MRI SAFETY

MRI Safety

MRI Safety

Introduction

Safety is paramount in magnetic resonance imaging. There is an ever present danger of ferromagnetic material unintentionally being brought inside the 0.5 mT isocontour and becoming a projectile in the main magnetic field. For this reason the local rules must be properly enforced at all times and staff trained to the required standard regarding safety. In the locality of the magnet, there will be an inner and an outer controlled area. The inner controlled area is typically the scanner room itself. This room must be clearly demarcated and access restricted by means of a lockable door. The outer controlled area would typically encompass the inner controlled area as well as the radiographer's control room, patients' changing room, a resuscitation room and in some cases reporting rooms. Access is also controlled by a door that is locked at all times and usually has a keypad entry mechanism. A plan of the layout of the MRI suite at my training centre is shown in figure 4-1 below. As a final control, patients are screened immediately before entering the inner controlled area. The other hazards involved in MRI are related to the strong magnetic gradients, RF and cryogens and these will also be addressed in this chapter.

Prevention of Projectile Effect Accidents

The "projectile effect" refers to the capability of the static magnetic field of an MRI system to attract a ferromagnetic object, drawing it rapidly into the scanner by considerable force. The projectile can affect metallic objects both outside and inside the patient. It can pose a significant risk to the patient inside the MRI system and/or anyone who is in the path of a projectile. Therefore, a strict policy should be established by the local MRI facility to detect metallic objects prior to allowing individuals or patients to enter the MRI environment in order to avoid accidents and potential injuries related to the projectile effect. The immediate area around the MRI system should be clearly demarcated, labelled with appropriate warning or danger signs, and secured by trained staff aware of proper MRI safety procedures. The area within the 0.5mT isocontour should be fully enclosed within the inner controlled area and the outer controlled area should be entered through a keypad-controlled door only by authorised personnel and supervised patients.

For patients preparing to undergo MRI procedures, all external metallic personal belongings (i.e., hearing aids, analogue watches, jewellery, etc.) and devices must be removed as well as clothing items that have metallic fasteners or other metallic components (e.g., belts, threads). Patients should wear a gown and any non-ambulatory patients must use a non-ferromagnetic wheelchair. Patients should also undergo appropriate MR safety screening as detailed in section 4.2 below.

While MRI safety guidelines and procedures are well known, accidents related to the projectile effect continue to occur. Local Safety Rules 1 are in force to minimise the probability of MRI accidents.

Experiment: Measurement of 0.5mT Isomagnetic Contour As a part of the routine QA procedures at Ninewells Hospital, I was required to determine the location of the ...