Christopher J. Yeung

RF safety of wires in interventional MRI: using a safety index.

With the rapid growth of interventional MRI, radiofrequency (RF) heating at the tips of guidewires, catheters, and other wire‐shaped devices has become an important safety issue. Previous studies have identified some of the variables that affect the relative magnitude of this heating but none could predict the absolute amount of heating to formulate safety margins. This study presents the first theoretical model of wire tip heating that can accurately predict its absolute value, assuming a straight wire, a homogeneous RF coil, and a wire that does not extend out of the tissue. The local specific absorption rate (SAR) amplification from induced currents on insulated and bare wires was calculated using the method of moments. This SAR gain was combined with a semianalytic solution to the bioheat transfer equation to generate a safety index. The safety index (°C/(W/kg)) is a measure of the in vivo temperature change that can occur with the wire in place, normalized to the SAR of the pulse sequence. This index can be used to set limits on the spatial peak SAR of pulse sequences that are used with the interventional wire. For the case of a straight resonant wire in a tissue with very low perfusion, only about 100 mW/kg/°C spatial peak SAR may be used at 1.5 T. But for ≤10‐cm wires with an insulation thickness ≥30% of the wire radius that are placed in well‐perfused tissues, normal operating conditions of 4 W/kg spatial peak SAR are possible at 1.5 T. Further model development to include the influence of inhomogeneous RF, curved wires, and wires that extend out of the sample are required to generate safety indices that are applicable to common clinical situations. We propose a simple way to ensure safety when using an interventional wire: set a limit on the SAR of allowable pulse sequences that is a factor of a safety index below the tolerable temperature increase. Magn Reson Med 47:187–193, 2002.

Multifunctional interventional devices for MRI: a combined electrophysiology/MRI catheter.

The design and application of a two‐wire electrophysiology (EP) catheter that simultaneously records the intracardiac electrogram and receives the MR signal for active catheter tracking is described. The catheter acts as a long loop receiver, allowing for visualization of the entire catheter length while simultaneously behaving as a traditional two‐wire EP catheter, allowing for intracardiac electrogram recording and ablation. The application of the device is demonstrated by simultaneously tracking the catheter and recording the intracardiac electrogram in canine models using 7 and 10 frame/sec real‐time imaging sequences. Using solely MR imaging, the entire catheter was visualized and guided from the jugular vein into the cardiac chambers, where the intracardiac electrogram was recorded. By combining several functions in a single, simple structure, the excellent tissue contrast and functional imaging capabilities of MR can be used to improve the efficacy of EP interventions. This catheter will facilitate MR‐guided interventions and demonstrates the design of multifunctional interventional devices for use in MRI. Magn Reson Med 47:594–600, 2002.

RF Heating Due to Conductive Wires During MRI Depends on the Phase Distribution of the Transmit Field

In many studies concerning wire heating during MR imaging, a “resonant wire length” that maximizes RF heating is determined. This may lead to the nonintuitive conclusion that adding more wire, so as to avoid this resonant length, will actually improve heating safety. Through a theoretical analysis using the method of moments, we show that this behavior depends on the phase distribution of the RF transmit field. If the RF transmit field has linear phase, with slope equal to the real part of the wavenumber in the tissue, long wires always heat more than short wires. In order to characterize the intrinsic safety of a device without reference to a specific body coil design, this maximum‐tip heating phase distribution must be considered. Finally, adjusting the phase distribution of the electric field generated by an RF transmit coil may lead to an “implant‐friendly” coil design. Magn Reson Med 48:1096–1098, 2002.

A Green's function approach to local rf heating in interventional MRI

Current safety regulations for local radiofrequency (rf) heating, developed for externally positioned rf coils, may not be suitable for internal rf coils that are being increasingly used in interventional MRI. This work presents a two-step model for rf heating in an interventional MRI setting: (1) the spatial distribution of power in the sample from the rf pulse (Maxwells equations); and (2) the transformation of that power to temperature change according to thermal conduction and tissue perfusion (tissue bioheat equation). The tissue bioheat equation is approximated as a linear, shift-invariant system in the case of local rf heating and is fully characterized by its Greens function. Expected temperature distributions are calculated by convolving (averaging) transmit coil specific absorption rate (SAR) distributions with the Greens function. When the input SAR distribution is relatively slowly varying in space, as is the case with excitation by external rf coils, the choice of averaging methods makes virtually no difference on the expected heating as measured by temperature change (deltaT). However, for highly localized SAR distributions, such as those encountered with internal coils in interventional MRI, the Greens function method predicts heating that is significantly different from the averaging method in current regulations. In our opinion, the Greens function method is a better predictor since it is based on a physiological model. The Greens function also elicits a time constant and scaling factor between SAR and deltaT that are both functions of the tissue perfusion rate. This emphasizes the critical importance of perfusion in the heating model. The assumptions made in this model are only valid for local rf heating and should not be applied to whole body heating.

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 wires 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 ≈2.4 m and ≈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. Magn Reson Med 58:1028–1034, 2007.

Intravascular extended sensitivity (IVES) MRI antennas

The design and application of an intravascular extended sensitivity (IVES) MRI antenna is described. The device is a loopless antenna design that incorporates both an insulating, dielectric coating and a winding of the antenna whip into a helical shape. Because this antenna produces a broad region of high SNR and also allows for imaging near the tip of the device, it is useful for imaging long, luminal structures. To elucidate the design and function of this device, the effects of both insulation and antenna winding were characterized by theoretical and experimental studies. Insulation broadens the longitudinal region over which images can be collected (i.e., along the lumen of a vessel) by increasing the resonant pole length. Antenna winding, conversely, allows for imaging closer to the tip of the antenna by decreasing the resonant pole length. Over a longitudinal region of 20 cm, the IVES imaging antenna described here produces a system SNR of approximately 40,000/r (mL–1Hz1/2), where r is the radial distance from the antenna axis in centimeters. As opposed to microcoil antenna designs, these antennas do not require exact positioning and allow for imaging over broad tissue regions. While focusing on the design of the IVES antenna, this work also serves to enhance our overall understanding of the properties and behavior of the loopless antenna design. Magn Reson Med 50:383–390, 2003.

RF Transmit Power Limit for the Barewire Loopless Catheter Antenna

The safety of the barewire loopless catheter antenna in transmit mode is addressed with respect to radiofrequency (RF) heating. Analytical expressions for electric field and specific absorption rate (SAR) distributions surrounding the antenna are postulated and experimentally verified. Limiting RF transmit power to 40-70 mW time-averaged power, depending on the specific antenna design, will ensure that the current regulatory guideline of SAR of 8 W/kg in any gram of tissue is not exceeded. These limits can act as guidelines for the design of RF pulses for use with this device. Further study is required to examine the safety of the antenna in receive mode.

Development of an Intravascular Heating Source Using an MR Imaging Guidewire

To develop a novel endovascular heating source using a magnetic resonance (MR) imaging guidewire (MRIG) to deliver controlled microwave energy into the target vessel for thermal enhancement of vascular gene transfection.

Measuring Local RF Heating in MRI: Simulating Perfusion in a Perfusionless Phantom

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.

RF safety of wires in interventional MRI: using a safety index

With the rapid growth of interventional MRI, radiofrequency (RF) heating at the tips of guidewires, catheters, and other wire-shaped devices has become an important safety issue. Previous studies have identified some of the variables that affect the relative magnitude of this heating but none could predict the absolute amount of heating to formulate safety margins. This study presents the first theoretical model of wire tip heating that can accurately predict its absolute value. The method of moments was used to find the induced currents on insulated and bare wires that were completely embedded in the tissue. The induced currents caused an amplification of the local specific absorption rate (SAR) distribution near the wire. This SAR gain was combined with a semi-analytic solution to the bioheat transfer equation to generate a safety index. The safety index is a measure of the worst case in vivo temperature change that can occur with the wire in place. It can be used to set limits on the spatial peak SAR of pulse sequences that are used with the interventional wire. Under worst-case conditions with resonant wires in a poorly perfused tissue, only about 100 mW/kg//spl deg/C spatial peak SAR may be used at 1.5 T. But for /spl les/10 cm wires with insulation thickness /spl ges/30% of the wire radius that are placed in well perfused tissues, normal operating conditions of 4 W/kg spatial peak SAR are possible at 1.5 T. We propose a simple way to ensure safety when using an interventional wire: set a limit on the SAR of allowable pulse sequences that is a factor of a safety index below the tolerable temperature increase.