**3.5. Operational principles of MRI**

As opposed to conventional x-rays and computed tomography (CT) scans, there is no ioniz‐ ing radiation used in MRI. However, MRI uses an extremely powerful static magnetic field, rapidly changing gradient magnetic fields and radiofrequency electromagnetic impulses to obtain detailed anatomic or functional images of any part of the body (Faulker, 2002; Berger, 2002). Currently, there is no evidence of a short or long term adverse effect due to exposure to field strengths of MRI and durations that is clinically used (Schenck, 2000).

creasingly stronger pull on objects they are wearing or carrying as they walk closer to the MR system, permitting them to retreat from the MR system before an accident occurs (Kanal

Assessment of Safety Standards of Magnetic Resonance Imaging at the Korle Bu Teaching Hospital…

Various forms of hazards occur in the MRI suite which can be categorized into translational force- missile effect, torque forces, induced magnetic fields, thermal heating and quenching (Colletti, 2004). In the translational force, the effect is manifested on the ferromagnetic mate‐ rials and the static field generated by the MR system usually in the form of the missile effect involving non-compatible objects and miscellaneous patient and visitor objects.A hair or pa‐ per clip within the 5-10 gauss line range could reach a velocity of 40 mph (about 70 kph) and

Just like the translational forces, the torque force is also associated with ferromagnetic mate‐ rials and the static field generated by the MR machine. Ferromagnetic objects that are at‐ tracted by the magnetic field react by aligning parallel to the magnetic lines of the force being created by the MRI machine. The centre of the MRI- generated fields has the highest torque force, creating a serious exposure for all contraindicated items and MRI- conditional items in the MRI suite, depending on the tesla rating of the MRI (Gould, 2008). When any metallic object is introduced into a high flux field, current will be induced if that object is perpendicular and moving to the lines of the force. The new current will create a secondary magnet field that will oppose the original field. This can cause patient discomfort and anxi‐ ety due to the reactive forces on the MRI safe medical implants and a life threatening condi‐

will be attracted to the centre of the lines of force of equal (Lahr and Rowan, 2004).

tion may be created under the five- gauss line (Kangarlau and Robitaille, 2000).

**Magnetic Field type Hazard Potential Adverse Effects**

Attraction of ferromagnetic objects to intense magnetic field. Rotational force/ torque: rotation of object to align with the magnetic field

Heating due to absorbed RF energy Electromagnetic interference

Induced current in electrical devices

Missile effect: acceleration of objects into the

http://dx.doi.org/10.5772/52699

61

Tearing of tissues, pain, and dislodgement of

Overheating, burns (thermal, electrical) Device malfunction; imaging artefact

Nerve and muscle stimulation Device malfunction/failure

bore of the magnet.

some implants.

Static magnetic field Translational force: power.

Gradient magnetic field Induced currents in conductive tissues

**Table 1.** Hazardous Magnetic Field Interactions

Adapted with permission from Centre for Device and Radiological Health of USA

*et al.,* 2002).

Radiofrequency electromagnetic fields

**3.8. Hazards in the MRI suite**

Despite the relative safety of MRI, there are potential hazards associated with its operations. Some of these are related to the physical properties of the MRI equipment and also to the challenges of maintaining physiologic stability of the individual undergoing the examina‐ tion. In a reported incident in 2001,a small boy undergoing an MRI following surgery to re‐ move a benign tumour was struck and killed by an oxygen tank inadvertently taken into the MRI suite (Emergency Care Research Institute, 2001). In most situations the MR systems cause the disaster due to it interactions with other properties around it.
