On the heating of linear conductive structures as guide wires and catheters in interventional MRI

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...

Section: Discussionmentioning

confidence: 73%

“…Numerous examinations have been reported concerning this subject (e.g., Ref. [2][3][4][5][6][7][8][9][10]. Regarding gradient switching, there are a few works that address the alteration of induced nerve stimulation near metallic implants (11,12).…”

mentioning

confidence: 99%

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

Graf

Steidle

Schick

2007

“…The coaxial cable in the catheter slightly increased the mechanical stiffness of the catheter, and to maintain the original properties of the catheter, cables (e.g., dedicated microstrip cables) with higher flexibility would be desirable. Unfortunately, a safety hazard of active catheter tracking is potential heating from rf coupling (26,27) and temperature increases of more than 50 K have been observed when the resonance length of the cable approaches a quarter wavelength of the rf field. Internal or external baluns have been proposed to shorten the resonance length of the connecting cables (28).…”

Section: Discussionmentioning

confidence: 99%

MR‐guided intravascular procedures: Real‐time parameter control and automated slice positioning with active tracking coils

Bock

Volz

Zühlsdorff

et al. 2004

Purpose: To implement and optimize a real-time pulse sequence and user interface to perform intravascular interventions using active catheter tracking. Materials and Methods:In magnetic resonance (MR)-guided interventions, small radio-frequency coils can be used to rapidly determine the device position (active tracking). In this work, active catheter tracking was combined with a dedicated real-time pulse sequence and user interface. The pulse sequence offered the imaging contrasts fast low angle shot (FLASH), true Fast imaging with steady state precession (TrueFISP), and projection MR digital subtraction angiography (MR-DSA), which could be selected by the radiologist from within the scanner room at any time during the intervention. Automatic slice positioning was added to the real-time pulse sequence so that the location of the tracking coils defined the image slice position and orientation. The technique was assessed in phantoms and animal experiments.Results: At a reaction time of 24 msec and a frame rate of three images per second, the movement of an active intravascular catheter could be monitored in the aorta and the renal arteries of a pig. With interactive contrast and orientation changes, the renal vasculature could be assessed by a fully MR-guided catheterization in less than 10 minutes. Conclusion:With carefully designed active catheters, a dedicated user interface, and an optimized pulse sequence intravascular interventions can successfully be performed by a single operator from within the MR scanner room.

“…The position of these coils along the catheter can be detected simultaneously without penalty in imaging time if each coil is connected to a separate receive channel of the MR system (9). A disadvantage of this method is the cable between the MR system and the coils, which can cause severe RF heating (51)(52)(53).…”

Section: Tracking and Profiling Rf Coilsmentioning

confidence: 99%

“…Although they do not cause any artifacts in the MR images as paraor ferromagnetic wires do, they can also be susceptible to severe heating (51,53,83). Any of the measures mentioned above to reduce coupling between the RF transmitting coil and the intravascular device that causes heating of the intravascular device would result in fundamental device modifications that could change the mechanical properties and would require new certification of approval.…”

Section: Safety Aspectsmentioning

confidence: 99%

MR‐guided intravascular interventions: Techniques and applications

Bock

Wacker

2008

Magnetic resonance imaging (MRI) offers several advantages over other imaging modalities that make it an attractive imaging tool for diagnostic and therapeutic procedures. This tremendous potential of MRI has provided the rationale for increased attention toward MR-guided endovascular interventions. MR guidance has been used recently to navigate endovascular catheters and deliver stents, vena cava filters, embolization materials, and septum closure devices. However, its potential goes beyond just copying existing procedures toward the development of new minimally invasive techniques that cannot be performed with conventional guiding techniques. Because of technical limitations and safety issues associated with some of the currently available devices, a limited number of clinical studies have been performed so far. The overall success for this developing field requires considerable interdisciplinary research within both the interventional and the MR community. Only through a combined effort can this complex technology find its way into clinical practice. This review discusses the hardware and software improvements that have helped to advance endovascular interventions under MR imaging guidance from a pure research tool to become a clinical reality. In addition, technical and safety issues specific to endovascular MR image guidance will be described and practical applications will be shown that take advantage of the benefits of MR for endovascular interventions.