## 1 Introduction  
The ability to perform comprehensive evaluations of anatomical, functional and molecular imaging data is important for early detection and accurate prognosis of cancer. In many cases, this information is acquired using several imaging modalities with various contrast agents being administered during multiple hospital visits. The assessment of these images is also performed separately because of difficulties in image registration, primarily due to patient repositioning and variations in organ position and shape. Also, detection and measurement of low amplitude signals in noisy, heterogeneous biological environments is a fundamental challenge in diagnostic tomographic applications. Combined imaging during a single examination would solve some of these difficulties and has been realized using x-ray computed tomography (CT) and positron emission tomography (PET), with PET/CT now fully integrated into the clinical environment. PET/CT was one of the first multi-modal diagnostic approaches that made significant strides towards meeting new tomographic challenges [1; 2; 3; 4; 5]. Clearly, tomography will continue to evolve beyond PET/CT and many other omni-tomography/multi-tomography approaches are envisioned [6]. Over the last several years a number of dual modality diagnostic approaches in addition to PET/CT have been reported including SPECT/CT, SPECT/MRI and PET/MRI, the latter which appears to have distinct advantages [7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. The dual imaging merger of PET/MRI has recently gained interest due to the combination of PET functional scanning with the anatomical and functional information provided by magnetic resonance imaging (MRI). For example, the PET/MRI combination has a distinct clinical advantage over CT because a lower level of ionizing radiation is utilized. In addition, the PET/MRI has the added advantage of combining biochemical soft tissue information (MRI) while the PET approach provides higher sensitivity with specific targeting dependent on the spatial location of the radiolabel. Unlike PET/CT, which requires sequential acquisitions involving patient table movement, current technological advances have led to a PET detector that is unaffected by magnetic fields allowing for true simultaneous image acquisition [16; 17; 18; 19; 20]. The feasibility of a dual-modality PET/MRI system has proven successful in both small animal studies and in imaging the human brain [16; 20]. As simultaneous PET/MR imaging progresses toward full clinical integration and application, the development of imaging agents with dual-modality properties will provide an important component to advancements in research and clinical applications in this area. Based on the variety of information that these imaging modalities can provide, it can be anticipated that simultaneous PET/MRI acquisition may yield complementary multiparametric imaging leading to new diagnostic paradigms. Therefore, the interest in development and characterization of dual imaging agents is apparent [21]. Fullerenes of various carbon content (e.g., C\({}_{60}\)/C\({}_{80}\)/C\({}_{82}\)) with an assortment of endohedral components including empty cages, single metal ions, and tri-metallic nitrides, have been increasingly employed in investigational animal studies as imaging contrast agents. Isolating the metal component from the biological system by placing it inside the carbon cage is a primary benefit of this nanoplatform. In regards to undesired biological effects, the toxicity of nanoparticles in general is more complex than with traditional bulk materials and highly variable, dependent on multiple parameters including size, shape, surface functionality, aggregation, surface charge, _etc._[22; 23; 24; 25]. In regard to the fullerene nanoparticles utilized in this application, the toxicities _in vitro_ and _in vivo_ have been studied extensively [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32].