Brain Magnetic Resonance Imaging Periprocedural Care

Updated: Aug 18, 2015
  • Author: Omar Islam, MD, FRCPC; Chief Editor: Gowthaman Gunabushanam, MD, FRCR  more...
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Periprocedural Care

Patient Education & Consent

Elements of Informed Consent

Patients should be informed that the MRI will assist with evaluation of the medical condition and will guide future management. Patients should also be informed with regards to what the scan will entail.

Certain biological effects might be temporarily experienced at magnetic fields higher than 3 tesla (T). [1] These include dizziness, upset stomach, metallic taste, focal areas of heat, and tingling sensations. Although cardiac functions are not affected, electrocardiography signals are affected owing to changes in surface currents under magnetic fields.

Following contrast administration, patients might experience a flushing sensation.

There are no documented long-term effects of MRI, even at high magnetic strengths. [1] Patients should also be informed as to the potential risks of contrast administration, namely allergic reactions or NSF/NFD. [11]

If necessary, patients should be administered any required sedatives only after informed consent has been obtained.

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Pre-Procedure Planning

The types of sequences to be conducted on the scan must be planned beforehand. Some conditions require particular sequences for sufficient diagnostic accuracy. Therefore, it is often helpful to indicate the results of a history and clinical examination on the requisition form. There are also standardized protocols for various conditions that can also be used, such as for stroke or Alzheimer disease. [13] The need for contrast administration must also be assessed in light of the suspected pathology, as well as the patient’s kidney function and allergies. [10]

If there is any uncertainty as to the presence of metal objects, plain radiography can be helpful.

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Equipment

Most MRI magnets employ a tunnel shaped system by using a cylindrical magnetic bore. This allows for the generation of a strong magnetic field. However, this creates physical restrictions for the patients, as well as any interventional or surgical procedures.

MRI Magnet MRI Magnet

Alternatively, open systems are more accessible and can allow for MRI scans in patients who fail to meet the physical requirements of tunnel shaped systems. Open systems also allow for interventional procedures as well, although they have weaker magnetic strengths. [1]

Several types of magnets can be used in MRI machines. The first type is the permanent magnet, which does not require any energy. However, its disadvantages include its limited field strength and its high weight.

Alternatively, resistive magnets can be used, in which a current is passed through a coil in order to generate a magnetic field. Although these can achieve high strengths, they are rarely used owing to the great amounts of heat they create. In some cases, both permanent and resistive magnets are used.

The third type of magnets used is the most common, namely superconducting magnets, which used cryogens (eg, helium and nitrogen) at low temperatures (approximately -270°C). Such low temperatures allow for the conduction of electricity and subsequently generate strong magnetic fields, although at high costs. [1]

Radiofrequency coils are also necessary in MRI in order to transmit radiofrequency signals to tissue and to function as an antenna in order to receive the generated signals. Coils can also function to alter the magnetic fields. Coils should be close to the size of the body part and should surround it. Specific to brain MRI, a head coil, which resembles a helmet, or a neurovascular coil is used, depending on the type of imaging. [1]

Head coil. Courtesy of Lee Ryan, PhD, McKnight Bra Head coil. Courtesy of Lee Ryan, PhD, McKnight Brain Institute, Department of Psychology, University of Arizona.

Strength of magnets

MRI machines are available in various magnetic strengths, from less than 1T up to 10T. Higher magnetic strengths allow for a greater signal-to-noise ratio (SNR), therefore allowing for higher-quality images or faster scans at the same quality. The most commonly used magnetic strengths are 1.5T and 3T. [4]

Contrast agents

Most MRI contrast agents are gadolinium-based, which is a paramagnetic metal that changes the T1 relaxation times of water protons. [4] Hence, it causes enhancement of various structures, including vessels, meninges, structures outside the blood brain barrier (pineal gland, pituitary gland, choroid plexus), and areas of an absent or leaky blood-brain barrier. Therefore, contrast is valuable in imaging neoplasms, infections, vascular diseases, and inflammatory diseases. [4] Gadolinium-based contrast agents are injected intravenously for brain imaging. [14]

Gadolinium-based contrast is associated with lower rates of nephrotoxicity than iodinating contrast agents used for radiography or CT scanning. Most gadolinium-based agents are eliminated through the kidney, although recent agents that are eliminated through the liver have also been developed. [10]

Common contrast agents include the following:

  • Gadopentetate dimeglumine (Magnevist)
  • Gadobenate dimeglumine (MultiHance)
  • Gadodiamide (Omniscan)
  • Gadoversetamide (OptiMARK)
  • Gadoteridol (ProHance) [10]
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Patient Preparation

Generally, there are no dietary restrictions and patients can continue medications normally.

Patients regularly complete a screening form with the assistance of a professional. Patients are subsequently instructed to change into a gown and to remove all accessories. Any piercings, jewelry or removable metals should also be taken off.

If a contrast will be administered, an intravenous line is established, and saline is administered until the contrast material is injected. Any required anesthesia will also be administered at this time, after informed consent is obtained.

Mild sedation is achieved by administering 1-2 mg of lorazepam (Ativan) orally in order to treat claustrophobia or anxiety.

Stronger sedation might be required depending on the patient’s circumstances, and support from an anesthesiologist should be requested.

The patient is asked to lie supine.

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Monitoring & Follow-up

High-risk patients should be monitored for the development of an allergic reaction or NSF/NFD if contrast is administered. [10]

Patients are often be advised by their own physician as to the next steps of management after the radiology report is obtained. It is possible that further imaging is required if different sequences are required, images with fewer motion artifacts are needed, or a different modality altogether is required.

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