## 3 Iron Oxide Nanoparticles (USPIO) and Inflammation  
USPIO accumulate at sites of inflammation; therefore, their detection with CMR allows for the identification of active myocardial inflammation. Following intravenous administration, the iron oxide nanoparticles remain in the intravascular space and are taken up by the macrophages of the reticuloendothelial system [20]. Both the size and the coating of the nanoparticles are crucial for their bio-availability, with the smaller and more hydrophilic particles being a lot less susceptible to phagocytosis [6,23]. While larger iron oxide nanoparticles are rapidly identified and engulfed by the macrophages in the lymphoreticular organs (liver, spleen, lymph nodes), USPIO, because of their smaller size, are much less readily recognised and can subsequently avoid instant phagocytosis [6]. In this way, USPIO remain in the bloodstream for much longer, as reflected by their longer half-life (up to 36 h) compared to larger iron oxide nanoparticles (up to 2 h) [6,27]. After being taken up by macrophages, they are subsequently carried by these cells to areas  
Figure 1: A schematic presentation of a SPION (superparamagnetic iron oxide nanoparticle): The core radius ranges from 5 to 15 nm, and the hydrodynamic radius (core with shell and water coat) is between 20 and 150 nm. Unless there is a magnetic field, magnetisation equals 0. As shown, SPIONs can be easily coupled with antibodies that facilitate the majority of the SPION applications discovered so far [19]. Reproduced with permission from [PERSON] et al. [19] under a Creative Commons Attribution 4.0 International License.  
of active inflammation [7]. In addition to this mechanism, it has also been suggested that iron oxide nanoparticles passively cross the endothelial barrier at sites of loss of integrity and with increased permeability [7,30]. It has been shown, however, that the main mechanism by which USPIO accumulate is predominantly via cellular infiltration within the myocardium [31].  
USPIO-tagged macrophages have been identified with MRI in murine models of myocardial ischaemia and infarction [32,33]. In a proof-of-principle trial that consisted of fourteen patients with acute myocardial infarction, it was shown that USPIO have the ability to detect infiltrating macrophages, identifying areas of active inflammation [30]. In another proof-of-concept study that included sixteen patients with acute myocardial infarction, USPIO uptake was evident in areas of the infarcted and remote myocardium [34]. More recently, [PERSON] et al. demonstrated that USPIO can identify areas of myocardial inflammation in a murine model, with USPIO having been shown to be present inside macrophages in areas of the infarcted myocardium [1]. The same study, however, also showed that USPIO can accumulate in areas of inflammation not only after being engulfed by the macrophages but can also be 'passively' present in the interstitium outside phagocytic cells [1]. This is a significant finding, as it shows that USPIO detection is not specific for macrophage activity, and therefore, the cardiac MRI techniques used should be able to differentiate USPIO enhancement due to active macrophage uptake from the passive presence of the nanoparticles in the myocardial interstitium [1].