# Direct Temperature Measurements of Cardiac Stent during MRI Examinations
## 4 Discussion  
Patients with a medical implant can be excluded for an MRI examination because of the many safety concerns that exist when introducing metal inside the MRI room [30; 31]. Short and compact passive metal implants, such as cardiac stent, do not appear to pose substantial contraindications for MRI, since only minor temperature changes have been reported and no cases of serious injuries have never been reported for this kind of implant. However, implant manufacturers have to test and label each device according to the standards: the evaluation of the device in terms of heating is particularly challenging due to the many factors that may impact the RF-related temperature increase (the type and the physical properties of the implant, its positioning inside the body and respect to the RF coil, the RF frequency generated by the MRI system and the type of RF coil used). Generally, the RF safety assessment of implants is performed using simplified scenarios because the full analysis is computationally too time-consuming. Moreover, actual MR thermometry technique is not suitable to obtain information about RF heating pattern on the stent surface, due to the MR artifacts induced by the stent [23]. In this study, we have adopted a simplified state, leading to conservative results to ensure safety, and using RF exposure conditions matching a real MR cardiac exam. In this way, the obtained results are not relative to only one specific sequence but give indications regarding the temperature rise due to the entire standard clinical protocol. The obtained results show that the maximum temperature rise (4.12 \({}^{\circ}\)C) is within the limit of 5 \({}^{\circ}\)C stated in the manual for 15 min of continued scanning with the specific conditions (MR conditional: static magnetic field of 3.0 and 1.5 T only, maximum whole-body SAR of <2 W/kg, maximum spatial gradient magnetic field of 23 T/m). As expected, the maximum temperature rise was in correspondence of the stent tips [23; 32]. Calculated SAR confirms the fact that two hotspots are present near the tips of the stent. Since the stent has a length shorter than \(\lambda/4\pi\) of the RF wavelength in tissues, we can use the local SAR at the tip of the device as surrogate metric for worst-case heating [32]. Regarding thermal dose assessment, for muscle tissue, CEM43\({}^{\circ}\)C > 80 min has been reported to cause chronic damage, whereas 41 < CEM43\({}^{\circ}\)C < 80 min was associated with acute but minor damages [7]. As can be observed in our study, CEM43\({}^{\circ}\)C remained well below 40. CEM43\({}^{\circ}\)C has a physical meaning when it is evaluated in the tissue: as an approximation, our calculation was based on the temperature measured on the stent assuming the temperature at the interface of the metal and tissue to be similar. From the literature, different studies were conducted on the stent during MRI examinations. The comparison with our study is not simple because of the different setup used, in particular, the strength of static magnetic field of the scanners, the exposure parameters and the different stents used. However, although numerical results cannot be compared, the trend of temperature increase (over time and along the entire length of the stent), is consistent with that found in both simulation and experimental literature studies. For example, in [23], the authors conducted experiments on RF heating induced by a coronary stent using a 7 T scanner. They found a maximum temperature increase of about 3 \({}^{\circ}\)C. This result cannot be directly compared with our results for temperature rise, due to the very different exposure conditions. In [26], the authors measured the RF heating of a stents implanted on pigs and their results showed an average temperature rise of 2.4 \({}^{\circ}\)C in vivo and 2.7 \({}^{\circ}\)C postmortem near the distal end of the stent. In this case, the a 3T MRI scanner was used with an exposure time of 10 min, so the exposure conditions were similar to those used in our study.