Brain Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a noninvasive technique used for diagnostic imaging. MRI is particularly useful for the imaging of soft tissues. Therefore, MRI allows for high-quality imaging of the brain with good anatomic detail and offers more sensitivity and specificity than other imaging modalities for many types of neurologic conditions. MRI also offers significant flexibility with the use of contrast agents and combinations of different sequence types.[1]

Image segmentation is one of the primary tasks in image analysis. In brain MRI analysis, image segmentation is commonly used for measuring and visualizing the brain's anatomic structures; analyzing brain changes; and delineating pathologic regions.[2]  Therapeutic uses of MRI in the brain also exist. For example, MRI-guided radiotherapy is frequently used for brain tumors.[3] MRI is also used for neurosurgical planning and neurointerventional radiologic procedures, although specialized nonmagnetic equipment is required for the latter.

MRI utilizes the varying content of hydrogen atoms contained within different tissues. Specifically, hydrogen atoms, which have their own magnetic field based on the direction of their spin, are aligned to a strong magnetic field generated by the MRI machine. The MRI machine subsequently generates an electromagnetic pulse that is at the adequate frequency to be absorbed by the hydrogen atoms.[4] This absorbed pulse causes the hydrogen atoms to enter an excited state, thereby changing the spin of the hydrogen atoms and misaligning them from the magnetic field. With time, hydrogen atoms return to their original relaxed state and become aligned with the magnetic field once again.[1] This happens at different rates depending on the tissue they are contained within. In returning to their relaxed state, hydrogen atoms also generate a radiofrequency pulse, which is detected and converted into an image that can be used for diagnostic purposes.[4]

Prior to the procedure, patients need to be assessed for the presence of any contraindications. This entails passing the safety criteria and clearing the physical requirements. Patients must not exceed the maximum weight that can be supported by the table and must also be able to fit into the magnetic bore, which often has a 60 cm diameter.[1]

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.[5, 6]  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.[7]

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

Tissue contrast in MRI may be based on the following:

• Water/fat/protein content

• Metabolic compounds (eg, choline, creatine, N-acetylaspartate, lactate)

• Magnetic properties of specific molecules (eg, hemoglobin)

• Proton density

• Diffusion of water

• Perfusion (capillary blood flow)

• Bulk flow (large vessels, cerebrospinal fluid [CSF])[4]

Faster acquisitions and development of advanced MRI sequences, such as magnetic resonance spectroscopy (MRS), diffusion tensor imaging (DTI), perfusion imaging, functional MR imaging (fMRI), and susceptibility-weighted imaging (SWI), as well as the use of higher magnetic field strengths, have made MRI an important tool for detailed evaluation of the developing brain.[8, 9]

Subsequent to the scans, anywhere from 100-1000 high-resolution 2-dimensional images are produced directly in multiple planes. Three-dimensional reconstruction is also possible. Quantitative analysis may also be conducted, allowing the identification of regions of interest (ROI).[4]  These images can reveal pathology or abnormal anatomy and can further guide management of the patient.

See also the Medscape articles on Brain Herniation Imaging, Brain Aneurysm Imaging, and Brain Abscess Imaging.

MRI is particularly useful for the imaging of soft tissues. Therefore, MRI allows for high-quality imaging of the brain with good anatomic detail and offers more sensitivity and specificity than other imaging modalities for many types of neurological conditions. MRI also offers significant flexibility with the use of contrast agents and combinations of different sequence types.[1]

MRI can be useful in evaluation of the following:

• Ischemia/infarct

• Vascular anomalies

• Hemorrhage

• Infection

• Tumors and masses

• Trauma and diffuse axonal injuries

• Neurodegenerative disorders and dementias

• Inflammatory conditions

• Congenital abnormalities

• Seizures

• Headaches

• Cranial neuropathies

• Fetal brain (see image below)

  Fetal Brain MRI (T2 sequence)

Because of the lack of ionizing radiation, MRI is often recommended as a safe alternative in radiosensitive populations, such as pregnant women and children.[10]

Therapeutic uses of MRI in the brain also exist. For example, MRI-guided radiotherapy is frequently used for brain tumors.[3] MRI is also used for neurosurgical planning and neurointerventional radiologic procedures, although specialized nonmagnetic equipment is required for the latter.

There are few contraindications to MRI. Because of strong magnetic fields, no metals or electronic devices should be brought into the scan room, as they can create a safety hazard and cause image artifacts. There is a risk of causing movement or turning, generating heat or a current, or causing malfunction of devices. Moreover, items may become projectiles or get stuck in the machine.[1]

The following are some items that might be contraindicated:

• Foreign bodies from trauma, mechanical heart valves, surgical implants, plates, screws, staples and clips, and prosthetics that contain metal

• Pacemakers, cochlear implants, drug infusion ports, insulin pumps, deep-brain stimulators, and other electrical devices

• Metal tooth implants and fillings

• Accessories such as keys, glasses, piercings, jewelry, hairpins, pagers, watches, wallets, identification badges, and pens

• Oxygen tanks, carts, chairs, IV poles, and other medical equipment

Generally, braces, intravascular stents and filters, surgical clips, staples, sutures, and orthopedic hardware are safe, especially newer ones. However, caution should be used with these items, and the radiologist should also be informed to determine any subsequent impact on the images.[1]

Physical limitations that prevent supine positioning, such as severe respiratory distress or marked kyphosis or kyphoscoliosis, can render patients unsuitable for an MRI. Inability to fit on the table or within the machine (eg, obese patients) also precludes MRI.[1]

Patients who are unable to lie still, such as many children,[11, 12] patients with movement disorders, or patients in severe pain, also might be unsuitable for an MRI and can require sedation or general anesthesia. Similarly, those with severe anxiety or claustrophobia might require mild sedation or anxiolytics.

MRI is also unsuitable for emergency situations because of its longer scan durations, unless necessary.

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]

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.

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]

Laboratory tests

While laboratory tests in advance of MRI are not usually required, a kidney function assessment and pregnancy test might be indicated if contrast is to be administered.

Serum creatinine levels should be measured to determine the extent of renal insufficiency and can be particularly important in determining the extent of AKI. Serum creatinine evaluation is also useful in chronic situations in which the eGFR is unknown in order to determine the stage of renal failure.

A qualitative beta-HCG test can be useful to confirm pregnancy in uncertain situations. Use of gadolinium-based contrast agents can be avoided accordingly.

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.

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

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 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]

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.

The basic types of sequences used in brain MRI create either T1-weighted or T2-weighted images.

In T1-weighted images, CSF and fluid appear dark. Gray matter is darker than white matter.

Axial T1

Sagittal T1

In T2-weighted images, CSF and fluid have a higher signal intensity than tissue and therefore appear bright.[4]

Axial T2

Other specialized sequences are also available and can be useful to demonstrate various pathologies. Research continues for the development of new specialized sequences. Some specialized sequences include the following:

• FLAIR (T2 with water suppression)

• T2 with fat suppression

• T1 with contrast

• Echoplanar

• Proton density

• MR spectroscopy (MRS)

• Functional MRI (fMRI)

• Perfusion MR

• MR angiography/venography (see image below)

• Diffusion and diffusion tensor MR[9]

• Diffusion-weighted imaging (good for small strokes)

• Gradient echo (GRE)

• Fast imaging employing steady-state acquisition (FIESTA)[1, 4]

  Magnetic Resonance Angiography

Patients are instructed to lie supine and to stay still with their hands at their side. Patients might be asked to hold their breath for certain short periods. Additionally, patients might also receive short breaks between scans.

Each scan takes 30 seconds to 3 minutes, and the procedure can take up to an hour.

A head coil helmet is placed around the patient’s head. The helmet allows the patient to see outside, thereby minimizing claustrophobia.[17]

The patient is able to communicate outside the room with a two-way intercom.

Some machines play music or display TV, hence minimizing symptoms of claustrophobia and reducing the loud scanner noises. Vibrations might be felt along with the scanner noises that are produced.

A panic button is also provided to the patient to abandon the procedure if he/she cannot continue further. Patients are also advised to inform the technician if they feel uncomfortable.[17]

The International Society for Magnetic Resonance in Medicine released guidelines on the clinical and research use of gadolinium-based contrast agents.[20]  The key recommendations included: 

• Caution is urged in the use of gadolinium-based contrast agents (GBCAs). Use of GBCAs should be avoided when not necessary.
• GBCAs should not be withheld from patients with a clinical indication for gadolinium-enhanced MRI. The physician responsible for the administration of a contrast agent should understand the benefits and risks of the agent.
• The clinical indication for which a GBCA is administered, the specific contrast agent used, its dosage, and other pertinent information should be documented in the patient's medical record.
• Some commercially available macrocyclic agents might deposit less gadolinium than some linear agents; however, evidence shows that gadolinium deposition in the brain can also occur after the administration of macrocyclic agents. Evidence suggests differences in gadolinium deposition rates among macrocyclic agents and among linear agents, although some data are discordant. Relaxivity differences between contrast agents and between the potentially deposited chemical species can complicate the interpretation of differences in signal intensity.
• No evidence shows any harmful effects from the deposition of gadolinium, and therefore whether use of macrocyclic agents should be favored over linear agents is unclear.
• When choosing a contrast agent, many factors should be considered, including pharmacokinetics, relaxivity, efficacy, potential side-effects (such as allergic reactions), patient age, probability of the need for repeated examinations, and cost. Institutions should weigh these factors and consider that some agents might have a greater propensity for deposition than others.