Power deposition into a metallic hip prosthesis exposed to switched gradient fields
Luca Zilberti1, Oriano Bottauscio1, Mario Chiampi2, Jeffrey Hand3, Hector Sanchez Lopez4, Rüdiger Brühl5, and Stuart Crozier6

1Istituto Nazionale di Ricerca Metrologica, Torino, Italy, 2Dipartimento Energia, Politecnico di Torino, Torino, Italy, 3Division of Imaging Sciences and Biomedical Engineering, King’s College London, London, United Kingdom, 4Department of Engineering, Universitas Dian Nuswantoro, Semarang, Indonesia, 5Physikalisch-Technische Bundesanstalt, Berlin, Germany, 6School of Information Technology and Electrical Engineering, University of Queensland, St. Lucia, Australia


Concern has been recently raised about the possible heating of massive metallic implants, in particular hip prostheses, due to the gradient fields used in MRI. Thus, this contribution discusses the computation of the power density deposited by the magnetic field into the implant, which represents the first step to estimate the thermal heating. The analysis is based on numerical simulations, performed through a computational formulation applied to an anatomical model of the body. The results provide evidence of the role of the three gradient coil axes and of the different harmonic components of the signals in this power deposition process.


Nowadays, scanner manufacturers remain cautious for the extension of Magnetic Resonance Imaging (MRI) to patients with orthopedic implants, unless the implant is labelled as MR conditionally safe. Focusing the attention on metallic hip prostheses, some reports have evaluated the heating effects due to the MRI radiofrequency fields1-5. In this case, the strong skin effect limits the field penetration in the metallic element and the potentially high temperatures in the body are due to “hot-spots” in the power directly deposed in the tissues. Conversely, gradient fields do not give rise to a direct heating of the tissues, but can introduce a significant power deposition in the metallic implant, thus producing a potentially harmful indirect thermal effect6,7. In order to investigate this problem, the present contribution evaluates and discusses how the electromagnetic power (the driving term for the thermal problem describing the heating) is deposited within a metallic hip prosthesis by standard gradient coils (GC).


The analysis exploits the high-resolution anatomical model “Duke”, modified to include a realistic unilateral right implant and segmented with a resolution of 2x2x2 mm3. The hip implant involves some metallic components (i.e., acetabular shell, femoral head and stem) made of a CoCrMo alloy, and a liner made of polyethylene. The metallic alloy has an electrical conductivity of 1.16 MS/m and a unitary permittivity; the liner has a negligible conductivity and a relative permittivity equal to 2.25. Such parameters are assumed to be temperature independent. The body model is placed within a set of conventional gradient coils for cylindrical bore MRI scanners, producing a gradient of 30 mT/m in a 500 mm DSV. The electromagnetic simulations are carried out in the frequency domain through a non-commercial code, based on a hybrid Finite Element – Boundary Element Method (FEM-BEM). The code, running in GPU environment, was preliminarily validated through experimental tests8. The frequencies involved by GC (fundamental frequency around 1 kHz, plus harmonics) practically confines the power deposition inside the metallic implants. Thus, for a given frequency of the GC field, the computation of the spatial distribution of the volume power density (Pem) is restricted to the prosthesis. Since the typical waveform of the gradient fields is trapezoidal, the simulations are repeated for the main harmonic components, namely the first harmonic (assumed at 1 kHz) and the third, fifth and seventh harmonics, whose magnitude would depend on the specific features of the trapezoidal signal. The DC component is not considered because it does not produce electromagnetic induction. In order to show quite general results, the computations have been performed by imposing the same current for all harmonics of a given coil. More specifically, all sinusoidal harmonics produce the nominal field gradient in correspondence of their peak. This clearly overestimates the power density produced by the higher harmonics (whose magnitude, actually, is just a fraction of the amplitude of the main harmonic). In order to obtain quite homogeneous values, the power density has been rescaled inversely with the square of the harmonic order (n = 1, 3, 5, 7).

Results and discussion

The results of a set of simulations are presented in Figure 1, where the spatial distribution of Pem is depicted over a coronal section. In the case under analysis, the position of the prosthesis is such that the top of the femoral head is at 300 mm from the isocentre (i.e., the MRI exam involves the abdomen). This exposure situation was found to be significant in a previous work7. The figure indicates the power density developed within the implant for the three GC (i.e., X, Y and Z) separately; for each axis, the results obtained with the different harmonics are reported. Moreover, the maximum value of the power density and the total power deposed within the prosthesis (adopting the same rescaling as for Fig.1) are given in Tables 1 and 2. As can be seen, the coils of the X and Z axes are responsible for the highest power deposition, with “hot spots” mainly localized in the acetabular shell. However, this result should not be taken as completely general, because it could change depending on the position of the body. In absence of skin effect, for a given axis the same distribution of Pem should be found for the different harmonics (having assumed the same amplitude and rescaled the results with n-2). Actually, a general reduction of Pem is observed when passing from n = 1 to n = 7, indicating a non-negligible role of the skin effect, which reduces the capability of the higher harmonics to transfer power into the implant.


No acknowledgement found.


1. Schaefers G, Kugel H. A basic investigation of heating effects on total hip prostheses in combination with a simulated skin contact of the inner thighs during magnetic resonance imaging (MRI) with an 1.5 Tesla MR system. In Proceedings of the ISMRM Workshop on MRI Safety: Update, Practical Information and Future Implications, McClean, Virginia, USA, 2005.

2. Stenschke J, Li D, Thomann M, Schaefers G, Zylka W. A numerical investigation of RF heating effect on implants during MRI compared to experimental measurements. Adv. Med. Eng. 2007;114:53–58.

3. Mohsin SA, Sheikh NM, Abbas W. MRI induced heating of artificial bone implants. J. Electromagnet. Wave. 2009;23(5):799–808.

4. Powell J, Papadaki A, Hand J, Hart A, McRobbie D. Numerical simulation of SAR induced around Co-Cr-Mo hip prostheses in situ exposed to RF fields associated with 1.5 and 3 T MRI body coils. Magn. Reson. Med. 2012;68(3):960–968.

5. Abbasi M, Schaefers G, Sánchez JD, Erni D. Worst-case analysis of RF-induced heating during MRI scanning in a generic multicomponent orthopedic medical implant applying the design of experiment method (DOE). In Proceedings of the Joint Annual Meeting ISMRM-ESMRMB, Milan, Italy, 2014. p.4866.

6. Graf H, Steidle G, Schick F. Heating of metallic implants and instruments induced by gradient switching in a 1.5 T whole body unit. J. Magn. Reson. Imaging. 2007;26(5):1328-1333.

7. Zilberti L, Bottauscio O, Chiampi M, Hand J, Sanchez Lopez H, Brühl R, Crozier S. Numerical Prediction of Temperature Elevation Induced around Metallic Hip Prostheses by Traditional, Split, and Uniplanar Gradient Coils. Magn. Res. Med. 2015;74(1):272-9, 2015.

8. Bottauscio O, Cassarà A M, Hand J W, Giordano D, Zilberti L, Borsero M, Chiampi M, Weidemann G. Assessment of computational tools for MRI RF dosimetry by comparison with measurements on a laboratory phantom. Phys. Med. Biol. 2015;60:5655–5680.


Figure 1 – Chromatic map of the volume power density over a coronal section of the body. The values are rescaled inversely with the square of the harmonic order.

Table 1 – Maximum volume power density, rescaled inversely with the square of the harmonic order.

Table 2 – Total power deposited in the prosthesis, rescaled inversely with the square of the harmonic order.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)