Brain Magnetic Resonance Imaging Periprocedural Care

Updated: Nov 06, 2019
  • Author: Omar Islam, MD, FRCPC; Chief Editor: Mahan Mathur, MD  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 because of 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. [14]   The FDA requires every patient be asked to read a Medication Guide with educational information before receiving a gadolinium-based contrast agent (GBCA). [16]

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

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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 -haped 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 because of 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 use 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 so as 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. [17]

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 there are agents that are eliminated through the liver. [7]

The presence of intrathecal gadolinium displays characteristic features on MRI of the brain and may mimic subarachnoid hemorrhage on susceptibility-weighted images. Identification of high-dose gadolinium in the CSF spaces is necessary so as to prevent diagnostic and therapeutic errors. [15]

Patients with advanced renal insufficiency who are administered gadolinium-based contrast agents are at risk for developing nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermatopathy (NFD). Therefore, patients with acute kidney injury (AKI) or stage 4 or higher chronic kidney disease (with an estimated glomerular filtration rate [GFR] < 30 mL/min/1.73 m²) should not receive contrast agents. [7]  In addition to end-stage renal disease, hepatorenal syndrome and a perioperative liver transplantation period are also risk factors for the development of NSF/NFD. [14]

Patients with moderate kidney disease should be administered contrast agents only cautiously by avoiding high doses, minimizing the number of times contrast is administered, and allowing for significant time between consecutive scans. Agents safer than gadodiamide, gadoversetamide and gadopentetate dimeglumine should be used. [7]

Caution should also be used in patients with a history of allergies and in children younger than one year. [7]  Allergic reactions to gadolinium contrast agents are rare.  [14]   Idiosyncratic reactions are more common.

While pregnancy is not a contraindication because of a lack of ionizing radiation, minimum use of MRI is still recommended. Gadolinium-based contrast agents are able to cross the placenta and should not be administered, particularly during the first trimester. [13]

The use of contrast material is not recommended in patients with advanced renal insufficiency, acute or chronic; therefore, an imaging modality other than MRI might be required. [7]

In 2015, the FDA asked that a label change be added to three of the linear agents, noting a gadolinium retention risk in patients with preexisting kidney failure. This population is also at risk for the painful skin disease nephrogenic systemic fibrosis (NSF). [18]   In 2017, the FDA reported that a new review showed that gadolinium is retained in organs, but no new adverse events were uncovered. Retention is greater for linear over macrocyclic GBCAs. [16]  In 2017, the European Medicines Agency (EMA) restricted use of the intravenous linear agents gadoxetic acid and gadobenic acid, as well as gadopentetic acid given intra-articularly. [19]

 

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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 orally 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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