Introduction
Safety issues and discussions about potential hazards associated with magnetic resonance imaging (MRI) systems and procedures have been extremely controversial over the past decade: partly because of the disputed assertions about the role of electromagnetic fields in carcinogenesis or the promotion of abnormalities in growth and development [1-3]; partly because the assumption that MRI was inherently a safe procedure had reduced the importance of the publication of negative results [4]. Since the introduction of MRI as a clinical modality in the early 1980s, more than 100,000,000 diagnostic procedures (estimated) have been completed worldwide, with relatively few major incidents [5,6].Most reported cases of MRI related injuries have been caused by misinformation related to the MR safety aspects of metallic objects, implants, and biomedical devices [7,8]. In fact, the MR environment may be unsafe for patients with certain implants, primarily due to movement or dislodgment of objects made from ferromagnetic materials [9], but also because of heating and induction of electrical currents, which may present risks to patients with implants or external devices [10]. These safety problems are typically associated with implants that have elongated configurations or that are electronically activated (e.g. neurostimulation systems, cardiac pacemakers, etc.). In the MR environment, magnetic field-related translational attraction and torque may cause hazards to patients and individuals with such implants. The risks are proportional to the strength of the static magnetic field, the strength of the spatial gradient, the mass of the object, its shape and its magnetic susceptibility. Furthermore, the intended in vivo use of the implant or device must be taken into consideration because existing counteracting forces may be present that effectively prevent movement or dislodgment of the object. To date, more than one thousand implants and objects have been tested for MR safety or compatibility. This information is readily available to MR healthcare professionals, though it requires heightened awareness by the MR community to continually review and update their policies and procedures pertaining to MR safety based on the information in the relevant medical literature [11]. Physicians are aware of the absolute contraindications to MRI with regard to implantable devices, less familiar is the potential for an MRI-induced thermal or electrical burn associated with induced currents in conductors in contact with the patient's body. Although detailed studies concerning the burn hazard in MRI have not yet been reported, recent reports have, however, indicated that direct electromagnetic induction in looped cables in contact with the patient may be responsible for excessive heating [12-14].
A comprehensive presentation and discussion of MR related hazardous effects is beyond the scope of this review, thus we will limit the discussion to bio-effects produced by MRI systems acting directly on the human body.
Several research studies have been conducted over the past thirty years in order to assess the potential dangerous bio-effects associated with exposure to MRI diagnostics. Because of the complexity and importance of this issue, most of these works are dedicated to separately examining biological effects produced by a particular magnetic or electromagnetic field source utilized in MRI. Moreover, the scientific literature proliferates in an ever-increasing number of studies concerning biological effects produced by the interactions of biological matter with electromagnetic fields. Thus, there is a need to integrate and summarize the current findings about this topic and, at the same time, provide the basic knowledge to understand the physics of the interactions between electromagnetic fields and biological systems.
In the present work, after an introduction on the basic principles of MRI systems and the electric and magnetic properties of biological tissues, the basic principles needed to understand the bio-effects caused by the three main sources of electromagnetic fields utilized in MRI procedures are described.
Basic principles of MRI procedures
Three different types of electromagnetic fields are utilized in creating an image based on magnetic resonance:1. the static magnetic field,
, which aligns the proton spins and generates a net magnetization vector 
in the human body;2. the gradient magnetic field, which produces different resonant frequencies for aligned protons, depending on their spatial positions on the gradient axes; these gradient fields allow for the spatial localization of bi-dimensional MRI slices and hence the reconstruction of three dimensional MRI images;
3. the radio-frequency electromagnetic wave, centered at the proton resonant frequency, which rotates the vector
out of the direction of the static magnetic field; the time during which the magnetization vector returns to the equilibrium is different for each tissue, and this results in the two main imaging parameters, T1 and T2, which directly relate to the image contrast.These three fields are essential features of MRI procedures, and each interacts with the electromagnetic properties of biological tissues.
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