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)

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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.