3-T MRI implant safety: heat induction with new dual-channel radiofrequency transmission technology

Recent implementations of metal-artifact reduction sequences in magnetic resonance imaging (MRI) have allowed the evaluation of anatomical structures close to metallic implants [1–3]. These sequences steadily become more important in daily clinical use given the strongly increasing rates of worldwide prosthesis implantations and of health issues in an aging population. The Food and Drug Administration now recommends to include metal-artifact reduction sequences in the preoperative evaluation of symptomatic patients with metal-on-metal hip implants [4].

However, there are safety issues associated with the examination of patients with metallic implants [5]. The principles of heat induction by MRI radiofrequency (RF) transmission are well known [6, 7]. The trend towards MRI scanning at high field (≥ 3 T), beyond challenges of optimizing image contrast and minimizing artifacts, also accentuates the problem of guaranteeing patient safety [8]. When comparing nominally equal image-acquisition pulse sequences, the RF energy irradiated at 3 T can be up to four times higher than at 1.5 T, while the limits for the estimated absorbed energy are the same at both field strengths [9]. The shorter RF wavelength at 3 T can cause spatially inhomogeneous image contrast. Thus, attempts were made to optimize the RF transmitting field by using a multi-transmit system, such as the TimTX TrueForm technology (Siemens Healthcare, Erlangen, Germany). It allows dual-channel parallel transmission via a two-port birdcage body coil and promises a more uniform RF field throughout the imaged body volume, with improved B₁ homogeneity (RF shimming) and, thus, also the potential to minimize standing-wave artifacts [10–13]. In part, the improvements are reached by (optionally) switching from the conventional circular field polarization to an elliptically polarized B₁ field, whereas single-channel RF transmission systems only allow circular RF polarization [14]. When scanning the thorax or abdomen, with their elliptic trans-sectional geometries, the technology potentially improves image quality by channel-specific, independent adaptation of the two RF channel amplitudes according to pre-established optimized patterns (elliptic TrueForm). Such adaptations have been shown to have consequences for RF-induced temperature increases [15, 16].

While manufacturers, must demonstrate that their systems reliably stay within the limits specified in the International Electrotechnical Commission 60601-2-33 standard [17], potential additional complications that may occur in the presence of metallic implants are not well investigated or documented. The MRI safety labels are defined by the American Society for Testing and Materials (ASTM) F2503-13 [18], which helps the implant manufacturer in guiding them which test for which implant is needed in respect to test for safety.

Radiofrequency heating tests are currently only done using circular polarization. To obtain an MRI-conditional label, implants need to be tested for RF heating at all field strengths, since the effects may vary, e.g., due to varying resonance effects at different resonance frequency. Ideally, the safety should be established for all RF transmission schemes at each field strength.

Therefore, the purpose of this phantom study was to comparatively assess RF heating effects at 3 T using the circularly polarized RF transmission (CPT) in comparison to the elliptically polarized RF transmission (EPT) setting in the presence of metallic hip implants.

In the 2-min baseline experiment without any implant in the phantom, a maximal temperature increase of 0.5 °C was measured with CPT, with a whole-body averaged specific absorption rate (wbSAR) of 1.042 W/kg, while with the EPT (wbSAR 0.750 W/kg) no relevant temperature increase was observed. The temperature increase on the left side (view from the patient table to the scanner) was higher compared to the right side (4.3 °C versus 1 °C; wbSAR 1.042 W/kg) using CPT.

In general, more severe heating was measured using the CPT than using the EPT setting (Table 2, Fig. 2), with the highest temperature increase measured at the greatest distance from the isocenter (6.2 °C versus 1.5 °C) (Table 2, Fig. 2). CoCr stem together with the stainless steel head and the titanium cup compared to the CoCr stem together with the ceramic head and the titanium cup showed slightly more severe heat induction when using the CPT setting (5.7 °C versus 5.4 °C; wbSAR 1.042 W/kg), while it showed slightly less heat induction when using the EPT setting (1.2 °C versus 1.5 °C; wbSAR 0.750 W/kg) at a 20-cm lateral offset from the isocenter on the left side.

Lateral offset from magnetic axis (cm)	RF transmission polarization mode	B1 + rms (μT)	ΔT (°C)	wbSAR (W/kg)
20	Circular	3.248	6.2	1.042
	Elliptic	3.335	1.5	0.75
16	Circular	3.248	4.2	1.042
	Elliptic	3.335	1.2	0.75
12	Circular	3.248	2	1.042
	Elliptic	3.335	0.5	0.75
8	Circular	3.248	0.9	1.042
	Elliptic	3.335	0.4	0.75
4	Circular	3.248	0.4	1.042
	Elliptic	3.335	0.3	0.75

RF radiofrequency; B ₁ + rms root-mean-square value of the magnetic resonance imaging (MRI) effective component of the RF magnetic (B₁) field, μT microtesla, ΔT induced temperature increase; °C degree Celsius; wbSAR whole-body averaged specific absorption rate, W/kg Watts per kilogram

In this study, we investigated whether different dual-channel transmission settings, such as the CPT versus the EPT, may determine different heat induction around an implant material dependent on its location within the magnetic field. While the heat induction steadily increased with the lateral offset from the isocenter for both settings, it was consistently higher for CPT than for EPT. To the best knowledge of the authors, this is the first study that reports a reduced heat induction through the use of an EPT in comparison to conventional CPT. It is also the first report to highlight that EPT reduces the heat induction near implants placed with a lateral offset from isocenter in comparison with CPT.

The main driving force for the development of dual-channel, parallel RF transmission technology was the reduction of standing-wave artifacts in MRI of the abdomen and thorax [12, 13]. However, the technique was also reported to reduce artifacts and improve image quality in other body regions, which was mainly explained with a better B₁-field homogeneity [10] or a decrease in locally induced RF currents resulting in decreased shading artifacts around metallic implants [19].

For MRI-conditional devices, the safety is tested for traditional, single-channel, RF body coils. In our measurements, conducted in a phantom tank and with implant orientations as recommended for RF heating tests according to the ASTM standard F2182-11a [18], the EPT resulted in consistently less heating than the CPT. However, our findings cannot be generalized to the general assumption that EPT would always be preferable to, and safer than, CPT. Actual electric field patterns in a patient’s body depend on many factors that are hard to control in clinical examination. Certain configurations may lead to higher heating with EPT than with CPT. For example, Murbach et al. demonstrated that, in the absence of implants, EPT may generate a higher temperature increase near the fetus in pregnant women than CPT [16].

However, the presented results are important not only for clinical daily use, where the EPT setting should be preferably used if scanning implants, but also for safety test measurements to avoid false underestimations of heat induction around metallic objects when are conducted with EPT instead of CPT. If in vitro RF heating tests are performed following the ASTM standard F2182-11a [18]. However, using EPT instead of CPT (as required), the results could strongly underestimate the potential RF heating. Special care is needed, since some dual-transmit MR system from other vendors might force the use of EPT setting, which cannot be overruled in clinical software.

Some study limitations need recognition. First, all the temperature measurements of implant-related heating were performed over a time period of 2 min. This is a shorter time than the duration of many clinical acquisition sequences. However, our goal was not to provide evidence for the certification of an MRI-conditional label for the used implants, but to relatively compare the effects of two RF transmission settings. The scan duration (2 min) was chosen because an assessment of the relative heat induction was considered sufficient for a comparison of the two RF transmission settings. The temperature increases reached after 2 min were large enough for a robust quantification of the effects but small enough to keep the required cool-down periods between two successive experiments sufficiently short to allow the realization of this study. A scan duration of 15 min would result in a temperature increase of about twice as high, but deposit 7.5 times more energy with thus much longer waiting time. This would only lead to a higher likelihood of alteration of the implant or the temperature sensors and, therefore, potentially generate larger errors. In addition, we only used the body coil for RF transmission. The use of transmit head or knee coils may produce different results, while the choice of the receive coil seems of minimum relevance in this context. Finally, this was only a phantom study. Studying such effects in living humans is very challenging, and the positioning of temperature probes next to the implant is not realistic.

In conclusion, in this phantom study, comparatively testing different dual-channel transmission settings (CPT versus EPT), we demonstrated that the heat induction around metallic implants increases with increasing lateral implant distance from the isocenter and is more severe using RF with CPT than with EPT. However, further studies, including simulations and evaluations on humans, are needed to verify and generalize our findings.