Perioperative Challenges During Diagnostic and Perioperative Magnetic Resonance Imaging (MRI)

Abstract

The majority of patients with a neurological condition will undergo magnetic resonance (MR) imaging at some stage of their clinical management. MR imaging is non-invasive, but the patient is required to lie motionless in a narrow noisy tunnel with the body part to be imaged encased in a receiver coil.

Glossary

Controlled area

The controlled area wholly contains the 0.5 mT magnetic field contour. Self-locking doors with coded locks control access to the controlled area. Only authorised personnel have unsupervised access to the controlled area. All patients, visitors and non-authorised staff should be medically screened before entering the controlled area.

Gauss

Gauss is a unit of magnetic field strength (the correct term is magnetic flux density). The SI Unit, Tesla, has mostly superseded it. 1 Gauss  =  0.1 milliTesla (mT). 1 Tesla  =  10,000 Gauss. The cardiac pacemaker field contour is often referred to as the 5 Gauss line.

Gradient magnetic fields

The technique of MRI uses magnetic field gradients to localise and encode the MR signals with spatial information to generate an image. The gradient fields are superimposed onto the static field but are only switched on for short periods (typically just a few milliseconds). The fast switching of the gradient fields generates rapidly varying magnetic fields. This may cause interference on the patient’s electrocardiogram and, on higher performance MR systems, may cause peripheral nerve stimulation. The electromagnet coils that generate the switched gradient fields vibrate, generating acoustic noise, which is often in excess of 100 dB. Hearing protection should always be provided to patients whether conscious or unconscious.

Inner controlled area

The inner controlled area is a term that is often used to describe the MR examination room that contains the MR system magnet. The attractive force of the fringe field is at its greatest in this room presenting a significant projectile hazard. The inner controlled area is always contained within the controlled area.

MR conditional

An item that has been demonstrated to pose no known hazards in a specified MR environment with specified conditions of use.

MR safe

Equipment is designated as MR safe if it presents no safety hazard to patients or personnel when it is taken into the MR examination room, provided that instructions concerning its use are correctly followed. This does not, however, guarantee that it will function normally and not interfere with the correct operation of the MR imaging equipment, with degradation of image quality.

Quenching

Quenching refers to the rapid, almost explosive, boil-off of liquid helium and the accompanying loss of superconductivity. The magnetic field is lost and a large volume of helium gas is generated and should normally be vented to the outside atmosphere through a quench pipe. If this pipe were to fracture it would present a potentially lethal hazard caused by the rapid build-up of helium gas in the MR examination room, leading to asphyxiation. Quenches can be spontaneous, usually occurring when the magnet is being powered up or down during installation or services, or can be deliberately activated in an emergency, such as fire.

Tesla

Tesla is the SI unit of magnetic field strength (the correct term for field strength is magnetic flux density). The cardiac pacemaker field contour is often referred to as the 0.5 mT line. Typical nominal field strengths of commercial MR systems are 0.5, 1.0 and 1.5 T. Higher field strength (e.g., 3 T) systems are becoming commercially available.