Multiplanar MR temperature-sensitive imaging of cerebral thermal treatment using interstitial ultrasound applicators in a canine model

Deployable ultrasound applicators for endoluminal delivery of volumetric hyperthermia

PURPOSE

To investigate the design of an endoluminal deployable ultrasound applicator for delivering volumetric hyperthermia to deep tissue sites as a possible adjunct to radiation and chemotherapy.

METHOD

This study considers an ultrasound applicator consisting of two tubular transducers situated at the end of a catheter assembly, encased within a distensible conical shaped balloon-based reflector that redirects acoustic energy distally into the tissue. The applicator assembly can be inserted endoluminally or laparoscopically in a compact form and expanded after delivery to the target site. Comprehensive acoustic and biothermal simulations and parametric studies were employed in generalized 3D and patient-specific pancreatic head and body tumor models to characterize the acoustic performance and evaluate heating capabilities of the applicator by investigating the device at a range of operating frequencies, tissue acoustic and thermal properties, transducer configurations, power modulation, applicator positioning, and by analyzing the resultant 40, 41, and 43 °C isothermal volumes and penetration depth of the heating volume. Intensity distributions and volumetric temperature contours were calculated to define moderate hyperthermia boundaries.

RESULTS

Parametric studies demonstrated the frequency selection to control volume and depth of therapeutic heating from 62 to 22 cm³ and 4 to 2.6 cm as frequency ranges from 1 MHz to 4.7 MHz, respectively. Width of the heating profile tracks closely with the aperture. Water cooling within the reflector balloon was effective in controlling temperature to 37 °C maximum within the luminal wall. Patient-specific studies indicated that applicators with extended OD in the range of 3.6-6.2 cm with 0.5-1 cm long and 1 cm OD transducers can heat volumes of 1.1-7 cm³, 3-26 cm³, and 3.3-37.4 cm³ of pancreatic body and head tumors above 43, 41, and 40 °C, respectively.

CONCLUSION

In silico studies demonstrated the feasibility of combining endoluminal ultrasound with an integrated expandable balloon reflector for delivering volumetric hyperthermia in regions adjacent to body lumens and cavities.

Modeling of Interstitial Ultrasound Ablation for Continuous Applicator Rotation With MR Validation

The primary objective of cancer intervention is the selective removal of malignant cells while conserving surrounding healthy tissues. However, the accessibility, size and shape of the cancer can make achieving appropriate margins a challenge. One minimally invasive treatment option for these clinical cases is interstitial needle based therapeutic ultrasound (NBTU). In this work, we develop a finite element model (FEM) capable of simulating continuous rotation of a directional NBTU applicator. The developed model was used to simulate the thermal deposition for different rotation trajectories. The actual thermal deposition patterns for the simulated trajectories were then evaluated using magnetic resonance thermal imaging (MRTI) in a porcine skin gelatin phantom. An MRI-compatible robot was used to control the rotation motion profile of the physical NBTU applicator to match the simulated trajectory. The model showed agreement when compared to experimental measurements with Pearson correlation coefficients greater than 0.839 when comparing temperature fields within an area of 12.6 mm radius from the ultrasound applicator. The average temperature error along a 6.3 mm radius profile from the applicator was 1.27 °C. The model was able to compute 1 s of thermal deposition by the applicator in 0.2 s on average with a 0.1 mm spatial resolution and 0.5 s time steps. The developed simulation demonstrates performance suitable for real-time control which may enable robotically-actuated closed-loop conformal tumor ablation.

A minimally invasive catheter-based ultrasound technology for therapeutic interventions in brain: initial preclinical studies

OBJECTIVE Minimally invasive procedures may allow surgeons to avoid conventional open surgical procedures for certain neurological disorders. This paper describes the iterative process for development of a catheter-based ultrasound thermal therapy applicator. METHODS Using an ultrasound applicator with an array of longitudinally stacked and angularly sectored tubular transducers within a catheter, the authors conducted experimental studies in porcine liver, in vivo and ex vivo, in order to characterize the device performance and lesion patterns. In addition, they applied the technique in a rodent model of Parkinson's disease to investigate the feasibility of its application in brain. RESULTS Thermal lesions with multiple shapes and sizes were readily achieved in porcine liver. The feasibility of catheter-based focused ultrasound in the treatment of brain conditions was demonstrated in a rodent model of Parkinson's disease. CONCLUSIONS The authors show proof of principle of a catheter-based ultrasound system that can create lesions with concurrent thermode-based measurements.

Robotic Assisted MRI-Guided Interventional Interstitial MR-Guided Focused Ultrasound Ablation in a Swine Model

BACKGROUND

Ablative lesions are current treatments for epilepsy and brain tumors. Interstitial magnetic resonance (MR) guided focused ultrasound (iMRgFUS) may be an alternate ablation technique which limits thermal tissue charring as compared to laser therapy (LITT) and can produce larger ablation patterns nearer the surface than transcranial MR guided focused ultrasound (tcMRgFUS).

OBJECTIVE

To describe our experience with interstitial focused ultrasound (iFUS) ablations in swine, using MR-guided robotically assisted (MRgRA) delivery.

METHODS

In an initial 3 animals, we optimized the workflow of the robot in the MR suite and made modifications to the robotic arm to allow range of motion. Then, 6 farm pigs (4 acute, 2 survival) underwent 7 iMRgFUS ablations using MRgRA. We altered dosing to explore differences between thermal dosing in brain as compared to other tissues. Imaging was compared to gross examination.

RESULTS

Our work culminated in adjustments to the MRgRA, iMRgFUS probes, and dosing, culminating in 2 survival surgeries; swine had ablations with no neurological sequelae at 2 wk postprocedure. Immediately following iMRgFUS therapy, diffusion-weighted imaging, and T1 weighted MR were accurate reflections of the ablation volume. T2 and fluid-attenuated inversion-recovery (FLAIR) images were accurate reflections of ablation volume 1-wk postprocedure.

CONCLUSION

We successfully performed MRgRA iFUS ablation in swine and found intraoperative and postoperative imaging to correlate with histological examination. These data are useful to validate our system and to guide imaging follow-up for thermal ablation lesions in brain tissue from our therapy, tcMRgFUS, and LITT.

Capacitive Micromachined Ultrasound Transducers for Interstitial High-Intensity Ultrasound Therapies

Capacitive micromachined ultrasound transducers (CMUTs) exhibit several potential advantages over conventional piezo technologies for use in therapeutic ultrasound (US) devices, including ease of miniaturization and integration with electronics, broad bandwidth (>several megahertz), and compatibility with magnetic resonance imaging (MRI). In this paper, the electroacoustic performance of CMUTs designed for interstitial high-intensity contact US (HICU) applications was evaluated and the feasibility of generating US-induced heating and thermal destruction of biological tissues was studied. One-dimensional CMUT linear arrays as well as a prism-shaped 2-D array composed of multiple 1-D linear arrays mounted on a cylindrical catheter were fabricated. The electromechanical and acoustic characteristics of the CMUTs were first studied at low intensity. Then, the acoustic output during continuous wave (CW) driving was studied while varying the bias voltage ( V_DC ) and driving voltage ( V_AC ). US heating was performed in tissue-mimicking gel phantoms under infrared (IR) or MR-thermometry monitoring. Acoustic intensities compatible with thermal ablation were obtained by driving the CMUTs in the collapse-snapback operation mode ( [Formula: see text]). Hysteresis in the acoustic output was observed with varying V_DC . IR- and MR-thermometry monitoring showed directional US-induced heating patterns in tissue-mimicking phantoms (frequency: 6-8 MHz and exposure time: 60-240 s) extending over 1.5-cm depth from the CMUT surface. Irreversible thermal damage was produced in turkey breast tissue samples ( [Formula: see text]). Multidirectional US-induced heating was also achieved in 3-D with the CMUT catheter. These studies demonstrate that CMUTs can be integrated into HICU devices and be used for heating and destruction of tissue under MR guidance.

Theoretical investigation of transgastric and intraductal approaches for ultrasound-based thermal therapy of the pancreas

Background

The goal of this study was to theoretically investigate the feasibility of intraductal and transgastric approaches to ultrasound-based thermal therapy of pancreatic tumors, and to evaluate possible treatment strategies.

Methods

This study considered ultrasound applicators with 1.2 mm outer diameter tubular transducers, which are inserted into the tissue to be treated by an endoscopic approach, either via insertion through the gastric wall (transgastric) or within the pancreatic duct lumen (intraductal). 8 patient-specific, 3D, transient, biothermal and acoustic finite element models were generated to model hyperthermia (n = 2) and ablation (n = 6), using sectored (210°–270°, n = 4) and 360° (n = 4) transducers for treatment of 3.3–17.0 cm³ tumors in the head (n = 5), body (n = 2), and tail (n = 1) of the pancreas. A parametric study was performed to determine appropriate treatment parameters as a function of tissue attenuation, blood perfusion rates, and distance to sensitive anatomy.

Results

Parametric studies indicated that pancreatic tumors up to 2.5 or 2.7 cm diameter can be ablated within 10 min with the transgastric and intraductal approaches, respectively. Patient-specific simulations demonstrated that 67.1–83.3% of the volumes of four sample 3.3–11.4 cm³ tumors could be ablated within 3–10 min using transgastric or intraductal approaches. 55.3–60.0% of the volume of a large 17.0 cm³ tumor could be ablated using multiple applicator positions within 20–30 min with either transgastric or intraductal approaches. 89.9–94.7% of the volume of two 4.4–11.4 cm³ tumors could be treated with intraductal hyperthermia. Sectored applicators are effective in directing acoustic output away from and preserving sensitive structures. When acoustic energy is directed towards sensitive structures, applicators should be placed at least 13.9–14.8 mm from major vessels like the aorta, 9.4–12.0 mm from other vessels, depending on the vessel size and flow rate, and 14 mm from the duodenum.

Conclusions

This study demonstrated the feasibility of generating shaped or conformal ablative or hyperthermic temperature distributions within pancreatic tumors using transgastric or intraductal ultrasound.

Influence of geometric and material properties on artifacts generated by interventional MRI devices: Relevance to PRF-shift thermometry

PURPOSE

Magnetic resonance imaging (MRI) is capable of providing valuable real-time feedback during medical procedures, partly due to the excellent soft-tissue contrast available. Several technical hurdles still exist to seamless integration of medical devices with MRI due to incompatibility of most conventional devices with this imaging modality. In this study, the effect of local perturbations in the magnetic field caused by the magnetization of medical devices was examined using finite element analysis modeling. As an example, the influence of the geometric and material characteristics of a transurethral high-intensity ultrasound applicator on temperature measurements using proton resonance frequency (PRF)-shift thermometry was investigated.

METHODS

The effect of local perturbations in the magnetic field, caused by the magnetization of medical device components, was examined using finite element analysis modeling. The thermometry artifact generated by a transurethral ultrasound applicator was simulated, and these results were validated against analytic models and scans of an applicator in a phantom. Several parameters were then varied to identify which most strongly impacted the level of simulated thermometry artifact, which varies as the applicator moves over the course of an ablative high-intensity ultrasound treatment.

RESULTS

Key design parameters identified as having a strong influence on the magnitude of thermometry artifact included the susceptibility of materials and their volume. The location of components was also important, particularly when positioned to maximize symmetry of the device. Finally, the location of component edges and the inclination of the device relative to the magnetic field were also found to be important factors.

CONCLUSIONS

Previous design strategies to minimize thermometry artifact were validated, and novel design strategies were identified that substantially reduce PRF-shift thermometry artifacts for a variety of device orientations. These new strategies are being incorporated into the next generation of applicators. The general strategy described in this study can be applied to the design of other interventional devices intended for use with MRI.

Catheter-based ultrasound technology for image-guided thermal therapy: current technology and applications

Catheter-based ultrasound (CBUS) is applied to deliver minimally invasive thermal therapy to solid cancer tumours, benign tissue growth, vascular disease, and tissue remodelling. Compared to other energy modalities used in catheter-based surgical interventions, unique features of ultrasound result in conformable and precise energy delivery with high selectivity, fast treatment times, and larger treatment volumes. We present a concise review of CBUS technology being currently utilized in animal and clinical studies or being developed for future applications. CBUS devices have been categorised into interstitial, endoluminal and endovascular/cardiac applications. Basic applicator designs, site-specific evaluations and possible treatment applications have been discussed in brief. Particular emphasis has been given to ablation studies that incorporate image guidance for applicator placement, therapy monitoring, feedback control, and post-procedure assessment. Examples of devices included here span the entire spectrum of the development cycle from preliminary simulation-based design studies to implementation in clinical investigations. The use of CBUS under image guidance has the potential for significantly improving precision and applicability of thermal therapy delivery.

MR-guided focused ultrasound surgery, present and future

MR-guided focused ultrasound surgery (MRgFUS) is a quickly developing technology with potential applications across a spectrum of indications traditionally within the domain of radiation oncology. Especially for applications where focal treatment is the preferred technique (for example, radiosurgery), MRgFUS has the potential to be a disruptive technology that could shift traditional patterns of care. While currently cleared in the United States for the noninvasive treatment of uterine fibroids and bone metastases, a wide range of clinical trials are currently underway, and the number of publications describing advances in MRgFUS is increasing. However, for MRgFUS to make the transition from a research curiosity to a clinical standard of care, a variety of challenges, technical, financial, clinical, and practical, must be overcome. This installment of the Vision 20∕20 series examines the current status of MRgFUS, focusing on the hurdles the technology faces before it can cross over from a research technique to a standard fixture in the clinic. It then reviews current and near-term technical developments which may overcome these hurdles and allow MRgFUS to break through into clinical practice.

A multi-element interstitial ultrasound applicator for the thermal therapy of brain tumors

Interstitial thermal therapy is a minimally invasive treatment modality that has been used clinically for ablating both primary and secondary brain tumors. Here a multi-element interstitial ultrasound applicator is described that allows for increased spatial control during thermal ablation of tumors as compared to existing clinical devices. The device consists of an array of 56 ultrasound elements operating at 6 MHz, oriented on the seven faces of a 3.2 mm flexible catheter. The device was first characterized using the acoustic holography method to examine the functioning of the array. Then experiments were performed to measure heating in tissue-mimicking gel phantoms and ex vivo tissue samples using magnetic resonance imaging-based thermometry. Experimental measurements were compared with results obtained using numerical simulations. Last, simulations were performed to study the feasibility of using the device for thermal ablation in the brain. Experimental results show that the device can be used to induce a temperature rise of greater than 20 °C in ex vivo tissue samples and numerical simulations further demonstrate that tumors with diameters of greater than 30-mm could potentially be treated.

Modelling of endoluminal and interstitial ultrasound hyperthermia and thermal ablation: applications for device design, feedback control and treatment planning

Endoluminal and catheter-based ultrasound applicators are currently under development and are in clinical use for minimally invasive hyperthermia and thermal ablation of various tissue targets. Computational models play a critical role in device design and optimisation, assessment of therapeutic feasibility and safety, devising treatment monitoring and feedback control strategies, and performing patient-specific treatment planning with this technology. The critical aspects of theoretical modelling, applied specifically to endoluminal and interstitial ultrasound thermotherapy, are reviewed. Principles and practical techniques for modeling acoustic energy deposition, bioheat transfer, thermal tissue damage, and dynamic changes in the physical and physiological state of tissue are reviewed. The integration of these models and applications of simulation techniques in identification of device design parameters, development of real time feedback-control platforms, assessing the quality and safety of treatment delivery strategies, and optimisation of inverse treatment plans are presented.

Visualization of thermal ablation lesions using cumulative dynamic contrast enhancement MRI

A novel robust and user friendly method for post-processing dynamic contrast enhanced (DCE) MRI data is presented, which provides reliable real-time delineation of the borders of thermal ablation lesions on low SNR images shortly after contrast agent injection without any model-based curve fitting. Some simple descriptors of the DCE process are calculated in a time efficient recursive manner and combined into a single image reflecting both current and previous enhancement states of each pixel, which allows robust discrimination between tissue areas with different perfusion properties. The resulting cumulative DCE (CDCE) images are shown to exhibit a strong correlation with histopathology and late gadolinium enhancement representations of the thermal damage in soft tissue. It is shown that the outer border of the non-perfused ablation lesion core on CDCE MRI corresponds to the histopathological lesion border. The described method has a potential not only to facilitate thermal ablation outcome assessment, but also to improve detection of infiltrative tumours and reduce the administered contrast agent dose in any DCE scans.

Multiple applicator hepatic ablation with interstitial ultrasound devices: theoretical and experimental investigation

PURPOSE

To evaluate multiple applicator implant configurations of interstitial ultrasound devices for large volume ablation of liver tumors.

METHODS

A 3D bioacoustic-thermal model using the finite element method was implemented to assess multiple applicator implant configurations for thermal ablation with interstitial ultrasound energy. Interstitial applicators consist of linear arrays of up to four 10 mm-long tubular ultrasound transducers, each under separate and dynamic power control, enclosed within a water-cooled delivery catheter (2.4 mm OD). The authors considered parallel implants with two and three applicators (clustered configuration), spaced 2-3 cm apart, to simulate open surgical placement. In addition, the authors considered two applicator implants with applicators converging and diverging at angles of ∼20°, 30°, and 45° to simulate percutaneous placement. Heating experiments (10-15 min) were performed and compared against simulations employing the same experimental parameters. To estimate the performance of parallel, multiple applicator configurations in an in vivo setting, simulations were performed taking into account a range of blood perfusion levels (0, 5, 12, and 15 kg m(-3) s(-1)) that may occur in tumors of varying vascularity. The impact of tailoring the power supplied to individual transducer elements along the length of applicators is explored for applicators inserted in non-parallel (converging and diverging) configurations. Thermal dose (t(43) > 240 min) and temperature thresholds (T > 52 °C) were used to define the ablation zones, with dynamic changes to tissue acoustic and thermal properties incorporated within the model.

RESULTS

Experiments in ex vivo bovine liver yielded ablation zones ranging between 4.0-5.6 cm × 3.2-4.9 cm, in cross section. Ablation zone dimensions predicted by simulations with similar parameters to the experiments were in close agreement (within 5 mm). Simulations of in vivo heating showed that 15 min heating and interapplicator spacing less than 3 cm are required to obtain contiguous, complete ablation zones. The ability to create complete ablation zone profiles for nonparallel implants was illustrated by tailoring applied power levels along the length of applicators.

CONCLUSIONS

Parallel implants consisting of three interstitial ultrasound applicators in a triangular configuration yield complete ablation zones measuring up to 6.2 cm × 5.7 cm after 15 min heating. At larger interapplicator spacing, the level of blood perfusion in the tumor may yield indentations along the periphery of the ablation zone. Tailoring applied power along the length of the applicator can accommodate for nonparallel implants, without compromising safety.

Accumulation of phase-shift nanoemulsions to enhance MR-guided ultrasound-mediated tumor ablation in vivo

Magnetic resonance-guided high intensity focused ultrasound (MRgHIFU) is being explored as a non-invasive technology to treat solid tumors. However, the clinical use of HIFU for tumor ablation applications is currently limited by the long treatment times required. Phase-shift nanoemulsions (PSNE), consisting of liquid perfluorocarbon droplets that can be vaporized into microbubbles, are being developed to accelerate HIFU-mediated heating. The purpose of this study was to examine accumulation of PSNE in intramuscular rabbit tumors in vivo. MR images were acquired before and after intravenous injection of gadolinium-containing PSNE. MR signal enhancement was observed in rabbit tumors up to six hours after injection, indicating that PSNE accumulated in the tumors. In addition, PSNE vaporization was detected in the tumor with B-mode ultrasound imaging, and MR thermometry measurements indicated that PSNE accelerated the rate of HIFU-mediated heating. These results suggest that PSNE could dramatically improve the efficiency and clinical feasibility of MRgHIFU.

Considerations for theoretical modelling of thermal ablation with catheter-based ultrasonic sources: implications for treatment planning, monitoring and control

PURPOSE

To determine the impact of including dynamic changes in tissue physical properties during heating on feedback controlled thermal ablation with catheter-based ultrasound. Additionally, we compared the impact of several indicators of thermal damage on predicted extents of ablation zones for planning and monitoring ablations with this modality.

METHODS

A 3D model of ultrasound ablation with interstitial and transurethral applicators incorporating temperature-based feedback control was used to simulate thermal ablations in prostate and liver tissue. We investigated five coupled models of heat dependent changes in tissue acoustic attenuation/absorption and blood perfusion of varying degrees of complexity. Dimensions of the ablation zone were computed using temperature, thermal dose, and Arrhenius thermal damage indicators of coagulative necrosis. A comparison of the predictions by each of these models was illustrated on a patient-specific anatomy in the treatment planning setting.

RESULTS

Models including dynamic changes in blood perfusion and acoustic attenuation as a function of thermal dose/damage predicted near-identical ablation zone volumes (maximum variation < 2.5%). Accounting for dynamic acoustic attenuation appeared to play a critical role in estimating ablation zone size, as models using constant values for acoustic attenuation predicted ablation zone volumes up to 50% larger or 47% smaller in liver and prostate tissue, respectively. Thermal dose (t(43) ≥ 240 min) and thermal damage (Ω ≥ 4.6) thresholds for coagulative necrosis are in good agreement for all heating durations, temperature thresholds in the range of 54°C for short (<5 min) duration ablations and 50°C for long (15 min) ablations may serve as surrogates for determination of the outer treatment boundary.

CONCLUSIONS

Accounting for dynamic changes in acoustic attenuation/absorption appeared to play a critical role in predicted extents of ablation zones. For typical 5-15 min ablations with this modality, thermal dose and Arrhenius damage measures of ablation zone dimensions are in good agreement, while appropriately selected temperature thresholds provide a computationally cheaper surrogate.

Quantitative comparison of thermal dose models in normal canine brain

PURPOSE

Minimally invasive thermal ablative therapies as alternatives to conventional surgical management of solid tumors and other pathologies is increasing owing to the potential benefits of performing these procedures in an outpatient setting with reduced complications and comorbidity. Magnetic resonance temperature imaging (MRTI) measurement allows existing thermal dose models to use the spatiotemporal temperature history to estimate the thermal damage to tissue. However, the various thermal dose models presented in the literature employ different parameters and thresholds, affecting the reliability of thermal dosimetry. In this study, the authors quantitatively compared three thermal dose models (Arrhenius rate process, CEM43, and threshold temperature) using the dice similarity coefficient (DSC).

METHODS

The DSC was used to compare the spatial overlap between the region of thermal damage as predicted by the models for in vivo normal canine brain during thermal therapy to the region of thermal damage as revealed by contrast-enhanced T1-weighted images acquired immediately after therapy (< 20 min). The outer edge of the hyperintense rim of the ablation region was used as the surrogate marker for the limits of thermal coagulation. The DSC was also used to investigate the impact of varying the thresholds on each models' ability to predict the zone of thermal necrosis.

RESULTS

At previously reported thresholds, the authors found that all three models showed good agreement (defined as DSC > 0.7) with post-treatment imaging. All three models examined across the range of commonly applied thresholds consistently showed highly accurate spatial overlap, low variability, and little dependence on temperature uncertainty. DSC values corresponding to cited thresholds were not significantly different from peak DSC values.

CONCLUSIONS

Thus, the authors conclude that the all three thermal dose models can be used as a reliable surrogate for postcontrast tissue damage verification imaging in rapid ablation procedures and can also be used to enhance the capability of MRTI to control thermal therapy in real time.

MRI-based thermal dosimetry and diffusion-weighted imaging of MRI-guided focused ultrasound thermal ablation of uterine fibroids

PURPOSE

To investigate tissue changes observed in diffusion-weighted imaging (DWI) and its relation to contrast imaging, thermal dosimetry, and changes in the apparent diffusion coefficient (ADC) after MRI-guided focused ultrasound surgery (MRgFUS) of uterine fibroids.

MATERIALS AND METHODS

Imaging data were analyzed from 45 fibroids in 42 women treated with MRgFUS. The areas of the hyperintense regions in DWI and of nonperfused regions in T1-weighted contrast enhanced imaging (both acquired immediately after treatment) were compared with each other and to thermal dosimetry based estimates. Changes in ADC were also calculated.

RESULTS

Hyperintense regions were observed in 35/45 fibroids in DWI. When present, the areas of these regions were comparable on average to the thermal dose estimates and to the nonperfused regions, except for in several large treatments in which the nonperfused region extended beyond the treated area. ADC increased in 19 fibroids and decreased in the others.

CONCLUSION

DWI changes, which includes changes in both in T2 and ADC, may be useful in many cases to delineate the treated region resulting from MRgFUS. However, clear DWI changes were not always observed, and in some large treatments, the extent of the nonperfused region was under estimated. ADC changes immediately after MRgFUS were unpredictable.

Interstitial devices for minimally invasive thermal ablation by high-intensity ultrasound

Interstitial ultrasound applicators have been proposed for treating deep-seated tumours that cannot be reached with extra-corporeal high-intensity focused ultrasound. In addition, interstitial ultrasound offers several advantages compared with conventional ablation technology (radiofrequency, microwaves, cryotherapy) in terms of penetration, speed of coagulation, ability to direct and control the thermal lesion and compatibility with image monitoring. The ultrasound source is brought as close as possible to the target in order to minimize the effects of attenuation and phase aberration along the ultrasound pathway. The present paper is a review of the interstitial applicators that were described during the last decade in the literature. It is presented in three sections. The technical aspects common to all applicators are first described. For example, most-described applicators are sideview applicators whose active element is water-cooled and operates at rather high frequency (above 3 MHz) in order to promote heating. Then the different potential techniques for monitoring treatment administered by the interstitial route are presented and illustrated through a review of image-guided interstitial thermal ablation. Three major techniques of imaging are used for guiding interstitial treatment: MRI, ultrasound and fluoroscopy. The third section goes in to further detail on diverse described medical applications.

Interstitial ultrasound applicators with dynamic angular control for thermal ablation of tumors under MR-guidance

Thermal ablation has been investigated as a treatment for a variety of cancers. Heat treatments have not gained large-scale clinical acceptance due to inconsistencies in controlling heat deposition in vivo and the lack of precise temperature measurement. Interstitial ultrasound provide a good method of controlling the radial depth of a thermal lesion and the applicator designs evaluated in this study allow for dynamic angular control of the shape of the lesion. A trisectored internally water-cooled applicator (TriAD) and a rotating catheter water-cooled applicator (RIUS) angularly controlled thermal dose to a target area. Both devices were small in diameter (1.8 mm-2.4 mm), making them clinically feasible for minimally invasive treatment in device size-sensitive tissues. A biothermal model accounting for changes in acoustic attenuation and perfusion as a function of thermal dose was used to evaluate and predict applicator performance. The MR susceptibility artifact of the applicators was examined with MR temperature imaging (MRTI) sequences at 1.5 T and 0.5 T. Ex vivo experiments in turkey and beef muscle with realtime MRTI correlated well with results from the biothermal model. These results display the feasibility of thermally treating tumors with controllable interstitial ultrasound applicators under real-time MRTI and bracket the applicators' predicted performance in vivo.

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.

Intradiscal thermal therapy using interstitial ultrasound: an in vivo investigation in ovine cervical spine

STUDY DESIGN

In vivo investigation of intradiscal ultrasound thermal therapy in ovine cervical spine model.

OBJECTIVE

To evaluate the potential of interstitial ultrasound for selective heating of intradiscal tissue in vivo.

SUMMARY OF BACKGROUND DATA

Application of heat in the spine using resistive wire and radiofrequency current heating devices is currently being used clinically for minimally invasive treatment of discogenic low back pain. Treatment temperatures are representative of those required for thermal necrosis of ingrowing nociceptor nerve fibers and disc cellularity alone, or with coagulation and restructuring of anular collagen in the high temperature case.

METHODS

Two interstitial ultrasound applicator design configurations with directional heating patterns were evaluated in vivo in ovine cervical intervertebral discs (n = 62), with up to 45-day survival periods. Two heating protocols were employed in which the temperature measured 5 mm away from the applicator was controlled to either <54 C (capable of nerve and cellular necrosis) or >70 C (for coagulation of collagen) for a 10-minute treatment period. Transient and steady state temperature maps, calculated thermal doses (t43), and histology were used to assess the thermal treatments.

RESULTS

These studies demonstrated the capability to control spatial temperature distributions within selected regions of the in vivo intervertebral disc and anular wall using interstitial ultrasound.

CONCLUSIONS

Ultrasound energy is capable of penetrating within the highly attenuating disc tissue to produce more extensive radial thermal penetration, lower maximum intradiscal temperature, and shorter treatment times than can be achieved with current clinical intradiscal heating technology. Thus, interstitial ultrasound offers potential as a more precise and faster heating modality for the clinical management of low back pain and studies of thermal effects on disc tissue in animal models.

Uterine leiomyomas: MR imaging-based thermometry and thermal dosimetry during focused ultrasound thermal ablation

PURPOSE

To retrospectively evaluate magnetic resonance (MR) imaging-based thermometry and thermal dosimetry during focused ultrasound treatments of uterine leiomyomas (ie, fibroids).

MATERIALS AND METHODS

All patients gave written informed consent for the focused ultrasound treatments and the current HIPAA-compliant retrospective study, both of which were institutional review board approved. Thermometry performed during the treatments of 64 fibroids in 50 women (mean age, 46.6 years +/- 4.5 [standard deviation]) was used to create thermal dose maps. The areas that reached dose values of 240 and 18 equivalent minutes at 43 degrees C were compared with the nonperfused regions measured on contrast material-enhanced MR images by using the Bland-Altman method. Volume changes in treated fibroids after 6 months were compared with volume changes in nontreated fibroids and with MR-based thermal dose estimates.

RESULTS

While the thermal dose estimates were shown to have a clear relationship with resulting nonperfused regions, the nonperfused areas were, on average, larger than the dose estimates (means of 1.9 +/- 0.7 and 1.2 +/- 0.4 times as large for areas that reached 240- and 18-minute threshold dose values, respectively). Good correlation was observed for smaller treatment volumes at the lower dose threshold (mean ratio, 1.0 +/- 0.3), but for larger treatment volumes, the nonperfused region extended to locations within the fibroid that clearly were not heated. Variations in peak temperature increase were as large as a factor of two, both between patients and within individual treatments. On average, the fibroid volume reduction at 6 months increased as the ablated volume estimated by using the thermal dose increased.

CONCLUSION

Study results showed good correlation between thermal dose estimates and resulting nonperfused areas for smaller ablated volumes. For larger treatment volumes, nonperfused areas could extend within the fibroid to unheated areas.

Multisectored interstitial ultrasound applicators for dynamic angular control of thermal therapy

Dynamic angular control of thermal ablation and hyperthermia therapy with current interstitial heating technology is limited in capability, and often relies upon nonadjustable angular power deposition patterns and/or mechanical manipulation of the heating device. The objective of this study was to investigate the potential of multisectored tubular interstitial ultrasound devices to provide control of the angular heating distribution without device manipulation. Multisectored tubular transducers with independent sector power control were incorporated into modified versions of internally cooled (1.9 mm OD) and catheter-cooled (2.4 mm OD) interstitial ultrasound applicators in this work. The heating capabilities of these multisectored devices were evaluated by measurements of acoustic output properties, measurements of thermal lesions produced in ex vivo tissue samples, biothermal simulations of thermal ablation and hyperthermia treatments, and MR temperature imaging of ex vivo and in vivo experiments. Acoustic beam measurements of each applicator type displayed a 35 degrees -40 degrees acoustic dead zone between each independent sector, with negligible mechanical or electrical coupling. Thermal lesions produced in ex vivo liver tissue with one, two, or three sectors activated ranged from 13-18 mm in radius with contiguous zones of coagulation between active sectors. The simulations demonstrated the degree of angular control possible by using variable power levels applied to each sector, variable duration of applied constant power to individual sectors, respectively, or a multipoint temperature controller to vary the power applied to each sector. Despite the acoustic dead zone between sectors, the simulations also showed that the variance from the maximum lesion radius with three elements activated is within 4%-13% for tissue perfusions from 1-10 kg m(-3) s(-1). Simulations of hyperthermia with maximum tissue temperatures of 45 degrees C and 48 degrees C displayed radial penetration up to 2 cm of the 40 degrees C steady-state contour. Thermal characterizations of trisectored applicators in ex vivo and in vivo muscle, using real-time MR thermal imaging, reinforced angular controllability and negligible radial variance of the heating pattern from the applicators, demonstrated effective heating penetration, and displayed MR compatibility. The multisectored interstitial ultrasound applicators developed in this study demonstrated a significant degree of dynamic angular control of a heating pattern without device manipulation, while maintaining heat penetration consistent with previously reported results from other interstitial ultrasound applicators.

Magnetic resonance temperature imaging for focused ultrasound surgery: a review

Magnetic resonance temperature imaging (MRTI) is an enabling technology that has recently demonstrated the potential to bring the emerging minimally invasive image-guided thermal therapy procedures, such as radiofrequency, microwave, laser, ultrasound, and cryosurgery, into the clinical setting with a level of safety and efficacy not previously possible. By coupling the wealth of soft tissue contrast mechanisms available with magnetic resonance imaging with its intrinsic temperature sensitivity, magnetic resonance imaging is in a unique position to provide image-guided treatment planning and verification and quantitative or qualitative feedback during treatment delivery, heightening of the control the physician has over the method, and enhancement of the ability to deliver conformal treatments. The basic principles behind MRTI technology and its application to minimally invasive thermal therapy during ultrasound thermal therapy delivery are reviewed in this study.

Partially parallel imaging with phase-sensitive data: Increased temporal resolution for magnetic resonance temperature imaging

Magnetic resonance temperature imaging can be used to monitor the progress of thermal ablation therapies, increasing treatment efficacy and improving patient safety. High temporal resolution is important when therapies rapidly heat tissue, but many approaches to faster image acquisition compromise image resolution, slice coverage, or phase sensitivity. Partially parallel imaging techniques offer the potential for improved temporal resolution without forcing such concessions. Although these techniques perturb image phase, relative phase changes between dynamically acquired phase-sensitive images, such as those acquired for MR temperature imaging, can be reliably measured through partially parallel imaging techniques using reconstruction filters that remain constant across the series. Partially parallel and non-accelerated phase-difference-sensitive data can be obtained through arrays of surface coils using this method. Average phase differences measured through partially parallel and fully Fourier encoded images are virtually identical, while phase noise increases with g(sqrt)L as in standard partially parallel image acquisitions..

Feasibility of using interstitial ultrasound for intradiscal thermal therapy: a study in human cadaver lumbar discs

Application of heat in the spine using resistive wire heating devices is currently being used clinically for minimally invasive treatment of discogenic low back pain. In this study, interstitial ultrasound was evaluated for the potential to heat intradiscal tissue more precisely by directing energy towards the posterior annular wall while avoiding vertebral bodies. Two single-element directional applicator design configurations were tested: a 1.5 mm OD direct-coupled (DC) applicator which can be implanted directly within the disc, and a catheter-cooled (CC) applicator which is inserted in a 2.4 mm OD catheter with integrated water cooling and implanted within the disc. The transducers were sectored to produce 90 degrees spatial heating patterns for directional control. Both applicator configurations were evaluated in four human cadaver lumbar disc motion segments. Two heating protocols were employed in this study in which the temperature measured 5 mm away from the applicator was controlled to either T=52 degrees C, or T>70 degrees C for the treatment period. These temperatures (thermal doses) are representative of those required for thermal necrosis of in-growing nociceptor nerve fibres and disc cellularity alone, or with coagulation and restructuring of annular collagen in the high-temperature case. Steady-state temperature maps, and thermal doses (t43) were used to assess the thermal treatments. Results from these studies demonstrated the capability of controlling temperature distributions within selected regions of the disc and annular wall using interstitial ultrasound, with minimal vertebral end-plate heating. While directional heating was demonstrated with both applicator designs, the CC configuration had greater directional heating capabilities and offered better temperature control than the DC configuration, particularly during the high-temperature protocol. Further, ultrasound energy was capable of penetrating within the highly attenuating disc tissue to produce more extensive radial thermal penetration, lower maximum intradiscal temperature, and shorter treatment times than can be achieved with current clinical intradiscal heating technology. Thus, interstitial ultrasound offers potential as a more precise and faster heating modality for the clinical management of low back pain.

MRI-guided interstitial ultrasound thermal therapy of the prostate: A feasibility study in the canine model

The feasibility of MRI-guided interstitial ultrasound thermal therapy of the prostate was evaluated in an in vivo canine prostate model. MRI compatible, multielement interstitial ultrasound applicators were developed using 1.5 mm diameter cylindrical piezoceramic transducers (7 to 8 MHz) sectored to provide 180 degrees of angular directional heating. Two in vivo experiments were performed in canine prostate. The first using two interstitial ultrasound applicators, the second using three ultrasound applicators in conjunction with rectal and urethral cooling. In both experiments, the applicators were inserted transperineally into the prostate with the energy directed ventrally, away from the rectum. Electrical power levels of 5-17 W per element (approximately 1.6-5.4 W acoustic output power) were applied for heating periods of 18 and 48 min. Phase-sensitive gradient-echo MR imaging was used to monitor the thermal treatment in real-time on a 0.5 T interventional MRI system. Contrast-enhanced T1-weighted images and vital-stained serial tissue sections were obtained to assess thermal damage and correlate to real-time thermal contour plots and calculated thermal doses. Results from these studies indicated a large volume of ablated (nonstained) tissue within the prostate, extending 1.2 to 2.0 cm from the applicators to the periphery of the gland, with the dorsal margin of coagulation well-defined by the applicator placement and directionality. The shape of the lesions correlated well to the hypointense regions visible in the contrast-enhanced T1-weighted images, and were also in good agreement with the contours of the 52 degrees C threshold temperature and t43 > 240 min. This study demonstrates the feasibility of using directional interstitial ultrasound in conjunction with MRI thermal imaging to monitor and possibly control thermal coagulation within a targeted tissue volume while potentially protecting surrounding tissue, such as rectum, from thermal damage.

Interleaved echo-planar imaging for fast multiplanar magnetic resonance temperature imaging of ultrasound thermal ablation therapy

PURPOSE

To develop a multiplanar magnetic resonance temperature imaging (MRTI) technique based on interleaved gradient-echo echo-planar imaging (EPI), verify in phantom, develop software tools to process and display data on a clinical scanner in near real-time, and demonstrate feasibility to monitor ultrasound thermal ablation therapy in vivo.

MATERIALS AND METHODS

Temperature estimation used complex phase-difference subtraction of the EPI MRTI data to indirectly measure the temperature-dependent water proton-resonance-frequency shift. Software tools were developed to run on a clinical 1.5-T MR scanner that processed and displayed relevant temperature and thermal dosimetry data during the course of thermal ablation treatments in canine brain and prostate in vivo.

RESULTS

EPI MRTI provided multi-planar acquisitions and increased temperature sensitivity and lipid suppression. Relative to a single-plane fast gradient-echo MRTI sequence at comparable spatial and temporal resolutions in phantom, EPI MRTI demonstrated a three-fold increase in sensitivity and slice coverage per TR. In vivo monitoring of ultrasound thermal ablation therapy in canine brain and prostate demonstrated the usefulness of the temperature and thermal dose information.

CONCLUSION

Multi-planar MRTI allowed progression of thermal damage to be monitored and treatment parameters adjusted in near real-time (less than five second delay). EPI MRTI is an effective multi-planar monitoring method during ultrasound thermal ablation procedures.

Minimum-time thermal dose control of thermal therapies

The problem of controlling noninvasive thermal therapies is formulated as the problem of directly controlling thermal dose of the target. To limit the damage to the surrounding normal tissue, the constraints on the peak allowable temperatures in the selected spacial locations are imposed. The developed controller has a cascade structure with a linear, constrained, model predictive temperature controller in the secondary loop. The temperature controller manipulates the intensity of the ultrasound transducer with saturation constraints, which noninvasively heats the spatially distributed target. The main nonlinear thermal dose controller dynamically generates the reference temperature trajectories for the temperature controller. The thermal dose controller is designed to force the treatment progression at either the actuation or temperature constraints, which is required to minimize the treatment time. The developed controller is applicable to high and low-intensity treatments, such as thermal ablation and thermoradiotherapy. The developed approach is tested using computer simulations for a one-dimensional model of a tumor with constraints on the maximum allowable temperature in the normal tissue and a constrained power output of the ultrasound transducer. The simulation results demonstrate that the proposed approach is effective at delivering the desired thermal dose in a near minimum time without violating constraints on the maximum allowable temperature in healthy tissue, despite significant plant-model mismatch introduced during numerical simulation. The results of in vitro and in vivo validation are reported elsewhere.

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.

Transurethral ultrasound applicators with directional heating patterns for prostate thermal therapy: in vivo evaluation using magnetic resonance thermometry

A catheter-based transurethral ultrasound applicator with angularly directional heating patterns has been designed for prostate thermal therapy and evaluated in canine prostate in vivo using MRI to monitor and assess performance. The ultrasound transducer array (3.5 mm diameter tubular transducers, 180 degrees active sectors, approximately 7.5 MHz) was integrated to a flexible delivery catheter (4 mm OD), and encapsulated within an expandable balloon (35 mm x 10 mm OD, 80 ml min(-1) ambient water) for coupling and cooling of the prostatic urethra. These devices were used to thermally coagulate targeted portions of the canine prostate (n = 2) while using MR thermal imaging (MRTI) to monitor the therapy. MRI was also used for target definition, positioning of the applicator, and evaluation of target viability post-therapy. MRTI was based upon the complex phase-difference mapping technique using an interleaved gradient echo-planar imaging sequence with lipid suppression. MRTI derived temperature distributions, thermal dose exposures, T1-contrast enhanced MR images, and histology of sectioned prostates were used to define destroyed tissue zones and characterize the three-dimensional heating patterns. The ultrasound applicators produced approximately 180 degrees directed zones of thermal coagulation within targeted tissue which extended 15-20 mm radially to the outer boundary of the prostate within 15 min. Transducer activation lengths of 17 mm and 24 mm produced contiguous zones of coagulation extending axially approximately 18 mm and approximately 25 mm from base to apex, respectively. Peak temperatures around 90 degrees C were measured, with approximately 50 degrees C-52 degrees C corresponding to outer boundary t43 = 240 min at approximately 15 min treatment time. These devices are MRI compatible, and when coupled with multiplanar MRTI provide a means for selectively controlling the length and sector angle of therapeutic thermal treatment in the prostate.

Two-dimensional acoustic attenuation mapping of high-temperature interstitial ultrasound lesions

Acoustic attenuation change in biological tissues with temperature and time is a critical parameter for interstitial ultrasound thermal therapy treatment planning and applicator design. Earlier studies have not fully explored the effects on attenuation of temperatures (75-95 degrees C) and times (5-15 min) common in interstitial ultrasound treatments. A scanning transmission ultrasound attenuation measurement system was devised and used to measure attenuation changes due to these types of thermal exposures. To validate the approach and to loosely define expected values, attenuation changes in degassed ex vivo bovine liver, bovine brain and chicken muscle were measured after 10 min exposures in a water bath to temperatures up to 90 degrees C. Maximum attenuation increases of approximately seven, four and two times the values at 37 degrees C were measured for the three tissue models at 5 MHz. By using the system to scan over lesions produced using interstitial ultrasound applicators, 2D contour maps of attenuation were produced. Attenuation profiles measured through the centrelines of lesions showed that attenuation was highest close to the applicator and decreased with radial distance, as expected with decreasing thermal exposure. Attenuation values measured in profiles through lesions were also shown to decrease with reduced power to the applicator. Attenuation increases in 2D maps of interstitial ultrasound lesions in ex vivo chicken breast, bovine liver and bovine brain were correlated with visible tissue coagulation. While regions of visible coagulation corresponded well to contours of attenuation increase in liver and chicken, no lesion was visible under the same experimental conditions in brain, due primarily to the heterogeneity of the tissue. Acoustic and biothermal simulations were employed to show that attenuation models taking into account these attenuation changes at higher temperatures and longer times were better able to fit experimental data than previous models. These simulations also indicated that the characterization of tissue acoustic and thermal properties over a large range of temperatures is critical for accurate treatment planning or design studies involving high-temperature interstitial ultrasound.

Highly directional transurethral ultrasound applicators with rotational control for MRI-guided prostatic thermal therapy

Transurethral ultrasound applicators with highly directional energy deposition and rotational control were investigated for precise treatment of benign prostatic hyperplasia (BPH) and adenocarcinoma of the prostate (CaP). Two types of catheter-based applicators were fabricated, using either 90 degrees sectored tubular (3.5 mm OD x 10 mm) or planar transducers (3.5 mm x 10 mm). They were constructed to be MRI compatible, minimally invasive and allow for manual rotation of the transducer array within a 10 mm cooling balloon. In vivo evaluations of the applicators were performed in canine prostates (n = 3) using MRI guidance (0.5 T interventional magnet). MR temperature imaging (MRTI) utilizing the proton resonance frequency shift method was used to acquire multiple-slice temperature overlays in real time for monitoring and guiding the thermal treatments. Post-treatment T1-weighted contrast-enhanced imaging and triphenyl tetrazolium chloride stained tissue sections were used to define regions of tissue coagulation. Single sonications with the 90 degrees tubular applicator (9-15 W, 12 min, 8 MHz) produced coagulated zones covering an 80 degrees wedge of the prostate extending from 1-2 mm outside the urethra to the outer boundary of the gland (16 mm radial coagulation). Single sonications with the planar applicator (15-20 W, 10 min, approximately 8 MHz) generated thermal lesions of approximately 30 degrees extending to the prostate boundary. Multiple sequential sonications (sweeping) of a planar applicator (12 W with eight rotations of 30 degrees each) demonstrated controllable coagulation of a 270 degrees contiguous section of the prostate extending to the capsule boundary. The feasibility of using highly directional transurethral ultrasound applicators with rotational capabilities to selectively coagulate regions of the prostate while monitoring and controlling the treatments with MRTI was demonstrated in this study.

MRI investigation of the threshold for thermally induced blood-brain barrier disruption and brain tissue damage in the rabbit brain

The ability of MRI-derived thermometry to predict thermally induced tissue changes in the brain was tested, and the thermal thresholds for blood-brain barrier (BBB) disruption and brain tissue damage were estimated. In addition, the ability of standard MRI to detect threshold-level effects was confirmed. These safety thresholds are being investigated to provide guidelines for clinical thermal ablation studies in the brain. MRI-monitored focused ultrasound heating was delivered to 63 locations in 26 rabbits. Tissue changes were detected in T(2)-weighted imaging and T(1)-weighted imaging (with and without contrast) and with light microscopy. The probability for tissue damage as a function of the accumulated thermal dose, the peak temperature achieved, the applied acoustic energy, and the peak acoustic power was estimated with probit regression. The discriminative abilities of these parameters were compared using the areas under the receiver operator characteristic (ROC) curves. In MRI, BBB disruption was observed in contrast-enhanced T(1)-weighted imaging shortly after the ultrasound exposures, sometimes accompanied by changes in T(2)-weighted imaging. Two days later, changes in T(2)-weighted imaging were observed, sometimes accompanied by changes in T(1)-weighted imaging. In histology, tissue damage was seen at every location where MRI changes were observed, ranging from small (diameter <1.0 mm) areas of tissue necrosis to severe vascular damage and associated hemorrhagic infarct. In one location, small (diameter: 0.8 mm) damage was not detected in MRI. The thermal dose and peak temperature thresholds were between 12.3-40.1 equivalent min at 43 degrees C and 48.0-50.8 degrees C, respectively, and values of 17.5 equivalent min at 43 degrees C and 48.4 degrees C were estimated to result in tissue damage with 50% probability. Thermal dose and peak temperature were significantly better predictors than the applied acoustic energy and peak acoustic power (P < 0.01). BBB disruption was always accompanied by tissue damage. The temperature information was better than the applied acoustic power or energy for predicting the damage than the ultrasound parameters. MRI was sensitive in detecting threshold-level damage.

Thermal therapy of canine cerebral tumors using a 980 nm diode laser with MR temperature-sensitive imaging feedback

BACKGROUND AND PURPOSE

The laser-induced thermal therapy (LITT) of cerebral tumors has conventionally been performed using Nd:YAG lasers and is associated with a risk of high focal temperatures potentially followed by cavitation that could result in boiling and/or explosive char. We have developed small diffusing laser fiber tips to better distribute the energy deposition and a computer controlled feedback system to monitor therapy and prevent excess temperature buildup. In this study, we evaluated the feasibility of using magnetic resonance temperature imaging (MRTI)-based feedback system for the thermal treatment of experimental intracerebral tumors using 980 nm laser irradiation delivered through these diffusing tips.

STUDY DESIGN/MATERIALS AND METHODS

Transmissible venereal tumors (TVTs) were grown via inoculation in the right cerebral hemisphere of seven canines. The laser fiber tips were inserted into a total of 10 independent TVT-suspected regions in the seven animals. Margins for the target area in each animal were prescribed on the basis of pretreatment MR images. MRTI-based feedback software was used to measure and regulate both temperature and the delivered thermal dose to achieve the desired thermal ablation and prevent excess heating. The effects of treatment were verified by results of histologic analyses.

RESULTS

Treatments resulted in contiguous areas of thermal necrosis in tumors and adjacent brain margin. The feedback software successfully cut off the laser power once the desired treatment volume was achieved, and prevented focal temperatures from exceeding predefined thresholds. Follow-up MRI studies showed 1.4- to 2.9-fold LITT-induced lesion expansion within 1-6 days after treatment.

CONCLUSIONS

Targeted thermal coagulation of small intracerebral tumors is feasible using MRTI-based feedback and diffused 980 nm diode laser light.