In vivo porcine liver radiofrequency ablation with simultaneous MR temperature imaging

PURPOSE: To demonstrate in vivo MR-guided temperature mapping during radiofrequency (RF) ablation of the liver with a commercially available RF generator modified to allow simultaneous RF treatment and MRI. MATERIALS AND METHODS: A commercial RF generator was modified using passive filtering to allow the continuous application of the treatment current during MRI studies. A total of six ablations were performed with the device in vivo in three porcine livers, and imaging was concurrently performed using one of two different temperature mapping strategies. RESULTS: MR images acquired during RF ablation demonstrated no noticeable interference from the RF ablation device, which was operated at clinically relevant power levels. Temperature maps showed areas of heating that were consistent with the dimensions of the RF ablation probe, with some asymmetry (likely depending on the orientation of the probe and heat propagation effects), and some differences in heating-spot area stability depending on the specific temperature mapping strategy used. Lesions were visualized on post-ablation imaging and sectioning. CONCLUSION: The feasibility of performing RF ablation with a modified commercial RF generator simultaneously with MRI was demonstrated. Interference-free MR temperature maps were produced with both variable respiratory motion and mechanical ventilation, and showed the extent of heating as the ablation progressed.     

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.     

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.     

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.     

Neurostimulation systems for deep brain stimulation: in vitro evaluation of magnetic resonance imaging-related heating at 1.5 tesla

PURPOSE: To assess magnetic resonance imaging (MRI)-related heating for a neurostimulation system (Activa Tremor Control System, Medtronic, Minneapolis, MN) used for chronic deep brain stimulation (DBS). MATERIALS AND METHODS: Different configurations were evaluated for bilateral neurostimulators (Soletra Model 7426), extensions, and leads to assess worst-case and clinically relevant positioning scenarios. In vitro testing was performed using a 1.5-T/64-MHz MR system and a gel-filled phantom designed to approximate the head and upper torso of a human subject. MRI was conducted using the transmit/receive body and transmit/receive head radio frequency (RF) coils. Various levels of RF energy were applied with the transmit/receive body (whole-body averaged specific absorption rate (SAR); range, 0.98-3.90 W/kg) and transmit/receive head (whole-body averaged SAR; range, 0.07-0.24 W/kg) coils. A fluoroptic thermometry system was used to record temperatures at multiple locations before (1 minute) and during (15 minutes) MRI. RESULTS: Using the body RF coil, the highest temperature changes ranged from 2.5 degrees-25.3 degrees C. Using the head RF coil, the highest temperature changes ranged from 2.3 degrees-7.1 degrees C.Thus, these findings indicated that substantial heating occurs under certain conditions, while others produce relatively minor, physiologically inconsequential temperature increases. CONCLUSION: The temperature increases were dependent on the type of RF coil, level of SAR used, and how the lead wires were positioned. Notably, the use of clinically relevant positioning techniques for the neurostimulation system and low SARs commonly used for imaging the brain generated little heating. Based on this information, MR safety guidelines are provided. These observations are restricted to the tested neurostimulation system.     

RF-induced temperature elevation along metallic wires in clinical magnetic resonance imaging: influence of diameter and length.

With the development of interventional MRI, heating of biological tissues along the metallic wires in the MRI scanner has become an important issue. To assess thermal response to RF exposure during MRI, we studied the temperature elevation near nonmagnetic metallic wires. All tests were performed on a 1.5 T clinical scanner. Four experiments were conducted to investigate the effects of the wire diameter, the excitation flip angle, the temperature distribution along the wire, and the wire length. Electromagnetic simulations of the experimental setup were made with the use of commercial method of moments (MoM) software and numerical simulations of Hallen's equations. Comparisons between measured and calculated values of the electric field are presented. This study demonstrates that 1) temperature decreases with the diameter of the wire,2) temperature increases quadratically with the excitation flip angle, 3) heating occurs not only at the tip but also along the wire, and 4) the heating peaks are not obtained for the classical resonant length multiple of lambda/4 (where lambda is the RF field wavelength). In addition, significant and rapid heating increases were observed in the close vicinity of the wire.     

Safety of MRI-guided endovascular guidewire applications

Magnetic radiofrequency (RF) fields applied during magnetic resonance imaging (MRI) may induce heating in devices made from conductive materials. The present paper reports on theoretical and experimental studies on the RF heating resonance phenomenon of an endovascular guidewire. A nitinol-based guidewire was inserted into a vessel phantom and imaged at 1.5 and 0.2 T with continuous temperature monitoring at the guidewire tip. The heating effects due to different experimental settings were examined. A model is developed for the resonant current and the associated electric field produced by the guidewire acting as an antenna. Temperature increases of up to 17 degrees C were measured while imaging the guidewire at an off-center position in the 1.5 T MR system. Power absorption produced by the resonating wire decreased as the repetition time was increased. No temperature rise was measured at 0.2 T. Considering the potential utility of low-field, open MR systems for MRI-guided endovascular interventions, it is important to be aware of the safety of such applications.     

Evaluation of RF heating on humerus implant in phantoms during 1.5T MR imaging and comparisons with electromagnetic simulation.

PURPOSE: To evaluate the effect of radiofrequency (RF) heating on a metallic implant during magnetic resonance imaging (MRI), temperatures at several positions of an implant were measured, and results are compared with electromagnetic simulations using a finite element method. METHODS: A humerus nail implant made of stainless steel was embedded at various depths of tissue-equivalent gel-phantoms with loop (loop phantom) and partially cut loop (loop-cut phantom), and the phantoms were placed parallel to the static magnetic field of a 1.5T MRI device. Scans were conducted at maximum RF for 15 min, and temperatures were recorded with 2 RF-transparent fiberoptic sensors. Finally, electromagnetic-field analysis was performed. RESULTS: Temperatures increased at both ends of the implants at various depths, and temperature increase was suppressed with increasing depth. The maximum temperature rise was 12.3 degrees C at the tip of the implant and decreased for the loop-cut phantom. These tendencies resembled the results of electromagnetic simulations. CONCLUSION: RF heating was verified even in a nonmagnetizing metal implant in a case of excessive RF irradiation. Particularly, rapid temperature rise was observed at both ends of the implant having large curvatures. The difference in temperature increase by depth was found to reflect the skin-depth effect of RF intensity. Electromagnetic simulation was extremely useful for visualizing the eddy currents within the loop and loop-cut phantoms and for evaluating RF heating of a metallic implant for MRI safety.     

Significant precision improvement for temperature mapping.

MR temperature measurements are important for applications such as the evaluation of thermal therapies and radiofrequency (RF) coil heating effects. In this work the spherical mean value (SMV) method has been applied to significantly improve the precision of MR temperature mapping in a homogeneous gel phantom. Temperature-increase maps of the phantom were obtained with three-dimensional (3D) MR phase difference mapping after heating with the RF coil. The temperature-increase distribution in most regions in the phantom is a harmonic function with the mean value property. Based on this property, the precision of temperature-increase maps was improved up to sixfold with the SMV method. Comparison of this method with conventional smoothing, further precision improvement, and the in vivo application of the SMV method are discussed.     

Calculation of MRI-induced heating of an implanted medical lead wire with an electric field transfer function

PURPOSE: To develop and demonstrate a method to calculate the temperature rise that is induced by the radio frequency (RF) field in MRI at the electrode of an implanted medical lead. MATERIALS AND METHODS: The electric field near the electrode is calculated by integrating the product of the tangential electric field and a transfer function along the length of the lead. The transfer function is numerically calculated with the method of moments. Transfer functions were calculated at 64 MHz for different lengths of model implants in the form of bare wires and insulated wires with 1 cm of wire exposed at one or both ends. RESULTS: Heating at the electrode depends on the magnitude and the phase distribution of the transfer function and the incident electric field along the length of the lead. For a uniform electric field, the electrode heating is maximized for a lead length of approximately one-half a wavelength when the lead is terminated open. The heating can be greater for a worst-case phase distribution of the incident field. CONCLUSION: The transfer function is proposed as an efficient method to calculate MRI-induced heating at an electrode of a medical lead. Measured temperature rises of a model implant in a phantom were in good agreement with the rises predicted by the transfer function. The transfer function could be numerically or experimentally determined.     

Rapid radiofrequency field mapping in vivo using single‐shot STEAM MRI

Higher field strengths entail less homogeneous RF fields. This may influence quantitative MRI and MRS. A method for rapidly mapping the RF field in the human head with minimal distortion was developed on the basis of a single‐shot stimulated echo acquisition mode (STEAM) sequence. The flip angle of the second RF pulse in the STEAM preparation was set to 60° and 100° instead of 90°, inducing a flip angle‐dependent signal change. A quadratic approximation of this trigonometric signal dependence together with a calibration accounting for slice excitation‐related bias allowed for directly determining the RF field from the two measurements only. RF maps down to the level of the medulla could be obtained in less than 1 min and registered to anatomical volumes by means of the T₂‐weighted STEAM images. Flip angles between 75% and 125% of the nominal value were measured in line with other methods. Magn Reson Med 60:739–743, 2008. © 2008 Wiley‐Liss, Inc.     

Measuring local RF heating in MRI: Simulating perfusion in a perfusionless phantom

PURPOSE: To overcome conflicting methods of local RF heating measurements by proposing a simple technique for predicting in vivo temperature rise by using a gel phantom experiment. MATERIALS AND METHODS: In vivo temperature measurements are difficult to conduct reproducibly; fluid phantoms introduce convection, and gel phantom lacks perfusion. In the proposed method the local temperature rise is measured in a gel phantom at a timepoint that the phantom temperature would be equal to the perfused body steady-state temperature value. The idea comes from the fact that the steady-state temperature rise in a perfused body is smaller than the steady-state temperature increase in a perfusionless phantom. Therefore, when measuring the temperature on a phantom there will be the timepoint that corresponds to the perfusion time constant of the body part. RESULTS: The proposed method was tested with several phantom and in vivo experiments. Instead, an overall average of 30.8% error can be given as the amount of underestimation with the proposed method. This error is within the variability of in vivo experiments (45%). CONCLUSION: With the aid of this reliable temperature rise prediction the amount of power delivered by the scanner can be controlled, enabling safe MRI examinations of patients with implants.     

Transesophageal cardiac pacing during magnetic resonance imaging: Feasibility and safety considerations

The feasibility and safety of transesophageal cardiac pacing during clinical MRI at 1.5 Tesla is considered. An MRI compatible pace catheter was developed. In vitro testing showed a normal performance of the pulse generator, image artifacts that extended less than 11 mm from the catheter, and a less than 5% increase in noise. Cardiac stimulation induced by MRI was not observed and, theoretically, is not expected. Potentially, tissue around the catheter tip may become heated. This heating (ΔT) was monitored. Eight dogs were exposed to MRI during pacing. For low RF radiation exposure, a time-averaged squared B₁ field below 0.08 pT² (SAR < 0.03 W/kg), ΔT was below 1°C. For high RF radiation exposure, but at normal RF radiation specific absorption rate (0.4 W/kg), ΔT was 5°C. Thus, transesophageal atrial pacing during MRI at low RF exposure seems to be possible to perform cardiac stress studies or to correct unstable heart rates.     

Tailored excitation using nonlinear B0-shims

In high-field MRI, RF flip angle inhomogeneity due to wavelength effects can lead to spatial variations in contrast and sensitivity. Improved flip angle homogeneity can be achieved through multidimensional excitation, but long RF pulse durations limit practical application. A recent approach to reduce RF pulse duration is based on parallel excitation through multiple RF channels. Here, an alternative approach to shorten multidimensional excitation is proposed that makes use of nonlinear spatial variations in the stationary (B(0)) magnetic field during a B(0)-sensitive excitation pulse. As initial demonstration, the method was applied to 2D gradient echo (GE) MRI of human brain at 7 T. Using B(0) shims with up to second-order spatial dependence, it is demonstrated that root-mean-squared flip angle variation can be reduced from 20 to 11% with RF pulse lengths that are practical for general GE imaging applications without requiring parallel excitation. The method is expected to improve contrast and sensitivity in GE MRI of human brain at high field.     

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.     

Active deep brain stimulation during MRI: a feasibility study.

The goal of this study was to evaluate the feasibility of active deep brain stimulation (DBS) during the application of standard clinical sequences for functional MRI (fMRI) in phantom measurements. During active DBS, we investigated induced voltage, temperature at the electrode tips and lead, forces on the electrode and lead, consequences of defective leads and loose connections, proper operation of the neurostimulator, and image quality. Sequences for diffusion- and perfusion-weighted imaging, fMRI, and morphologic MRI were used. The DBS electrode and lead were placed in a NaCl solution-filled phantom. The results indicate that there are severe potential hazards for patients. Strong heating, high induced voltage, and even sparking at defects in the connecting cable could be observed. However, it was demonstrated that under certain conditions, safe MR examinations during active DBS are feasible. Certain safety precautions are recommended in this report.     

Pacemaker lead tip heating in abandoned and pacemaker-attached leads at 1.5 Tesla MRI

Purpose

To assess the risk of RF‐induced heating in pacemaker‐attached and abandoned leads using in vitro temperature measurements at 1.5 Tesla as a function of lead length.

Materials and Methods

Five custom lead lengths, 20–60 cm, were exposed to a uniform magnitude and phase radiofrequency electric field to examine the effect of lead length on pacemaker lead tip heating for pacemaker‐attached and abandoned pacemaker leads.

Results

Abandoned and pacemaker‐attached leads show resonant heating behavior and maximum heating occurs at different lead lengths due to the differences in termination conditions. For clinical lead lengths (40–60 cm) abandoned leads exhibited greater lead tip heating compared with pacemaker‐attached leads.

Conclusion

Current recommendations for MRI pacemaker safety should highlight the possible increased risk for patients with abandoned leads as compared to pacemaker‐attached leads. J. Magn. Reson. Imaging 2011;33:426–431. © 2011 Wiley‐Liss, Inc.     

Implant hyperthermia resonant circuit produces heat in response to MRI unit radiofrequency pulses

A small resonant circuit was investigated for its potential for producing hyperthermia to treat cancer. Implant hyperthermia has been performed using tiny elements implanted inside the body that are heated by an external energy source. We assessed the effect on heat generation of a resonant circuit used as an implant hyperthermia device by MRI unit radiofrequency (RF) pulses with different imaging sequences. The resonant circuit used as a heating device consisted of a closed connection between a coil and a capacitor. The resonant frequency was set to 63.9 MHz so that the circuit would react and generate heat in response to the RF pulses of a 1.5 T MRI unit. The resonant circuit was placed in the MRI unit with an optical thermometer enclosed by insulating material, and the temperature rise was monitored during imaging sequences. Standard imaging MRI sequences--fast low angle shot gradient echo (FLASH), T1 weighted spin echo image (T1WI) and rapid acquisition with refocused echoes (RARE)--were used to produce RF pulses that affected the resonant circuit. This circuit was gradually heated during all MRI sequences. The temperature rise ranged from 7.2 degrees C to 12.6 degrees C. The highest temperature rise was obtained with RARE, followed by FLASH and T1WI. Thus, this apparatus may have potential for implant hyperthermia, which could provide minimally invasive anticancer therapy.     

Heating around intravascular guidewires by resonating RF waves

We examined the unwanted radiofrequency (RF) heating of an endovascular guidewire frequently used in interventional magnetic resonance imaging (MRI). A Terumo guidewire was partly immersed in an oblong saline bath to simulate an endovascular intervention. The temperature rise of the guidewire tip during an FFE sequence [average specific absorption rate (SAR) = 3.9 W/kg] was measured with a Luxtron fluoroscopic fiber. Starting from 26 degrees C, the guidewire tip reached temperatures up to 74 degrees C after 30 seconds of scanning. Touching the guidewire may cause sudden heating at the point of contact, which in one instance caused a skin burn. The excessive heating of a linear conductor like the guidewire can only be explained by resonating RF waves. The capricious dependencies of this resonance phenomenon on environmental factors have severe consequences for predictability and safety guidelines.     

Minimizing RF heating of conducting wires in MRI.

Performing interventions using long conducting wires in MRI introduces the risk of focal RF heating at the wire tip. Comprehensive EM simulations are combined with carefully measured experimental data to show that method-of-moments EM field modeling coupled with heat transfer modeling can adequately predict RF heating with wires partially inserted into the patient-mimicking phantom. The effects of total wire length, inserted length, wire position in the phantom, phantom position in the scanner, and phantom size are examined. Increasing phantom size can shift a wire's length of maximum tip heating from about a half wave toward a quarter wave. In any event, with wires parallel to the scanner bore, wire tip heating is minimized by keeping the patient and wires as close as possible to the central axis of the scanner bore. At 1.5T, heating is minimized if bare wires are shorter than 0.6 m or between approximately 2.4 m and approximately 3.0 m. Heating is further minimized if wire insertion into phantoms equivalent to most aqueous soft tissues is less than 13 cm or greater than 40 cm (longer for fatty tissues, bone, and lung). The methods demonstrated can be used to estimate the absolute amount of heating in order to set RF power safety thresholds.