1

What is magnetic resonance imaging?

MRI incorporates the use of static and gradient magnetic fields with radiofrequency (RF) pulses to produce images of the body. Magnetic field strengths range from 0.15–3.0 tesla (T). The tesla is a measure of magnetic field strength (1 T = 10,000 gauss). Image quality depends on the strength of the magnetic field.

Hydrogen is the element most commonly used in MRI because it is the most abundant element in human tissue and it can be magnetized. Atoms, such as hydrogen, with an unpaired number of protons or neutrons respond to and align themselves within the magnetic field of the MRI scanner.

After placement of the patient within the cylindrical bore of the magnet, a steady state is established in which hydrogen atoms are in alignment. RF pulses are introduced and deflect the orientation of the atoms. When the RF pulses are eliminated, the hydrogen atoms return to their original position of alignment. As these atoms establish a resting state, the energy emitted is used to produce a resulting image.

2

What are the magnetic field problems associated with the magnetic resonance imaging?

MRI magnets exert a substantial pull on ferromagnetic objects. The major concern is that the magnet could convert ferromagnetic objects into missiles that can lead to injury or lethal outcomes to patients or personnel. Care must be taken to avoid the use of or to have in one’s possession objects such as pens, scissors, clamps, stethoscopes, nonlithium batteries, ferromagnetic compressed gas cylinders, and other objects that are ferromagnetic. Before bringing any piece of equipment into the MRI suite, it should be checked to ensure that it is MRI-compatible.

Patients with implanted ferromagnetic devices or objects, which include pacemakers, tissue expanders with metallic ports, implantable cardioverter defibrillators, implantable infusion pumps, cochlear implants, and certain types of intracranial aneurysm clips, are generally excluded from MRI studies. The magnetic fields of MRI scanners can potentially affect the function and safety of these devices. Newer aneurysm clips contain nonferrous material and are not considered to be problematic. However, a thorough investigation of the type of aneurysm clip is required before proceeding with an MRI study. Metallic-based substances such as eye makeup or tattoos can produce local skin irritation during MRI scanning ( Box 67-1 ). Metals that are known to be safe include stainless steel, titanium, alloys, and nickel. Plastic equipment is preferred for use in proximity to MRI magnets.

3

What are the specific problems encountered with physiologic monitors and equipment in the magnetic resonance imaging suite?

The use of conventional electrocardiogram (ECG) monitors in the MRI suite can distort the image because wire leads act as antennas. Additionally, ECG monitors may be unable to distinguish myocardial electrical potentials from background static magnetic field and RF pulses. Voltage induced in the wire leads can cause electrical shock hazards and burns to the patient.

Magnetic fields produced by MRI scanners can cause interference with and possibly inactivation of conventional operating room pulse oximeters. Either nonferrous or fiberoptic cabled pulse oximeters should be used. The pulse oximeter probe is best positioned on a distal extremity, as far from the site to be scanned as possible. In this way, interference and possible scan artifact can be minimized.

The oscillometric method is optimal for noninvasive blood pressure monitoring in the MRI suite because it is not affected by magnetic fields. Fiberoptic systems in conjunction with invasive blood pressure monitoring have been used successfully. In addition, central venous pressure can be monitored if necessary. There are several transducers that lack ferrous components and can be used in the MRI suite.

The use of side-stream capnography with a long sampling line allows for monitoring ventilation, anesthetic gas concentrations, and circuit disconnection during MRI scanning. However, the long sampling line may create a greater lag time between the actual event and the time of detection.

Several MRI-compatible anesthesia machines are available commercially. Only oxygen, air, and nitrous oxide cylinders made of aluminum can be used in the MRI suite. Ferromagnetic compressed gas cylinders are attracted by the magnet and have resulted in injury and death to patients and personnel. Infusion pumps should be checked for MRI compatibility as well.