mri Flashcards
How does an MRI scanner work?
The MRI scanner makes use of a physics concept called the dipole moment. This refers to the ability of the nucleus of a hydrogen atom to act as a magnet. When placed within a powerful magnetic field, the spin of the hydrogen’s lone electron
will align itself parallel to the magnetic field. The patient’s watercontaining tissues are then subjected to radiofrequency impulses. When the proton in the hydrogen is stimulated by this
impulse it emits radiofrequency signals that can be interpreted as three-dimensional spatial information.
How do the strong magnetic fields in an MRI scanner affect the patient?
Because of its iron content, the patient’s blood flow will be altered and the blood pressure will exhibit a compensatory increase. Some patients report dizziness, headache, nausea, and visual sensations.
The American College of Radiology divides the MRI
area into four zones. What are they?
Zone 1 is the area with public access and requires no supervision. The waiting room and hallway outside the MRI suite are an example of Zone 1. Zone 2 has public access but limited supervision. The entrance into the MRI suite is an example of Zone 2. Zone 3 has limited access and stringent
supervision. The area immediately outside the MRI room is an example of Zone 3. Zone 4 is the room where the MRI scanner is located and is under constant supervision.
Why might sedation or anesthesia be required for an MRI scan?
The patient must remain completely still for an hour or more during an MRI. Patients that cannot remain still or suffer from claustrophobia may require sedation, or in exceptional cases, require a general anesthetic.
Why is the presence of ferromagnetic (ironcontaining) objects contraindicated in the MRI suite?
Because of the strength of the magnet used, iron-containing objects such as pens, cell phones, pagers, jewelry, some anesthesia equipment, and even credit cards can become missiles that could strike the patient or damage the machine.
How can a pacemaker be affected by an MRI scan?
The pacemaker can be reset or reprogrammed, it may revert to asynchronous mode, become hot, or even become dislodged.
What is the composition of MRI contrast dye and
what are the side effects of its administration?
MRI contrast is a chelated form of gadolinium that is administered intravenously. A common side effect is nausea.
You are performing a general anesthetic for a patient undergoing an MRI. You are using an endotracheal tube with an extra-long circuit and extension tubing for the end-tidal CO2 sampling line. How would this affect the capnograph waveform?
Lengthening the ETCO2 sampling cable will result in a prolonged upslope of the capnography waveform.
Can a pregnant anesthetist be present in an MRI
suite?
Pregnant anesthesia can be in the MRI room during all of the required anesthesia preparations, but the American College of Radiology recommends that personnel should not be in the MRI scanner with the patient during the actual scan. Pregnant anesthesia personnel should discuss this with their obstetrician prior to MRI exposure.
How does the anesthetic equipment and monitoring
setup differ in an MRI suite versus the operating
room?
All of the standard equipment necessary to deliver anesthesia in any other environment is still mandatory for delivery of an anesthetic in an MRI suite, including all monitoring and emergency equipment. An oxygen supply is necessary, but is
usually provided by a non-ferrous wall oxygen flowmeter. Pressurized oxygen tanks may be composed of ferrous material that is not allowed in the MRI suite itself.
What are the most important preparations to make
prior to beginning IV sedation for an MRI?
As in all sedation procedures, there is a risk that the patient may need some form of emergency airway management. An emergency plan should be developed for this procedure because the patient is secluded in the MRI machine making
observation more difficult. Should an emergency arise, it is necessary to be able to communicate the issue with the technician who can then, immediately stop the procedure and remove the patient from the MRI. If intubation is necessary and
MRI safe laryngoscope blades are not available, it may be necessary to have a stretcher ready to remove the patient from the suite entirely.
Why should monitor leads be maintained in a
straight line when placed on a patient undergoing
anesthesia for an MRI? What other anesthesia
items may be precluded by this fact?
Loops in the monitor leads can cause thermal injury during the MRI. The magnetic field can induce the flow of current in coiled leads or tubing, causing them to become hot enough to burn the
patient. For this reason, flexible LMAs and anode endotracheal tubes that contain wire winding should not be used. Even tissue loops, such as the curve caused by the patient’s hand touching his/her hip can induce current flow and result in injury.
