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A circuit model for the inductive coupling of a resonant wire to a resonant MRI coil. b: As the strength of the coupling increases, the single spectral peak splits into two, and c: current in the wire increases.
Source publication
PurposeThe concept of a “radiofrequency safety prescreen” is investigated, wherein dangerous interactions between radiofrequency fields used in MRI, and conductive implants in patients are detected through impedance changes in the radiofrequency coil. TheoryThe behavior of coupled oscillators is reviewed, and the resulting, observable impedance cha...
Contexts in source publication
Context 1
... coupled systems, whether mechanical, optical, or electrical, the combined oscillation will result in normal modes, each with a slightly different frequency, thus producing a distinct excitation spectrum. To see this, consider the model circuit presented in Figure 1a. The birdcage coil is modeled by the loop on the left, as a voltage source driving a series resonant circuit (28). ...
Context 2
... M, L, and C are values for the circuit elements shown in Figure 1, representing mutual inductance, inductance, and capacitance, respectively. The figure shows the current resulting in both resonant loops (i.e., the coil and the wire) as a function of excitation fre- quency, and for different coupling strengths. ...
Context 3
... the model presented in Figure 1 is an appropriate model for the wire/coil system, it is expected that the excitation spectrum of the coil will show distinctive fea- tures readily identifiable using a low-power scan with a network analyzer. In addition to these "red flag" features, it would also be desirable to have some measure of the coupling strength in order to assess the level of risk. ...
Context 4
... approach which monitors the coupling through impedance changes of pickup coils has been presented in (18)(19)(20), but here we suggest that the optimal monitor- ing frequency may not be the imaging frequency. Figure 2 illustrates the change in reflected power for the circuit model given in Figure 1 as the coupling to the resonant load is increased. The behavior is different at different frequencies. ...
Context 5
... at some frequencies, although that frequency range varies. These correlation results are summarized for the bench top experiments in Figure 10, where the correlation coefficient is shown as a function of monitor- ing frequency. The Pearson R coefficient, which is calcu- lated between wire current and the change in S- parameter magnitude (vs. the reference), has been plotted to highlight the change in both the strength and sign of the correlation. ...
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Citations
... Approaches to generalize the results of safety testing of medical implants exist and can be categorized in simulation based approaches, [12][13][14] electromagnetic measurements 15,16 MR-based measurements [17][18][19][20][21][22][23] or a combination of those. 3 Despite these important contributions to the field, we are still far away from a patient and exam specific assessment of implant safety in terms of accurate hazard predictions. ...
... This has been demonstrated previously using pick-up coil measurements to detect RF coupling of a guidewire 49,50 or current sensors on the guidewire. 16,26,28,51 Here we use a time-domain E-field sensor close to the guidewire (radial distance ≈ 1 mm) but 65 mm away from its tip in z (distal) direction. The distance between tip and sensor was chosen to guarantee an emersion of the E-field sensor tip in the phantom liquid during measurements in order to avoid spurious field contributions. ...
Purpose
To implement a modular, flexible, open‐source hardware configuration for parallel transmission (pTx) experiments on medical implant safety and to demonstrate real‐time mitigation strategies for radio frequency (RF) induced implant heating based on sensor measurements.
Methods
The hardware comprises a home‐built 8‐channel pTx system (scalable to 32‐channels), wideband power amplifiers and a positioning system with submillimeter precision. The orthogonal projection (OP) method is used to mitigate RF induced tip heating and to maintain sufficient B1+ for imaging. Experiments are performed at 297MHz and inside a clinical 3T MRI using 8‐channel pTx RF coils, a guidewire substitute inside a phantom with attached thermistor and time‐domain E‐field probes.
Results
Repeatability and precision are ~3% for E‐field measurements including guidewire repositioning, ~3% for temperature slopes and an ~6% root‐mean‐square deviation between B1+ measurements and simulations. Real‐time pTx mitigation with the OP mode reduces the E‐fields everywhere within the investigated area with a maximum reduction factor of 26 compared to the circularly polarized mode. Tip heating was measured with ~100 μK resolution and ~14 Hz sampling frequency and showed substantial reduction for the OP vs CP mode.
Conclusion
The pTx medical implant safety testbed presents a much‐needed flexible and modular hardware configuration for the in‐vitro assessment of implant safety, covering all field strengths from 0.5‐7 T. Sensor based real‐time mitigation strategies utilizing pTx and the OP method allow to substantially reduce RF induced implant heating while maintaining sufficient image quality without the need for a priori knowledge based on simulations or in‐vitro testing.
... A metallic implant with strong coupling to the RF coil causes significant impedance changes. These can be measured, via the coil's current distribution or scattering (S) parameters, and related to the induced implant current strengths and even the location of the implant if a multichannel transmission system is used [79][80][81] (Fig. 8). Such an "implant detector" at the MR-system level would solve many problems, only it will be very difficult to ensure that every hazardous implant is detected. ...
During an MRI scan, the radiofrequency field from the scanner's transmit coil, but also the switched gradient fields, induce currents in any conductive object in the bore. This makes any metallic medical implant an additional risk for an MRI patient, because those currents can heat up the surrounding tissues to dangerous levels. This is one of the reasons why implants are, until today, considered a contraindication for MRI; for example, by scanner manufacturers. Due to the increasing prevalence of medical implants in our aging societies, such general exclusion is no longer acceptable. Also, it should be no longer needed, because of a much‐improved safety‐assessment methodology, in particular in the field of numerical simulations. The present article reviews existing literature on implant‐related heating effects in MRI. Concepts for risk assessment and quantification are presented and also some first attempts towards an active safety management and risk mitigation.
Level of Evidence
5
Technical Efficacy
Stage 5
... 55 Graesslin et al 56 demonstrated detection of unsafe interactions using pick-up coil signals from each element of an 8-coil transmit array and modified a parallel transmit array (pTx) system to reduce RF heating by excluding the elements with unsafe device coupling levels. In a previous work, 57 a similar result was demonstrated on a 2-channel system using a low-power prescreen. Other previous reports have described a 2-channel system at 3T to modify the electric field distribution to minimize implant heating 58,59 and a 4-channel pTx at 1.5T to control induced currents in guidewires. ...
Purpose
To dynamically minimize radiofrequency (RF)‐induced heating of an active catheter through an automatic change of the termination impedance.
Methods
A prototype wireless module was designed that modifies the input impedance of an active catheter to keep the temperature rise during MRI below a threshold, ΔTmax. The wireless module (MR safety watchdog; MRsWD) measures the local temperature at the catheter tip using either a built‐in thermistor or external data from a fiber‐optical thermometer. It automatically changes the catheter input impedance until the temperature rise during MRI is minimized. If ΔTmax is exceeded, RF transmission is blocked by a feedback system.
Results
The thermistor and fiber‐optical thermometer provided consistent temperature data in a phantom experiment. During MRI, the MRsWD was able to reduce the maximum temperature rise by 25% when operated in real‐time feedback mode.
Conclusion
This study demonstrates the technical feasibility of an MRsWD as an alternative or complementary approach to reduce RF‐induced heating of active interventional devices. The automatic MRsWD can reduce heating using direct temperature measurements at the tip of the catheter. Given that temperature measurements are intrinsically slow, for a clinical implementation, a faster feedback parameter would be required such as the RF currents along the catheter or scattered electric fields at the tip.
... 19 Another pre-scan method senses impedance perturbations of the transmit coil that are indicative of significant coupling. 20 Although initial results have been demonstrated using these methods, none have yet achieved widespread clinical adoption. ...
Purpose
We explore the use of thermo‐acoustic ultrasound (TAUS) to monitor temperature at the tips of conductive device leads during MRI.
Theory
In TAUS, rapid radiofrequency (RF) power deposition excites an acoustic signal via thermoelastic expansion. Coupling of the MRI RF transmit to device leads causes SAR amplification at lead tips, allowing MRI RF transmitters to excite significant lead tip TAUS signals. Because the amplitude of the TAUS signal depends on temperature, it becomes feasible to monitor the lead tip temperature during MRI by tracking the TAUS amplitude.
Methods
The TAUS temperature dependence was characterized in a phantom and in tissue. To perform TAUS acquisitions in an MRI scanner, amplitude modulated RF chirps were transmitted by the body coil, and the lead tip TAUS signal was detected by an ultrasonic transducer. The TAUS signal level was correlated with the RF current induced on the lead and the associated B1 artifacts in MRI. TAUS signals acquired during RF‐induced heating were used to estimate the lead tip temperature.
Results
The TAUS signal exhibited strong dependence on temperature, increasing over 30% with 10∘C of heating both in the phantom and in tissue. A lead tip TAUS signal was observed for a 100 mA rms current induced on a lead. During RF‐induced heating, the TAUS signal appeared to accurately approximate the peak lead tip temperature.
Conclusions
TAUS allows for noninvasive monitoring of lead tip temperature in an MRI environment. With further development, TAUS opens new avenues to improve RF device safety during MRI scans.
... Determining the actual range and number of lead-case impedance values that provide safe scanning requires an extensive study that is beyond the scope of this article. Detecting coil-implant coupling by impedance measurement of RF coil has been investigated 38 previously, which may be used for developing a method to determine the required impedance values outside of an MRI scanner. However, it requires extensive analysis, so it would be a focus for future studies. ...
Purpose
To introduce a prototype active implantable medical device (AIMD) for which the induced radiofrequency currents can be controlled wirelessly.
Methods
The modified transmission line method is used to formulate how the lead‐case impedance of an AIMD affects the temperature rise around the electrode. A prototype AIMD is designed with the aim of controlling the unwanted temperature rise around its electrode during an MRI examination by altering the impedance between the lead and the case of the implant. MRI experiments were conducted with this prototype implant, which also has a built‐in temperature sensor at its electrode. During the experiment, the implant’s lead‐case impedance was controlled using Bluetooth communication with a remote computer, and the lead tip temperature was recorded.
Results
Ten different lead‐case impedance values and their corresponding tip temperature rises were examined during MRI experiments. The experimental results confirmed that the tip temperature rise can be controlled by varying the lead‐case impedance wirelessly.
Conclusion
The feedback from the temperature at the AIMD tip, together with variable lead‐case impedance, enables control of the safety profile of the AIMD during an MRI examination.
... In a real iMRI procedure, we would tailor the RF excitation based on the minimization of W and adopt a real-time device safety monitoring method during excitation. 11,[25][26][27][28] FDTD methods offer more accurate predictions of the fields for a given wire and phantom configuration but require more calculation power (that could take many hours to days of simulations) which currently is not practical for near-real-time monitoring during an intervention. Conversely, the routine runs in just few seconds, and the relative position of the wire to the array (obtained through imaging) could be updated in the algorithm to get a new set of optimum phase and amplitude values that minimize heating. ...
Purpose:
To provide a rapid method to reduce the radiofrequency (RF) E-field coupling and consequent heating in long conductors in an interventional MRI (iMRI) setup.
Methods:
A driving function for device heating (W) was defined as the integration of the E-field along the direction of the wire and calculated through a quasistatic approximation. Based on this function, the phases of four independently controlled transmit channels were dynamically changed in a 1.5 T MRI scanner. During the different excitation configurations, the RF induced heating in a nitinol wire immersed in a saline phantom was measured by fiber-optic temperature sensing. Additionally, a minimization of W as a function of phase and amplitude values of the different channels and constrained by the homogeneity of the RF excitation field (B1) over a region of interest was proposed and its results tested on the benchtop. To analyze the validity of the proposed method, using a model of the array and phantom setup tested in the scanner, RF fields and SAR maps were calculated through finite-difference time-domain (FDTD) simulations. In addition to phantom experiments, RF induced heating of an active guidewire inserted in a swine was also evaluated.
Results:
In the phantom experiment, heating at the tip of the device was reduced by 92% when replacing the body coil by an optimized parallel transmit excitation with same nominal flip angle. In the benchtop, up to 90% heating reduction was measured when implementing the constrained minimization algorithm with the additional degree of freedom given by independent amplitude control. The computation of the optimum phase and amplitude values was executed in just 12 s using a standard CPU. The results of the FDTD simulations showed similar trend of the local SAR at the tip of the wire and measured temperature as well as to a quadratic function of W, confirming the validity of the quasistatic approach for the presented problem at 64 MHz. Imaging and heating reduction of the guidewire were successfully performed in vivo with the proposed hardware and phase control.
Conclusions:
Phantom and in vivo data demonstrated that additional degrees of freedom in a parallel transmission system can be used to control RF induced heating in long conductors. A novel constrained optimization approach to reduce device heating was also presented that can be run in just few seconds and therefore could be added to an iMRI protocol to improve RF safety.
... In a real iMRI procedure, we would tailor the RF excitation based on the minimization of W and adopt a real-time device safety monitoring method during excitation. 11,[25][26][27][28] FDTD methods offer more accurate predictions of the fields for a given wire and phantom configuration but require more calculation power (that could take many hours to days of simulations) which currently is not practical for near-real-time monitoring during an intervention. Conversely, the ?????? routine runs in just few seconds, and the relative position of the wire to the array (obtained through imaging) could be updated in the algorithm to get a new set of optimum phase and amplitude values that minimize heating. ...
To extend the concept of deflecting the tip of a catheter with the magnetic force created in an MRI system through the use of an array of independently controllable steering coils located in the catheter tip, and to present methods for visualization of the catheter and/or surrounding areas while the catheter is deflected.
An array of steering coils made of 42-gauge wire was built over a 2.5 Fr (0.83 mm) fiber braided microcatheter. Two of the coils were 70 turn axial coils separated by 1 cm, and the third was a 15-turn square side coil that was 2 x 4 mm2. Each coil was driven independently by a pulse width modulation (PWM) current source controlled by a microprocessor that received commands from a MATLAB routine that dynamically set current amplitude and direction for each coil. The catheter was immersed in a water phantom containing 1% Gd-DTPA that was placed at the isocenter of a 1.5 T MRI scanner. Deflections of the catheter tip were measured from image-based data obtained with a real-time radio frequency (RF) spoiled gradient echo sequence (GRE). The small local magnetic fields generated by the steering coils were exploited to generate a hyperintense signal at the catheter tip by using a modified GRE sequence that did not include slice-select rewinding gradients. Imaging and excitation modes were implemented by synchronizing the excitation of the steering coil array with the scanner by ensuring that no current was driven through the coils during the data acquisition window; this allowed visualization of the surrounding tissue while not affecting the desired catheter position.
Deflections as large as 2.5 cm were measured when exciting the steering coils sequentially with a 100 mA maximum current per coil. When exciting a single axial coil, the deflection was half this value with 30% higher current. A hyperintense catheter tip useful for catheter tracking was obtained by imaging with the modified GRE sequence. Clear visualization of the areas surrounding the catheter was obtained by using the excitation and imaging mode even with a repetition time (TR) as small as 10 ms.
A new system for catheter steering is presented that allows large deflections through the use of an integrated array of steering coils. Additionally, two imaging techniques for tracking the catheter tip and visualization of surrounding areas, without interference from the active catheter, were shown. Together the demonstrated steerable catheter, control system and the imaging techniques will ultimately contribute to the development of a steerable system for interventional MRI procedures.
To protect implant carriers in MRI from excessive radiofrequency heating it has previously been suggested to assess that hazard via sensors on the implant. Other work recommended parallel transmission (pTx) to actively mitigate implant‐related heating. Here, both ideas are integrated into one comprehensive safety concept where native pTx safety (without implant) is ensured by state‐of‐the‐art field simulations and the implant‐specific hazard is quantified in situ using physical sensors. The concept is demonstrated by electromagnetic simulations performed on a human voxel model with a simplified spinal‐cord implant in an 8‐channel pTx body coil at 3 T. To integrate implant and native safety, the sensor signal must be calibrated in terms of an established safety metric, e.g., SAR. Virtual experiments show that E‐field and implant‐current sensors are well suited for this purpose, while temperature sensors require some caution, and B1 probes are inadequate. Based on an implant sensor matrix Qs, constructed in situ from sensor readings, and precomputed native SAR limits, a vector space of safe RF excitations is determined where both global (native) and local (implant‐related) safety requirements are satisfied. Within this safe‐excitation subspace, the solution with best image quality in terms of B1+ magnitude and homogeneity is then found by a straightforward optimization algorithm. In the investigated example, the optimized pTx shim provides a three‐fold higher meanB1+ magnitude compared to circular polarized excitation for a maximum implant‐related temperature increase ∆Timp≤1K. So far, sensor‐equipped implants interfaced to a pTx scanner exist as demonstrator items in research labs, but commercial devices are not yet within sight. The paper aims to demonstrate the significant benefits of such an approach and how this could impact implant related RF safety in MRI. Today, the responsibility for safe implant scanning lies with the implant manufacturer and the MRI operator; within the sensor concept, the MRI manufacturer would assume much of the operator’s current responsibility.
Patients who have implanted medical devices with long conductive leads are often restricted from receiving MRI scans due to the danger of RF-induced heating near the lead tips. Phantom studies have shown that this heating varies significantly on a case-by-case basis, indicating that many patients with implanted devices can receive clinically useful MRI scans without harm. However, the difficulty of predicting RF-induced lead tip heating prior to scanning prevents numerous implant recipients from being scanned. Here, we demonstrate that thermo-acoustic ultrasound (TAUS) has the potential to be utilized for a prescan procedure assessing the risk of RF-induced lead tip heating in MRI. A system was developed to detect TAUS signals by four different TAUS acquisition methods. We then integrated this system with an MRI scanner and detected a peak in RF power absorption near the tip of a model lead when transmitting from the scanner’s body coil. We also developed and experimentally validated simulations to characterize the thermo-acoustic signal generated near lead tips. These results indicate that TAUS is a promising method for assessing RF implant safety, and with further development, a TAUS pre-scan could allow many more patients to have access to MRI scans of significant clinical value.
Purpose:
To develop a new method capable of directly measuring specific absorption rate (SAR) deposited in tissue using the thermoacoustic signal induced by short radiofrequency (RF) pulse excitation.
Theory:
A detailed model based on the thermoacoustic wave generation and propagation is presented.
Methods:
We propose a new concept for direct measurement of SAR, to be used as a safety assessment/monitoring tool for MRI. The concept involves the use of short bursts of RF energy and the measurement of the resulting thermoacoustic excitation pattern by an array of ultrasound transducers, followed by image reconstruction to yield the 3D SAR distribution. We developed a simulation framework to model this thermoacoustic SAR mapping concept and verified the concept in vitro.
Results:
Simulations show good agreement between reconstructed and original SAR distributions with an error of 4.2, 7.2, and 8.4% of the mean SAR values in axial, sagittal, and coronal planes and support the feasibility of direct experimental mapping of SAR distributions in vivo. The in vitro experiments show good agreement with theory (r(2) = 0.52).
Conclusions:
A novel thermoacoustic method for in vivo mapping of local SAR patterns in MRI has been proposed and verified in simulation and in a phantom experiment. Magn Reson Med, 2016. © 2016 International Society for Magnetic Resonance in Medicine.
