Abstract
This work focuses on the in silico evaluation of the energy deposed by MRI switched gradient fields in bulk metallic implants and the consequent temperature increase in the surrounding tissues. An original computational strategy, based on the subdivision of the gradient coil switching sequences into sub-signals and on the time-harmonic electromagnetic field solution, allows to realistically simulate the evolution of the phenomena produced by the gradient coils fed according to any MRI sequence. Then, Pennes’ bioheat equation is solved through a Douglas–Gunn time split scheme to compute the time-dependent temperature increase. The procedure is validated by comparison with laboratory results, using a component of a realistic hip implant embedded within a phantom, obtaining an agreement on the temperature increase better than 5%, lower than the overall measurement uncertainty. The heating generated inside the body of a patient with a unilateral hip implant when undergoing an Echo-Planar Imaging (EPI) MRI sequence is evaluated and the role of the parameters affecting the thermal results (body position, coil performing the frequency encoding, effects of thermoregulation) is discussed. The results show that the gradient coils can generate local increases of temperature up to some kelvin when acting without radiofrequency excitation. Hence, their contribution in general should not be disregarded when evaluating patients’ safety.
Highlights
The replacement of joints, in particular hip and knee, with orthopedic implants is a growing practice within the population
The procedure is validated by comparison with laboratory results, using a component of a realistic hip implant embedded within a phantom, obtaining an agreement on the temperature increase better than 5%, lower than the overall measurement uncertainty
The specific case of a hip implant exposed to the RF field of a magnetic resonance imaging (MRI) scanner has been analyzed by Powell et al (2012), who showed the effect of hip implant positioning within the MRI scanner limiting the attention on the local specific absorption rate (SAR), and by Mohsin et al (2009), who showed that the local SAR can exceed the average whole-body SAR significantly and the heating concentrates below the tip of the implant stem
Summary
The replacement of joints, in particular hip and knee, with orthopedic implants is a growing practice within the population. Further work on a hip implant was presented by Destruel et al (2019), who considered the continuous exposure to a 8-channel RF pTx hip coil inside a 7 T scanner for 30 min, investigating, in particular, the spatial correlation between SAR and temperature elevation Their simulations, performed for an anatomical body including thermoregulation, put in evidence hot-spots close to the tip of the stem and the screw of the acetabular cup (that was not present in the model used by Mohsin) and, for a local SAR (averaged over 10 g) below 10 W/kg, found a maximum final temperature exceeding the limit prescribed by Standard IEC 60601-2-33 (2010), i.e. 39 °C
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