Simulations of thermal tissue coagulation and their value for the planning and monitoring of laser-induced interstitial thermotherapy (LITT)

Modeling and in vivo experimental validation of 1,064 nm laser interstitial thermal therapy on brain tissue

Introduction

Laser interstitial thermal therapy (LITT) at 1064 nm is widely used to treat epilepsy and brain tumors; however, no numerical model exists that can predict the ablation region with careful in vivo validation.

Methods

In this study, we proposed a model with a system of finite element methods simulating heat transfer inside the brain tissue, radiative transfer from the applicator into the brain tissue, and a model for tissue damage.

Results

To speed up the computation for practical applications, we also validated P1-approximation as an efficient and fast method for calculating radiative transfer by comparing it with Monte Carlo simulation. Finally, we validated the proposed numerical model in vivo on six healthy canines and eight human patients with epilepsy and found strong agreement between the predicted temperature profile and ablation area and the magnetic resonance imaging-measured results.

Discussion

Our results demonstrate the feasibility and reliability of the model in predicting the ablation area of 1,064 nm LITT, which is important for presurgical planning when using LITT.

Thermal Damage Estimate Artifact Following Antecedent Biopsy: A Case Report

MR-guided laser interstitial therapy (MRgLITT) is becoming more commonly used for minimal access approaches to intracranial lesions of all etiologies. The short-term safety profile of MRgLITT is favorable compared with sweeping incisions and open craniotomies, especially for lesions located in deep, periventricular, and highly eloquent areas. The Visualase software (Medtronic Inc., Minneapolis, MN, USA) has multiple adaptations to assist with this safety margin, including the thermal damage estimate (TDE), which applies predictive mathematical modeling to a two-dimensional (2D) graphical representation. TDE has been shown to highly correlate with actual tissue destruction in a priori MRgLITT cases and to anecdotally be imprecise when MRgLITT is combined with biopsy. We present a case regarding a 17-year-old male patient with intractable focal epilepsy. He underwent stereotactic biopsy and then ablation where it was shown that TDE is ~35% larger in the coronal plane than in the actual ablation zone. Air may have caused this artifact in the biopsy cavity, which affected the proton resonance frequency (PRF) and caused TDE pigment deposition. We believe in the need for a more comprehensive understanding and investigation regarding this TDE artifact. Future prospective studies into MRgLITT should attend carefully in cases where it is combined with biopsy.

Validating a simulation model for laser-induced thermotherapy using MR thermometry

OBJECTIVES

We want to investigate whether temperature measurements obtained from MR thermometry are accurate and reliable enough to aid the development and validation of simulation models for Laser-induced interstitial thermotherapy (LITT).

METHODS

Laser-induced interstitial thermotherapy (LITT) is applied to ex-vivo porcine livers. An artificial blood vessel is used to study the cooling effect of large blood vessels in proximity to the ablation zone. The experimental setting is simulated using a model based on partial differential equations (PDEs) for temperature, radiation, and tissue damage. The simulated temperature distributions are compared to temperature data obtained from MR thermometry.

RESULTS

The overall agreement between measurement and simulation is good for two of our four test cases, while for the remaining cases drift problems with the thermometry data have been an issue. At higher temperatures local deviations between simulation and measurement occur in close proximity to the laser applicator and the vessel. This suggests that certain aspects of the model may need some refinement.

CONCLUSION

Thermometry data is well-suited for aiding the development of simulations models since it shows where refinements are necessary and enables the validation of such models.

Preclinical evaluation of a clinical prototype transrectal diffuse optical tomography system for monitoring photothermal therapy of focal prostate cancer

SIGNIFICANCE

Our work demonstrates in preclinical models that continuous-wave transrectal diffuse optical tomography (TRDOT) can be used to accurately monitor photothermal therapy (PTT) and, in particular, the progression of the photocoagulation boundary toward the rectum. When used in patients, this should prevent rectal damage during PTT, thereby achieving maximum treatment efficacy while ensuring safety, using a technology platform suitable for wide dissemination.

AIM

We aim to validate that TRDOT measurements analyzed using a shape-based image-reconstruction algorithm (SBDOT) allow localization of the photocoagulation boundary during PTT within ±1  mm toward the rectum in the transverse plane.

APPROACH

TRDOT measurements were performed in tissue-simulating phantoms, ex vivo tissues, and an in vivo canine prostate model. The accuracy and sensitivity of reconstructing the size and location of the coagulation zone were determined, based on changes in the tissue absorption and reduced scattering coefficients upon photocoagulation. The reconstruction also yields the native and coagulated tissue optical properties.

RESULTS

The TRDOT measurements and SBDOT reconstruction algorithm were confirmed to perform sufficiently well for clinical translation in PTT monitoring, recovering the location of the coagulation boundary within ±1  mm compared to the true value as determined by direct visualization postexcision and/or MRI.

CONCLUSIONS

Implementing previously described TRDOT instrumentation and SBDOT image reconstruction in different tissue models confirms the potential for clinincal translation, including required refinements of the system and reconstruction algorithm.

Effects of Intraoperative Magnetic Resonance Thermal Imaging Signal Artifact During Laser Interstitial Thermal Therapy on Thermal Damage Estimate and Postoperative Magnetic Resonance Imaging Ablative Area Concordance

BACKGROUND

Magnetic resonance-guided laser interstitial thermal therapy (MRgLITT) is a minimally invasive procedure that utilizes intraoperative magnetic resonance thermal imaging (MRTI) to generate a thermal damage estimate (TDE) of the ablative area. In select cases, the MRTI contains a signal artifact or defect that distorts the ablative region. No study has considered the impact of this artifact on TDE accuracy.

OBJECTIVE

To determine the effect of intraoperative MRTI signal artifact on postoperative magnetic resonance imaging (MRI)-predicted ablative area.

METHODS

All ablations were performed using the Visualase MRI-Guided Laser Ablation System (Medtronic). Patients were grouped based on whether the intraoperative MRTI contained signal artifact that distorted the ablative region. Cross-sectional area of the ablative lesion from the MRI image was measured, and the difference between intraoperative TDE and postoperative MRI cross-sectional area was calculated and compared between groups with and without intraoperative MRTI artifact.

RESULTS

A total of 91 patients undergoing MRgLITT for various surgical indications were examined. MRTI artifact was observed in 43.9% of cases overall. The mean absolute difference between TDE and the postoperative MRI cross-sectional area was 94.8 mm2 (SEM = 11.6) in the group with intraoperative MRTI artifact and 54.4 mm2 (SEM = 5.5) in the nonartifact group.

CONCLUSION

MRTI signal artifact is common during LITT. The presence of signal artifact during intraoperative MRTI results in higher variation between intraoperative TDE and postoperative MRI cross-sectional ablative area. In cases in which intraoperative MRTI artifact is observed, there may be a larger degree of variation between observed intraoperative TDE and measured postoperative MRTI ablative area.

Laser interstitial thermotherapy (LITT) for breast cancer: dosimetry optimization and numerical simulation

Surgical treatment is standard for the treatment of small breast cancers. Due to the pain and esthetic sequelae that can follow surgery, minimally invasive treatments are under investigation. Our aim was to conduct a dosimetry study of laser interstitial thermotherapy. Turkey tissue was used as an ex vivo model, and mammary glands from ewes were used as in vivo models. We used two different wavelength lasers (805 nm and 980 nm). Two types of fiber from two different manufacturers were used: bare fibers with a diameter of 600 μm and diffusing fiber. The diffusing fibers were 5 mm and 10 mm in length. We also used a computerized model to predict thermal damage and to correlate with the ex vivo and in vivo procedures using a constant and variable coefficient. The mathematical model was based on the finite element method for solving light distribution, bio-heat, and thermal damage equations. Based on our ex vivo and in vivo experiments, we found that the optimal configuration for this treatment was the use of the 980-nm laser at 4 W with bare fibers for a minimum treatment time of 150 s. We also developed a predictive mathematical model that showed good predictability of necrosis in line with the experimental data. Laser treatment is a promising therapy for small breast lesions. However, further development of treatment guidance is necessary to support its use in clinical practice.

Techniques for Temperature Monitoring of Myocardial Tissue Undergoing Radiofrequency Ablation Treatments: An Overview

Cardiac radiofrequency ablation (RFA) has received substantial attention for the treatment of multiple arrhythmias. In this scenario, there is an ever-growing demand for monitoring the temperature trend inside the tissue as it may allow an accurate control of the treatment effects, with a consequent improvement of the clinical outcomes. There are many methods for monitoring temperature in tissues undergoing RFA, which can be divided into invasive and non-invasive. This paper aims to provide an overview of the currently available techniques for temperature detection in this clinical scenario. Firstly, we describe the heat generation during RFA, then we report the principle of work of the most popular thermometric techniques and their features. Finally, we introduce their main applications in the field of cardiac RFA to explore the applicability in clinical settings of each method.

Effect of optical energy modulation on the thermal response of biological tissue: computational and experimental validations

This study develops an energy modulation technique to attain a constant interstitial tissue temperature and to induce the predetermined thermal coagulation without carbonization in tissue. An optical diffuser was employed to deliver 1064 nm light to the biological tissue. The combined mode maintained the interstitial temperature at 70 °C for longer durations compared to the continuous wave mode. Coagulation volumes increased linearly with the time and met the predetermined treatment volume range (0.32-0.52 cm³) after the combined treatment for 100 s. The combined modulation can be a feasible modality to induce the predetermined extent of thermal coagulation for treating papillary thyroid microcarcinoma.

Characterization of Magnetic Resonance Thermal Imaging Signal Artifact During Magnetic Resonance Guided Laser-Induced Thermal Therapy

BACKGROUND

Magnetic resonance-guided laser interstitial thermal therapy (MRgLITT) is a minimally invasive procedure that utilizes intraoperative magnetic resonance thermal imaging (MRTI) to generate a thermal damage estimate (TDE) of the ablative area. In select cases, the MRTI contains a signal artifact or defect that distorts the ablative region. No study has attempted to characterize this artifact.

OBJECTIVE

To characterize MRTI signal the artifact in select cases to better understand its potential relevance and impact on the ablation procedure.

METHODS

All ablations were performed using the Visualase magnetic resonance imaging-guided laser ablation system (Medtronic). Patients were included if the MRTI contained signal artifact that distorted the ablative region during the first thermal dose delivered. Ablation artifact was quantified using MATLAB version R2018a (Mathworks Inc, Natick, Massachusetts).

RESULTS

A total of 116 patients undergoing MRgLITT for various surgical indications were examined. MRTI artifact was observed in 37.0% of cases overall. Incidence of artifact was greater at higher powers (P < .001) and with longer ablation times (P = .024), though artifact size did not correlate with laser power or ablation duration.

CONCLUSION

MRTI signal artifact is common during LITT. Higher powers and longer ablation times result in greater incidence of ablation artifact, though artifact size is not correlated with power or duration. Future studies should aim to evaluate effects of artifact on postoperative imaging and, most notably, patient outcomes.

The Yin and Yang of PDT and PTT

In Chinese philosophy, yin and yang ("dark-bright," "negative-positive") describe how seemingly opposite or contrary forces may actually be complementary, interconnected and interdependent. This paper provides this perspective on photodynamic and photothermal therapies, with a focus on the treatment of solid tumors. The relative strengths and weaknesses of each modality, both current and emerging, are considered with respect to the underlying biophysics, the required technologies, the biological effects, their translation into clinical practice and the realized or potential clinical outcomes. For each specific clinical application, one or the other modality may be clearly preferred, or both are effectively equivalent in terms of the various scientific/technological/practical/clinical trade-offs involved. Alternatively, a combination may the best approach. Such combined approaches may be facilitated by the use of multifunctional nanoparticles. It is important to understand the many factors that go into the selection of the optimal approach and the objective of this paper is to provide guidance on this.

A thermometry software tool for monitoring laser-induced interstitial thermotherapy

The purpose of this study was to develop a thermometry software tool for temperature monitoring during laser-induced interstitial thermotherapy (LITT). C++ programming language and several libraries including DICOM Toolkit, Grassroots DICOM library, Insight Segmentation and Registration Toolkit, Visualization Toolkit and Quasar Toolkit were used. The software's graphical user interface creates windows displaying the temperature map and the coagulation extent in the tissue, determined by the magnetic resonance imaging (MRI) thermometry with the echo planar imaging sequence and a numerical simulation based on the radiation and heat transfer in biological tissues, respectively. The software was evaluated applying the MRI-guided LITT to ex vivo pig liver and simultaneously measuring the temperature through a fiber-optic thermometer as reference. Using the software, the temperature distribution determined by the MRI method was compared with the coagulation extent simulation. An agreement was shown between the MRI temperature map and the simulated coagulation extent. Furthermore, the MRI-based and simulated temperatures agreed with the measured one - a correlation coefficient of 0.9993 and 0.9996 was obtained, respectively. The precision of the MRI temperature amounted to 2.4°C. In conclusion, the software tool developed in the present study can be applied for monitoring and controlling the LITT procedure in ex vivo tissues.

Does the Thermal Damage Estimate Correlate With the Magnetic Resonance Imaging Predicted Ablation Size After Laser Interstitial Thermal Therapy?

BACKGROUND

Magnetic resonance guided laser induced thermal therapy (LITT) is a minimally invasive method to treat a wide range of intracranial pathologies. The Arrhenius model is used to generate a thermal damage estimate (TDE) predicting ablation extent.

OBJECTIVE

Evaluation and correlation of the TDE to magnetic resonance imaging (MRI)-estimated ablation extent in human cases.

METHODS

The Medtronic Visualase system (Medtronic Inc, Dublin, Ireland) was utilized. Postablation axial T1-contrast enhanced images were acquired and intraoperative TDE image was obtained from the Visualase console. OsiriX DICOM Viewer (Pixmeo Inc, Bernex, Switzerland) was utilized to calculate cross-sectional area on MRI. ImageJ (National Institutes of Health, Bethesda, Maryland) was utilized for TDE area. Two blinded raters performed all measures. Statistical testing included Pearson correlation and the Student's t-test.

RESULTS

Twenty-two cases including tumor and epilepsy were evaluated. Average MRI predicted tumor ablation area was 4.72 ± 2.22 cm2 and average predicted epilepsy ablation area was 4.12 ± 1.89 cm2. Average tumor TDE was 4.02 ± 1.95 cm2 and average epilepsy TDE was 4.36 ± 2.21 cm2. Rater 1’s ablation areas and TDEs correlated with r = 0.89 (P &lt; .0001) and no significant difference (P &gt; .5). Rater 2’s ablation areas and TDEs correlated with r = 0.91 (P &lt; .0001) and no significant difference (P &gt; .7). Rater 1 vs Rater 2 showed a strong correlation for TDE (r = 0.98, P &lt; .000001) and ablation area (r = 0.96, P &lt; .0001) and no significant difference (P &gt; .5).

CONCLUSION

The TDE is an accurate and reliable measure of ablated area in LITT in human cases as assessed on postoperative MRI. Future studies should be larger and assess reliability of the TDE when multiple lasers and planes are used.

Validation of a mathematical model for laser-induced thermotherapy in liver tissue

The purpose of the study was to develop a simulation approach for laser-induced thermotherapy (LITT) that is based on mathematical models for radiation transport, heat transport, and tissue damage. The LITT ablation was applied to ex vivo pig liver tissue. Experiments were repeated with different laser powers, i.e., 22-34 W, and flow rates of the cooling water in the applicator system, i.e., 47-92 ml/min. During the procedure, the temperature was measured in the liver sample at different distances to the applicator as well as in the cooling circuit using a fiber optic thermometer. For validation, the simulation results were compared with the results of the laser ablation experiments in the ex vivo pig liver samples. The simulated and measured temperature curves presented a relatively good agreement. The Bland-Altman plot showed an average of temperature differences of -0.13 ^∘C and 95%-limits-of-agreement of ±7.11 ^∘C. The standard deviation amounted to ±3.63 ^∘C. The accuracy of the developed simulation is comparable with the accuracy of the MR thermometry reported in other clinical studies. The simulation showed a significant potential for the application in treatment planning.

Techniques for temperature monitoring during laser-induced thermotherapy: an overview

Laser-induced thermotherapy (LITT) is a hyperthermic procedure recently employed to treat cancer in several organs. The amount of coagulated tissue depends on the temperature distribution around the applicator, which plays a crucial role for an optimal outcome: the removal of the whole neoplastic tissue, whilst preventing damage to the surrounding healthy tissue. Although feedback concerning tissue temperature could be useful to drive the physician in the adjustment of laser settings and treatment duration, LITT is usually performed without real-time monitoring of tissue temperature. During recent decades, many thermometric techniques have been developed to be used during thermal therapies. This paper provides an overview of techniques and sensors employed for temperature measurement during tissue hyperthermia, focusing on LITT, and an investigation of their performances in this application. The paper focuses on the most promising and widespread temperature monitoring techniques, splitting them into two groups: the former includes invasive techniques based on the use of thermocouples and fibre-optic sensors; the second analyses non-invasive methods, i.e. magnetic resonance imaging-, computerised tomography- and ultrasound-based thermometry. Background information on measuring principle, medical applications, advantages and weaknesses of each method are provided and discussed.

Stereotactic laser induced thermotherapy (LITT): a novel treatment for brain lesions regrowing after radiosurgery

Since the inception of radiosurgery, the management of brain metastases has become a common problem for neurosurgeons. Although the use of stereotactic radiosurgery and/or whole brain radiation therapy serves to control the majority of disease burden, patients who survive longer than 6-8 months sometimes face the problem of symptomatic radiographically regrowing lesions with few treatment options. Here we investigate the feasibility of use of MRI-guided stereotactic laser induced thermotherapy (LITT) as a novel treatment option for these lesions. Six patients who had previously undergone gamma knife stereotactic radiosurgery for brain metastases were selected. All patients had an initial favorable response to radiosurgery but subsequently developed regrowth of at least one lesion associated with recurrent edema and progressive neurological symptoms requiring ongoing steroids for symptom control. All lesions were evaluated for craniotomy, but were deemed unresectable due to deep location or patient's comorbidities. Stereotactic biopsies were performed prior to the thermotherapy procedure in all cases. LITT was performed using the Visualase system and follow-up MRI imaging was used to determine treatment response. In all six patients biopsy results were negative for tumor and consistent with adverse radiation effects also known as radiation necrosis. Patients tolerated the procedure well and were discharged from the hospital within 48 h of the procedure. In 4/6 cases there was durable improvement of neurological symptoms until death. In all cases steroids were weaned off within 2 months. One patient died from systemic causes related to his cancer a month after the procedure. One patient had regrowth of the lesion 3 months after the procedure and required re-initiation of steroids and standard craniotomy for surgical resection. There were no complications directly related to the thermocoagulation procedure. Stereotactic laser induced thermotherapy is a feasible alternative for the treatment of symptomatic regrowing metastatic lesions after radiosurgery. The procedure carries minimal morbidity and, in this small series, shows some effectiveness in the symptomatic relief of edema and neurological symptoms paralleled by radiographic lesional control. Further studies are necessary to elucidate the safety of this technology.

MR-based thermometry of laser induced thermotherapy: temperature accuracy and temporal resolution in vitro at 0.2 and 1.5 T magnetic field strengths

PURPOSE

To evaluate MR-thermometry using fast MR sequences for laser induced interstitial thermotherapy (LITT) at 0.2 and 1.5 T systems.

METHODS & MATERIALS

In-vitro experiments were performed using Agarose gel mixture and lobes of porcine liver. MR-thermometry was performed by means of longitudinal relaxation time (T1) and proton resonance frequency shift (PRF) methods under acquisition of amplitude and phase shift images. Four different sequences were used for T1 thermometry: A gradient-echo (GRE), a True Fast Imaging with Steady Precession (TRUFI), a Saturation Recovery Turbo-FLASH (SRTF), and an Inversion Recovery Turbo-FLASH (IRTF) sequence (FLASH-Fast Low Angle Shot). PRF was measured with four sequences: Two fast-spoiled GRE sequences (one as WIP sequence), a Turbo-FLASH (TFL) sequence (WIP sequence), and a multiecho-TrueFISP sequence. Temperature was controlled and verified using a fiber-optic Luxtron device. The temperature was correlated with the MR measurement.

RESULTS

All sequences showed a good linear correlation R(2) = 0.97-0.99 between the measured temperature and the MR-thermometry measurements. The only exception was the TRUFI sequence in the Agarose phantom that showed a non-linear calibration curve R(2) = 0.39-0.67. At 1.5 T, the Agarose experiments revealed similar temperature accuracies of 4-6°C for all sequences excluding TRUFI. During experiments with the liver, the PRF sequences showed better performance than the T1, with accuracies of 5-12°C, contrary to the T1 sequences at 14-18°C. The accuracy of the Siemens PRF-FLASH sequence was 5.1°C. At 0.2 T, the Agarose experiments provided the highest accuracy of 3.3°C for PRF measurement. At the liver experiments the T1 sequences SRTF and FLASH revealed the best accuracies at 6.4 and 7.0°C.

CONCLUSION

The accuracy and speed of MR temperature measurements are sufficient for controlling the temperature-based tumor destruction. For 0.2 T systems SRTF and FLASH sequences are recommended. For 1.5 T systems SRTF and FLASH are the most accurate.

Injury depth control from combined wavelength and power tuning in scanned beam laser thermal therapy

Laser thermal therapy represents a possible method to treat premalignant epithelial lesions of the esophagus. Dynamically conforming the thermal injury profile to a specific lesion boundary is expected to improve the efficacy of such a treatment and avoid complications. In this work, we investigated wavelength tuning as a mechanism to achieve this aimed control over injury depth by using the strong variation of water absorption close to 1900 nm. We developed a numerical model simulating in steps the photon propagation in the tissue, the diffusion of the absorbed heat, and the resulting tissue damage. The model was compared with experimental results on porcine esophageal specimens ex vivo and showed good agreement. Combined with power tuning, the wavelength agility in the range of 1860 to 1895 nm extends the injury range compared to a fixed wavelength source beyond 1 mm, while at the same time improving control over shallow depths and avoiding vaporization at the tissue surface. The combination of two or three discrete wavelengths combined at variable ratios provides similar control, and may provide an improved strategy for the treatment of endothelial lesions.

In vivo validation of a therapy planning system for laser-induced thermotherapy (LITT) of liver malignancies

PURPOSE

In situ ablation is increasingly being used for the treatment of liver malignancies. The application of these techniques is limited by the lack of a precise prediction of the destruction volume. This holds especially true in anatomically difficult situations, such as metastases in the vicinity of larger liver vessels. We developed a three-dimensional (3D) planning system for laser-induced thermotherapy (LITT) of liver tumors. The aim of the study was to validate the system for calculation of the destruction volume.

METHODS

LITT (28 W, 20 min) was performed in close contact to major hepatic vessels in six pigs. After explantation of the liver, the coagulation area was documented. The liver and its vascular structures were segmented from a pre-interventional CT scan. Therapy planning was carried out including the cooling effect of adjacent liver vessels. The lesions in vivo and the simulated lesions were compared with a morphometric analysis.

RESULTS

The volume of lesions in vivo was 6,568.3 ± 3,245.9 mm(3), which was not different to the simulation result of 6,935.2 ± 2,538.5 mm(3) (P = 0.937). The morphometric analysis showed a sensitivity of the system of 0.896 ± 0.093 (correct prediction of destructed tissue). The specificity was 0.858 ± 0.090 (correct prediction of vital tissue).

CONCLUSIONS

A 3D computer planning system for the prediction of thermal lesions in LITT was developed. The calculation of the directional cooling effect of intrahepatic vessels is possible for the first time. The morphometric analysis showed a good correlation under clinical conditions. The pre-therapeutic calculation of the ablation zone might be a valuable tool for procedure planning.

Current devices for high-performance whole-body hyperthermia therapy

For late-stage cancer, whole-body hyperthermia (WBH) is highly regarded by physicians as a promising alternative to conventional therapies. Although WBH is still under scrutiny due to potential toxicity, its benefits are incomparable, as diversified devices and very promising treatment protocols in this area are advanced into Phase II and III clinical trials. Following the introduction of the WBH principle, this paper comprehensively reviews the state-of-art high-performance WBH devices based on the heat induction mechanisms - radiation, convection and conduction. Through analyzing each category's physical principle and heat-induction property, the advantages and disadvantages of the devices are evaluated. Technical strategies and critical scientific issues are summarized. For future developments, research directions worth pursuing are presented in this article.

Conductive interstitial thermal therapy device for surgical margin ablation:In vivoverification of a theoretical model

PURPOSE

To demonstrate the efficacy and predictability of a new conductive interstitial thermal therapy (CITT) device to ablate surgical margins.

METHOD

The temperature distributions during thermal ablation of CITT were calculated with finite element modelling in a geometrical representation of perfused tissue. The depth of ablation was derived using the Arrhenius and the Sapareto and Dewey (S&D) models for the temperature range of 90 to 150 degrees C. The female pig animal model was used to test the validity of the mathematical model. Breast tissues were ablated to temperatures in the range of 79-170 degrees C, in vivo. Triphenyltetrazolium chloride viability stain was used to delineate viable tissue from ablated regions and the ablation depths were measured using digital imaging.

RESULTS

The calculations suggest that the CITT can be used to ablate perfused tissues to a 10-15 mm width within 20 minutes. The measured and calculated depths of ablation were statistically equivalent (99% confidence intervals) within +/- 1mm at 170 degrees C. At lower temperatures the equivalence between the model and the observations was within +/- 2 mm.

CONCLUSION

The CITT device can reliably and uniformly ablate a 10-15 mm wide region of soft tissue. Thus, it can be used to secure negative margins following the resection of a primary tumor, which could impede local recurrences in the treatment of local diseases such as early staged, non-metastatic, breast cancer.

Analysis of the spatial and temporal accuracy of heating in the prostate gland using transurethral ultrasound therapy and active MR temperature feedback

A new MRI-guided therapy is being developed as a minimally invasive treatment for localized prostate cancer utilizing high-intensity ultrasound energy to generate a precise region of thermal coagulation within the prostate gland. The purpose of this study was to evaluate in vivo the capability to produce a spatial heating pattern in the prostate that accurately matched the shape of a target region using transurethral ultrasound heating and active MR temperature feedback. Experiments were performed in a canine model (n = 9) in a 1.5 T MR imager using a prototype device comprising a single planar transducer operated under rotational control. The spatial temperature distribution, measured every 5 s with MR thermometry, was used to adjust the acoustic power and rotation rate in order to achieve a temperature of 55 degrees C along the outer boundary of the target region. The results demonstrated the capability to produce accurate spatial heating patterns within the prostate gland. An average temperature of 56.2 +/- 0.6 degrees C was measured along the outer boundary of the target region across all experiments in this study. The average spatial error between the target boundary and the 55 degrees C isotherm was 0.8 +/- 0.7 mm (-0.2 to 3.2 mm), and the overall treatment time was < or =20 min for all experiments. Excellent spatial agreement was observed between the temperature information acquired with MRI and the pattern of thermal damage measured on H&E-stained tissue sections. This study demonstrates the benefit of adaptive energy delivery using active MR temperature feedback, and an excellent capability to treat precise regions within the prostate gland with this technology.

Determination of the optical properties of turbid media using relative interstitial radiance measurements: Monte Carlo study, experimental validation, and sensitivity analysis

Interstitial quantification of the optical properties of tissue is important in biomedicine for both treatment planning of minimally invasive laser therapies and optical spectroscopic characterization of tissues, for example, prostate cancer. In a previous study, we analyzed a method first demonstrated by Dickey et al., [Phys. Med. Biol. 46, 2359 (2001)] to utilize relative interstitial steady-state radiance measurements for recovering the optical properties of turbid media. The uniqueness of point radiance measurements were demonstrated in a forward sense, and strategies were suggested for improving performance under noisy experimental conditions. In this work, we test our previous conclusions by fitting the P3 approximation for radiance to Monte Carlo predictions and experimental data in tissue-simulating phantoms. Fits are performed at: 1. a single sensor position (0.5 or 1 cm), 2. two sensor positions (0.5 and 1 cm), and 3. a single sensor position (0.5 or 1 cm) with input knowledge of the sample's effective attenuation coefficient. The results demonstrate that single sensor radiance measurements can be used to retrieve optical properties to within approximately 20%, provided the transport albedo is greater than approximately 0.9. Furthermore, compared to the single sensor fits, employing radiance data at two sensor positions did not significantly improve the accuracy of recovered optical properties. However, with knowledge of the effective attenuation coefficient of the medium, optical properties can be retrieved experimentally to within approximately 10% for an albedo greater or equal to 0.5.

Real time monitoring of radiofrequency ablation based on MR thermometry and thermal dose in the pig liver in vivo

To evaluate the feasibility and accuracy of MR thermometry based on the thermal dose (TD) concept for monitoring radiofrequency (RF) ablations, 13 RF ablations in pig livers were performed under continuous MR thermometry at 1.5 T with a filtered clinical RF device. Respiratory gated fast gradient echo images were acquired simultaneously to RF deposition for providing MR temperature maps with the proton resonant frequency technique. Residual motion, signal to noise ratio (SNR) and standard deviation (SD) of MR temperature images were quantitatively analyzed to detect and reject artifacted images in the time series. SD of temperature measurement remained under 2 degrees C. Macroscopic analysis of liver ablations showed a white zone (Wz) surrounded by a red zone (Rz). A detailed histological analysis confirmed the ongoing nature of the coagulation necrosis in both Wz and Rz. Average differences (+/-SD) between macroscopic size measurements of Wz and Rz and TD predictions of ablation zones were 4.1 (+/-1.93) mm and -0.71 (+/-2.47) mm, respectively. Correlation values between TD and Wz and TD and Rz were 0.97 and 0.99, respectively. MR thermometry monitoring based on TD is an accurate method to delineate the size of the ablation zone during the RF procedure and provides a clinical endpoint.

MRI-guided Thermal Ablation Therapy: Model and Parameter Estimates to Predict Cell Death from MR Thermometry Images

Solid tumors and other pathologies can be treated using laser thermal ablation under interventional magnetic resonance imaging (iMRI) guidance. A model was developed to predict cell death from magnetic resonance (MR) thermometry measurements based on the temperature-time history, and validated using in vivo rabbit brain data. To align post-ablation T2-weighted spin-echo MR lesion images to gradient-echo MR images, from which temperature is derived, a registration method was used that aligned fiducials placed near the thermal lesion. The outer boundary of the hyperintense rim in the post-ablation MR lesion image was used as the boundary for cell death, as verified from histology. Model parameters were simultaneously estimated using an iterative optimization algorithm applied to every interesting voxel in 328 images from multiple experiments having various temperature histories. For a necrotic region of 766 voxels across all lesions, the model provided a voxel specificity and sensitivity of 98.1 and 78.5%, respectively. Mislabeled voxels were typically within one voxel from the segmented necrotic boundary with median distances of 0.77 and 0.22 mm for false positives (FP) and false negatives (FN), respectively. As compared to the critical temperature cell death model and the generalized Arrhenius model, our model typically predicted fewer FP and FN. This is good evidence that iMRI temperature maps can be used with our model to predict therapeutic regions in real-time during treatment.

Perturbative diffusion theory formalism for interpreting temporal light intensity changes during laser interstitial thermal therapy

In an effort to understand dynamic optical changes during laser interstitial thermal therapy (LITT), we utilize the perturbative solution of the diffusion equation in heterogeneous media to formulate scattering weight functions for cylindrical line sources. The analysis explicitly shows how changes in detected interstitial light intensity are associated with the extent and location of the volume of thermal coagulation during treatment. Explanations for previously reported increases in optical intensity observed early during laser heating are clarified using the model and demonstrated with experimental measurements in ex vivo bovine liver tissue. This work provides an improved understanding of interstitial optical signal changes during LITT and indicates the sensitivity and potential of interstitial optical monitoring of thermal damage.

64-element intraluminal ultrasound cylindrical phased array for transesophageal thermal ablation under fast MR temperature mapping: an ex vivo study

This work was undertaken to investigate the feasibility of using a cylindrical phased array for transoesophaeal thermal ablation under magnetic resonance (MR) imaging guidance. Sixty-four transducers (0.45 mm wide by 15 mm tall), operating at 4.6 MHz, were spread around the periphery of a 10.6-mm-diam cylinder. The head of the applicator was covered with a 65-microm thick latex balloon attached using watertight seals. This envelope was inflated with degassed water to provide acoustic coupling between the transducer and the tissues. The underlying operating principle of this applicator is to rotate a plane ultrasound beam electronically. For this purpose, eight adjacent transducers were excited with appropriate delay times so as to generate a plane wave. The exposure direction was changed by exciting a different set of eight elements. Ex vivo experiments conducted on 47 samples of pig liver under MR temperature monitoring demonstrated the ability of this applicator to generate cylindrical or sector-based coagulation necroses at depths up to 19 mm with excellent angular precision by applying 20 W/cm2. MR thermometry was performed in "real-time" with segmented echo-planar imaging gradient echo sequences. The temporal resolution was approximately 3 s/ image. The average value for the temperature baseline in liver tissue close to the applicator was 0.3 degrees C (+/- 0.6 degrees C). The thermal dose delivered in tissues was computed on-line during temperature imaging. Excellent MR compatibility was demonstrated, all MR acquisitions were performed without susceptibility artifacts or radio-frequency interferences with the ultrasound device. Thermal lesions identified on post-treatment follow up showed good correlation with online MR thermometry data. The individual differences between measurements performed visually and using MRI thermal dose maps were about 11% of volume. This study demonstrated the feasibility of thermal ablation using a phased array intraluminal ultrasound applicator and on-line MR monitoring.

Intraluminal high intensity ultrasound treatment in the esophagus under fast MR temperature mapping: in vivo studies

New curative and palliative treatments are needed to respond to the poor prognosis of esophageal cancer. The purpose of this study was to determine whether magnetic resonance imaging (MRI) and MR thermometry can be used to monitor the thermal ablation induced by an intraluminal high-intensity ultrasound applicator positioned in the esophagus. Experiments were performed in vivo in 2 pig esophagi (25 thermal lesions per pig). Respiratory gated or cardiac gated MR thermometry was performed with segmented echo-planar imaging gradient echo sequences. All MR acquisitions were performed without susceptibility artifacts or radiofrequency interference with the ultrasound device. The experimental procedure proposed for accurate measurement of temperature in the esophagus was found to achieve an SD of +/- 1.5 degrees C for respiratory gating and +/- 3.1 degrees C for cardiac gating. Gd-enhanced T(1)-weighted images were used to depict coagulation necrosis. Autopsy was performed immediately after the treatment. Ultrasound effects were inspected visually, and the dimensions of the lesions in the liver neighboring the esophagus were compared with those determined on the MRI images. The visually assessed thermal lesions showed good correlation with the MRI data (10% mean volume difference). The feasibility of esophageal thermal ablation using intraluminal high-intensity ultrasound and of on-line MR temperature monitoring was demonstrated.

Analysis of factors important for transurethral ultrasound prostate heating using MR temperature feedback

The feasibility of using MR thermometry for temperature feedback to control a transurethral ultrasound heating applicator with planar transducers was investigated. The sensitivity of a temperature-based feedback algorithm to spatial (control point area, slice thickness, angular alignment) and non-spatial (imaging time, temperature uncertainty) parameters was evaluated through numerical simulations. The angular alignment of the control point with the ultrasound beam was an important parameter affecting the average spatial error in heat delivery. The other spatial parameters were less influential, thus providing an opportunity to reduce spatial resolution for increased SNR in the MR imaging. The update time was the most important non-spatial parameter determining the performance of the control algorithm. Combined non-spatial and spatial parameters achieved acceptable performance with a voxel size of 3 mm x 3 mm, a 10 mm slice thickness and a 5 s update time. Temperature uncertainty of up to 2 degrees C had little effect on the performance of the control algorithm but did reduce the average error slightly due to a systematic, noise-induced overestimation of the boundary temperature. These simulations imply that MR thermometry performed on clinical 1.5 T imaging systems is of sufficient quality for use as thermal feedback for conformal prostate thermal therapy with transurethral ultrasound heating applicators incorporating planar transducers.

Method for MRI-guided conformal thermal therapy of prostate with planar transurethral ultrasound heating applicators

A method for conformal prostate thermal therapy using transurethral ultrasound heating applicators incorporating planar transducers is described. The capability to shape heating patterns to the geometry of the prostate gland from a single element in a multi-element heating applicator was evaluated using Bioheat transfer modelling. Eleven prostate geometries were obtained from patients who underwent MR imaging of the prostate gland prior to radical prostatectomy. Results indicate that ultrasound heating applicators incorporating multi-frequency planar transducers (4 x 20 mm, f = 4.7 MHz, 9.7 MHz) are capable of shaping thermal damage patterns to the geometry of individual prostates. A temperature feedback control algorithm has been developed to control the frequency, rotation rate and applied power level from transurethral heating applicators based on measurements of the boundary temperature during heating. The discrepancy between the thermal damage boundary and the target boundary was less than 5 mm, and the transition distance between coagulation and normal tissue was less than 1 cm. Treatment times for large prostate volumes were less than 50 min, and perfusion did not have significant impact on the control algorithm. Rectal cooling will play an important role in reducing undesired heating near the rectal wall. Experimental validation of the simulations in a tissue-mimicking gel phantom demonstrated good agreement between the predicted and generated patterns of thermal damage.

Inter-frame motion correction for MR thermometry

Noninvasive temperature measurement is feasible with MRI to monitor changes in thermal therapy. Phase shift based MR thermometry gives an estimate of the relative temperature variation between thermal and baseline images. This technique is limited, however, when applied on targets under inter-frame motion. Simple image registration and subtraction are not adequate to recover the temperature properly since the phase shift due to temperature changes is corrupted by an unwanted phase shift. In this work, the unwanted phase shift is predicted from the raw registered phase shift map itself. To estimate the unwanted phase shift, a thin plate smoothing spline is fitted to the values outside the heated region. The spline value in the heated area serves as an estimate for the offset. The estimation result is applied to correct errors in the temperature maps of an ex-vivo experiment.

High-dose Gd-DTPA vs. Bis-Gd-mesoporphyrin for monitoring laser-induced tissue necrosis

PURPOSE

To compare Bis-Gd-mesoporphyrin (Bis-Gd-MP), a contrast agent with a reported high affinity to necrotic tissue, with high-dose gadopentate dimeglumin (Gd-DTPA) for defining laser-induced muscle and liver necrosis by contrast-enhanced (CE) MRI.

MATERIALS AND METHODS

Laser-induced interstitial thermotherapy (LITT) was performed in the muscle and liver tissue of New Zealand White rabbits (1500 J and 2100 J; n=80 lesions). The animals were randomly assigned to a group that received 0.3 mmol/kg bw Gd-DTPA or a group that received 0.05 mmol/kg bw Bis-Gd-MP. Following contrast injection, dynamic MRI was performed on muscle lesions with a T1-weighted, two-dimensional, fast low-angle shot (FLASH) sequence. The liver and muscle lesions were then repeatedly imaged for six hours after contrast injection using a T1-weighted spin-echo (SE) sequence. Central and peripheral lesion enhancement was determined and correlated with gross pathology and microscopy findings.

RESULTS

Both contrast agents allowed precise determination of lesion diameters with an average accuracy of 6.8%+/-1.3%. Rim enhancement during dynamic MRI was superior for Gd-DTPA (P<0.001) and revealed slightly higher lesion diameters compared to the results of follow-up MR studies. A persistent enhancement of necrotic liver and muscle tissue was observed for both contrast agents throughout the observation period, suggesting that simple diffusion-type processes may underlie the supposed affinity of Bis-Gd-MP for tissue necrosis.

CONCLUSION

Bis-Gd-MP and Gd-DTPA are equally well suited for postinterventional lesion assessment in LITT.

Transport lattice models of heat transport in skin with spatially heterogeneous, temperature-dependent perfusion

BACKGROUND

Investigation of bioheat transfer problems requires the evaluation of temporal and spatial distributions of temperature. This class of problems has been traditionally addressed using the Pennes bioheat equation. Transport of heat by conduction, and by temperature-dependent, spatially heterogeneous blood perfusion is modeled here using a transport lattice approach.

METHODS

We represent heat transport processes by using a lattice that represents the Pennes bioheat equation in perfused tissues, and diffusion in nonperfused regions. The three layer skin model has a nonperfused viable epidermis, and deeper regions of dermis and subcutaneous tissue with perfusion that is constant or temperature-dependent. Two cases are considered: (1) surface contact heating and (2) spatially distributed heating. The model is relevant to the prediction of the transient and steady state temperature rise for different methods of power deposition within the skin. Accumulated thermal damage is estimated by using an Arrhenius type rate equation at locations where viable tissue temperature exceeds 42 degrees C. Prediction of spatial temperature distributions is also illustrated with a two-dimensional model of skin created from a histological image.

RESULTS

The transport lattice approach was validated by comparison with an analytical solution for a slab with homogeneous thermal properties and spatially distributed uniform sink held at constant temperatures at the ends. For typical transcutaneous blood gas sensing conditions the estimated damage is small, even with prolonged skin contact to a 45 degrees C surface. Spatial heterogeneity in skin thermal properties leads to a non-uniform temperature distribution during a 10 GHz electromagnetic field exposure. A realistic two-dimensional model of the skin shows that tissue heterogeneity does not lead to a significant local temperature increase when heated by a hot wire tip.

CONCLUSIONS

The heat transport system model of the skin was solved by exploiting the mathematical analogy between local thermal models and local electrical (charge transport) models, thereby allowing robust, circuit simulation software to obtain solutions to Kirchhoff's laws for the system model. Transport lattices allow systematic introduction of realistic geometry and spatially heterogeneous heat transport mechanisms. Local representations for both simple, passive functions and more complex local models can be easily and intuitively included into the system model of a tissue.

Interventional and intraoperative MR: review and update of techniques and clinical experience

The concept of interventional magnetic resonance imaging (MRI) is based on the integration of diagnostic and therapeutic procedures, favored by the combination of the excellent morphological and functional imaging characteristics of MRI. The spectrum of MRI-assisted interventions ranges from biopsies and intraoperative guidance to thermal ablation modalities and vascular interventions. The most relevant recently published experimental and clinical results are discussed. In the future, interventional MRI is expected to play an important role in interventional radiology, minimal invasive therapy and guidance of surgical procedures. However, the associated high costs require a careful evaluation of its potentials in order to ensure cost-effective medical care.

Intraluminal ultrasound applicator compatible with magnetic resonance imaging "real-time" temperature mapping for the treatment of oesophageal tumours: an ex vivo study

High intensity ultrasound has shown considerable ability to produce precise and deep thermal coagulation necrosis. Focused, cylindrical, spherical or plane transducers have been used to induce high temperatures in tissues to coagulate proteins and kill cells. Recently magnetic resonance imaging (MRI) has been used, with extracorporeal or intracavitary focused transducers and cylindrical interstitial applicators, to monitor temperature distribution and provide feedback during heating procedures. If intraluminal applicators are used, the active part is in contact with the region of interest and it is essential to provide an accurate view of heat deposition and the extent of coagulation necrosis close to the transducer. The purpose of this study was to develop a 10 mm diameter intraluminal ultrasound applicator, designed to treat oesophageal cancers and compatible with MRI "real-time" temperature mapping. The active part of the ultrasound applicator, covered by a latex balloon, is a 15 X 8 mm2 plane transducer, which is in contact with the tumours during treatment. Each ultrasound exposure generates coagulation necrosis, in an area with the approximate shape of a rectangular parallelepiped up to 10 mm deep. When the exposures were repeated by rotating the applicator on its axis, sector-based or cylindrical volumes of necrosis could be produced, matching the shape of oesophageal cancers. Ex vivo trials were performed to demonstrate the applicator's compatibility with a clinical MRI scanner (1.5 T). MRI signals were acquired without any magnetic susceptibility distortion, even close to the applicator. Fast (0.72 images per second) 2D temperature mapping was performed during ultrasound exposure, using temperature-related proton resonance frequency shift at a resolution of 0.5 degrees C. Coagulation necrosis viewed with inversion recovery sequences, were in good agreement with the qualitative macroscopic observations made for the few cases tested in this study.

Multi-modality tissue-mimicking phantom for thermal therapy

A tissue-mimicking phantom material has been developed for use with thermal therapy devices and techniques. This material has magnetic resonance properties (primarily T2) which change drastically upon thermal coagulation, enabling its use for device characterization and treatment verification using simple T2-weighted imaging techniques. The coagulation temperature of the phantom can be changed from 50-60 degrees C by adjusting the pH from 4.3 to 4.7. The energy absorption properties can be adjusted to match the acoustical and optical properties of tissues. T2 relaxation measurements are provided as a function of temperature, along with T2-weighted MR images to illustrate the visualization of heating patterns. A complete recipe for fabricating phantoms is provided.