# Direct Temperature Measurements of Cardiac Stent during MRI Examinations
## 1 Introduction  
Magnetic Resonance Imaging (MRI) represents a diagnostic technique furnishing images of the human body without applying ionizing radiation [1]; for this reason, we can consider it a non-invasive diagnostic tool. But, this does not mean that MRI has no side effects. The major risks are related to (a) the static magnetic field, which can contribute to projectile effects; (b) the gradient magnetic field, which can cause current induction; and (c) the radio frequency (RF) field, used for nuclei excitation and interact with patient tissues causing heat [2; 3; 4]. In general, MRI staff are exposed to static magnetic field and,occasionally, to a radiofrequency field and time varying gradient fields [5], while patients during the MRI scan are exposed to all the three kind of magnetic fields [1]. The rate of RF deposition is well determined in terms of the Specific Absorption Ratio (SAR). Moreover, the accepted metric for thermal dose assessment that correlates well with thermal damage in a variety of tissues is cumulative equivalent minutes at 43 \({}^{\circ}\)C (CEM43\({}^{\circ}\)C). First proposed by [PERSON] and [PERSON] [6], this metric quantifies thermal exposure in terms of the number of minutes of heating at 43 \({}^{\circ}\)C needed to obtain equivalent effects in biological tissues. CEM43\({}^{\circ}\)C thermal dose thresholds have been proposed as a potential guide for magnetic resonance radiofrequency exposure levels [7]. It is important to underline that MRI is now considered the gold standard for imaging the brain, spinal cord, musculoskeletal system, head and neck, and complex congenital heart malformations [8; 9]; consequentially, the number of MRI scans in patients with implantable electronic devices has simultaneously increased [10]. Today, millions of patients have implanted devices. Initially, implantable medical devices were a firm contraindication for MRI [11], although advances in the technology have made sure that metallic implants such as mechanical heart valves, coronary stents, and sternal sutures are compatible with MRI because they are not ferromagnetic. On the other hand, these devices are electrically conducting structures that may interact with the electromagnetic fields (RF) of the scanner. They can concentrate the RF power, potentially causing high-power depositions in the vicinity of these objects. Severe MRI-related RF heating of electrically conducting structures and associated risks have been shown for interventional guide wires [12], implanted electrodes for deep brain stimulators [13] and cardiac pacemaker leads [14]. The European Medical Device Regulation [15] identifies that electromagnetic immunity is an essential requirement for both non-implantable and implantable medical devices. Manufacturers follow harmonized standards (not mandatory) to obtain the presumption of conformity [16]. Technical specifications for passive implanted medical devices and for active ones are available, ASTM F2182 and ISO/TS 10974, respectively [17; 18]. From the above tests, an implantable device can obtain a label stating it is MRI safe, MRI conditional or MRI unsafe. Cardiac stents are labelled as MRI Conditional and limitations are placed during MRI scans (e.g., RF power). The RF field used during MRI examination can induce currents and these induced currents cause charge accumulation, creating areas where the temperature is drastically increased, causing tissue burns and damages [12]. Precise in vivo measurements of MRI-related heating are hard to achieve with any technique currently available. Usually, these kinds of tests are conducted on phantoms during periodic quality controls procedures [19; 20] to ensure patients' safety and maintain acceptable diagnostic equipment performances [21]. Up to now, not too many studies were published about the specific analysis of the temperature changes and SAR on a cardiac stent during MRI examination [22; 23; 24; 25; 26].