstitial laser thermotherapy (ILT) is gaining acceptance for the treatment of irresectable liver tumours and as a potential alternative to surgery. An understanding of the principles of therapy and review of clinical outcomes may allow better use of this technology.

METHOD

An electronic search using the Medline database was performed for studies on the treatment of hepatic malignancy published between January 1983 and February 2003.

RESULTS

Current information on the efficacy of ILT is based on prospective studies. ILT appears to be a safe and minimally invasive technique that consistently achieves tumour destruction. The extent of destruction depends on the fibre design, delivery system, tumour size and tumour biology. Real-time magnetic resonance imaging provides the most accurate assessment of laser-induced tumour necrosis. In selected patients with HCC and colorectal cancer liver metastases, ILT achieves complete tumour necrosis, provides long-term local control, and improves survival, compared with the natural history of the disease. In addition, ILT has survival benefits for patients with other tumour types, especially those with isolated liver metastases from a breast cancer primary.

CONCLUSION

ILT improves overall survival in specific patients with liver tumours. Advances in laser technology and refinements in technique, and a better understanding of the processes involved in laser-induced tissue injury, may allow ILT to replace surgery as the procedure of choice in selected patients with liver malignancies.

Thermal ablative therapy for malignant liver tumors: a critical appraisal

The management of primary and secondary malignant liver tumors poses a great challenge to clinicians. Although surgical resection is the gold-standard treatment, most patients have unresectable malignant liver tumors. Over the past decade, various modalities of loco-regional therapy have gained much interest. Among them, thermal ablative therapy, including cryotherapy, microwave coagulation, interstitial laser therapy, and radiofrequency ablation (RFA), have been proven to be safe and effective. Despite the effective tumor eradication achieved within cryotherapy, the underlying freeze/thaw mechanism has resulted in serious complications that include bleeding from liver cracking and the 'cryoshock' phenomenon. Thermal ablation using microwave and laser therapy for malignant liver tumors is curative and is associated with minimal complications. However, this treatment modality is effective only for tumors <3 cm diameter. Radiofrequency ablation seems to be the most promising form of thermal ablative therapy in terms of a lower complication rate and a larger volume of ablation. However, its use is restricted by the difficulty encountered when using imaging studies to monitor the areas of ablation during and after the procedure. Moreover, the techniques of RFA need to be refined in order to achieve the same oncological radicality of malignant liver tumors as achieved by surgical resection. As each of the loco-regional therapies has its own advantages and limitations, a multidisciplinary approach using a combination of therapies will be the future trend for the management of malignant liver tumors.

Other thermal ablation techniques: microwave and interstitial laser ablation of liver tumors

BACKGROUND

Thermal ablation of hepatic malignancies is becoming a widespread treatment approach. In addition to radiofrequency ablation, microwave coagulation (MCT) and laser-induced interstitial thermotherapy (LITT) are being used clinically to treat patients with liver cancers.

METHODS

The principles and clinical indications for MCT and LITT are described. Treatment approaches and results from published clinical studies are reviewed. The evolution of these thermal treatment modalities and limitations of currently available equipment is provided.

RESULTS

The interstitial probes and equipment used for MCT and LITT for liver tumors are undergoing changes to improve treatment efficacy. Both MCT and LITT have been limited by the relatively small zone of coagulation produced with a single probe placement. Both techniques can be performed safely, and local recurrence and long-term survival rates are being established.

CONCLUSIONS

MCT and LITT are two alternative thermal ablation techniques being used to treat patients with primary and metastatic hepatic malignancies. The utility of these two treatments has been limited by the relatively small area of thermal necrosis produced around the interstitial probes, but design modifications and new equipment may improve these limitations.

Ultrasound properties of human prostate tissue during heating

Changes in the ultrasound (US) properties of tissue during heating affect the delivery of US thermal therapy and may provide a basis for US image monitoring of thermal therapy. The US attenuation coefficient and backscatter power of fresh human prostate tissue were measured as the tissue was heated. Samples of human prostate were obtained directly from autopsies and heated rapidly to final temperatures of 45 degrees C, 50 degrees C, 55 degrees C, 60 degrees C and 65 degrees C. A 5.0-MHz transducer was scanned in a raster pattern over the tissue and radiofrequency (RF) data were collected at 36 uncorrelated positions. Both attenuation and backscatter were measured over the frequency range 3.5 to 7.0 MHz at each min of a 30-min heating. Little change was observed in attenuation or backscatter at 55 degrees C or less. The attenuation coefficient and backscatter power increased by factors of 1.25 and 5, respectively, during the 60 degrees C heating. During the 65 degrees C heating, the same properties showed increases by factors of 2.7 and 9.

Ultrasound nucleolysis: an in vitro study

Thermal intradiscal therapy for chronic low back pain, using a catheter inserted into the intervertebral disc, is becoming more popular in the treatment of low back pain. The aim of this study was to investigate the possibility of heating the nucleus pulposus of the intervertebral disc with high-intensity focused ultrasound (US) or HIFU. Two specific situations were considered, invasive transducers that would be in contact with the annulus fibrosus of the disc, and noninvasive transducers that could be used externally. Theoretical simulations were performed to find the optimal parameters of US transducers and then experimental studies were done using transducers made to these specifications. These experiments confirmed that it was possible to heat the discs with HIFU. Two orthogonal transducers resulted in a superior temperature distribution than using just one transducer. It is, therefore, feasible to consider thermal treatment of the nucleus pulposus of the disc using noninvasive US.

Interstitial microwave thermal therapy for prostate cancer: method of treatment and results of a phase I/II trial

PURPOSE

Interstitial microwave thermal therapy is experimental treatment for prostate cancer with the goal of curing disease, while causing fewer complications than standard treatment options. We present a method for delivering interstitial microwave thermal therapy using microwave radiating helical antennae inserted percutaneously under transrectal ultrasound guidance. We report the results of a trial of this method in 25 patients in whom primary external beam radiation therapy had previously failed. This patient group currently has limited curative options that are associated with a high complication rate. However, these recurrent tumors often remain localized to the prostate, and so they may be amenable to localized therapy.

MATERIALS AND METHODS

Patients with proved prostatic adenocarcinoma were candidates for treatment when prostate specific antigen (PSA) was 15 ng./ml. or less and prostate volume was 50 cc. or less. Followup included PSA measurement, digital rectal examination, urinalysis, and documentation of adverse events at 4, 8, 12 and 24 weeks. Sextant biopsy was performed at week 24. The procedure involved the insertion of 5 antennae percutaneously through a modified brachytherapy template. The antenna arrangement was determined based on computer simulated predictions of temperature throughout the prostate. The prostate was dissected away from the rectum by an injection of sterile saline to provide a thermal barrier that protected the rectum from thermal damage. Temperatures were monitored using interstitial mapping thermistor probes that were also inserted through the template. A minimum peripheral target temperature of 55C but less than 70C was maintained for 15 to 20 minutes, while the urethra, rectum and hydrodissection space remained below 42C. The urethra and rectum were actively cooled in addition to hydrodissection.

RESULTS

Peripheral target temperatures of 55C were achieved. The urethra and rectum remained at a safe temperature. The procedure, including setup and treatment, required approximately 2.5 hours of operating room time. At 24 weeks the PSA nadir was 0.5 ng./ml. or less in 52% of patients and 0.51 to 4 ng./ml. was achieved in an additional 40%. The negative biopsy rate at 24 weeks was 64%, assuming that 3 patients lost to followup would have had positive results. No major complications were observed and in most cases minor complications resolved within 3 months.

CONCLUSIONS

Interstitial microwave thermal therapy for prostate cancer was developed to heat the prostate safely to a cytotoxic temperature. Experience with 25 patients in whom external beam radiation therapy for prostate cancer had failed indicates that the treatment is safe. Although our series indicates that this therapy may be effective, further studies and longer followup are required in larger patient groups to confirm the potential role of this therapy as an option for recurrent and primary prostate cancer.

Changes in ultrasound properties of porcine kidney tissue during heating

Changes in the ultrasound (US) attenuation and backscatter of fresh pig kidney were measured as the tissue was heated. The objective was to use these changes to predict how an US image would change in real-time with a view to its use as a monitoring tool for minimally invasive thermal therapy (MITT). Separate samples of fresh pig kidney were heated from 37 degrees C to temperatures of 45 degrees, 50 degrees, 55 degrees, 60 degrees and 65 degrees with warm water. Measurements were made over the frequency range from 3.5 MHz to 7.0 MHz during 30-min heating experiments. A general increase in attenuation magnitude (dB/cm) and slope (dB/cm-MHz) was observed at temperatures of 55 degrees C or greater. Little change in backscatter power was observed during heating to 45 degrees C. At higher temperatures, the changes in backscatter showed a more complex pattern throughout the experiments, but still showed a trend of increase to a greater value at the end of heating than at the start. This backscatter increase was greater at higher temperatures. The net effect of the changes in US properties suggests that it may be possible to use diagnostic US to monitor, in real-time, MITT in kidney.

Interstitial laser thermotherapy in the treatment of colorectal liver metastases

Metastatic liver disease is the commonest cause of death in patients with colorectal cancer. A small proportion of these patients (10%) may be treated by surgical resection with five year survival approaching 35-40%. Alternative treatment modalities for localised hepatic disease include in situ ablative techniques that have the advantages of percutaneous application and minimal morbidity. These include Interstitial Laser Thermotherapy (ILT), Radio Frequency Ablation, Percutaneous Microwave therapy, and Focussed Ultrasound Therapy. This article focuses specifically on the development and utilisation of ILT in the treatment of colorectal liver metastases. It provides a review of the pathophysiological factors involved, present status of clinical studies, and future directions. ILT is a safe technique for the treatment of colorectal liver metastases. It may be delivered by minimally invasive techniques to lesions considered unresectable by present criteria. Limitations include the extent and completeness of tumour necrosis achieved as well as imaging techniques. Clinical problems include a lack of controlled studies. Assessment of long-term survival in prospective randomised trials is needed to assess the efficacy of this procedure.

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

High-intensity focused ultrasound (HIFU) and conventional B-mode ultrasound (US) imaging were synchronized to develop a system for real-time visualization of HIFU treatment. The system was tested in vivo in pig liver. The HIFU application resulted in the appearance of a hyperechoic spot at the focus that faded gradually after cessation of HIFU exposure. The duration of HIFU exposure needed for a hyperechoic spot to appear, was inversely related to the HIFU intensity. The threshold intensity required to produce a hyperechoic spot in liver in < 1 s was 970 W/cm(2), in situ. At this HIFU dose, no immediate cellular damage was observed, providing a potential for pretreatment targeting. The real-time visualization method was used in hemostasis of actively bleeding internal pelvic vessels, allowing targeting and monitoring of successful treatment. Real-time US imaging may provide a useful tool for image-guided HIFU therapy.

Comparison of thermal damage calculated using magnetic resonance thermometry, with magnetic resonance imaging post-treatment and histology, after interstitial microwave thermal therapy of rabbit brain

Clinical application of high-temperature thermal therapy as a treatment for solid tumours requires an accurate and close to real-time method for assessing tissue damage. Imaging methods that detect structural changes during heating may underestimate the extent of thermal damage. This is due to the occurrence of delayed damage manifested at tissue locations exposed to temperatures lower than those required to cause immediate structural changes. An alternative approach is to measure temperature and then calculate the expected damage based on the temperature history at each tissue location. Magnetic resonance (MR) imaging methods now allow temperature maps of the target and surrounding tissues to be generated in almost real-time. The aim of this work was to evaluate whether thermal damage zones calculated on the basis of MR thermometry maps measured during heating correspond to actual tissue damage as measured after treatment by histological methods and MR imaging. Four male rabbits were treated with high-temperature thermal therapy delivered in the brain by a single microwave antenna operating at 915 MHz. MR scanning was performed before, during and after treatment in a 1.5 T whole-body scanner. Temperature maps were produced using the proton resonance frequency (PRF) shift method of MR thermometry. In addition, conventional T1-weighted and T2-weighted spin-echo images were acquired after treatment. Thermal damage zones corresponding to cell death, microvascular blood flow stasis and protein coagulation were calculated using an Arrhenius analysis of the MR temperature/time course data. The calculated zones were compared with the lesions seen on histopathological examination of the brains which were removed within 6-8 h of treatment. The results showed that calculated damage zones based on MR thermometry agreed well with areas of damage as assessed using histology after heating was completed. The data suggest that real-time calculations of final expected thermal damage based on an Arrhenius analysis of MR temperature data may provide a useful method of real-time monitoring of thermal therapy when combined with conventional T2-weighted images taken after treatment.