Jianfeng Zheng

An Efficient Approach to Estimate MRI RF Field Induced In Vivo Heating for Small Medical Implants

In this paper, a quick and efficient approach is proposed to estimate in vivo RF-induced heating from small medical implants under MRI procedure. An efficient methodology is developed to correlate the incident electric field to the induced temperature rise for stents of arbitrary shapes. A 10-cm-long titanium rod and a 6-cm-long intestine stent are used as examples to validate the approach under in vitro testing conditions. In vivo temperature rise estimations are then performed on an anatomically corrected adult male model. Numerical and experimental studies demonstrate the efficiency and accuracy of the proposed method.

Numerical study of SAR for multi-component orthopaedic hip replacement system during MRI

In this study we present numerical simulations of the Specific Absorption Rate (SAR) for multi-component orthopaedic hip replacement systems. The SAR is used to evaluate the radio frequency (RF)-induced heating of the devices during magnetic resonance imaging (MRI). Because multi-component orthopaedic hip replacement systems have many combinations of components with various designs and sizes, it is computationally intensive, and almost impossible, to evaluate the SAR and the corresponding temperature rise for each possible combination and configuration. In this study, an effective searching strategy and a computational simulation model are developed to evaluate the factors associated with induced SAR in the tissue near an orthopaedic hip replacement system, and to find the “worst case” peak SAR for all possible combinations. The finite-difference time-domain (FDTD) was used to calculate the peak SAR for a typical hip replacement system inside the American Society for Testing and Materials (ASTM) phantom for both 1.5 Tesla (T) and 3T MRI systems. The results indicate that the stem and screw lengths are the most important factors influencing the peak SAR for both field strengths, 1.5T/64 MHz and 3T/128 MHz, respectively. The peak 1 gram averaged SAR reaches 216 W/kg and 103 W/kg for 64 MHz and 128 MHz, respectively. We also found that shortest stems, and the longest screws, typically induce higher peak SAR.

MRI induced heating for fully implanted, partially implanted and minimally implanted medical electrode leads

One of the critical MRI safety concerns for patients with medical implants is the RF coil induced heating in the vicinity of electrode lead tip. Many works have been carried out to assess such risk for fully implanted leads. However studies on partially implanted, as well as minimally implanted electrodes have not been explored. In addition, effects of statures, weight, ages and many other factors may vary the RF heating dramatically. In this paper fully implanted, partially implanted and minimally implanted medical leads are studied in both adult male and boy models to demonstrate the heating variations due to pathway types and human bodies.

Using scaling approach to estimate MRI RF field induced heating for small medical implant

In this paper, a quick and efficient approach is proposed to estimate the in-vivo RF induced heating of small medical implants under MRI procedure. The method is based on scaling the in-vivo incident electric fields. Numerical and experimental studies demonstrate the efficiency and accuracy of the proposed method.

A Novel Design of Implantable Esophageal Stent to Reduce the MRI RF-Induced Heating

In this work, the magnetic resonance imaging (MRI) radio frequency (RF)-induced heating mechanism for an esophageal stent is discussed from the prospective of forward scattering and the reciprocity theorem. We conclude that the RF-induced heating largely depends on the induced currents on the surface of the stent. Based on this understanding, a novel implantable segmented stent structure is developed to reduce the RF-induced heating of an esophageal stent under MRI procedures. By using a segmented structure, the specific absorption rate (SAR) for a 10-mm-radius and 98-mm-length stent decreased from 53.0 to 14.2 W/kg at a whole-body average SAR of 2 W/kg, and the maximum temperature rise over 15-min MRI exam is reduced from 4.52 to 1.72

A novel design of implantable medical stent for reducing the MRI RF-induced heating

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MRI heating reduction for external fixation devices using absorption material

C. The simulated results and experimental measurements agree with each other within the combined uncertainty and demonstrate the effectiveness of the structured stent design. Related to RF-induced heating, this novel design can be used to develop inherently safe esophageal stents.

Evaluation of MRI RF electromagnetic field induced heating near leads of cochlear implants

In this work, a novel implantable medical stent is designed to reduce the radio frequency (RF)-induced heating from implantable medical stent under magnetic resonance imaging (MRI) procedure. By using segmented structure, the induced specific absorption rate (SAR) for a 100 mm length stent is reduced from 54.4 mW/g to 13.4 mW/g. Numerical simulation demonstrates the effectiveness of the designed structure.

A Transmission Line Model for the Evaluation of MRI RF-Induced Fields on Active Implantable Medical Devices

Using absorption materials to reduce RF heating during MRI procedure is studied in this paper. Materials with different electromagnetic absorption characteristics are used to alter the EM and current distributions on the device. Consequently, the RF induced heating behaviors of external fixation devices can be reduced. Numerical and experimental studies are provided to demonstrate the potentials of reducing the RF heating for external fixation devices of using lossy materials.

Comparison study of RF-induced heating in leg phantom with circular external fixator for TEM and birdcage coils at 3 T

A typical cochlear implant system under magnetic resonance imaging (MRI) procedures may couple with the radio frequency (RF) electromagnetic field (EMF) and results in an intensified electric field at the lead tip. As a result, the RF energy deposited in human tissues around the lead tip may induce heating and cause tissue damage concerns. The purpose of this work is to evaluate the RF-EMF-induced heating for cochlear implant system in 1.5 T MRI coil and highlight the factors that have significant effects on the heating. The potential factors involved in the RF-EMF-induced heating including the lead type, lead trajectory, human model and MRI landmark. In this paper, the RF-EMF-induced heating for three types of leads is evaluated in two virtual human models. A total of 24 lead trajectories and 23 anatomical landmark positions are studied using the transfer function method. The average temperature rise for all the studied cases in the human models is 0.79 °C, and the maximum value is 2.80 °C for a maximum whole-body average specific absorption rate of 2 W kg-1 in an RF body coil. It is found that the lead trajectory and MRI landmark are two primary influencing factors. The maximum temperature rises for different lead trajectories can vary from 0.82 to 2.80 °C. A difference in heating of 2.80 °C is observed when the landmark changes from  -100 to 700 mm. This work demonstrates that it is necessary to take these factors into account when evaluating the RF-EMF-induced heating for implanted medical devices.