Evaluation of specific absorption rate as a dosimeter of MRI‐related implant heating

Baker, Kenneth B.; Tkach, Jean A.; Nyenhuis, J.A.; Phillips, Michael; Shellock, Frank G.; González-Martínez, Jorge; Rezai, Ali R.

doi:10.1002/jmri.20103

Cited by 208 publications

(195 citation statements)

References 8 publications

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“…From Eq. [7] it follows that for larger implants power-matching results at higher resistance, and the deposited energy also increases. A shortening of the gradient ramp time at constant amplitude leads to higher values R match as well (proportional to 1/ ) at constant E(R match ), i.e., power-matching for an implant or instrument is shifted toward a worse conducting material.…”

Section: Discussionmentioning

confidence: 87%

Heating of metallic implants and instruments induced by gradient switching in a 1.5‐Tesla whole‐body unit

Graf

Steidle

Schick

2007

Magnetic Resonance Imaging

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Purpose: To examine gradient switching-induced heating of metallic parts. Materials and Methods:Copper and titanium frames and sheets (Ϸ50 ϫ 50 mm 2 , 1.5 mm thick, frame width ϭ 3 mm) surrounded by air were positioned in the scanner perpendicular to the static field horizontally 20 cm off-center. During the execution of a sequence (three-dimensional [3D] true fast imaging with steady precession [True-FISP], TR ϭ 6.4 msec) exploiting the gradient capabilities (maximum gradient ϭ 40 mT/m, maximum slew rate ϭ 200 T/m/ second), heating was measured with an infrared camera. Radio frequency (RF) amplitude was set to zero volts. Heating of a copper frame with a narrowing to 1 mm over 20 mm at one side was examined in air and in addition surrounded by several liters of gelled saline using fiber-optic thermography. Further heating studies were performed using an artificial hip made of titanium, and an aluminum replica of the hip prosthesis with the same geometry. Results:For the copper specimens, considerable heating (Ͼ10°C) in air and in gelled saline (Ͼ1.2°C) could be observed. Heating of the titanium specimens was markedly less (Ϸ1°C in air). For the titanium artificial hip no heating could be detected, while the rise in temperature for the aluminum replica was approximately 2.2°C. Conclusion:Heating of more than 10°C solely due to gradient switching without any RF irradiation was demonstrated in isolated copper wire frames. Under specific conditions (high gradient duty cycle, metallic loop of sufficient inductance and low resistance, power matching) gradient switching-induced heating of conductive specimens must be considered. ONE IMPORTANT ASPECT in MR safety testing of medical implants and instruments is the inspection of whether hazardous heating can occur in the electromagnetic environment of an MR scanner. In the standard ASTM F 2182-02a (1), heating resulting from the interaction with the radio frequency (RF) magnetic field is considered. Numerous examinations have been reported concerning this subject (e.g., Ref. 2-10). Regarding gradient switching, there are a few works that address the alteration of induced nerve stimulation near metallic implants (11,12). Two publications (13,14) ascribe heat sensations of patients with larger metallic implants to vibrations of these implants caused by gradient switching.In contrast to ASTM F 2182-02a and to those works, the present study is concerned with direct gradient switching-induced heating of medical implants or instruments made of electrically conducting material. Following Faraday's law, the change of the magnetic flux through such a device induces eddy currents in the device and the metal subsequently converts electric energy into thermal energy. This effect increases with distance from isocenter. To obtain insight into the order of magnitude of possible effects, quadratic wire frames and sheets made of copper and titanium were examined. Most experiments were performed in air, since the underlying cause for the heating is pure magnetic induction during gradien...

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Section: Discussionmentioning

confidence: 87%

Heating of metallic implants and instruments induced by gradient switching in a 1.5‐Tesla whole‐body unit

Graf

Steidle

Schick

2007

Magnetic Resonance Imaging

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“…The acquisition sequence used in the post-operative MRI was designed to control for heating in accordance with safety studies conducted at the Cleveland Clinic (Baker et al, 2004). Detailed localization of the electrode lead and four contacts was performed by isosurfacing the halo around the electrode shaft in the postoperative MRI.…”

Section: Image Coregistration and Brain Atlas Warpingmentioning

confidence: 99%

Patient-specific analysis of the volume of tissue activated during deep brain stimulation

et al. 2007

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Despite the clinical success of deep brain stimulation (DBS) for the treatment of movement disorders, many questions remain about its effects on the nervous system. We have developed a methodology to predict the volume of tissue activated (VTA) by DBS on a patient-specific basis. Our goals are to identify the intersection between the VTA and surrounding anatomical structures and to compare activation of these structures with clinical outcomes. The model system consists of three fundamental components: 1) a 3D anatomical model of the subcortical nuclei and DBS electrode position in the brain, each derived from magnetic resonance imaging (MRI); 2) a finite element model of the DBS electrode and electric field transmitted to the brain, with tissue conductivity properties derived from diffusion tensor MRI; 3) VTA prediction derived from the response of myelinated axons to the applied electric field, which is a function of the stimulation parameters (contact, impedance, voltage, pulse width, frequency). We used this model system to analyze the effects of subthalamic nucleus (STN) DBS in a patient with Parkinson's disease. Quantitative measurements of bradykinesia, rigidity, and corticospinal tract (CST) motor thresholds were evaluated over a range of stimulation parameter settings. Our model predictions showed good agreement with CST thresholds. Additionally, stimulation through electrode contacts that improved bradykinesia and rigidity generated VTAs that overlapped the zona incerta / fields of Forel (ZI/H2). Application of DBS technology to various neurological disorders has preceded scientific characterization of the volume of tissue directly affected by the stimulation. Synergistic integration of clinical analysis, neuroimaging, neuroanatomy, and neurostimulation modeling provides the opportunity to address wide ranging questions on the factors linked with the therapeutic benefits and side effects of DBS.

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“…Simulation studies (e.g. see reference [48]) may help to quantify these differences. This, combined with the RF coil specific patterns which themselves are radically altered by the introduction of implants makes the determination of absolute 'worst case' or 'typical' implant positioning difficult [33,42].…”

Section: Experimental Considerationsmentioning

confidence: 99%

“…Variability in maximum heating over all locations due to cable length and termination was a factor of 2 (sets B-C) on both scanners although heating was lower for the 3 T Tim trio system. This may be due to the scanner-dependent SAR estimation methods leading to different heating for nominally identical experimental arrangements [48,49]. Therefore, SAR limits for safe MRI operation in the presence of intracranial electrodes cannot be generalised across MRI scanners without careful cross-scanner calibration [48,49].…”

Section: Generalisation Of Findingsmentioning

confidence: 99%

“…This may be due to the scanner-dependent SAR estimation methods leading to different heating for nominally identical experimental arrangements [48,49]. Therefore, SAR limits for safe MRI operation in the presence of intracranial electrodes cannot be generalised across MRI scanners without careful cross-scanner calibration [48,49]. In particular we note that even if calorimetric measurements were performed to cross-calibrate scanner-reported average SAR between systems, hardware-dependent variations in interactions between the MRI RF field and conducting implants might still lead to significant variations in the resulting tissue temperature increases.…”

Section: Generalisation Of Findingsmentioning

confidence: 99%

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Feasibility of simultaneous intracranial EEG-fMRI in humans: A safety study

Carmichael¹,

Thornton

Rodionov³

et al. 2010

NeuroImage

111

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In epilepsy patients who have electrodes implanted in their brains as part of their presurgical assessment, simultaneous intracranial EEG and fMRI (icEEG-fMRI) may provide important localising information and improve understanding of the underlying neuropathology. However, patient safety during icEEG-fMRI has not been addressed.Here the potential health hazards associated with icEEG-fMRI were evaluated theoretically and the main risks identified as: mechanical forces on electrodes from transient magnetic effects, tissue heating due to interaction with the pulsed RF fields and tissue stimulation due to interactions with the switched magnetic gradient fields.These potential hazards were examined experimentally in-vitro on a Siemens 3 T Trio, 1.5 T Avanto and a GE 3 T Signa Excite scanner using a Brain Products MR compatible EEG system. No electrode flexion was observed. Temperature measurements demonstrated thatheating well above guideline limits can occur. However heating could be kept within safe limits (<1.0°C) by using a head transmit RF coil, ensuring EEG cable placement to exit the RF coil along its central z-axis, using specific EEG cable lengths and limiting MRI sequence specific absorption rates (SARs). We found that the risk of tissue damage due to RF-induced heating is low provided implant and scanner specific SAR limits are observed with a safety margin used to account for uncertainties (e.g. in scanner-reported SAR). The observed scanner gradientswitching induced current (0.08mA) and charge density (0.2μC/cm 2 ) were well within safety limits (0.5mA and 30μC/cm 2 , respectively).3

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Evaluation of specific absorption rate as a dosimeter of MRI‐related implant heating

Cited by 208 publications

References 8 publications

Heating of metallic implants and instruments induced by gradient switching in a 1.5‐Tesla whole‐body unit

Heating of metallic implants and instruments induced by gradient switching in a 1.5‐Tesla whole‐body unit

Patient-specific analysis of the volume of tissue activated during deep brain stimulation

Feasibility of simultaneous intracranial EEG-fMRI in humans: A safety study

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