Computational and experimental studies of an orthopedic implant: MRI‐related heating at 1.5‐T/64‐MHz and 3‐T/128‐MHz

Purpose:

To use numerical modeling to predict the worst‐case of magnetic resonance imaging (MRI)‐induced heating of an orthopedic implant of different sizes under 1.5‐T/64‐MHz and 3‐T/128‐MHz conditions and to apply the experimental test to validate the numerical results for worst‐case heating.

Materials and Methods:

Investigations of specific absorption rate (SAR) and the temperature rise of an orthopedic implant of different sizes within a standard phantom were accomplished by numerical finite‐difference time‐domain modeling and experimental measurements. MRI‐related heating experiments were performed using standardized techniques at 1.5‐T/64‐MHz and 3‐T/128‐MHz.

Results:

The numerical modeling results indicated that the induced energy deposition is almost linearly related to the dimension of the orthopedic implant when it is less than 100 mm for 1.5‐T/64‐MHz and 3‐T/128‐MHz conditions. At 3‐T/128‐MHz, when the dimension is greater than 100 mm, the linear relation does not exist, which suggests a wavelength effect at higher frequency. Higher temperature rises occurred at 1.5‐T/64‐MHz MRI than at 3‐T/128‐MHz for both numerical modeling and experimental studies.

Conclusion:

The numerical technique predicted which device size had maximum heating and its location. Temperature rise data agreed well with thermal simulation results. The presented method proved to be suitable to assess MRI‐induced heating of complex medical implants. J. Magn. Reson. Imaging 2013;37:491–497. © 2012 Wiley Periodicals, Inc.     

Automatic spatial and temporal temperature control for MR-guided focused ultrasound using fast 3D MR thermometry and multispiral trajectory of the focal point.

Of the different modalities to induce local hyperthermia, focused ultrasound is the only noninvasive technology available at the moment. In addition to the 3D localization of the target region, it has been shown that MRI can provide real-time thermometry and allows online, automatic control of temperature evolution of the focal point. Treatment of a large tissue volume (as compared to the focal spot size, i.e., the ultrasound wavelength) can be achieved rapidly by moving the focal point along an inside-out spiral trajectory. It has been shown previously that under linear conditions of energy deposition versus temperature, the spatial profile of the temperature within a large area can be controlled. In this study, a proportional, integral, and derivative (PID) spatial-and-temporal controller is described for the control of the temperature evolution within the target region under more variable conditions. The aim was to reach a predefined temperature profile after a few successive trajectories. Heat conduction in tissue is exploited to obtain a uniform temperature increase in a volume using discrete sonications without any waiting time. Input data sets consisted of 3D temperature maps provided online by a MR scanner. For each new trajectory, the controller recalculates the number of sonications per surface unit (spatial density of points describing the trajectory) and the applied power. Its performance was tested ex vivo and in vivo. Diameters of the target region ranged from 9 mm to 19 mm. Targeted temperature increase ranged from +8 degrees C to +18 degrees C. Spatiotemporal temperature control showed good stability and fast convergence, for both circular and elliptic ROIs.     

Automatic temperature control for MR‐guided interstitial ultrasound ablation in liver using a percutaneous applicator: Ex vivo and in vivo initial studies

Image‐guided thermal ablation offers minimally invasive options for treating hepatocellular carcinoma and colorectal metastases in liver. Here, the feasibility and the potential benefit of active temperature control for MR‐guided percutaneous ultrasound ablation was investigated in pig liver. An MR‐compatible interstitial ultrasound applicator (flat transducer), a positioning system with rotation‐translation guiding frame, and an orbital ring holder were developed. Step‐by‐step rotated elementary lesions were produced, each being formed by directive heating of a flame‐shaped volume of tissue. In vivo feasibility of automatic temperature control was investigated on two pigs. Proton Resonance Frequency Shift (PRFS)‐based MR thermometry was performed on a 1.5‐T clinical scanner, using SENSE acceleration and respiratory gating. MR follow‐up of animals and macroscopic analysis were performed at 3 and, respectively, 4 days postprocedure. No sonication‐related radiofrequency artifacts were detected on MR images. The temperature controller converged to the target elevation within ±2°C unless the requested power level exceeded the authorized limit. Large variability of the controller's applied powers from one sonication to another was found both ex vivo and in vivo, indicating highly anisotropic acoustic coupling and/or tissue response to identical beam pattern along different radial directions. The automatic control of the temperature enabled reproducible shape of lesions (15 ± 2 mm radial depth). Magn Reson Med 63:667–679, 2010. © 2010 Wiley‐Liss, Inc.     

Local hyperthermia with MR-guided focused ultrasound: spiral trajectory of the focal point optimized for temperature uniformity in the target region

The objective of hyperthermia treatment is to deliver a similar therapeutic thermal dose throughout the target volume within a minimum amount of time. We describe a noninvasive approach to this goal based on magnetic resonance imaging (MRI)-guided focused ultrasound (FUS) with a spherical transducer that can be moved along two directions inside the bed of a clinical MR imager and that has an adjustable focal length in the third dimension. Absorption of FUS gives rise to a highly localized thermal buildup, which then spreads by heat diffusion and blood perfusion. A uniform temperature within a large target volume can be obtained using a double spiral trajectory of the transducer focal point together with constant and maximum FUS power. Differences between the real and target temperatures during the first spiral are evaluated in real time with temperature MRI and corrected for during the second spiral trajectory employing FUS focal point velocity modulation. Once a uniform temperature distribution is reached within the entire volume, FUS heating is applied only at the region's boundaries to maintain the raised temperature levels. Heat conduction, together with the design and timing of the trajectories, therefore ensures a similar thermal dose for the entire target region. Good agreement is obtained between theory and experimental results in vitro on gel phantoms, ex vivo on meat samples, and in vivo on rabbit thigh muscle. Edema in muscle was visible 1 hour after hyperthermia as a spatially uniform rise of the signal intensity in T(2)-weighted images.     

MR imaging during endovascular procedures: An evaluation of the potential for catheter heating

MRI in catheterized patients is considered unsafe due to the potential for focal heating. This concern arises from the continuous metallic braid that is incorporated into catheters to provide their desired physical properties. The potential for catheter heating during MR scanning was assessed in an in vitro model simulating a patient undergoing a neurovascular procedure in which MR scans of the brain will be performed. Heating adjacent to endovascular devices was assessed with fluoroptic temperature probes in a polyacrylamide gel. The effect of variable immersion lengths, lateral and longitudinal offsets, position along the endovascular device, physical MR system, and specific absorption rate (SAR) level were studied to determine their effect on catheter heating. A rapid temperature rise was evident next to endovascular devices during MR scanning and varied moderately with immersed length, position within the bore, measurement point on the device, and MR system used. Peak heating rates were less than 1°C/min with maximal SAR exposure and anatomically realistic geometries. Heating scaled linearly with SAR and SAR values below 0.2 W/kg produced negligible heating near catheters. For the evaluated application, substantial SAR restrictions, coupled with limited imaging durations, are proposed as sufficient to permit MRI without concern for thermal injury. Magn Reson Med 61:45–53, 2009. © 2008 Wiley‐Liss, Inc.     

MRI thermometry: Fast mapping of RF‐induced heating along conductive wires

Conductive implants are in most cases a strict contraindication for MRI examinations, as RF pulses applied during the MRI measurement can lead to severe heating of the surrounding tissue. Understanding and mapping of these heating effects is therefore crucial for determining the circumstances under which patient examinations are safe. The use of fluoroptic probes is the standard procedure for monitoring these heating effects. However, the observed temperature increase is highly dependent on the positioning of such a probe, as it can only determine the temperature locally. Temperature mapping with MRI after RF heating can be used, but cooling effects during imaging lead to a significant underestimation of the heating effect. In this work, an MRI thermometry method was combined with an MRI heating sequence, allowing for temperature mapping during RF heating. This technique may provide new opportunities for implant safety investigations. Magn Reson Med, 2008. © 2008 Wiley‐Liss, Inc.     

Measuring RF‐induced currents inside implants: Impact of device configuration on MRI safety of cardiac pacemaker leads

Radiofrequency (RF)‐related heating of cardiac pacemaker leads is a serious concern in magnetic resonance imaging (MRI). Recent investigations suggest such heating to be strongly dependent on an implant's position within the surrounding medium, but this issue is currently poorly understood. In this study, phantom measurements of the RF‐induced electric currents inside a pacemaker lead were performed to investigate the impact of the device position and lead configuration on the amount of MRI‐related heating at the lead tip. Seven hundred twenty device position/lead path configurations were investigated. The results show that certain configurations are associated with a highly increased risk to develop MRI‐induced heating, whereas various configurations do not show any significant heating. It was possible to precisely infer implant heating on the basis of current intensity values measured inside a pacemaker lead. Device position and lead configuration relative to the surrounding medium are crucial to the amount of RF‐induced heating in MRI. This indicates that a considerable number of implanted devices may incidentally not develop severe heating in MRI because of their specific configuration in the body. Small variations in configuration can, however, strongly increase the risk for such heating effects, meaning that hazardous situations might appear during MRI. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Noninvasive temperature mapping with MRI using chemical shift water‐fat separation

Tissues containing both water and lipids, e.g., breast, confound standard MR proton reference frequency‐shift methods for mapping temperatures due to the lack of temperature‐induced frequency shift in lipid protons. Generalized Dixon chemical shift–based water‐fat separation methods, such as GE's iterative decomposition of water and fat with echo asymmetry and least‐squares estimation method, can result in complex water and fat images. Once separated, the phase change over time of the water signal can be used to map temperature. Phase change of the lipid signal can be used to correct for non‐temperature‐dependent phase changes, such as amplitude of static field drift. In this work, an image acquisition and postprocessing method, called water and fat thermal MRI, is demonstrated in phantoms containing 30:70, 50:50, and 70:30 water‐to‐fat by volume. Noninvasive heating was applied in an Off1‐On‐Off2 pattern over 50 min, using a miniannular phased radiofrequency array. Temperature changes were referenced to the first image acquisition. Four fiber optic temperature probes were placed inside the phantoms for temperature comparison. Region of interest (ROI) temperature values colocated with the probes showed excellent agreement (global mean ± standard deviation: ?0.09 ± 0.34°C) despite significant amplitude of static field drift during the experiments. Magn Reson Med 63:1238–1246, 2010. © 2010 Wiley‐Liss, Inc.     

Consideration of physiological response in numerical models of temperature during MRI of the human head

Purpose

To examine the thermal effects of the physiological response to heating during exposure to radiofrequency (RF) electromagnetic fields in magnetic resonance imaging (MRI) with a head‐specific volume coil.

Materials and Methods

Numerical methods were used to calculate the temperature elevation in MRI of the human head within volume coils from 64–400 MHz at different power levels both with and without consideration of temperature‐induced changes in rates of metabolism, perspiration, radiation, and perfusion.

Results

At the highest power levels currently allowed in MRI for head volume coils, there is little effect from the physiological response as predicted with existing methods. This study does not rule out the possibility that at higher power levels or in different types of coils (such as extremity or whole‐body coils) the physiological response may have more significant effects.

Conclusion

In modeling temperature increase during MRI of the human head in a head‐sized volume coil at up to 3.0 W/kg head‐average specific energy absorption rates, it may not be necessary to consider thermally induced changes in rates of metabolism, perfusion, perspiration, and radiation. J. Magn. Reson. Imaging 2008;28:1303–1308. © 2008 Wiley‐Liss, Inc.     

The effects of spatial sampling choices on MR temperature measurements

The purpose of this article is to quantify the effects that spatial sampling parameters have on the accuracy of magnetic resonance temperature measurements during high intensity focused ultrasound treatments. Spatial resolution and position of the sampling grid were considered using experimental and simulated data for two different types of high intensity focused ultrasound heating trajectories (a single point and a 4‐mm circle) with maximum measured temperature and thermal dose volume as the metrics. It is demonstrated that measurement accuracy is related to the curvature of the temperature distribution, where regions with larger spatial second derivatives require higher resolution. The location of the sampling grid relative temperature distribution has a significant effect on the measured values. When imaging at 1.0 × 1.0 × 3.0 mm³ resolution, the measured values for maximum temperature and volume dosed to 240 cumulative equivalent minutes (CEM) or greater varied by 17% and 33%, respectively, for the single‐point heating case, and by 5% and 18%, respectively, for the 4‐mm circle heating case. Accurate measurement of the maximum temperature required imaging at 1.0 × 1.0 × 3.0 mm³ resolution for the single‐point heating case and 2.0 × 2.0 × 5.0 mm³ resolution for the 4‐mm circle heating case. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Safety of localizing epilepsy monitoring intracranial electroencephalograph electrodes using MRI: radiofrequency-induced heating

Purpose

To investigate heating during postimplantation localization of intracranial electroencephalograph (EEG) electrodes by MRI.

Materials and Methods

A phantom patient with a realistic arrangement of electrodes was used to simulate tissue heating during MRI. Measurements were performed using 1.5 Tesla (T) and 3T MRI scanners, using head‐ and body‐transmit RF‐coils. Two electrode‐lead configurations were assessed: a “standard” condition with external electrode‐leads physically separated and a “fault” condition with all lead terminations electrically shorted.

Results

Using a head‐transmit–receive coil and a 2.4 W/kg head‐average specific absorption rate (SAR) sequence, at 1.5T the maximum temperature change remained within safe limits (<1°C). Under “standard” conditions, we observed greater heating (≤2.0°C) at 3T on one system and similar heating (<1°C) on a second, compared with the 1.5T system. In all cases these temperature maxima occurred at the grid electrode. In the “fault” condition, larger temperature increases were observed at both field strengths, particularly for the depth electrodes. Conversely, with a body‐transmit coil at 3T significant heating (+6.4°C) was observed (same sequence, 1.2/0.5 W/kg head/body‐average) at the grid electrode under “standard” conditions, substantially exceeding safe limits. These temperature increases neglect perfusion, a major source of heat dissipation in vivo.

Conclusion

MRI for intracranial electrode localization can be performed safely at both 1.5T and 3T provided a head‐transmit coil is used, electrode leads are separated, and scanner‐reported SARs are limited as determined in advance for specific scanner models, RF coils and implant arrangements. Neglecting these restrictions may result in tissue injury. J. Magn. Reson. Imaging 2008;28:1233–1244. © 2008 Wiley‐Liss, Inc.     

Spectrally selective pencil‐beam navigator for motion compensation of MR‐guided high‐intensity focused ultrasound therapy of abdominal organs

MR‐guided high‐intensity focused ultrasound (MR‐HIFU) is a noninvasive technique for depositing thermal energy in a controlled manner deep within the body. However, the MR‐HIFU treatment of mobile abdominal organs is problematic as motion‐related thermometry artifacts need to be corrected and the focal point position must be updated in order to follow the moving organ to avoid damaging healthy tissue. In this article, a fat‐selective pencil‐beam navigator is proposed for real‐time monitoring and compensation of through‐plane motion. As opposed to the conventional spectrally nonselective navigator, the fat‐selective navigator does not perturb the water–proton magnetization used for proton resonance frequency shift thermometry. This allows the proposed navigator to be placed directly on the target organ for improved motion estimation accuracy. The spectral and spatial selectivity of the proposed navigator pulse is evaluated through simulations and experiments, and the improved slice tracking performance is demonstrated in vivo by tracking experiments on a human kidney and on a human liver. The direct motion estimation provided by the fat‐selective navigator is also shown to enable accurate motion compensated MR‐HIFU therapy of in vivo porcine kidney, including motion compensation of thermometry and beam steering based on the observed three‐dimensional kidney motion. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Implementation of vascular‐space‐occupancy MRI at 7T

Vascular‐space‐occupancy (VASO) MRI exploits the difference between blood and tissue T₁ to null blood signal and measure cerebral blood volume changes using the residual tissue signal. VASO imaging is more difficult at higher field because of sensitivity loss due to the convergence of tissue and blood T₁ values and increased contamination from blood‐oxygenation‐level‐dependent (BOLD) effects. In addition, compared to 3T, 7T MRI suffers from increased geometrical distortions, e.g., when using echo‐planar‐imaging, and from increased power deposition, the latter especially problematic for the spin‐echo‐train sequences commonly used for VASO MRI. Third, non‐steady‐state blood spin effects become substantial at 7T when only a head coil is available for radiofrequency transmit. In this study, the magnetization‐transfer‐enhanced‐VASO approach was applied to maximize tissue‐blood signal difference, which boosted signal‐to‐noise ratio by 149% ± 13% (n = 7) compared to VASO. Second, a 3D fast gradient‐echo sequence with low flip‐angle (7°) and short echo‐time (1.8 ms) was used to minimize the BOLD effect and to reduce image distortion and power deposition. Finally, a magnetization‐reset technique was combined with a motion‐sensitized‐driven‐equilibrium approach to suppress three types of non‐steady‐state spins. Our initial functional MRI results in normal human brains at 7T with this optimized VASO sequence showed better signal‐to‐noise ratio than at 3T. Magn Reson Med 69:1003–1013, 2013. © 2012 Wiley Periodicals, Inc.     

Parallel‐transmission‐enabled three‐dimensional T2‐weighted imaging of the human brain at 7 Tesla

Purpose: A promise of ultra high field MRI is to produce images of the human brain with higher spatial resolution due to an increased signal to noise ratio. Yet, the shorter radiofrequency wavelength induces an inhomogeneous distribution of the transmit magnetic field and thus challenges the applicability of MRI sequences which rely on the spin excitation homogeneity. In this work, the ability of parallel‐transmission to obtain high‐quality T₂‐weighted images of the human brain at 7 Tesla, using an original pulse design method is evaluated. Methods: Excitation and refocusing square pulses of a SPACE sequence were replaced with short nonselective transmit‐SENSE pulses individually tailored with the gradient ascent pulse engineering algorithm, adopting a k_T‐point trajectory to simultaneously mitigate B₁⁺ and ΔB₀ nonuniformities. Results: In vivo experiments showed that exploiting parallel‐transmission at 7T with the proposed methodology produces high quality T₂‐weighted whole brain images with uniform signal and contrast. Subsequent white and gray matter segmentation confirmed the expected improvements in image quality. Conclusion: This work demonstrates that the adopted formalism based on optimal control, combined with the k_T‐point method, successfully enables three‐dimensional T₂‐weighted brain imaging at 7T devoid of artifacts resulting from B₁⁺ inhomogeneity. Magn Reson Med 73:2195–2203, 2015. © 2014 Wiley Periodicals, Inc.     

Spatial distribution of RF‐induced E‐fields and implant heating in MRI

The purpose of this study was to assess the distribution of RF‐induced E‐fields inside a gel‐filled phantom of the human head and torso and compare the results with the RF‐induced temperature rise at the tip of a straight conductive implant, specifically examining the dependence of the temperature rise on the position of the implant inside the gel. MRI experiments were performed in two different 1.5T MR systems of the same manufacturer. E‐field distribution inside the liquid was assessed using a custom measurement system. The temperature rise at the implant tip was measured in various implant positions and orientations using fluoroptic thermometry. The results show that local E‐field strength in the direction of the implant is a critical factor in RF‐related tissue heating. The actual E‐field distribution, which is dependent on phantom/body properties and the MR‐system employed, must be considered when assessing the effects of RF power deposition in implant safety investigations. Magn Reson Med 60:312–319, 2008. © 2008 Wiley‐Liss, Inc.     

Role of the lead structure in MRI‐induced heating: In vitro measurements on 30 commercial pacemaker/defibrillator leads

MRI‐induced heating on endocardial leads is a serious concern for the safety of patients with implantable pacemakers or cardioverter‐defibrillator. The lead heating depends on many factors and its amount is largely variable. In this study, we investigated the role of those structural properties of the lead that are reported on the accompanying documents of the device: ( 1 ) fixation modality (active vs. passive); ( 2 ) number of electrodes (unipolar vs. bipolar); ( 3 ) length; ( 4 ) tip surface; and ( 5 ) tip and ring resistance. In vitro temperature and specific absorption rate measurements on 30 leads (27 pacemakers, three implantable cardioverter‐defibrillator leads) exposed to the radiofrequency field typical of a 1.5 T MRI scanner are presented. The data show that each lead has its own attitude to radiofrequency‐induced heating and that the information that is available in the accompanying documents of the pacemaker is not sufficient to explain such attitude. Even if combined with that of the implant geometry, this information is still not sufficient to estimate the amount of heating due to the exposure to the radiofrequency field during MRI examination. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Diffusion‐weighted MRI for monitoring tumor response to photodynamic therapy

Purpose:

To examine diffusion‐weighted MRI (DW‐MRI) for assessing the early tumor response to photodynamic therapy (PDT).

Materials and Methods:

Subcutaneous tumor xenografts of human prostate cancer cells (CWR22) were initiated in athymic nude mice. A second‐generation photosensitizer, Pc 4, was delivered to each animal by a tail vein injection 48 h before laser illumination. A dedicated high‐field (9.4 Tesla) small animal MR scanner was used to acquire diffusion‐weighted MR images pre‐PDT and 24 h after the treatment. DW‐MRI and apparent diffusion coefficients (ADC) were analyzed for 24 treated and 5 control mice with photosensitizer only or laser light only. Tumor size, prostate specific antigen (PSA) level, and tumor histology were obtained at different time points to examine the treatment effect.

Results:

Treated mice showed significant tumor size shrinkage and decrease of PSA level within 7 days after the treatment. The average ADC of the 24 treated tumors increased 24 h after PDT (P < 0.001) comparing with pre‐PDT. The average ADC was 0.511 ± 0.119 × 10^?3 mm²/s pre‐PDT and 0.754 ± 0.181 × 10^?3 mm²/s 24 h after the PDT. There is no significant difference in ADC values pre‐PDT and 24 h after PDT in the control tumors (P = 0.20).

Conclusion:

The change of tumor ADC values measured by DW‐MRI may provide a noninvasive imaging marker for monitoring tumor response to Pc 4‐PDT as early as 24 h. J. Magn. Reson. Imaging 2010;32:409–417. © 2010 Wiley‐Liss, Inc.

     

Use of ultrasmall superparamagnetic particles of iron oxide (USPIO)‐enhanced MRI to demonstrate diffuse inflammation in the normal‐appearing white matter (NAWM) of multiple sclerosis (MS) patients: An exploratory study

Purpose

To explore ultrasmall superparamagnetic particles of iron oxide (USPIO) as a marker for diffuse inflammation in multiple sclerosis (MS) normal‐appearing white matter (NAWM), using quantitative MRI. Disease activity in the NAWM of MS patients partly explains why MRI lesion burden correlates only moderately with disability. USPIO have been shown to visualize the cellular component of inflammation in focal MS lesions. In this study, we aimed to explore USPIO as a marker for the more diffuse inflammation in MS NAWM, using quantitative MRI.

Materials and Methods

In this prospective MRI study, 16 MS patients (eight relapsing‐remitting MS [RRMS] and eight primary‐progressive MS [PPMS] cases) and five healthy control (HC) subjects were included. Using a flip‐angle (FA) array, B1‐corrected T1 maps were generated before and 24 hours after USPIO (SHU555C) injection. White‐matter (WM) T1 histogram and region‐of‐interest (ROI) characteristics were compared between both time points using Wilcoxon signed‐rank test.

Results

Both NAWM ROI and histogram analyses showed T1 shortening after USPIO injection in MS patients (P < 0.01), but not in HCs (P = 0.68).

Conclusion

This exploratory study suggests that USPIO‐enhanced MRI may be a new potential marker for subtle inflammatory activity in MS NAWM. Further studies should focus on relating diffuse inflammation to clinical disease activity and treatment efficacy. J. Magn. Reson. Imaging 2009;29:774–779. © 2009 Wiley‐Liss, Inc.     

Reconstruction of fully three‐dimensional high spatial and temporal resolution MR temperature maps for retrospective applications

Many areas of MR‐guided thermal therapy research would benefit from temperature maps with high spatial and temporal resolution that cover a large three‐dimensional volume. This article describes an approach to achieve these goals, which is suitable for research applications where retrospective reconstruction of the temperature maps is acceptable. The method acquires undersampled data from a modified three‐dimensional segmented echo‐planar imaging sequence and creates images using a temporally constrained reconstruction algorithm. The three‐dimensional images can be zero‐filled to arbitrarily small voxel spacing in all directions and then converted into temperature maps using the standard proton resonance frequency shift technique. During high intensity focused ultrasound heating experiments, the proposed method was used to obtain temperature maps with 1.5 mm × 1.5 mm × 3.0 mm resolution, 288 mm × 162 mm × 78 mm field of view, and 1.7 s temporal resolution. The approach is validated to demonstrate that it can accurately capture the spatial characteristics and time dynamics of rapidly changing high intensity focused ultrasound‐induced temperature distributions. Example applications from MR‐guided high intensity focused ultrasound research are shown to demonstrate the benefits of the large coverage fully three‐dimensional temperature maps, including characterization of volumetric heating trajectories and near‐ and far‐field heating. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

1H and hyperpolarized 3He MR imaging of mouse with LPS‐induced inflammation

Purpose

To evaluate the lipopolysaccharide (LPS) model of chronic obstructive pulmonary disease (COPD) in mouse with ¹H and hyperpolarized (HP) ³He MR imaging.

Materials and Methods

Axial slices of the lung volume were acquired with HP ³He and ¹H MRI at 4, 24, and 48 h after LPS exposure. A quantitative ventilation index was calculated from two HP ³He acquisitions. A bronchoalveolar lavage (BAL) for a cell count was performed following magnetic resonance imaging (MRI).

Results

The LPS exposure resulted in a significant increase of cells in BAL, with maximum at 48 h. Lesions on ³He images were characterized by ventilation defects, whereas lesions on ¹H images were hyperintense and were attributed to edema. The number of lesions was at maximum at 48 h. At this time point, and for both ³He and ¹H MRI, the volume of the lesions was significantly higher for LPS‐exposed mice compared to controls. At 4, 24, and 48 h the ventilation index from the ³He data was significantly smaller for the LPS‐exposed animals compared to controls.

Conclusion

The time point 48 h after LPS exposure was advantageous for MRI evaluation. Functional read‐out with ³He MRI seems to be more sensitive than conventional ¹H MRI. J. Magn. Reson. Imaging 2009;29:977–981. © 2009 Wiley‐Liss, Inc.