Magnetic Resonance Thermal Imaging Research Articles

SYST-17 GOLD NANOSTARS ENHANCE THE EFFICACY AND SAFETY OF LASER INTERSTITIAL THERAPY FOR THE TREATMENT OF INTRACRANIAL TUMORS

Abstract Laser interstitial thermal therapy (LITT) is a minimally-invasive technique for the treatment of intracranial tumors that uses a stereotactically-guided laser for ablation. Magnetic resonance thermal imaging is used to monitor ablation progress by measuring the cumulative heat energy exposure in both the tumor and the surrounding tissue to calculate thermal damage thresholds.Previous applications of GNS in laser ablation of tumors have studied extracranial tumors only and utilized externally administered near infrared radiation that cannot penetrate the cranial compartment. Herein, we present a novel platform for employing GNS in intracranial tumors. We developed GNS that exhibit maximum photothermal effects at the 1064 nm laser wavelength used in the FDA-approved clinical Neuroblate® LITT system. Monte Carlo simulations demonstrate that our GNS can accelerate and focus thermal energy distributions. Using ex vivo tumor phantom models, GNS-infused phantoms demonstrate more rapid heating, expanded ablation zones, and improved margin conformation, with surrounding areas exposed to lower temperatures than controls. In in vivo intracranial tumor models, we show that GNS selectively accumulate and are retained in CT2A glioma and B16F0 melanoma tumors at 24 and 72 hours post-injection. Using our in vivo heterotopic CT2A model, we demonstrate that systemically-delivered GNS dramatically augment laser ablation, with higher temperatures achieved in the tumor and more efficient temperature ramping. These results support the ability of GNS to optimize LITT for the treatment of intracranial tumors by improving the efficiency and safety of this platform.

What happens to brain outside the thermal ablation zones? An assessment of needle-based therapeutic ultrasound in survival swine

Background In stereotactic radiosurgery, isodose lines must be considered to determine how surrounding tissue is affected. In thermal ablative therapy, such as laser interstitial thermal therapy (LITT), transcranial MR-guided focused ultrasound (tcMRgFUS), and needle-based therapeutic ultrasound (NBTU), how the surrounding area is affected has not been well studied. Objective We aimed to quantify the transition zone surrounding the ablation core created by magnetic resonance-guided robotically-assisted (MRgRA) delivery of NBTU using multi-slice volumetric 2-D magnetic resonance thermal imaging (MRTI) and subsequent characterization of the resultant tissue damage using histopathologic analysis. Methods Four swine underwent MRgRA NBTU using varying duration and wattage for treatment delivery. Serial MRI images were obtained, and the most representative were overlaid with isodose lines and compared to brain tissue acquired postmortem which underwent histopathologic analysis. These results were also compared to predicted volumes using a finite element analysis model. Contralateral brain tissue was used for control data. Results Intraoperative MRTI thermal isodose contours were characterized and comprehensively mapped to post-operative MRI images and qualitatively compared with histological tissue sections postmortem. NBTU 360° ablations induced smaller lesion volumes (33.19 mm3; 120 s, 3 W; 30.05 mm3, 180 s, 4 W) versus 180° ablations (77.20 mm3, 120 s, 3 W; 109.29 mm3; 180 s; 4 W). MRTI/MRI overlay demonstrated the lesion within the proximal isodose lines. The ablation-zone was characterized by dense macrophage infiltration and glial/neuronal loss as demonstrated by glial fibrillary acidic protein (GFAP) and neurofilament (NF) absence and avid CD163 staining. The transition-zone between lesion and normal brain demonstrated decreased macrophage infiltration and measured ∼345 microns (n − 3). We did not detect overt hemorrhages or signs of edema in the adjacent spared tissue. Conclusion We successfully performed MRgRA NBTU ablation in swine and demonstrated minimal histologic changes extended past the ablation-zone. The lesion was characterized by macrophage infiltration and glial/neuronal loss which decreased through the transition-zone.

Design of a wideband metamaterial absorber for optical wavelength applications based on the quantum-inspired Hadamard matrix

Attaining broadband absorption with a simple-designed photonic absorber is still considered a complex and daunting task. The Hadamard matrix has been a well-known concept for designing quantum gates in quantum mechanics and computing. Despite a series of noteworthy works being done with regard to the design of metamaterial absorbers, very few works exist in literature where a quantum-inspired design has been employed to design a metamaterial-based photonic absorber. In this work, we have demonstrated a polarization-insensitive ultrathin and wideband metamaterial (MM) absorber based on the pattern distribution of the Hadamard matrix. Within the optical domain, the demonstrated MM absorber showcases absorption of over 91% for both transverse electric (TE) mode and transverse magnetic (TM) mode. To explore the angular dependence on absorption features of our Hadamard matrix meta-absorber (HMMA), both TE and TM modes have been used at numerous incident-angles. Finite integration technique has been utilized to simulate the demonstrated MM absorber design and validated using the interference theory model to assure the simulated data. Moreover, electric and magnetic field characteristics, current distributions, and a plethora of parametric sweeps have also been investigated in order to better understand the suggested HMMA absorption mechanism. Because of its wideband absorption and polarization-insensitive characteristic, this MM absorber based on the Hadamard matrix arrangement permits a variety of applications such as light detectors, optical-sensors, magnetic resonance imaging, plasmonic-sensors, and thermal imaging applications.

Polarization Insensitive Broadband Zero Indexed Nano-Meta Absorber for Optical Region Applications

Broadband response metamaterial absorber (MMA) remains a challenge among researchers. A nanostructured new zero-indexed metamaterial (ZIM) absorber is presented in this study, constructed with a hexagonal shape resonator for optical region applications. The design consists of a resonator and dielectric layers made with tungsten and quartz (Fused). The proposed absorbent exhibits average absorption of more than 0.8972 (89.72%) within the visible wavelength of 450–600 nm and nearly perfect absorption of 0.99 (99%) at 461.61 nm. Based on computational analysis, the proposed absorber can be characterized as ZIM. The developments of ZIM absorbers have demonstrated plasmonic resonance characteristics and a perfect impedance match. The incidence obliquity in typically the range of 0°–90° both in TE and TM mode with maximum absorbance is more than 0.8972 (∼89.72%), and up to 45° angular stability is suitable for solar cell applications, like exploiting solar energy. The proposed structure prototype is designed and simulated by studying microwave technology numerical computer simulation (CST) tools. The finite integration technique (FIT) based simulator CST and finite element method (FEM) based simulator HFSS also helps validate the numerical data of the proposed ZIM absorber. The proposed MMA design is appropriate for substantial absorption, wide-angle stability, absolute invisible layers, magnetic resonance imaging (MRI), color images, and thermal imaging applications.

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 12.6mm radius from the ultrasound applicator. The average temperature error along a 6.3mm radius profile from the applicator was 1.27 °C. The model was able to compute 1s of thermal deposition by the applicator in 0.2s on average with a 0.1mm spatial resolution and 0.5s time steps. The developed simulation demonstrates performance suitable for real-time control which may enable robotically-actuated closed-loop conformal tumor ablation.

Predicting ablation zones with multislice volumetric 2-D magnetic resonance thermal imaging

Background High-intensity focused ultrasound (HIFU) serves as a noninvasive stereotactic system for the ablation of brain metastases; however, treatments are limited to simple geometries and energy delivery is limited by the high acoustic attenuation of the calvarium. Minimally-invasive magnetic resonance-guided robotically-assisted (MRgRA) needle-based therapeutic ultrasound (NBTU) using multislice volumetric 2-D magnetic resonance thermal imaging (MRTI) overcomes these limitations and has potential to produce less collateral tissue damage than current methods. Objective To correlate multislice volumetric 2-D MRTI volumes with histologically confirmed regions of tissue damage in MRgRA NBTU. Methods Seven swine underwent a total of 8 frontal MRgRA NBTU lesions. MRTI ablation volumes were compared to histologic tissue damage on brain sections stained with 2,3,5-triphenyltetrazolium chloride (TTC). Bland-Altman analyses and correlation trends were used to compare MRTI and TTC ablation volumes. Results Data from the initial and third swine’s ablations were excluded due to sub-optimal tissue staining. For the remaining ablations (n = 6), the limits of agreement between the MRTI and histologic volumes ranged from −0.149 cm3 to 0.252 cm3 with a mean difference of 0.052 ± 0.042 cm3 (11.1%). There was a high correlation between the MRTI and histology volumes (r 2 = 0.831) with a strong linear relationship (r = 0.868). Conclusion We used a volumetric MRTI technique to accurately track thermal changes during MRgRA NBTU in preparation for human trials. Improved volumetric coverage with MRTI enhanced our delivery of therapy and has far-reaching implications for focused ultrasound in the broader clinical setting.

Numerical investigation of polarization-insensitive multiband metamaterial for terahertz solar absorber

This study presents a nanostructured multiband metamaterial absorber for the optical regime application. The proposed structure is exhibited 0° to 90° polarization insensitivity and up to 45° angular stability with more than 90% peak absorption in between 300 THz800 THz frequency range. Proposed MMA structure is acquired perfect absorption of 99.99% at 571THz, 99.50% at 488.26 THz, 99.32% at 598 THz frequency by adjusting geometrical parameters. Due to perfect impedance matching with plasmonic resonance characteristics, these structures achieved an average absorption of 95.30%, 91.96%, 97.25%, 97.65%, 91%,90%,90.23% in between (337.5 THz-346.5 THz), (471 THz-478 THz), (497.5 THz-505 THz), (519 THz-530.5 THz), 564.5 THz-577 THz), (604 THz-673 THz), (686 THz-708 THz), respectively. The near field pattern of proposed MMA is used to explain the absorption mechanism at resonance frequency point and the geometric parameters are explored and analyzed to demonstrate the performance of the proposed structure. Moreover, CST-HFSS interference is validated the simulation data with the help of the finite element method (FEM). Polarization insensitivity and wide angular stability in terahertz (THz) frequency regime make this structure suitable for the application of magnetic resonance imaging (MRI), color images, thermal imaging and solar cell applications like exploitation of solar energy.

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

Magnetic resonance-guided laser interstitial thermal therapy (MRgLITT) is a minimally invasive procedure that utilizes intraoperative magnetic resonance thermal imaging (MRTI) to generate a thermal damage estimate (TDE) of the ablative area. In select cases, the MRTI contains a signal artifact or defect that distorts the ablative region. No study has attempted to characterize this artifact. To characterize MRTI signal the artifact in select cases to better understand its potential relevance and impact on the ablation procedure. All ablations were performed using the Visualase magnetic resonance imaging-guided laser ablation system (Medtronic). Patients were included if the MRTI contained signal artifact that distorted the ablative region during the first thermal dose delivered. Ablation artifact was quantified using MATLAB version R2018a (Mathworks Inc, Natick, Massachusetts). A total of 116 patients undergoing MRgLITT for various surgical indications were examined. MRTI artifact was observed in 37.0% of cases overall. Incidence of artifact was greater at higher powers (P<.001) and with longer ablation times (P=.024), though artifact size did not correlate with laser power or ablation duration. MRTI signal artifact is common during LITT. Higher powers and longer ablation times result in greater incidence of ablation artifact, though artifact size is not correlated with power or duration. Future studies should aim to evaluate effects of artifact on postoperative imaging and, most notably, patient outcomes.

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

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

Design and parametric analysis of a wide-angle polarization-insensitive metamaterial absorber with a star shape resonator for optical wavelength applications

Optical wavelengths considered as the key source of electromagnetic waves from the sun, and metamaterial absorber (MMA) enables various applications for this region like real invisible cloaks, color imaging, magnetic resonance imaging, light trapping, plasmonic sensor, light detector, and thermal imaging applications. Contemplated those applications, a new wide-angle, polarization-insensitive MMA is presented in this study. The absorber was formatted with three layers that consisted of a sandwiched metal-dielectric-metal structure. This formation of metamaterial absorber showed a good impedance match with plasmonic resonance characteristics. The structure was simulated using the FIT and validated with the FEM. A variety of parametric studies were performed with the design to gain best physical dimension. The mechanism of absorption also explained immensely by various significant analysis. The design had average 96.77% absorption from wavelengths of 389.34 nm to 697.19 nm and a near-perfect absorption of 99.99% at a wavelength of 545.73 nm for TEM mode. For an ultra-wide bandwidth of 102 nm, the design exhibited above 99% absorbance. The proposed is wide-angle independent up to 60° for both TE and TM mode, which is useful for solar energy harvesting, solar cell, and solar thermophotovoltaics (STPV). This MMA can be used for an optical sensor or as a light detector. Moreover, this proposed design can be employed in some applications mentioned above.

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

Magnetic resonance-guided laser interstitial thermal therapy (MRgLITT) is a minimally invasive procedure that utilizes intraoperative magnetic resonance thermal imaging (MRTI) to generate a thermal damage estimate (TDE) of the ablative area. In select cases, the MRTI contains a signal artifact or defect that distorts the ablative region. No study has considered the impact of this artifact on TDE accuracy. To determine the effect of intraoperative MRTI signal artifact on postoperative magnetic resonance imaging (MRI)-predicted ablative area. All ablations were performed using the Visualase MRI-Guided Laser Ablation System (Medtronic). Patients were grouped based on whether the intraoperative MRTI contained signal artifact that distorted the ablative region. Cross-sectional area of the ablative lesion from the MRI image was measured, and the difference between intraoperative TDE and postoperative MRI cross-sectional area was calculated and compared between groups with and without intraoperative MRTI artifact. A total of 91 patients undergoing MRgLITT for various surgical indications were examined. MRTI artifact was observed in 43.9% of cases overall. The mean absolute difference between TDE and the postoperative MRI cross-sectional area was 94.8 mm2 (SEM=11.6) in the group with intraoperative MRTI artifact and 54.4 mm2 (SEM=5.5) in the nonartifact group. MRTI signal artifact is common during LITT. The presence of signal artifact during intraoperative MRTI results in higher variation between intraoperative TDE and postoperative MRI cross-sectional ablative area. In cases in which intraoperative MRTI artifact is observed, there may be a larger degree of variation between observed intraoperative TDE and measured postoperative MRTI ablative area.

An Integrated Robotic System for MRI-Guided Neuroablation: Preclinical Evaluation.

Treatment of brain tumors requires high precision in order to ensure sufficient treatment while minimizing damage to surrounding healthy tissue. Ablation of such tumors using needle-based therapeutic ultrasound (NBTU) under real-time magnetic resonance imaging (MRI) can fulfill this need. However, the constrained space and strong magnetic field in the MRI bore restricts patient access limiting precise placement of the NBTU ablation tool. A surgical robot compatible with use inside the bore of an MRI scanner can alleviate these challenges. We present preclinical trials of a robotic system for NBTU ablation of brain tumors under real-time MRI guidance. The system comprises of an updated robotic manipulator and corresponding control electronics, the NBTU ablation system and applications for planning, navigation and monitoring of the system. The robotic system had a mean translational and rotational accuracy of 1.39 ±0.64mm and 1.27[Formula: see text] in gelatin phantoms and 3.13 ±1.41mm and 5.58[Formula: see text] in 10 porcine trials while causing a maximum reduction in signal to noise ratio (SNR) of 10.3%. The integrated robotic system can place NBTU ablator at a desired target location in porcine brain and monitor the ablation in realtime via magnetic resonance thermal imaging (MRTI). Further optimization of this system could result in a clinically viable system for use in human trials for various diagnostic or therapeutic neurosurgical interventions.

Brown Adipose Tissue - role in metabolic disorders

Brown adipose tissue, a thermogenic organ, previously thought to be present in only small mammals and children has recently been identified in adult humans. Located primarily in the supraclavicular and cervical area, it produces heat by uncoupling oxidative phosphorylation due to the unique presence of uncoupling protein 1 by a process called nonshivering thermogenesis. BAT activity depends on many factors including age, sex, adiposity and outdoor temperature. Positron-emission tomography using 18F-fluorodeoxyglucose and computed tomography (18F-FDG PET–CT), magnetic resonance imaging (MRI) and thermal imaging (IRT) are among several methods used to detect BAT in humans. The importance of BAT is due to its role in whole body energy expenditure and fuel metabolism. Thus it is postulated that it may be useful in the treatment of metabolic diseases. However, there are still many unanswered questions to the clinical usefulness of this novel tissue.&#x0D; IMC J Med Sci 2019; 13(1): 002

Real-time mapping of heat generation and distribution in a laser irradiated agar phantom loaded with gold nanoparticles using MR temperature imaging.

Gold nanoparticles (AuNPs) have shown potential strength in photothermal therapy of cancer. Several techniques have been developed to investigate local heat generation by AuNPs. However, a sensitive thermal imaging technology with high temporal resolution, minimum invasiveness and high spatial resolution is still lacking. In this research study, by using magnetic resonance thermal imaging (MRTI), we reported a technique for monitoring of heat generation and distribution in an AuNPs loaded agar phantom irradiated by laser. Three different agar phantoms with various AuNPs concentrations (0, 8 and 16 μg/ml) were produced and studied. The phantoms were exposed to an external laser [532 nm; 4 min] under MRTI. For real-time temperature monitoring, we employed the theory of proton resonance frequency (PRF) shift. Infrared (IR) camera was employed to measure the actual temperature of each point on the surface of irradiated agar gel. Finally, the correlation between the temperatures obtained by IR camera and MRTI was evaluated. We observed that temperature of the gels loaded by AuNPs at concentration of 0, 8 and 16 μg/ml reached 27.2, 37.8, 45 °C with a total area of heat distribution of 94.98, 452.16, and 907.34 mm2 (from the point of irradiation). During the process of laser irradiation, we observed: (i) a significant rise in temperature, (ii) a dependency between the rate of temperature rise and concentration of AuNPs, and (iii) a direct correlation between temperature change and MR image phase. In addition, statistical analysis showed that the variation of temperatures measured by IR camera and temperatures computed by MRTI had acceptable correlation (R > 0.9). In conclusion, MRTI has a good sensitivity and precision that can be employed for nano-photothermal therapy planning and may be considered for real-time mapping of heat generation and distribution in a laser irradiated tissue loaded by AuNPs.

Heat equation inversion framework for average SAR calculation from magnetic resonance thermal imaging.

Deposition of radiofrequency (RF) energy can be quantified via electric field or temperature change measurements. Magnetic resonance imaging has been used as a tool to measure three dimensional small temperature changes associated with RF radiation exposure. When duration of RF exposure is long, conversion from temperature change to specific absorption rate (SAR) is nontrivial due to prominent heat-diffusion and conduction effects. In this work, we demonstrated a method for calculation of SAR via an inversion of the heat equation including heat-diffusion and conduction effects. This method utilizes high-resolution three dimensional magnetic resonance temperature images and measured thermal properties of the phantom to achieve accurate calculation of SAR. Accuracy of the proposed method was analyzed with respect to operating frequency of a dipole antenna and parameters used in heat equation inversion. Bioelectromagnetics. 37:493-503, 2016. © 2016 Wiley Periodicals, Inc.

TU‐A‐9A‐04: Development of a Thermally Stable Phantom for Photoacoustic and Magnetic Resonance Temperature Imaging

Purpose:Photoacoustic‐ultrasonic (PAUS) imaging, which utilizes an ultrasound transducer to provide co‐registered photoacoustic and pulse‐echo ultrasound images, is capable of measuring temperature non‐invasively while simultaneously providing anatomical images. The sub‐millimeter resolution and centimeter‐order penetration depths achievable with PAUS imaging have the potential to deliver active monitoring of both a targeted tumor microenvironment and nearby healthy tissue during thermal ablation. These characteristics make PAUS imaging a promising new technique for guidance and monitoring during photothermal ablations of solid tumors. In order to assess the potential clinical role of PAUS imaging, the technique was validated against the clinically accepted magnetic resonance thermal imaging approach (MRTI).Methods:To facilitate co‐registration between modalities, the phantom had inclusions of gold nanoshells encapsulating super‐paramagnetic iron oxide (SPIO) particles, where gold enhances the PA signal and SPIOs provide negative contrast on MRI. Several phantom designs were assessed for resilience to heating. PA images were acquired on a Vevo LAZR (FUJIFILM VisualSonics Inc., Toronto, Ontario) PAultrasound small‐animal imaging system (21MHz) operating at 710nm. MRTI experiments were performed using a 6‐channel flex coil (GE Healthcare, Waukesha, WI) on a 3T MRI scanner (Discovery MR750, GE Healthcare, Waukesha, WI) using a fast multi gradient echo acquisition (16 echoes, 128×128 acquisition matrix, 25.6×25.6cm field of view, 3mm slice thickness, 60ms TR, 20° flip angle, 2.9ms minimum TE and 1.6ms echo spacing). The accuracy and spatio‐temporal resolution of PA thermography was cross‐validated with both MRTI and a fluoroptic temperature sensor (LumaSense Technologies, Santa Clara, CA) in the custom‐designed phantom.Results:A thermally stable, dual‐modality phantom was created for cross‐validation of photoacoustic thermography and MRTI. Axial and lateral resolutions of PA images were sub‐millimeter with a temporal resolution of 0.2s, which will accommodate precise real‐time guidance and monitoring.Conclusion:These results indicate that a PA thermography technique offers tremendous promise for real‐time thermal monitoring of ablative therapy.Funding support provided by Cancer Prevention Research Institute of Texas and Julia Jones Matthews family. No disclosures or conflicts of interest.

Real-Time Magnetic Resonance-Guided Stereotactic Laser Amygdalohippocampotomy for Mesial Temporal Lobe Epilepsy

Open surgery effectively treats mesial temporal lobe epilepsy, but carries the risk of neurocognitive deficits, which may be reduced with minimally invasive alternatives. To describe technical and clinical outcomes of stereotactic laser amygdalohippocampotomy with real-time magnetic resonance thermal imaging guidance. With patients under general anesthesia and using standard stereotactic methods, 13 adult patients with intractable mesial temporal lobe epilepsy (with and without mesial temporal sclerosis [MTS]) prospectively underwent insertion of a saline-cooled fiberoptic laser applicator in amygdalohippocampal structures from an occipital trajectory. Computer-controlled laser ablation was performed during continuous magnetic resonance thermal imaging followed by confirmatory contrast-enhanced anatomic imaging and volumetric reconstruction. Clinical outcomes were determined from seizure diaries. A mean 60% volume of the amygdalohippocampal complex was ablated in 13 patients (9 with MTS) undergoing 15 procedures. Median hospitalization was 1 day. With follow-up ranging from 5 to 26 months (median, 14 months), 77% (10/13) of patients achieved meaningful seizure reduction, of whom 54% (7/13) were free of disabling seizures. Of patients with preoperative MTS, 67% (6/9) achieved seizure freedom. All recurrences were observed before 6 months. Variances in ablation volume and length did not account for individual clinical outcomes. Although no complications of laser therapy itself were observed, 1 significant complication, a visual field defect, resulted from deviated insertion of a stereotactic aligning rod, which was corrected before ablation. Real-time magnetic resonance-guided stereotactic laser amygdalohippocampotomy is a technically novel, safe, and effective alternative to open surgery. Further evaluation with larger cohorts over time is warranted.

Applicators for Magnetic Resonance–Guided Ultrasonic Ablation of Benign Prostatic Hyperplasia

The aims of this study were to evaluate in a canine model applicators designed for ablation of human benign prostatic hyperplasia (BPH) in vivo under magnetic resonance imaging (MRI) guidance, including magnetic resonance thermal imaging (MRTI), determine the ability of MRI techniques to visualize ablative changes in prostate, and evaluate the acute and longer term histologic appearances of prostate tissue ablated during these studies. An MRI-compatible transurethral device incorporating a tubular transducer array with dual 120° sectors was used to ablate canine prostate tissue in vivo, in zones similar to regions of human BPH (enlarged transition zones). Magnetic resonance thermal imaging was used for monitoring of ablation in a 3-T environment, and postablation MRIs were performed to determine the visibility of ablated regions. Three canine prostates were ablated in acute studies, and 2 animals were rescanned before killing at 31 days postablation. Acute and chronic appearances of ablated prostate tissue were evaluated histologically and were correlated with the MRTI and postablation MRI scans. It was possible to ablate regions similar in size to enlarged transition zone in human BPH in 6 to 18 minutes. Regions of acute ablation showed a central "heat-fixed" region surrounded by a region of more obvious necrosis with complete disruption of tissue architecture. After 31 days, ablated regions demonstrated complete apparent resorption of ablated tissue with formation of cystic regions containing fluid. The inherent cooling of the urethra using the technique resulted in complete urethral preservation in all cases. Prostatic ablation of zones of size and shape corresponding to human BPH is possible using appropriate transurethral applicators using MRTI, and ablated tissue may be depicted clearly in contrast-enhanced magnetic resonance images. The ability accurately to monitor prostate tissue heating, the apparent resorption of ablated regions over 1 month, and the inherent urethral preservation suggest that the magnetic resonance-guided techniques described are highly promising for the in vivo ablation of symptomatic human BPH.

Magnetic resonance thermal imaging combined with SMASH navigators in the presence of motion

This study develops and tests an MR thermometry method combined with SMASH navigators in phantom experiments mimicking human liver motion with the purpose of detecting and correcting motion artifacts in thermal MR images. Experimental data were acquired on a 3T MRI scanner. Motion artifacts of mobile phantoms mimicking human liver motion were detected and corrected using the SMASH navigators and then MR temperature maps were obtained using a proton resonant frequency (PRF) shift method with complex image subtraction. Temperature acquired by MR thermal imaging was compared to that measured via thermocouples. MR thermal imaging combined with the SMASH navigator technique resulted in accurate temperature maps of the mobile phantoms compared to temperatures measured using the thermocouples. The differences between the obtained and measured temperatures varied from 8.2°C to 14.2°C and 2.2°C to 4.9°C without and with motion correction, respectively. Motion correction improved the temperature acquired by MR thermal imaging by >55%. The combination of the MR thermal imaging and SMASH navigator technique will enable monitoring and controlling heat distribution and temperature change in tissues during thermal therapies and will be a very important tool for cancer treatment in mobile organs.PACS number: 87.57.‐s

SU‐E‐J‐53: A Phantom Validation Study of a 3D Background Phase Model for MR Thermometry

The proton resonance frequency (PRF) shift method is the most widely accepted method for magnetic resonance thermal imaging to provide real-time treatment monitoring of thermal therapies. However, the PRF shift technique involves the subtraction of a reference phase map, which causes the technique to be easily perturbed by tissue motion and other background contaminations. In this study, a three-dimensional background phase is estimated in order to create a phase reference for each time point. A magnetic resonance spectroscopy (MRS) sphere was scanned within a 3T MRI scanner employing a 3D fast SPGR sequence. Real and imaginary images were acquired to obtain phase images as the control. The ability to predict the background phase was investigated by systematically removing phase information from the control data set. Data was initially removed from a spherical region of interest (ROI) to simulate a region where ablativeheating would take place. In a second case, the same spherical ROI was removed as well as every other slice to further reduce the amount of existing data. A 3D finite element model was implemented to solve the Dirichlet problem given a measured phase on the boundary of the simulated available data. Line profiles taken through the phantom indicate phase estimates to compare well with actual phase measurements. The phase estimation still shows good agreement when reducing the amount of data to every other slice. The 3D multi-slice temperature estimate potentially provides a robust technique that is not as susceptible to through-plane or in-plane motion-induced temperature artifacts as compared to thecurrent PRF shift method. The research in this paper was supported in part through 1R21EB010196-01.