Synopsis
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.Introduction
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 fields
1-5.
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 effect
6,7. 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).
Method
The
analysis exploits the high-resolution anatomical model “Duke”, modified to
include a realistic unilateral right implant and segmented with a resolution of
2x2x2 mm
3.
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 tests
8. 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 work
7. 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.
Acknowledgements
No acknowledgement found.References
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