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PubMed2005

Effects of spatial and temporal resolution for MR image-guided thermal ablation of prostate with transurethral ultrasound.

PURPOSE: To describe approaches for determining optimal spatial and temporal resolutions for the proton resonance frequency shift method of quantitative magnetic resonance temperature imaging (MRTI) guidance of transurethral ultrasonic prostate ablation. MATERIALS AND METHODS: Temperature distributions of two transurethral ultrasound applicators (90 degrees sectored tubular and planar arrays) for canine prostate ablation were measured via MRTI during in vivo sonication, and agree well with two-dimensional finite difference model simulations at various spatial resolutions. Measured temperature distributions establish the relevant signal-to-noise ratio (SNR) range for thermometry in an interventional MR scanner, and are reconstructed at different resolutions to compare resultant temperature measurements. Various temporal resolutions are calculated by averaging MRTI frames. RESULTS: When noise is added to simulated temperature distributions for tubular and planar applicators, the minimum root mean squared (RMS) error is achieved by reconstructing to pixel sizes of 1.9 and 1.7 mm, respectively. In in vivo measurements, low spatial resolution MRTI data are shown to reduce the noise without significantly affecting thermal dose calculations. Temporal resolution of 0.66 frames/minute leads to measurement errors of more than 12 degrees C during rapid heating. CONCLUSION: Optimizing MRTI pixel size entails balancing large pixel SNR gain with accuracy in representing underlying temperature distributions.

Animals↗
PubMed2005

Local delivery of magnetic resonance (MR) contrast agent in kidney using thermosensitive liposomes and MR imaging-guided local hyperthermia: a feasibility study in vivo.

PURPOSE: To investigate the feasibility of local delivery of a magnetic resonance (MR) contrast agent in vivo using paramagnetic thermosensitive liposomes and infrared (IR) laser-induced local hyperthermia under real-time MR thermometry on rabbit kidney. MATERIALS AND METHODS: Respiratory gated, radio frequency (RF)-spoiled gradient-echo sequences were used for precise MR temperature mapping (SD = 1 degrees C). In vivo heating experiments confirmed local release of MR contrast agent from liposomes. RESULTS: T1 decreased from 800 msec to about 500 msec, as measured after tissue cooling, in those locations where the renal parenchyma was heated above the phase transition temperature of the liposome membrane. CONCLUSION: The release of MR contrast agent has been demonstrated in rabbit kidney in vivo. This may be used as a reporter for simultaneous release of therapeutic agents.

Animals↗
PubMed2006

Real-time monitoring of radiofrequency ablation of rabbit liver by respiratory-gated quantitative temperature MRI.

PURPOSE: To evaluate the feasibility and precision of magnetic resonance imaging (MRI) thermometry for monitoring radiofrequency (RF) liver ablation in vivo and predicting the size of the ablation zone. MATERIALS AND METHODS: At 1.5T, respiratory-triggered real-time MR temperature mapping (the proton resonance frequency (PRF) method) was used to monitor RF ablation in rabbit liver (N = 6) under free breathing. The size of the ablation zones, as assessed by histological analyses, was compared with that predicted from MR thermal dose (TD) maps or derived from conventional T1-weighted (T1w), T2-weighted (T2w), and T1w gadolinium (Gd)-enhanced (T1w-Gd) images acquired immediately after the ablation, and on days 4 and 8 postprocedure. RESULTS: MR temperature uncertainty remained under 1-2 degrees C even during RF deposition. The TD maps were shown to be more predictive and precise than the other MR images, with an average predictive precision for the final ablation zone size of about 1 mm as compared to the histologically proven lesion on day 8. CONCLUSION: Quantitative temperature MRI during RF ablation is feasible and offered a precise indication of the ablation zone size in this preclinical study based on the lethal dose threshold.

Animals↗
PubMed2006

Variability in RF-induced heating of a deep brain stimulation implant across MR systems.

PURPOSE: To compare the MRI-related heating per unit of specific absorption rate (SAR) profile of a conductive implant between two 1.5-Tesla/64 MHz MR systems using a transmit/receive (t/r) head coil configuration. MATERIALS AND METHODS: Deep brain stimulation (DBS) leads were configured within a gel-filled phantom of the human head and torso. Temperature variation at each of four contacts of the bilaterally-placed leads was monitored using fluoroptic thermometry. MRI was performed using the t/r head coils of two different-generation 1.5-Tesla MR systems from the same manufacturer. Temperature changes were normalized to SAR values for the head (DeltaT/SAR-H), and the slope of this DeltaT/SAR-H by time relationship was compared between the two scanners. RESULTS: The DeltaT/SAR-H for the implant ranged from 3.5 to 5.5 times higher on one MR system as compared to the other (P < 0.01) depending on the measurement site. CONCLUSION: The findings support previous observations that console-reported SAR does not constitute a reliable index of heating for elongated, conductive implants, such as the DBS hardware system tested. In contrast to our previous findings using a t/r body coil, the data presented here reveal marked differences between two MR systems using t/r head coils (the coil configuration was consistent with the implant manufacturer's imaging guidelines). J. Magn. Reson. Imaging 2006. (c) 2006 Wiley-Liss, Inc.

Body Temperature↗
PubMed1990

Nd:YAG laser-induced interstitial hyperthermia using a long frosted contact probe.

The heating potential of a closed loop interstitial hyperthermia system employing 1,064 nm laser light in conjunction with a long frosted contact probe was investigated in hind limb muscle of anesthetized dogs. The laser system was an Nd:YAG surgical laser modified with a single channel thermometry unit, a computer, a printer, and a computer-controlled laser exposure shutter. The long frosted laser probe was implanted into the muscle, and 3.12-5.00 Watts of laser power was delivered interstitially. Temperature distribution was measured in three dimensions around the frosted probe. The temperature distributions generated by this technique were satisfactory for producing desired hyperthermia temperatures in an approximately 3.5 cm3 cylindrical tissue volume. A multiple laser delivery system is needed to induce interstitial hyperthermia in large tumors. A significant potential for the long frosted contact probe may be its use in combining interstitial hyperthermia and interstitial photodynamic therapy. Using this technique, both modalities may be delivered while employing the same treatment setup.

Animals↗
PubMed1991

Nd:YAG laser hyperthermia treatment of rat mammary adenocarcinoma in conjunction with surface cooling.

Electromagnetic radiation ranging from radiofrequency to microwave has classically been used to induce hyperthermia for treatment of cancer. This paper presents a new technique using near infrared radiation from an Nd:YAG laser in conjunction with surface cooling to induce hyperthermia in a rat tumor model. A CW Nd:YAG laser hyperthermia system was used to induce hyperthermic temperatures in chemically (DMBA) induced rat mammary adenocarcinomas. The laser was interfaced to a computer and a thermometry unit that provided feedback to control the tumor temperature between 43.2-43.5 degrees C. A thermocouple was placed at the base of the tumor and its temperature was used to control laser exposure. All tumors were 10 to 20 mm in diameter. Surface cooling methods investigated included forced air flow from a fan, forced oxygen flow plus an IV drip, and forced moist oxygen flow from a nebulizer. Twelve rat mammary adenocarcinomas have been treated with Nd:YAG laser hyperthermia. In 4 treatments, no surface cooling was employed. In one treatment the surface was cooled using oxygen flow plus IV drip. In 7 treatments the skin was cooled using the nebulizer technique. The nebulization provided the most effective and reproducible surface cooling. Nd:YAG laser hyperthermia delivered in conjunction with nebulizer surface cooling produced efficient heating of rat mammary adenocarcinomas. A mean temperature of 42.1 degrees C was obtained at the base of the tumors while the mean surface temperature was 37.0 degrees C.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed2003

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

MRI information is widely used for the monitoring of thermal therapies, such as laser-induced interstitial thermotherapy (LITT). However, a detailed knowledge about the relationship between time/temperature exposure and resulting tissue thermal damage is needed to estimate the lesion extent. The aims of this work were to model the thermal response of laser-irradiated tissue and to assess the value of Monte Carlo simulation (MCS) for tumor therapy planning and monitoring. MCS yielded true 3D distributions of laser-induced temperature and thermal damage. These results were compared with 2D MR thermometry data from in vitro experiments performed on animal liver tissue over an ordinary range of laser powers (10-25 W) and irradiation times (5-25 min). Clinical reference data were available from MR-guided liver LITT procedures. MCS correctly predicted the shape of temperature and damage distributions. The quantitative agreement between simulated and measured lesion diameters was within 10% and 15% for the in vitro and in vivo cases, respectively. The simulated 53 degrees C isotherm best estimated the boundary of the final thermal damage (6% variance). This study shows that MCS of interstitial laser coagulation provides unique information that can be of use in the empirical correlation of temperature with thermal damage, and can assist greatly in thermal treatment planning and monitoring.

Animals↗
PubMed2004

On the calculation and interpretation of signal intensity in echo-shifted sequences.

Echo-shifted sequences have been shown to be useful in applications where strong T*2-weighting and short repetition times are wanted, such as BOLD-contrast fMRI, MR thermometry, and perfusion studies. However, a full understanding of signal formation with such methods, which is mandatory to optimize sequence parameters for particular applications, has still not been achieved. Here, two methods are proposed to calculate the steady-state signal intensity in coherent TR-periodic and TR-shifted gradient-echo sequences. The integration method, which consists of averaging the steady-state magnetization over all isochromats in a voxel, is shown to be a particularly efficient way of obtaining the analytical expression of the measurable signal. The partition method, based on a physical decomposition of the steady-state magnetization into a sum of contributions from past excitation pulses, reveals that the net transverse magnetization results from a destructive interference between the wanted component and a series of stimulated echoes. The analysis includes off-resonance effects and is illustrated by phantom measurements. Relationships with previous publications on this subject are discussed.

Echo-Planar Imaging↗
PubMed2001

Unifying linear prior-information-driven methods for accelerated image acquisition.

In the ongoing quest for faster imaging and higher spatial resolution, several methods have been developed to speed up data acquisition by incorporating prior information about the object being imaged. This study shows that many of these methods can be integrated into a single common equation. The unified framework provides a conceptual link that facilitates comparison among these methods to reveal their strengths and weaknesses. By considering the limitations of existing methods, a new member in this class of methods was developed. The broad-use linear acquisition speed-up technique (BLAST) uses the estimated amount of change within the FOV as prior information. BLAST has the flexibility of incorporating a variable amount of prior information to avoid the misleading appearance of "phantom features," which arise from overconstraining the reconstruction. Examples from dynamic imaging and MR thermometry are shown.

Body Temperature↗
PubMed2001

Temperature mapping of frozen tissue using eddy current compensated half excitation RF pulses.

Cryosurgery has been shown to be an effective therapy for prostate cancer. Temperature monitoring throughout the cryosurgical iceball could dramatically improve efficacy, since end temperatures of at least -40 degrees C are required. The results of this study indicate that MR thermometry based on tissue R(*)(2) has the potential to provide this information. Frozen tissue appears as a complete signal void on conventional MRI. Ultrashort echo times (TEs), achievable with half pulse excitation and a short spiral readout, allow frozen tissue to be imaged and MR characteristics to be measured. However, half pulse excitation is highly sensitive to eddy current distortions of the slice-select gradient. In this work, the effects of eddy currents on the half pulse technique are characterized and methods to overcome these effects are developed. The methods include: 1) eddy current compensated slice-select gradients, and 2) a correction for the phase shift between the first and second half excitations at the center of the slice. The effectiveness of these methods is demonstrated in R(*)(2) maps calculated within the frozen region during cryoablation.

Animals↗
PubMed1989

Exposure to a 1.5-T static magnetic field does not alter body and skin temperatures in man.

The literature has conflicting reports concerning the effect of static magnetic fields on body and skin temperatures in mammals. Since temperature changes induced by static magnetic fields would have important safety implications for clinical magnetic resonance imaging body (sublingual pocket) and skin (abdomen, forehead, chest, upper arm, forearm, thigh, and calf) temperatures were determined in six normal subjects using a fluoroptic thermometry system during a 20-min exposure to a 1.5-T static magnetic field. Ambient conditions were controlled and held constant. An analysis of variance for repeated measures revealed that there were no statistically significant changes in body or any of the skin temperatures recorded. We conclude that exposure for 20 min to a 1.5-T static magnetic field does not alter body and skin temperatures in man.

Air Movements↗
PubMed1997

Determination of laser-induced temperature distributions using echo-shifted TurboFLASH.

An echo-shifted TurboFLASH sequence implemented on a clinical whole body MR scanner was used to determine thermal changes in tissue. With this snapshot-like data acquisition, temperature-related phase shifts were measured with a temporal resolution of 1.3 s. For different types of tissue (postmortem porcine brain, liver, and muscle) the temperature coefficients of the proton chemical shift were recorded during uniform heating of the specimen in a water bath. The specific temperature-dependent frequency shifts appeared similar to the proton chemical shift of free water (-0.01 ppm/degrees C). With this method, laser-induced ablation in postmortem porcine brain was monitored by temperature mapping. Comparison of the induced temperature profiles measured with NiCrNi-thermocouples with the MR calculated profiles demonstrated excellent temperature sensitivity and accuracy for this method of MR thermometry, with a maximum deviation of the determined temperatures of only 1.8 degrees C. This investigation was designed as a feasibility study for this rapid version of the phase mapping method, and no in vivo studies were performed.

Animals↗
PubMed1998

Time and temperature dependence of MR parameters during thermal coagulation of ex vivo rabbit muscle.

Detailed measurements of the T1-weighted, T2-weighted, and MT-weighted signal were performed for ex vivo muscle samples heated to various temperatures for different times. Consistent, monotonic increases in signal intensity were observed with progressive thermal coagulation, corresponding to an increase in T2 relaxation time and an increase in MT-weighted signal for temperatures above 60 degrees C. The relationship for T1 relaxation was more complex, showing a decrease in T1 relaxation from 40 to 60 degrees C and an increase above 60 degrees C. These techniques provide a more direct measure of tissue thermal coagulation than that provided by MR thermometry and suggest MR imaging strategies for the optimization and monitoring of thermal coagulation therapy protocols that create thermal damage in target tissues.

Animals↗
PubMed2004

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

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

Animals↗
PubMed2004

500-element ultrasound phased array system for noninvasive focal surgery of the brain: a preliminary rabbit study with ex vivo human skulls.

The aim of this study was to test a prototype MRI-compatible focused ultrasound phased array system for trans-skull brain tissue ablation. Rabbit thigh muscle and brain were sonicated with a prototype, hemispherical 500-element ultrasound phased array operating at frequencies of 700-800 kHz. An ex vivo human skull sample was placed between the array and the animal tissue. The temperature elevation during 20-30-sec sonications was monitored using MRI thermometry. The induced focal lesions were observed in T2 and contrast-enhanced T1-weighted fast spin echo images. Whole brain histology evaluation was performed after the sonications. The results showed that sharp temperature elevations can be produced both in the thigh muscle and in the brain. High-power sonications (600-1080 W) produced peak temperatures up to 55 degrees C and focal lesions that were consistent with thermal tissue damage. The lesion size was found to increase with increasing peak temperature. The device was then modified to operate in the orientation that will be used in the clinic and successfully tested in phantom experiments. As a conclusion, this study demonstrates that it is possible to create ultrasound-induced lesions in vivo through a human skull under MRI guidance with this large-scale phased array.

Animals↗
PubMed1992

Temperature mapping in the canine prostate during transurethrally-applied local microwave hyperthermia.

Although the therapeutic effects of heat on tumors have been known for more than a century, only recently has hyperthermia been applied in the treatment of various solid tumors in a scientific fashion. In preliminary clinical trials, heat applied by transurethrally or transrectally placed microwave antennas has been used in the treatment of benign prostatic hyperplasia (BPH) with some success. However, basic information about prostatic temperature distribution and the cellular effects of hyperthermia is lacking. In an attempt to establish the safety of local hyperthermia of the prostate and the precise temperature distribution within the gland, we performed temperature mapping studies in canine prostates. Eight mongrel dogs were anesthetized, and a 16 Fr catheter with three helical coil microwave antennas was placed in the bladder through a perineal urethrostomy. Laparotomy was performed and the bladder opened. The antennas were placed under direct control in the prostatic urethra. Linear array and single point thermometers (Clini-Therm TS1200 thermometry system) were placed (1) within the catheter alongside the antennas, (2) alongside the Foley catheter in the urethra, and (3) longitudinally and radially in the prostate for mapping of tissue temperature. Heating was performed with a 915-MHz ISM frequency Z-80 microprocessor controlled microwave power generator (Prostek 3000, Clini-Therm Corporation, Dallas, TX) using 2-9 watts per channel (6-27 watts total) for 30 min to 1 hr. Baseline body temperatures varied between the individual dogs from 37 degrees to 38 degrees C, but temperature distribution within the prostate was even prior to heating. With relatively low power (6 watts total), temperatures of greater than or equal to 45 degrees C were reached within the catheter. Therapeutic temperatures of approximately 43 degrees C were achieved in the periurethral prostate. Intraurethral temperatures were in general 1-2 degrees C lower than within the catheter. The radial temperature dropoff in the prostate was sharp and about 1 degrees C per 3-mm distance from the catheter. This would limit the area of effectiveness to a periurethral zone of 1 cm in diameter in this model. No unsafe temperature peaks were noted either intraurethrally or between the prostate and the rectum during steady-state conditions. Histologic studies demonstrated an intact rectal wall and varying degrees of prostatic inflammation and/or necrosis despite uniform treatment regimens administered. These studies demonstrate the short-term safety of microwave hyperthermia in the canine prostate. Further studies will be necessary to determine the clinical efficacy and toxicity in men with BPH.

Animals↗
PubMed2000

Biological effects of electromagnetic fields--mechanisms for the effects of pulsed microwave radiation on protein conformation.

Microwave exposure under "athermal" conditions occurs when no temperature rise can be measured by conventional thermometry. The existence of biological effects arising from the athermal exposure is still controversial, partly because of a lack of the linear dose response relation. We propose a model in which pulsed microwave radiation causes a triggering of the heat shock or stress response by altering the conformation of proteins through a transient heating of the protein and its close environment. We support this by modelling using the heat diffusion equation and show that pulsed exposure even when athermal can lead to transient temperature excursions outside the normal range. We propose that the power window phenomenon in which biological effects are observed at low power levels may be caused by an incomplete triggering of the heat shock response.

Animals↗
PubMed2003

MR safety in patients with implanted deep brain stimulation systems (DBS).

INTRODUCTION: While it is desirable to perform MRI examinations in patients with deep brain stimulators (DBS), a major safety concern exists regarding the potential for excessive heating secondary to magnetically induced electrical currents. This study was designed to determine the safety of MRI and DBS. METHODS: Standard configurations of DBS systems were tested. In vitro testing was performed using a 1.5-Tesla MR system, a gel-filled phantom, and the body and head RF coils with varying levels of RF energy (SAR). A fluoroptic thermometry system was used to record temperatures. RESULTS: Using the 1.5-T MRI and body RF transmit coil, the temperature changes ranged from 2.5 to 25.3 degrees C. Using the 1.5-T MRI and head RF transmit coil, the temperature changes ranged from 2.3 to 7.1 degrees C. CONCLUSIONS: Excessive heating does occur with certain MR imaging conditions. Under certain conditions determined in this study, patients with DBS may safely undergo anatomical MR imaging. In the future, standardized testing and more comprehensive studies will be needed to ensure the MR safety of neurostimulation systems.

Body Temperature↗