Brain edema development after MRI-guided focused ultrasound treatment

Advances in Focused Ultrasound for the Treatment of Brain Tumors

Employing the full arsenal of therapeutics to treat brain tumors is limited by the relative impermeability of the blood-brain and blood-tumor barriers. In physiologic states, the blood-brain barrier serves a protective role by passively and actively excluding neurotoxic compounds; however, this functionality limits the penetrance of therapeutics into the tumor microenvironment. Focused ultrasound technology provides a method for overcoming the blood-brain and blood-tumor barriers through ultrasound frequency to transiently permeabilize or disrupt these barriers. Concomitant delivery of therapeutics has allowed for previously impermeable agents to reach the tumor microenvironment. This review details the advances in focused ultrasound in both preclinical models and clinical studies, with a focus on its safety profile. We then turn towards future directions in focused ultrasound-mediated therapies for brain tumors.

A Review of Imaging Methods to Assess Ultrasound-Mediated Ablation

Ultrasound ablation techniques are minimally invasive alternatives to surgical resection and have rapidly increased in use. The response of tissue to HIFU ablation differs based on the relative contributions of thermal and mechanical effects, which can be varied to achieve optimal ablation parameters for a given tissue type and location. In tumor ablation, similar to surgical resection, it is desirable to include a safety margin of ablated tissue around the entirety of the tumor. A factor in optimizing ablative techniques is minimizing the recurrence rate, which can be due to incomplete ablation of the target tissue. Further, combining focal ablation with immunotherapy is likely to be key for effective treatment of metastatic cancer, and therefore characterizing the impact of ablation on the tumor microenvironment will be important. Thus, visualization and quantification of the extent of ablation is an integral component of ablative procedures. The aim of this review article is to describe the radiological findings after ultrasound ablation across multiple imaging modalities. This review presents readers with a general overview of the current and emerging imaging methods to assess the efficacy of ultrasound ablative treatments.

AAV Vector-Mediated Antibody Delivery (A-MAD) in the Central Nervous System

In the last four decades, monoclonal antibodies and their derivatives have emerged as a powerful class of therapeutics, largely due to their exquisite targeting specificity. Several clinical areas, most notably oncology and autoimmune disorders, have seen the successful introduction of monoclonal-based therapeutics. However, their adoption for treatment of Central Nervous System diseases has been comparatively slow, largely due to issues of efficient delivery resulting from limited permeability of the Blood Brain Barrier. Nevertheless, CNS diseases are becoming increasingly prevalent as societies age, accounting for ~6.5 million fatalities worldwide per year. Therefore, harnessing the full therapeutic potential of monoclonal antibodies (and their derivatives) in this clinical area has become a priority. Adeno-associated virus-based vectors (AAVs) are a potential solution to this problem. Preclinical studies have shown that AAV vector-mediated antibody delivery provides protection against a broad range of peripheral diseases, such as the human immunodeficiency virus (HIV), influenza and malaria. The parallel identification and optimization of AAV vector platforms which cross the Blood Brain Barrier with high efficiency, widely transducing the Central Nervous System and allowing high levels of local transgene production, has now opened a number of interesting scenarios for the development of AAV vector-mediated antibody delivery strategies to target Central Nervous System proteinopathies.

Feasibility of ultrasound-induced blood-brain barrier disruption with a single-element transducer under three different frequencies in two non-human primates in vivo: Case report

BACKGROUND

Single-element focused transducers applied in blood-brain barrier (BBB) disruption experiments to optimize intravascular therapies in CNS diseases have the advantage of low cost and portability. Most of the in vivo studies on non-human primates report the use of single-element transducers with an annular spherical shape and a central frequency of 500 kHz. High-frequency ultrasound has smaller focal area and less standing-wave effect but lower transcranial penetration efficiency. Our study reports the feasibility and safety concerns of BBB opening by single-element spherical transducers with central frequencies of 300, 650 and 800 kHz on two rhesus macaques.

METHODS

Pulsed ultrasound exposure (3-minute duration, 0.5-1% duty cycle) combined with microbubble injection (SonoVue, 0.2uL/g) was used to disrupt the BBB of the monkeys under the magnetic resonance imaging (MRI) guidance. Gadolinium contrast-enhanced MRI was used to confirm and evaluate the BBB opening after sonication. T2-weighted fast spin echo and T2 * -weighted gradient echo sequences were used to check the post-sonication complications, such as edema and micro-bleeding.

RESULTS

Contrast enhancement was found on the post-sonication T1 weighted images for all experiments, showing that the BBB was successfully opened under all the three frequencies on both monkeys. The enhanced area was largest at the lowest frequency. No obvious hypo-intensity or hyper-intensity was observed on either the T2 * weighted gradient echo images or T2-weighted fast-spin echo images, implying the safety of the opening procedure. However, signal enhancement was also observed in the subarachnoid space of the sulci for all frequencies, indicating that the BBB was also disrupted in the propagation path outside the focal area.

CONCLUSIONS

The feasibility of BBB opening with single-element transducer under frequencies ranging from 300 kHz to 800 kHz was confirmed by experiments in two non-human primates in vivo. Further investigation into the off-target effects and transducer configurations is needed for safety optimization.

MR-guided blood-brain barrier opening induced by rapid short-pulse ultrasound in non-human primates

Background

Opening the blood-brain barrier (BBB) with focused ultrasound and microbubbles (MBs) has potential use in non-invasive targeted therapy for central nervous system (CNS) diseases. Rapid short-pulse (RaSP) ultrasound with a microsecond sequence has been proposed as a minimally disruptive and efficient method for opening the BBB. This work aimed to test the feasibility and safety of BBB opening in a non-human primate model using combined RaSP ultrasound sequence and MBs.

Methods

The BBB of 2 rhesus macaques were opened with RaSP and the commonly used 10 millisecond long pulse (LP), combined with microbubble (SonoVueTM, 0.2 µL/g) injection in a bolus. The transducer's central frequency was 300 kHz, and the acoustic pressure was set to 0.56 MPa calibrated in water. The BBB opening procedure was guided and evaluated with contrast-enhanced magnetic resonance imaging. The relative signal enhancement was compared between RaSP and LP sonication. T2-weighted fast-spin echo (FSE) and T2*-weighted gradient echo (GRE) sequences were scanned to evaluate edema and micro-bleeding at the end of the procedure.

Results

The relative signal enhancement was significantly higher (P<0.01) in the focal area compared to a similar area of the opposite hemisphere at all time points after sonication in each monkey, indicating the successful opening of the BBB. The relative signal enhancement in RaSP reached more than 60% of that with LP in our experiment, while the energy deposition was only 6% of LP. No edema or hemorrhage was found on magnetic resonance images after RaSP.

Conclusions

Combined RaSP ultrasound and MBs for the BBB opening is a practical method in large animal models.

A preclinical study of diffusion-weighted MRI contrast as an early indicator of thermal ablation

PURPOSE

Intraoperative T₂ -weighted (T2-w) imaging unreliably captures image contrast specific to thermal ablation after transcranial MR-guided focused ultrasound surgery, impeding dynamic imaging feedback. Using a porcine thalamotomy model, we test the unproven hypothesis that intraoperative DWI can improve dynamic feedback by detecting lesioning within 30 minutes of transcranial MR-guided focused ultrasound surgery.

METHODS

Twenty-five thermal lesions were formed in six porcine models using a clinical transcranial MR-guided focused ultrasound surgery system. A novel diffusion-weighted pulse sequence monitored the formation of T2-w and diffusion-weighted lesion contrast after ablation. Using postoperative T2-w contrast to indicate lesioning, apparent intraoperative image contrasts and diffusion coefficients at each lesion site were computed as a function of time after ablation, observed peak temperature, and observed thermal dose. Lesion sizes segmented from imaging and thermometry were compared. Image reviewers estimated the time to emergence of lesion contrast. Intraoperative image contrasts were analyzed using receiver operator curves.

RESULTS

On average, the apparent diffusion coefficient at lesioned sites decreased within 5 minutes after ablation relative to control sites. In-plane lesion areas on intraoperative DWI varied from postoperative T2-w MRI and MR thermometry by 9.6 ± 9.7 mm² and - 4.0 ± 7.1 mm² , respectively. The 0.25, 0.5, and 0.75 quantiles of the earliest times of observed T2-w and diffusion-weighted lesion contrast were 10.7, 21.0, and 27.8 minutes and 3.7, 8.6, and 11.8 minutes, respectively. The T2-w and diffusion-weighted contrasts and apparent diffusion coefficient values produced areas under the receiver operator curve of 0.66, 0.80, and 0.74, respectively.

CONCLUSION

Intraoperative DWI can detect MR-guided focused ultrasound surgery lesion formation in the brain within several minutes after treatment.

Volumetric analysis of magnetic resonance-guided focused ultrasound thalamotomy lesions

OBJECTIVE Magnetic resonance-guided focused ultrasound (MRgFUS) thalamotomy was recently approved for use in the treatment of medication-refractory essential tremor (ET). Previous work has described lesion appearance and volume on MRI up to 6 months after treatment. Here, the authors report on the volumetric segmentation of the thalamotomy lesion and associated edema in the immediate postoperative period and 1 year following treatment, and relate these radiographic characteristics with clinical outcome. METHODS Seven patients with medication-refractory ET underwent MRgFUS thalamotomy at Brigham and Women's Hospital and were monitored clinically for 1 year posttreatment. Treatment effect was measured using the Clinical Rating Scale for Tremor (CRST). MRI was performed immediately postoperatively, 24 hours posttreatment, and at 1 year. Lesion location and the volumes of the necrotic core (zone I) and surrounding edema (cytotoxic, zone II; vasogenic, zone III) were measured on thin-slice T2-weighted images using Slicer 3D software. RESULTS Patients had significant improvement in overall CRST scores (baseline 51.4 ± 10.8 to 24.9 ± 11.0 at 1 year, p = 0.001). The most common adverse events (AEs) in the 1-month posttreatment period were transient gait disturbance (6 patients) and paresthesia (3 patients). The center of zone I immediately posttreatment was 5.61 ± 0.9 mm anterior to the posterior commissure, 14.6 ± 0.8 mm lateral to midline, and 11.0 ± 0.5 mm lateral to the border of the third ventricle on the anterior commissure-posterior commissure plane. Zone I, II, and III volumes immediately posttreatment were 0.01 ± 0.01, 0.05 ± 0.02, and 0.33 ± 0.21 cm³, respectively. These volumes increased significantly over the first 24 hours following surgery. The edema did not spread evenly, with more notable expansion in the superoinferior and lateral directions. The spread of edema inferiorly was associated with the incidence of gait disturbance. At 1 year, the remaining lesion location and size were comparable to those of zone I immediately posttreatment. Zone volumes were not associated with clinical efficacy in a statistically significant way. CONCLUSIONS MRgFUS thalamotomy demonstrates sustained clinical efficacy at 1 year for the treatment of medication-refractory ET. This technology can create accurate, predictable, and small-volume lesions that are stable over time. Instances of AEs are transient and are associated with the pattern of perilesional edema expansion. Additional analysis of a larger MRgFUS thalamotomy cohort could provide more information to maximize clinical effect and reduce the rate of long-lasting AEs.

Accumulated thermal dose in MRI-guided focused ultrasound for essential tremor: repeated sonications with low focal temperatures

OBJECTIVE

The object of this study was to correlate lesion size with accumulated thermal dose (ATD) in transcranial MRI-guided focused ultrasound (MRgFUS) treatments of essential tremor with focal temperatures limited to 50°C-54°C.

METHODS

Seventy-five patients with medically refractory essential tremor underwent MRgFUS thalamotomy at the authors' institution. Intraoperative MR thermometry was performed to measure the induced temperature and thermal dose distributions (proton resonance frequency shift coefficient = -0.00909 ppm/°C). In 19 patients, it was not possible to raise the focal temperature above 54°C because of unfavorable skull characteristics and/or the pain associated with cranial heating. In this patient subset, sonications with focal temperatures between 50°C and 54°C were repeated (5.1 ± 1.5, mean ± standard deviation) to accumulate a sufficient thermal dose for lesion formation. The ATD profile sizes (17, 40, 100, 200, and 240 cumulative equivalent minutes at 43°C [CEM43]) calculated by combining axial MR thermometry data from individual sonications were correlated with the corresponding lesion sizes measured on axial T1-weighted (T1w) and T2-weighted (T2w) MR images acquired 1 day posttreatment. Manual corrections were applied to the MR thermometry data prior to thermal dose accumulation to compensate for off-resonance-induced spatial-shifting artifacts.

RESULTS

Mean lesion sizes measured on T2w MRI (5.0 ± 1.4 mm) were, on average, 28% larger than those measured on T1w MRI (3.9 ± 1.4 mm). The ATD thresholds found to provide the best correlation with lesion sizes measured on T2w and T1w MRI were 100 CEM43 (regression slope = 0.97, R2 = 0.66) and 200 CEM43 (regression slope = 0.98, R2 = 0.89), respectively, consistent with data from a previous study of MRgFUS thalamotomy via repeated sonications at higher focal temperatures (≥ 55°C). Two-way linear mixed-effects analysis revealed that dominant tremor subscores on the Fahn-Tolosa-Marin Clinical Rating Scale for Tremor (CRST) were statistically different from baseline at 3 months and 1 year posttreatment in both low-temperature (50°C-54°C) and high-temperature (≥ 55°C) patient cohorts. No significant fixed effect on the dominant tremor scores was found for the temperature cohort factor.

CONCLUSIONS

In transcranial MRgFUS thalamotomy for essential tremor, repeated sonications with focal temperatures between 50°C and 54°C can accumulate a sufficient thermal dose to generate lesions for clinically relevant tremor suppression up to 1 year posttreatment, and the ATD can be used to predict the size of the resulting ablation zones measured on MRI. These data will serve to guide future clinical MRgFUS brain procedures, particularly those in which focal temperatures are limited to below 55°C.

Predicting lesion size by accumulated thermal dose in MR-guided focused ultrasound for essential tremor

PURPOSE

To correlate the accumulated thermal dose (ATD) with lesion size in magnetic resonance (MR)-guided focused ultrasound (MRgFUS) thalamotomy to help guide future clinical treatments.

MATERIALS AND METHODS

Thirty-six patients with medication-refractory essential tremor were treated using a commercial MRgFUS brain system (ExAblate 4000, InSightec) in a 3T MR scanner (MR750, GE Healthcare). Intraoperative MR-thermometry was performed to measure the induced temperature and thermal dose distributions (thermal coefficient = -0.00909 ppm/°C). The ATD was calculated over multiple sonications with appropriate corrections for spatial-shifting artifacts. The ATD profile sizes obtained for dose values of 17, 40, 100, 200, and 240 cumulative equivalent minutes at 43°C (CEM) were correlated with the corresponding lesion sizes measured via axial T1- and T2-weighted MR images acquired 1 day post-treatment.

RESULTS

Of a total of 232 included sonications, 83 required corrections for off-resonance-induced spatial-shifting artifacts (correction range = [1.1,2.2] mm). The mean lesion sizes measured on T2-weighted MR images (6.2 ± 1.3 mm, mean ± SD) were 15% larger than those measured on corresponding T1-weighted MR images (5.3 ± 1.2 mm, mean ± SD). The ATD values that provided the best correlations with the measured lesion sizes on T2- and T1-weighted MR images were 100 and 200 CEM, respectively.

CONCLUSION

The ATD was correlated with lesion size measured 1 day following MRgFUS thalamotomy for essential tremor. These data provide useful information for predicting brain lesion size and determining treatment endpoints in future clinical MRgFUS procedures.

Fluorine MR Imaging Monitoring of Tumor Inflammation after High-Intensity Focused Ultrasound Ablation

Purpose To investigate whether high-intensity focused ultrasound (HIFU)-induced macrophage infiltration could be longitudinally monitored with fluorine 19 (¹⁹F) magnetic resonance (MR) imaging in a quantitative manner. Materials and Methods BALB/c mice were subcutaneously inoculated with 4T1 cells and were separated into three groups: untreated mice (control, n = 9), HIFU-treated mice (HIFU, n = 9), and HIFU- and clodronate-treated mice (HIFU+Clod, n = 9). Immediately after HIFU treatment, all mice were intravenously given perfluorocarbon (PFC) emulsion. MR imaging examinations were performed 2, 4, 7, 10, and 14 days after HIFU treatment. Two-way repeated measures analysis of variance was used to analyze the changes in ¹⁹F signal over time and differences between groups. Histologic examinations were performed to confirm in vivo data. Results Fluorine 19 signals were detected at the rims of tumors and the peripheries of ablated lesions. Mean ¹⁹F signal in tumors was significantly higher in HIFU-treated mice than in control mice up to day 4 (0.82 ± 0.26 vs 0.42 ± 0.17, P < .001). Fluorine 19 signals were higher in the HIFU+Clod group than in the control group from day 4 (0.82 ± 0.23, P < .001) to day 14 (0.55 ± 0.16 vs 0.28 ± 0.06, P < .05). Histologic examination revealed macrophage infiltration around ablated lesions. Immunofluorescence staining confirmed PFC labeling of macrophages. Conclusion Fluorine 19 MR imaging can longitudinally capture and quantify HIFU-induced macrophage infiltration in preclinical tumor models. ^© RSNA, 2018 Online supplemental material is available for this article.

Image-guided ultrasound phased arrays are a disruptive technology for non-invasive therapy

Focused ultrasound offers a non-invasive way of depositing acoustic energy deep into the body, which can be harnessed for a broad spectrum of therapeutic purposes, including tissue ablation, the targeting of therapeutic agents, and stem cell delivery. Phased array transducers enable electronic control over the beam geometry and direction, and can be tailored to provide optimal energy deposition patterns for a given therapeutic application. Their use in combination with modern medical imaging for therapy guidance allows precise targeting, online monitoring, and post-treatment evaluation of the ultrasound-mediated bioeffects. In the past there have been some technical obstacles hindering the construction of large aperture, high-power, densely-populated phased arrays and, as a result, they have not been fully exploited for therapy delivery to date. However, recent research has made the construction of such arrays feasible, and it is expected that their continued development will both greatly improve the safety and efficacy of existing ultrasound therapies as well as enable treatments that are not currently possible with existing technology. This review will summarize the basic principles, current statures, and future potential of image-guided ultrasound phased arrays for therapy.

MRI methods for the evaluation of high intensity focused ultrasound tumor treatment: Current status and future needs

Thermal ablation with high intensity focused ultrasound (HIFU) is an emerging noninvasive technique for the treatment of solid tumors. HIFU treatment of malignant tumors requires accurate treatment planning, monitoring and evaluation, which can be facilitated by performing the procedure in an MR-guided HIFU system. The MR-based evaluation of HIFU treatment is most often restricted to contrast-enhanced T1 -weighted imaging, while it has been shown that the non-perfused volume may not reflect the extent of nonviable tumor tissue after HIFU treatment. There are multiple studies in which more advanced MRI methods were assessed for their suitability for the evaluation of HIFU treatment. While several of these methods seem promising regarding their sensitivity to HIFU-induced tissue changes, there is still ample room for improvement of MRI protocols for HIFU treatment evaluation. In this review article, we describe the major acute and delayed effects of HIFU treatment. For each effect, the MRI methods that have been-or could be-used to detect the associated tissue changes are described. In addition, the potential value of multiparametric MRI for the evaluation of HIFU treatment is discussed. The review ends with a discussion on future directions for the MRI-based evaluation of HIFU treatment.

Different magnetic resonance imaging patterns after transcranial magnetic resonance–guided focused ultrasound of the ventral intermediate nucleus of the thalamus and anterior limb of the internal capsule in patients with essential tremor or obsessive-compulsive disorder

OBJECT

The authors report different MRI patterns in patients with essential tremor (ET) or obsessive-compulsive disorder (OCD) after transcranial MR-guided focused ultrasound (MRgFUS) and discuss possible causes of occasional MRgFUS failure.

METHODS

Between March 2012 and August 2013, MRgFUS was used to perform unilateral thalamotomy in 11 ET patients and bilateral anterior limb capsulotomy in 6 OCD patients; in all patients symptoms were refractory to drug therapy. Sequential MR images were obtained in patients across a 6-month follow-up period.

RESULTS

For OCD patients, lesion size slowly increased and peaked 1 week after treatment, after which lesion size gradually decreased. For ET patients, lesions were visible immediately after treatment and markedly reduced in size as time passed. In 3 ET patients and 1 OCD patient, there was no or little temperature rise (i.e., < 52°C) during MRgFUS. Successful and failed patient groups showed differences in their ratio of cortical-to-bone marrow thickness (i.e., skull density).

CONCLUSIONS

The authors found different MRI pattern evolution after MRgFUS for white matter and gray matter. Their results suggest that skull characteristics, such as low skull density, should be evaluated prior to MRgFUS to successfully achieve thermal rise.

A magnetic resonance imaging, histological, and dose modeling comparison of focused ultrasound, radiofrequency, and Gamma Knife radiosurgery lesions in swine thalamus

Object

The purpose of this study was to use MRI and histology to compare stereotactic lesioning modalities in a large brain model of thalamotomy.

Methods

A unilateral thalamotomy was performed in piglets utilizing one of 3 stereotactic lesioning modalities: focused ultrasound (FUS), radiofrequency, and radiosurgery. Standard clinical lesioning parameters were used for each treatment; and clinical, MRI, and histological assessments were made at early (< 72 hours), subacute (1 week), and later (1–3 months) time intervals.

Results

Histological and MRI assessment showed similar development for FUS and radiofrequency lesions. T2-weighted MRI revealed 3 concentric lesional zones at 48 hours with resolution of perilesional edema by 1 week. Acute ischemic infarction with macrophage infiltration was most prominent at 72 hours, with subsequent resolution of the inflammatory reaction and coalescence of the necrotic zone. There was no apparent difference in ischemic penumbra or “sharpness” between FUS or radiofrequency lesions. The radiosurgery lesions presented differently, with latent effects, less circumscribed lesions at 3 months, and apparent histological changes seen in white matter beyond the thalamic target. Additionally, thermal and radiation lesioning gradients were compared with modeling by dose to examine the theoretical penumbra.

Conclusions

In swine thalamus, FUS and radiosurgery lesions evolve similarly as determined by MRI, histological examination, and theoretical modeling. Radiosurgery produces lesions with more delayed effects and seemed to result in changes in the white matter beyond the thalamic target.

Prostate swelling and shift during high intensity focused ultrasound: implication for targeted focal therapy

PURPOSE

We quantified prostate swelling and the intraprostatic point shift during high intensity focused ultrasound using real-time ultrasound.

MATERIALS AND METHODS

The institutional review board approved this retrospective study. Whole gland high intensity focused ultrasound was done in 44 patients with clinically localized prostate cancer. Three high intensity focused ultrasound sessions were required to cover the entire prostate, including the anterior zone (session 1), middle zone (session 2) and posterior zone (session 3). Computer assisted 3-dimensional reconstructions based on 3 mm step-section images of intraoperative transrectal ultrasound were compared before and after each session.

RESULTS

Most prostate swelling and intraprostatic point shifts occurred during session 1. The median percent volume increase was 18% for the transition zone, 9% for the peripheral zone and 13% for the entire prostate. The volume percent increase in the transition zone (p <0.001), peripheral zone (p = 0.001) and entire prostate (p = 0.001) statistically depended on the volume of each area measured preoperatively. The median 3-dimensional intraprostatic shift was 3.7 mm (range 0.9 to 13) in the transition zone and 5.5 mm (range 0.2 to 14) in the peripheral zone. A significant negative linear correlation was found between the preoperative presumed circle area ratio, and the percent increase in prostate volume (p = 0.001) and shift (p = 0.01) during high intensity focused ultrasound.

CONCLUSIONS

We quantified significant prostate swelling and shift during high intensity focused ultrasound. Smaller prostates and a smaller preoperative presumed circle area ratio were associated with greater prostate swelling and intraprostatic shifts. Real-time intraoperative adjustment of the treatment plan impacts the achievement of precise targeting during high intensity focused ultrasound, especially in prostates with a smaller volume and/or a smaller preoperative presumed circle area ratio.

Future potential of MRI-guided focused ultrasound brain surgery

Magnetic resonance image-guided focused ultrasound surgery (MRgFUS) has surfaced as a viable noninvasive image-guided therapeutic method that integrates focused ultrasound (FUS), the therapeutic component, with magnetic resonance imaging (MRI), the image guidance module, into a real-time therapy delivery system with closed-loop control of energy delivery. The main applications for MRgFUS of the brain are thermal ablations for brain tumors and functional neurosurgery, and nonthermal, nonablative uses for disruption of the blood brain barrier (BBB) or blood clot and hematoma dissolution by liquification. The disruption of the BBB by FUS can be used for targeted delivery of chemotherapy and other therapeutic agents. MRI is used preoperatively for target definition and treatment planning, intraoperatively for procedure monitoring and control, and postoperatively for validating treatment success. Although challenges still remain, this integrated noninvasive therapy delivery system is anticipated to change current treatment paradigms in neurosurgery and the clinical neurosciences.

Preliminary assessment of one-dimensional MR elastography for use in monitoring focused ultrasound therapy

The purpose of this work is to assess a fast technique that measures tissue stiffness and temperature during focused ultrasound thermal therapy (FUS). A one-dimensional (1D) MR elastography (MRE) pulse sequence was evaluated for the purpose of obtaining rapid measurements of thermally induced changes in tissue stiffness and temperature for monitoring FUS treatments. The accuracy of the 1D measurement was studied by comparing tissue displacements measured by 1D MRE with those measured by the well-established 2D MRE pulse sequence. The reproducibility of the 1D MRE measurement was assessed, in gel phantoms and ex vivo porcine tissue, for varied FUS intensity levels (31.5-199.9 W cm(-2)) and over a range of displacements at the focus (0.1-1 microm). Temperature elevations in agarose gel phantoms were measured using 1D MRE and calibrated using fiberoptic-thermometer-based measurements. The 1D MRE displacement measurements are highly correlated with those obtained with the 2D technique (R(2) = 0.88-0.93), indicating that 1D MRE can successfully measure tissue displacement. Ten repeated trials at each FUS power level yielded a minimum detectable displacement change of 0.2 microm in phantoms and 0.4 microm in tissue (at 95% confidence level). The 1D MRE temperature measurements correlated well with temperature changes measured simultaneously with fiberoptic thermometers (R(2) = 0.97). The 1D MRE technique is capable of detecting tissue displacements as low as 0.4 microm, which is an order of magnitude smaller than 5 microm displacements expected during FUS therapy (Le et al 2005 AIP Conf. Proc.: Ther. Ultrasound 829 186-90). Additionally, 1D MRE was shown to provide adequate measurements of temperature elevations in tissue. These findings indicate that 1D MRE may be an effective tool for monitoring FUS treatments.

MAGNETIC RESONANCE IMAGING-GUIDED FOCUSED ULTRASOUND FOR THERMAL ABLATION IN THE BRAIN

INTRODUCTION

Magnetic resonance imaging (MRI)-guided focused ultrasound is a novel technique that was developed to enable precise, image-guided targeting and destruction of tumors by thermocoagulation. The system, ExAblate2000, is a focused ultrasound delivery system embedded within the MRI bed of a conventional diagnostic MRI scanner. The device delivers small volumetric sonications from an ultrasound phased array transmitter that converge energy to selectively destroy the target. Temperature maps generated by the MRI scanner verify the location and thermal rise as feedback, as well as thermal destruction. To assess the safety, feasibility, and precision of this technology in the brain, we have used the ExAblate system to create predefined thermal lesions in the brains of pigs.

METHODS

Ten pigs underwent bilateral craniectomy to provide a bone window for the ultrasound beams. Seven to 10 days later, the animals were anesthetized and positioned in the ExAblate system. A predefined, 1-cm frontal para ventricular region was delineated as the target and treated with multiple sonications. MRI was performed immediately and 1 week after treatment. The animals were then sacrificed and the brains removed for pathological study. The size of individual sonication points and the location of the lesion were compared between the planned dose maps, posttreatment MRI scans, and pathological specimen.

RESULTS

High-energy sonications led to precise coagulation necrosis of the specified targets as shown by subsequent MRI, macroscopic, and histological analysis. The thermal lesions were sharply demarcated from the surrounding brain with no anatomic or histological abnormalities outside the target.

CONCLUSION

MRI-guided focused ultrasound proved a precise and an effective means to destroy anatomically predefined brain targets by thermocoagulation with minimal associated edema or damage to adjacent structures. Contrast-enhanced T1-, T2-, and diffusion-weighted MRI scans may be used for real-time assessment of tissue destruction.

Uterine fibroids: diffusion-weighted MR imaging for monitoring therapy with focused ultrasound surgery--preliminary study

PURPOSE

To prospectively determine the feasibility of using diffusion-weighted (DW) imaging and apparent diffusion coefficient (ADC) mapping before (baseline) and after treatment and at 6-month follow-up to monitor magnetic resonance (MR) image-guided focused ultrasound surgical ablation of uterine fibroids.

MATERIALS AND METHODS

Informed consent was obtained from patients before treatment with our study protocol, as approved by the institutional review board, and the study complied with the Health Insurance Portability and Accountability Act. Fourteen patients (mean age, 46 years +/- 5 [standard deviation]) who underwent DW imaging were enrolled in this study, and 12 of 14 completed the inclusive MR examination with DW imaging at 6-month follow-up. Treatment was performed by one radiologist with a modified MR image-guided focused ultrasound surgical system coupled with a 1.5-T MR imager. Pre- and posttreatment and 6-month follow-up MR images were obtained by using phase-sensitive T1-weighted fast spoiled gradient-recalled acquisition, T1-weighted contrast material-enhanced, and DW imaging sequences. Total treatment time was 1-3 hours. Trace ADC maps were constructed for quantitative analysis. Regions of interest localized to areas of hyperintensity on DW images were drawn on postcontrast images, and quantitative statistics were obtained from treated and nontreated uterine tissue before and after treatment and at 6-month follow-up. Statistical analysis was performed with analysis of variance. Differences with P < .05 were considered statistically significant.

RESULTS

T1-weighted contrast-enhancing fibroids selected for treatment had no hyperintense or hypointense signal intensity changes on the DW images or ADC maps before treatment. Considerably increased signal intensity changes that were localized within the treated areas were noted on DW images. Mean baseline ADC value in fibroids was 1504 mm(-6)/sec2 +/- 290. Posttreatment ADC values for nontreated fibroid tissue (1685 mm(-6)/sec2 +/- 468) differed from posttreatment ADC values for fibroid tissue (1078 mm(-6)/sec2 +/- 293) (P = .001). A significant difference (P < .001) between ADC values for treated (1905 mm(-6)/sec2 +/- 446) and nontreated (1437 mm(-6)/sec2 +/- 270) fibroid tissue at 6-month follow-up was observed.

CONCLUSION

DW imaging and ADC mapping are feasible for identification of ablated tissue after focused ultrasound treatment of uterine fibroids.

Blood perfusion and thermal conduction effects in Gaussian beam, minimum time single-pulse thermal therapies

A previous analytical study has shown that the minimum obtainable treatment time for a single pulse that delivers a given thermal dose to a specified point at a specified time occurs when the temperature at that point is rapidly raised to its maximum allowable value. The present study extends that result by investigating the spatial distribution of thermal effects of a single Gaussian shaped focal zone pulse that reaches that maximum allowable temperature at the center point of the focal zone. Analytical solutions are obtained that separately include the effects of perfusion and conduction. This situation is analyzed for a conservative treatment strategy in which the desired thermal dose is delivered when the tumor cools down to basal conditions. The results show that for a specified thermal dose delivered by a spherical Gaussian beam with focal widths below approximately 4 mm, the maximum allowable temperature, the minimum obtainable treatment time, and the size of the treatment zone (as a percentage of the size of the Gaussian beam) are all independent of the tissue blood perfusion, and are only functions of the focal zone size. Conversely, for focal widths above approximately 20 cm, these results are independent of the focal width and are only functions of blood perfusion. Between these two sizes (where most practical treatments will occur, since single pulses with widths of <4 mm and >20 cm will be uncommon in practice) a transition zone exists in which both perfusion and conduction effects are important. Thus while it is possible to implement a truly perfusion-independent, single pulse thermal treatment by using focal widths of <4 mm, in practice many such pulses will be needed to treat most tumors. This is especially true since the nonlinear temperature/thermal dose relationship causes the width of the delivered dose distribution to be only approximately 25%-30% of the width of the focal zone. However, shorter overall treatment times can be obtained when multiple pulses are linked together by using larger focal zone sizes, but this gain in treatment time is accompanied by increased effects of perfusion, illustrating the conflict between attaining both perfusion-independence and minimal treatment time for multiple-pulse thermal treatments.

Emerging technologies in therapeutic ultrasound: thermal ablation to gene delivery

Ultrasound is used today in medicine as a modality for diagnostic imaging. Recently, there have been numerous reports on the application of thermal and nonthermal ultrasound energy for treating various diseases. In addition to thermal ablation of tumors, non-thermal ultrasound combined with drugs and genes have led to much excitement especially for cancer treatment, vascular diseases, and regenerative medicine. Ultrasound energy can enhance the effects of thrombolytic agents such as urokinase for treatment of stroke and acute myocardial infarction. New ultrasound technologies have resulted in advanced devices such as a) ultrasound catheters, b) Non-invasive methods as high intensity focused ultrasound (HIFU) in conjunction with MRI and CT is already being applied in the clinical field, c) Chemical activation of drugs by ultrasound energy for treatment of tumors is another new field recently termed "Sonodynamic Therapy", and d) Combination of genes and microbubble have induced great hopes for ideal gene therapy (sonoporation). Various examples of ultrasound combined modalities are under investigation which could lead to revolutionary therapy.

Prediction of subtle thermal histopathological change using a novel analysis of Gd-DTPA kinetics

PURPOSE

To investigate Gd-DTPA kinetics as predictors of histopathological changes following focused ultrasound (FUS) thermal ablation for improved planning and assessment.

MATERIALS AND METHODS

Twenty-nine FUS lesions were created in the thigh muscle of eight rabbits under MR-guidance at 1.5 Tesla. Three rabbits were killed at four hours; and 11 lesions were analyzed with histopathology. Temperature-sensitive MRI using proton-resonant frequency-shift was used for time-dependent temperature measurements. Analysis of the uptake kinetics of Gd-DTPA was performed after Gd-DTPA injection, within 20 minutes after heating and again at two hours after heating. The resulting kinetic maps, permeability (K(trans)) and leakage space (v(e)), were correlated to peak temperatures, T(2)-weighted MR, and histopathology.

RESULTS

Images of K(trans) and v(e) reveal regions of histopathological change not visible on conventional post-therapy MR. At early times after heating, v(e) predicts the area of injury more accurately than T(2) (7 +/- 2% vs. 25 +/- 6% underestimation). A circular region of extensive structural/vascular disruption is indicated only on K(trans) maps. The sharp decrease in K(trans) at the boundary of this region occurs at 47.5 +/- 0.5 degrees C, and may be a better estimate of cell death than the conventional method of temperature threshold (55 degrees C for coagulation) used in therapy planning.

CONCLUSION

Our results suggest Gd-DTPA kinetics can predict different histopathological changes following FUS ablation and may be valuable for early prediction.

High intensity focused ultrasound ablation of kidney guided by MRI

The effectiveness of magnetic resonance imaging (MRI) to monitor therapeutic protocols of high-intensity focused ultrasound (HIFU), in freshly excised pig kidney cortex is investigated. For high quality imaging, the pulse sequence fast spin echo (FSE) T1- and T2-weighted, and proton density were evaluated. For fast imaging, the pulse sequence T1-weighted fast spoiled gradient (FSPGR) was used. The main goal was to evaluate the MRI detection of large lesions (bigger than 1 cm x 1 cm x 1 cm) that is achieved by moving the transducer in a predetermined pattern. The contrast between lesion and kidney tissue is excellent with either T1-weighted or T2-weighted FSE. With T1-weighted FSE, the best contrast is observed for recovery time (TR) between 200 ms and 400 ms. With T2-weighted FSE best contrast can be achieved for echo time (TE) between 16 and 32 ms. T2-weighted FSE was proven as the best pulse sequence to detect cavitational activity. This advantage is attributed to the significant difference in signal intensity between air spaces and necrotic tissue. Air spaces appear brighter than thermal lesions. Therefore, for therapeutic protocols created using cavitational mode, T2-weighted FSE may be the optimum pulse sequence to use. The proton density pulse sequence does not provide any advantage over the T1- and T2-weighted pulse sequences. Using T1-weighted FSPGR, acquisition time as low as 5 s could be achieved. Good contrast and signal-to-noise ratio (SNR) are achieved with TR = 100 ms and flip angle between 75 to 90 degrees. The above techniques were very successful in detecting large lesion volumes.

Non-invasive transcranial high intensity focused ultrasound (HIFUS) under MRI thermometry and guidance in the treatment of brain lesions

Non-invasive transcranial high intensity focused ultrasound (HIFUS) therapy given under MRI thermometry and image guidance to awake patients lying within the bore of a 1.5 T MRI scanner (a) to thermally ablate brain lesions such as metastases, (b) to cause precise ablative brain lesions in functional disorders, or (c) to locally open the blood-brain-barrier for targeted therapeutic construct delivery--without the radiation risks of stereotactic radiotherapy--may sound science fiction. Kullervo Hynynen, a Finnish-born ultrasound and MRI physicist, and Ferenc Jolesz, a Hungarian-born neurosurgeon and visionary of image guided surgery, have joined forces at Radiology, Brigham & Women's Hospital, Boston, and they have taken every step to realize the vision above, in highly successful collaboration with the industry (GE, InSightec, TxSonics). The sophisticated transcranial HIFUS instrumentation, supported by profound research data from experimental animals and by the clinical experience from extracranial HIFUS targets (breast fibroadenoma, uterine fibroid), is now coming to a phase I clinical trial in cerebral metastases. It remains to be seen whether transcranial HIFUS will find applications in diffuse gliomas such as (a) thermal ablation of selected areas of glioma tissue, (b) opening the blood-brain-barrier for therapeutic constructs to enter selected areas, or (c) activating such constructs in desired areas. The prophecy of Dr. Jolesz, "this technology will put neurosurgeons out of business", may not fulfill during our lifetime.

Usefulness of contrast kinetics for predicting and monitoring tissue changes in muscle following thermal therapy in long survival studies

PURPOSE

To investigate Gd-DTPA kinetics as indicators of subacute and subchronic histopathological changes following focused ultrasound (FUS) thermal therapy for improved evaluation.

MATERIALS AND METHODS

A total of 18 FUS lesions were created in the thigh muscle of five rabbits under magnetic resonance (MR) guidance at 1.5 Tesla. The rabbits were killed at different times: 40 hours, three days, and seven days. All lesions were analyzed histologically. An analysis of the uptake kinetics of Gd-DTPA, injected within two hours postheating and before sacrifice, was performed. The resulting kinetic maps, permeability (K(trans)) and leakage space (v(e)), were correlated to T(2)-weighted MR and histology.

RESULTS

Images of K(trans) and v(e) better differentiate subacute and subchronic changes not visible on conventional MR in the days following therapy and are consistent with the histopathology observed. In particular, the border between nonviable and viable tissue is well demarcated. The extent of damage is best indicated on v(e), whereas the borders of inflammation are shown on K(trans). The total lesion extent is relatively stable over the 7 days posttherapy and can be predicted by v(e) or T(2)-weighted MR at early times after heating.

CONCLUSION

Our results suggest that Gd-DTPA kinetics can complement conventional MR for improved evaluation of FUS thermal therapy by providing finer differentiation of necrotic states, inflammation, and repair processes.

MRI-guided focused ultrasound surgery in the brain: tests in a primate model

MRI-guided focused ultrasound was tested in the brains of rhesus monkeys. Locations up to 4.8 cm deep were targeted. Focal heating was observed in all cases with MRI-derived temperature imaging. Subthreshold heating was observed at the focus when the ultrasound beam was targeted with low power sonications, and in the ultrasound beam path during high-power exposures. Lethal temperature values and histologically confirmed tissue damage were confined to the focal zone (e.g., not in the ultrasound beam path), except when the focus was close to the bone. In that case, damage to the neighboring brain tissue was observed. Focal lesions were observed on histological examination and, in some cases, in MR images acquired immediately after the ultrasound exposures. The capabilities demonstrated in this study will be of benefit for clinical ultrasound therapies in the brain.

Intraoperative magnetic resonance imaging and magnetic resonance imaging-guided therapy for brain tumors

Since their introduction into surgical practice in the mid 1990s, intraoperative MRI systems have evolved into essential, routinely used tools for the surgical treatment of brain tumors in many centers. Clear delineation of the lesion, "under-the-surface" vision, and the possibility of obtaining real-time feedback on the extent of resection and the position of residual tumor tissue (which may change during surgery due to "brain-shift") are the main strengths of this method. High-performance computing has further extended the capabilities of intraoperative MRI systems, opening the way for using multimodal information and 3D anatomical reconstructions, which can be updated in "near real time." MRI sensitivity to thermal changes has also opened the way for innovative, minimally invasive (LASER ablations) as well as noninvasive therapeutic approaches for brain tumors (focused ultrasound). Although we have not used intraoperative MRI in clinical applications sufficiently long to assess long-term outcomes, this method clearly enhances the ability of the neurosurgeon to navigate the surgical field with greater accuracy, to avoid critical anatomic structures with greater efficacy, and to reduce the overall invasiveness of the surgery itself.

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

PURPOSE

To study the feasibility of an interleaved gradient-echo, echo-planar imaging (iGE-EPI) sequence for multiplanar magnetic resonance temperature imaging (MRTI) to monitor intracerebral thermal treatment three-dimensionally using multielement ultrasound applicators.

MATERIALS AND METHODS

Transmissible venereal tumor (TVT) fragments were injected into the right cerebral hemisphere of five dogs. Guided by MRI, an interstitial ultrasound applicator was inserted into the tumor or normal brain tissue. The iGE-EPI sequence was used to estimate temperature changes by computing the complex phase-difference induced by temperature-dependent shifts in the proton resonance frequency of water. The thermal dose maps were updated every 6-8 seconds for five to seven image planes during treatment. The results of MRTI were compared with those of post-treatment MRI and histologic analysis.

RESULTS

The multiplanar MRTI monitored temperature and thermal dose distributions in tumor and normal brain tissue over the entire user-defined treatment volume. The ultrasound applicators produced contiguous areas of coagulative necrosis, resulting in 1.5-4.0 cm(3) volumes of tissue necrosis. MRTI-based assessments of thermal-dose distributions were consistent with the results of post-treatment MRI and histologic analysis.

CONCLUSION

Multiplanar MRTI is feasible for measuring necrosing thermal doses during intracerebral thermal delivery by interstitial ultrasound applicators.

Ultrasound nucleolysis: an in vitro study

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

Study of laser ablation in the in vivo rabbit brain with MR thermometry

PURPOSE

To investigate the peak temperature and thermal dose (T(43)) as tissue damage indicators for thermal therapy.

MATERIALS AND METHODS

The proton resonant frequency (PRF) shift thermal coefficient was calibrated on six in vivo rabbit brains during interstitial laser ablation. The peak temperature and T(43) were correlated with the lesion boundary observed on T2-weighted spin-echo (SE) MRI at 4 hours post-heating in seven thermal lesions using direct MR measurement and analysis based on a binary discriminate model.

RESULTS

The peak temperature and T(43) were 48.3 +/- 1.7 degrees C and 191 +/- 219 minutes, respectively, from the direct MR measurement. The values derived by the binary discriminate analysis were 47.8 +/- 2.2 degrees C and 28 +/- 41 minutes, respectively.

CONCLUSION

Our results suggest that tissue damage in rabbit brain 4 hours after thermal ablation can be predicted reliably from a threshold temperature of approximately 48 degrees C.

Intraoperative magnetic resonance: the future of surgery

Intraoperative magnetic resonance imaging (iMRI) is a new development in medicine that bridges the specialties of surgery and radiology. Deficiencies in the visualization of anatomical architecture and the perception of tumour boundaries in conventional open surgery have led to the integration of imaging within surgery. The superior soft tissue and multiplanar imaging features of magnetic resonance (MR) make this imaging modality superior to that of alternatives. The unique properties of MR to detect heat change and perfusion, and diffusion characteristics of tissue enhance the usefulness of this medium. Concurrent developments in computer aided image guidance and thermoablative technology, herald the era of minimally invasive tumour ablation. Applications have been developed for areas such as neurosurgery, general surgery, gynaecology and urology.

Fabrication and characterization of controlled release poly(D,L-lactide-co-glycolide) millirods

A compression-heat molding procedure was developed to fabricate poly(D,L-lactide-co-glycolide) (PLGA) controlled release drug delivery devices for the local treatment of tumors. The drug delivery devices were designed in the shape of a cylindrical millirod (1.6-mm diameter, 10-mm length), which allows them to be implanted by a modified 14-gauge tissue biopsy needle into tumor tissues via image-guided interventional procedures. In this study, the prototype trypan blue-containing PLGA millirods were fabricated under a compression pressure of 4.6 x 10(6) Pa and different fabrication temperatures for 2 h. The scanning electron microscopy results showed complete polymer annealing for millirods fabricated at 80 and 90 degrees C, while the cross sections of the 60 and 70 degrees C millirods showed incompletely annealed PLGA microspheres and trypan blue powders. The density, flexural modulus, and release properties of the PLGA millirods were also characterized and compared. The average values of the density and flexural modulus of the millirods increased with an increase in fabrication temperature. The flexural modulus values of most PLGA millirods were above 1 x 10(8) Pa, which provides sufficient stiffness for implantation within the tumor tissue. In addition, a Delta c(p) method was developed to determine the loading density of trypan blue in the PLGA millirods by differential scanning calorimetry. Results from the Delta c(p) measurement showed that trypan blue was homogeneously distributed in the millirod. Release studies in phosphate-buffered saline showed that the release rate decreased for the millirods fabricated at higher temperatures. The times for the release of 50% trypan blue were 5, 25, 25, and 25 h for millirods fabricated at 60, 70, 80, and 90 degrees C, respectively. Millirods fabricated at 90 degrees C had the most reproducible release profiles. The results from this study established compression--heat molding as an effective method to fabricate controlled release PLGA millirods with sufficient mechanical strength and reproducible release profiles for local cancer therapy.

MRI study of immediate cell viability in focused ultrasound lesions in the rabbit brain

The purpose of this study was to evaluate cell viability in MR imaged focused ultrasound (FUS) lesions using cell-viability staining with triphenyl tetrazolium chloride (TTC) and both light and electron microscopy. Ten paired ultrasonic lesions were created in 5 rabbit brains in vivo with an ultrasound beam of 1.5 MHz electrical power input to the transducer of 50 W and exposure duration of 15 seconds. T2-weighted fast spin-echo (FSE) MRI was performed to detect the FUS lesions in the brain 4 hours after treatment, after which the animals were immediately euthanized. Lesion sizes were measured on TTC-stained specimens, histological sections stained with hematoxylin and eosin (H&E), and T2-weighted MR images. The differences between the lesion diameters measured with the three methods were within the range of 0.1--0.7 mm. The lesion sizes measured from MRI correlated well with those seen from H&E sections. The measurements from MRI slightly overestimated lesion sizes on TTC-stained wet tissues by approximately one MRI pixel (0.31 mm). Electron microscopy demonstrated nuclear and cytoplasmic ultrastructural damage within the grey-white, non-TTC-stained lesion zone, whereas the TTC-stained normal tissue showed preservation of neuronal ultrastructure. Therefore, MR-imaged lesions represent a cell-death zone in rabbit brain 4 hours after FUS ablation, with slight overestimation by approximately one MRI pixel. J. Magn. Reson. Imaging 2001;13:23-30.

MRI detection of the thermal effects of focused ultrasound on the brain

This study tested the hypothesis that MRI thermometry can be correlated with the different degrees of tissue damage observed after focused ultrasound (US) exposure of brain. The brains of 6 rabbits were sonicated to calibrate the MRI proton resonant shift with temperature. In addition, 13 rabbits were sonicated at acoustic powers ranging from 3.5 to 17.5 W. The experiments were performed in a 1.5-T MRI scanner with the temperature-sensitive phase imaging used during the sonications of 4-5 different locations in each rabbit. MR images were obtained 2 h and 2 days after the sonications, depending on when the animals were sacrificed. Whole brain histologic evaluation was performed by sectioning the brain and performing a microscopic investigation. The MRI-derived temperature elevation was found to correlate well with the degree of tissue damage. In addition to the common histology findings, apoptotic cells were observed in the lesions. The T1-weighted contrast enhanced and T2-weighted scans both detected the brain damage. The applied acoustic power did not correlate well with the degree of damage. As a conclusion, the results showed that the measurement of temperature elevations by MRI during sonications can improve the accuracy and safety of clinical US brain surgery.

Study of focused ultrasound tissue damage using MRI and histology

This paper reports on an experimental study of in vivo tissue damage in the rabbit brain with focused ultrasound (FUS) using magnetic resonance imaging (MRI) and histopathological analysis. Ten ultrasonic lesions (tissue damage) were created in five rabbits using a focused ultrasound beam of 1.5 MHz, electrical power input to the transducer of 70-85 W, and an exposure duration of 15-20 seconds. T1- and T2-weighted fast spin-echo (FSE) and Fluid attenuated inversion recovery (FLAIR) sequences were used to detect the ultrasonic lesions after treatment. Imaging was performed for 4-8 hours after treatment, after which the animals were immediately sacrificed. Ultrasonic lesion diameter was measured on MRI and histological sections after correction for tissue shrinkage during the histological processing. The T1-weighted images showed lesions poorly, whereas both T2-weighted and FLAIR images showed lesions clearly. The lesion diameters on both T2 and FLAIR imaging correlated well with measurements from histology. The time delay before lesions appeared on T2-weighted imaging was 15 minutes to 1 hour, depending on the exposure location in the brain. J. Magn. Reson. Imaging 1999;10:146-153.