HAZARDS IN THE MRI SUITE

TRANSLATIONAL FORCES –  THE MISSILE EFFECT

This effect is generally attendant upon ferromagnetic materials and the static field generated by an MRI system, and often manifests as the missile effect,which can involve non-compatible objects, such as wheel chairs, transport stretchers, file cabinets, electrical equipmentand tools, powered and unpowered hand tools, stethoscopes, medical equipment carts, medical equipment (pulse oximeters, EKG, IV pumps, etc), communications devices, physician beepers, and miscellaneous (and often forgotten) patient and visitor objects that 

include hairpins, paper clips and pens.

  

TORQUE FORCES

Torque forces runparallel to the translational force. Both are associated with ferromagnetic materials and the static field generated by an MRI machine.Attracted objects react by aligning parallel to the magnetic lines of force being generated by the MRI machine. The center of the MRI-generated field 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. This effect has created life-threatening conditions for patients with some medical implants.

INDUCED MAGNETIC FIELDS

Any metallic object introduced into a high flux field will induce acurrent if that object is moving and perpendicular to the force lines. That new induced current will create a secondary magnetic field that will oppose the original field. The same metallic object introduced parallel to the force lines will generate no effect. While not as apparent as translational forces, induced magnetic fields can cause patients discomfort or anxiety due toreactive forces on MRI-safe medical implants.

All pacemakers and implantable cardioverter/defibrillators should be considered contraindicated under any circumstance. When these devices are exposed to an MRI environment, a life-threatening condition may be created within the five-gauss line. Intravascular catheters, intubation equipment, infusion pumps, orthopedic implants, stents and other devices should be verified as MRI-safe in the prescreening. The transportation of a patient should be designed to minimize potential torque from secondary magnetic fields by maintaining a parallel path to the lines of force. Any MRI-safe device must be verified as acceptable for the MRI tesla-rated machine and maximum closest safe distance from the machine.

THERMAL HEATING

The static magnetic MRI field will tend to induce currents in any conductive materials, including those that maybe non-ferrous. However, the human body is electrically conductive by nature and small RF fields will generate current that will be absorbed by the body as heat. The specific absorption rate (SAR) for the patient will depend on weight and body radius. The heating will be more prominent at the periphery of the body than at its core. This effect will also vary with the type of MRI scanning being done (angle, pulse frequency, fast spin echo, etc.).  

The most serious exposure is located in the bore of the MRI machine and in the axis points, as they possess the highest potential torque forces. The use of extremity coils could aggravate the risk, but canbe avoided through the use of MRI-safe polymeric foam padding. In addition, monitoring patient position in the MRI machine and proximity to its inner wall should be included in patient care protocol. The most common sources of thermal exposure tend to be looped/un-looped medical equipment leads, MRI accessories, and sensors. These include conductive loops touching the patient or crossing the extremities, clothing, and metalized drug delivery patches (OTC orprescription).