MR heating tests of MR critical implants (2007)

This article focuses on possible problems of MR heating tests for MR critical implants and gives some suggestions on how these problems could be mitigated. Critical implant length, MR heating tests in phantoms for 1.5T and 3.0T systems, whole-body averaged specific absorption rate provided by the MR system, and tissue damage assessment are briefly discussed. The U.S. Food and Drug Administration (FDA) is confronted with numerous MRI safety applications for implants. These MRI claims usually need to be supported by test results for force and torque, effects of the gradient fields, heating tests, and, in case of active implantable medical devices, by electromagnetic compatibility (EMC) test. For all types of implants, active and passive, heating tests are necessary to ensure the safety of the patient when exposed to the radio frequency field of the MR scanner. Unfortunately, there is currently misunderstanding about possible pitfalls in performing MR heating tests, in interpreting the results, and in assessing the measurement uncertainty. Most MR heating tests refer to the ASTM Standard F2182-02a (1), which is for passive implants only. Recent investigations revealed several problems associated with ASTM F2182-02a. Therefore, the ASTM Committee F04 on Medical and Surgical Materials and Devices, Sub-Committee F04.15 on Material Test Methods is currently revising F2182-02a. The suggestions outlined below summarize the FDA's current knowledge of MR-induced heating for MR critical implants, which is based on numerous MR heating test reports submitted to the FDA. Research in this area is ongoing, and future results will show how testing of MR-critical implants need to be revised and properly performed. active implantable medical devices (AIMDs) semiactive implants, i.e., implants powered from outside of the body elongated metallic structures that are in the range of the critical length (as defined below). For MR heating safety, the length and dimensions of the implant (i.e., lead length, stent length, etc.) in relation to the wavelength of the MR radio frequency field inside the patient or the phantom is critical. To become resonant, the length of the implant must be in the range of an odd number of half wavelengths of the electromagnetic field inside the patient or phantom (2). Once resonant with the electromagnetic field, the implant heating could become dangerously high. The half-wavelengths of the electromagnetic field inside a patient for 1.5T systems are about 25 cm, and for 3.0T MR systems they are about 12 cm. However, even for implants that do not have the exact resonant length, heating may occur and could become unacceptably high. It is currently unknown how much the length needs to differ from the resonant length to avoid unacceptable high heating. Preliminary and yet unpublished results of the ASTM SAR Intercomparison Protocol (3) show that for an insulated 20-cm-long straight wire with 1-cm-long bare ends, the heating can be up to 48°C (temperature rise or ΔT) in the normal operating mode of a 1.5T MR systems with a maximum whole-body averaged SAR of 2 W/kg. The field distribution inside a phantom strongly depends on the frequency, the shape of the phantom, and the position of the phantom inside the MR birdcage coil. Based on numerical simulations, we know that for the ASTM phantom (ASTM 2182 (1)), centered in a MR birdcage coil, the electric field inside the phantom has its minimum in the center of the phantom (4). In the center of the ASTM phantom body the electric field is almost zero. Therefore, heating of implants is also expected to be minimal in the center of the ASTM phantom. The electric field inside the ASTM phantom is also asymmetric and this asymmetry depends on the direction of the B1 field rotation. The B1 field rotation can differ in individual units of the same model, depending on how the static magnet was ramped during the installation (5). Therefore, it is important to realize that for MR-critical implants, the anatomical placement of the implant inside the patient cannot be translated to the phantom for realistic or worst-case MR heating tests. This means that placing an implant in the phantom in an anatomically equivalent position does not necessarily result in a realistic or worst-case testing for the whole patient population. For realistic heating assessment the local electric and magnetic field distribution inside the phantom needs to mimic the exposure situation of the implant inside the patient in such a way that the heating of the implant in the phantom is comparable to the heating inside the patient. Once the worst-case local field distribution inside the patient is known, the implant can be appropriately placed inside the phantom. The evaluation of the local field distribution inside the patient needs to be evaluated for the whole patient population under worst-case assumptions. Computer modeling, using anatomically-correct models of the whole patient population, can be used to evaluate the local field distribution inside the patient for the specific implant location. The whole-body averaged specific absorption rate (WB-SAR) displayed on MR scanner consoles is a conservative estimate intended to give an upper bound of the patient's WB-SAR. This WB-SAR number is intended only for patients and not for phantoms. Preliminary results of the SAR Intercomparison Protocol (3) show that using this WB-SAR number for scaling the heating results of implants, as required by ASTM 2182, could underestimate the heating by as much as a factor of 7. To avoid errors in using the WB-SAR displayed on the MR scanner console when performing phantom testing for implants, the International Eletctrotechnical Committee (IEC), Sub-Committee 62B, Maintenance Team 40 is considering a proposal to display on the MR scanner console a more direct measurement of the radio frequency (RF) electromagnetic field, B1rms, which can be used to evaluate implant heating. Meanwhile, calorimetry can be used to measure the true WB-SAR for phantoms. To assess possible tissue damage, the Cumulative Equivalent Minutes at 43°C (CEM 43) concept (6) could be used. CEM 43 is a well-established concept and values are available in the literature for most tissues. If literature values are not available for a specific tissue, animal studies could be used to evaluate the CEM 43 values for that tissue type. Because of the intrinsic differences for 1.5T and 3.0T systems, heating tests performed in one of the two systems cannot be translated to a MR system of the other field strength. Preliminary results of the ASTM SAR Intercomparison Protocol (3) show heating differences for a well-standardized generic implant between MR systems of the same field strength, and the same landmark of the phantom, of a factor of up to 11. The reason for these differences is currently unknown. Nevertheless, the possible large differences between MR systems could largely increase the MR heating measurement uncertainty. Also, other uncertainty factors (e.g., temperature probe placement, implant location, scanning position, patient differences, patient orientation in the coil, etc.) contribute to the overall uncertainty of the heating assessment and should be taken into account. Information on thorough uncertainty assessment can be found in Refs.7 and8. Testing should also take into account the heating effect of different types of RF emitting coils; e.g., body coil, head coil, local coils, etc.