Comments on "Ensuring Safety of Implanted Devices Under MRI Using Reversed Polarization"

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Improving RF safety in MRI by modifying the electric field distribution

In this work we demonstrate that the radiofrequency (RF) electric field in magnetic resonance imaging (MRI) can be modified in order to enhance patient safety. The heating of metallic devices in MRI is directly related to electric field distribution. On the other hand the MR image homogeneity is related to forward polarized component of the magnetic field (transmit sensitivity). In order to prevent heating, electric field-free zones should be generated in the body without significantly altering the transmit sensitivity. For this purpose the linearly polarized birdcage coil is proposed as a metallic device friendly MRI coil. The zero electric field plane of the linear birdcage coil is coincided with the location of the metallic device and the heating is reduced as shown by simulations and experiments. One disadvantage of this approach is, the linear coils generate twice as much whole body average SAR when compared to quadrature birdcage coils. In order to solve this problem simulations are performed to find electromagnetic field solutions with reduced average SAR and uniform transmit sensitivity. © 2011 IEEE

Design of internal MRI coils using ultimate intrinsic SNR

Object: Internal MRI coils have important applications in diagnostic and interventional studies. Since they can be placed very close to the region of interest in the body, they are favored over external coils in applications where high-resolution images are required. In this paper it is demonstrated that ultimate intrinsic SNR (UISNR) and the optimum coil sensitivity solutions can be used to make new coil designs with higher intrinsic SNR. Materials and methods: In this study, UISNR, which is the maximum attainable value of the intrinsic SNR, is used as a measure of performance and as a design criterion. As an example, a novel endorectal MRI coil is designed. The design is tested with phantom and patient studies. Results: An endorectal coil is built to demonstrate the effectiveness of the design strategy. ISNR of the endorectal coil approximates the UISNR to 72%. Conclusion: An internal coil design method that takes advantage of the UISNR and optimum coil sensitivity calculations was presented. This method can also be used to design better internal MRI coils for different applications. © 2009 ESMRMB

Reduction of the Radiofrequency Heating of Metallic Devices Using a Dual-Drive Birdcage Coil

Cataloged from PDF version of article.In this work, it is demonstrated that a dual-drive birdcage coil can be used to reduce the radiofrequency heating of metallic devices during magnetic resonance imaging. By controlling the excitation currents of the two channels of a birdcage coil, the radiofrequency current that is induced near the lead tip could be set to zero. To monitor the current, the image artifacts near the lead tips were measured. The electric field distribution was controlled using a dual-drive birdcage coil. With this method, the lead currents and the lead tip temperatures were reduced substantially [<0.3 C for an applied 4.4 W/kg SAR compared to >4.9 C using quadrature excitation], as demonstrated by phantom and animal experiments. The homogeneity of the flip angle distribution was preserved, as shown by volunteer experiments. The normalized root-mean-square error of the flip angle distribution was less than 10% for all excitations. The average specific absorption rate increased as a trade-off for using different excitation patterns. Magn Reson Med 69:845–852, 2013. VC 2012 Wiley Periodicals, In

Miniaturized fiber-optic transmission system for MRI signals

Conventional MRI instruments transmit received MRI signals through electrical cables. Although this design has proved to be effective over the years, we report a fiber-optic system that addresses the needs of recent developments in MRI technology. One of these technologies is phased array coils with a high number of elements, where total size of interconnections is a primary problem, and other problem is internal MRI coils, where there is a need for improvements in safety. The Miniature Fiber-Optic Transmission (FOT) System was developed to address these issues. The system consists of a receiver coil with active detuning, a low-noise preamplifier, and a laser diode connected to a photodetector with fiber-optic cabling. The overall noise figure of the system is lower than 1 dB. Total power consumption is 50 mW, and the device is switchable with another fiber-optic line, which can also control active detuning. A prototype device was tested in a GE 1.5 Tesla MRI scanner, and several images were acquired with a signal to noise ratio similar to coaxial cabling. We believe that this design will reduce the cabling problems of arrays and enable placement of internal coils into body cavities with no safety hazard to the patient, such as electrical shock or burns. © 2007 Wiley-Liss, Inc

Reduction of the radiofrequency heating of metallic devices using a dual-drive birdcage coil

In this work, it is demonstrated that a dual-drive birdcage coil can be used to reduce the radiofrequency heating of metallic devices during magnetic resonance imaging. By controlling the excitation currents of the two channels of a birdcage coil, the radiofrequency current that is induced near the lead tip could be set to zero. To monitor the current, the image artifacts near the lead tips were measured. The electric field distribution was controlled using a dual-drive birdcage coil. With this method, the lead currents and the lead tip temperatures were reduced substantially [4.9°C using quadrature excitation], as demonstrated by phantom and animal experiments. The homogeneity of the flip angle distribution was preserved, as shown by volunteer experiments. The normalized root-mean-square error of the flip angle distribution was less than 10% for all excitations. The average specific absorption rate increased as a trade-off for using different excitation patterns. Copyright © 2012 Wiley Periodicals, Inc

Evaluation of internal MRI coils using ultimate intrinsic SNR

The upper bounds of the signal-to-noise ratio (also known as the "ultimate intrinsic signal-to-noise ratio" (UISNR)) for internal and external coils were calculated. In the calculation, the body was modeled as a dielectric cylinder with a small coaxial cylindrical cavity in which internal coils could be placed. The calculated UISNR values can be used as reference solutions to evaluate the performance of internal MRI coils. As examples, we evaluated the performance of a loopless antenna and an endourethral coil design by comparing their ISNR with the UISNR. © 2004 Wiley-Liss, Inc

Reduction of implant RF heating through modification of transmit coil electric field

In this work, we demonstrate the possibility to modify the electric-field distribution of a radio frequency (RF) coil to generate electric field-free zones in the body without significantly altering the transmit sensitivity. Because implant heating is directly related to the electric-field distribution, implant-friendly RF transmit coils can be obtained by this approach. We propose a linear birdcage transmit coil with a zero electric-field plane as an example of such implant-friendly coils. When the zero electric-field plane coincides with the implant position, implant heating is reduced, as we demonstrated by the phantom experiments. By feeding RF pulses with identical phases and shapes but different amplitudes to the two orthogonal ports of the coil, the position of the zero electric-field plane can also be adjusted. Although implant heating is reduced with this method, a linear birdcage coil results in a whole-volume average specific absorption rate that is twice that of a quadrature birdcage coil. To solve this issue, we propose alternative methods to design implant-friendly RF coils with optimized electromagnetic fields and reduced whole-volume average specific absorption rate. With these methods, the transmit field was modified to reduce RF heating of implants and obtain uniform transmit sensitivity. Magn Reson Med, 2011. © 2010 Wiley-Liss, Inc