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Medical magnetic resonance imaging and spectroscopy

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Deep brain stimulation lead design to reduce radio-frequency heating in MRI
Author(s): Changqing Jiang, Xiaolong Mo, Jianqi Ding, Yantao Dong, Feng Zhang, Hongwei Hao, Luming Li
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Radio-frequency (RF) induced heating at the electrodes of the deep brain stimulation lead is a major concern when used in magnetic resonance imaging (MRI). A novel lead design is proposed to address this problem. By adopting a coil with non-uniform diameters, the maximal temperature rise measured in a standard phantom is reduced from 4.6 to 2.0°C, with an average reduction of 59%. The suppression of RF heating is probably achieved because of the alteration of the transmission line parameters of the lead by the novel design. The details of the design, experiment validation and discussion of the mechanisms are presented.

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Study of deep brain stimulation lead resonant length in 3.0 T MRI RF magnetic field
Author(s): Xiaolong Mo, Changqing Jiang, Jianqi Ding, Feng Zhang, Luming Li
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RF induced heating is a major concern when active implantable medical devices such as deep brain stimulation devices are present in MRI. The lead resonant length is an important factor as the heating would increase severely when the lead resonates with the RF field. Numerical simulations and in vitro tests were carried out to study the relationship between the lead design and the resonant length in 3.0 T MRI. It was found that the electrical conductivity of the conductor had little influence on lead resonant length, while the insulation thickness, the insulation relative electric permittivity, and the shape and structure of conductor had significant impact. A positive correlation between lead resonant length and heating magnitude were observed.

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Role of MRI coils in radio frequency heating of implantable medical devices
Author(s): Jianqi Ding, Changqing Jiang, Feng Zhang, Linze Li, Yinghua Liu, Luming Li
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MRI for patients with implanted medical devices is usually restricted due to the heating hazard arising from the interaction between the device and the electromagnetic field generated by the RF coil. RF coils used clinically have diverse dimensions and their heating effect remains unexamined. By numerical simulation, the similarity of the phantom magnetic field distribution of various coils was characterised by regression, and the heating around a standard implant was examined by correlation analysis. It was shown that the magnetic field distributions in the phantom were similar (>0.95) in the central area. This similarity was more significantly affected by the coil length rather than the diameter. As for the RF heating, the temperature distributions at the implant tips were much more similar (>0.9999) across all the coils. Although there were some differences in the temperature rise per unit RF magnetic field, these differences were within 20%, especially in the central region. These data reveal the influence of coil dimensions on the RF heating.

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Evaluation of local B 1 field as dosimeter of RF heating for implant in MRI
Author(s): Sen Wan, Changqing Jiang, Jianqi Ding, Luming Li
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Radio frequency (RF) heating is a serious risk for a patient with an elongated implant during MRI scan. The conventional dosimeter of specific absorption rate (SAR) and the recently proposed B 1+rms may not accurately indicate the implant heating. The local B 1 field around the tip of a lead and its relationship with the RF heating were studied by numerical simulations. It was found that regardless of the route of the lead, the angle or position of placement, there was a strong linear correlation between the averaged square of local B 1 field and the rise in temperature. The evaluation range had an effect on the linearity. Generally, the local B 1 field performed better as a dosimeter of RF heating than SAR or B 1+rms at the isocentre.

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