# RF Exposure Assessment for Various Poses of Patient Assistant in Open MRI Environment
## 1 Introduction  
Diagnostic medical devices based on magnetic resonance imaging (MRI) have taken a major share of the global medical device market owing to their unique features; for example, the technology provides various contrast information, particularly for soft biological tissues, has a high capability to provide 3D cross-sectional visualization of the human body, and has no associated ionizing radiation hazards. For these reasons, MRI systems are widely used for clinical and research purposes. A high magnetic field strength (B\({}_{0}\) in Tesla) is a preferred feature, particularly for research purposes, because a higher B\({}_{0}\) strength generally promises a better image quality and a variety of advanced imaging techniques. However, the increased amount of RF exposure and inhomogeneous MR images owing to the shortened wavelength in high-field MRI systems causes several challenges for RF engineers and researchers [1]. By contrast, budgeting for the costs of purchasing and operating an MRI system is a more important aspect for clinical purposes, and thus local or veterinary hospitals tend to favor MRI systems with a low field strength.  
The magnet shape of a low-field MRI system (lower than 1.5 Tesla) is usually an open-magnet type, whereas that of high-field MRI is a closed type. A closed magnet (i.e., a long and cylindrical shape) may cause patients to experience claustrophobia, particularly for younger subjects. The magnets of an open MRI system are located on the top and bottom, and thus the four sides are open. Although a lower image quality is generally expected in a low-field MRI, the open-magnet shape provides a unique MR imaging environment. Because a patient assistant has easy access to a (young) patient during an MRI scan, the patient assistant can set the patient's mind at rest by holding their hands or touching the patient's body while the MRI scans are running. This assistance actually enhances theimage quality and shortens the MRI scan time because of the minimized motion artifacts on the images, and consequently removes the need for repeated scans.  
General guidelines for human exposure to an electromagnetic field (EMF) are provided by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [2; 3; 4], governmental regulations, and standards. Radio-frequency (RF) safety during MRI scans for patients is regulated by the ICNIRP, which produces and implements RF safety standards and guidelines [5]. The safety of patients and laborers related to MRI systems are also covered by the international standard IEC60601-2-33 [6].  
Several studies have assessed the RF safety during MRI scans for workers [7; 8; 9] and patients such as adults with implanted medical devices [10; 11; 12], the general public [13; 14; 15; 16], and pregnant women and their fetuses [17; 18; 19]. However, there have been few studies on the EMF exposure of patient assistants during MRI scans. Because open MRI systems allow patients to be examined with a patient assistant, it is necessary to analyze the EMF exposure of patient assistants when using open MRI systems. In this study, we numerically investigated the RF energy exposure of a patient assistant to an EMF using RF coil models of a 0.3 Tesla open MRI system. A couple of patient assistant poses were modeled, and an analysis of the specific absorption rate (SAR) of a patient assistant for each pose was then conducted. The results of this research provide a basis for us to inform patient assistants about the postures that lead to more exposure to EMFs from the RF coils during MRI scans.