Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI) is a neuroimaging technique that uses strong magnetic fields, magnetic field gradients, and radio waves to generate anatomical images. MRI is particularly useful at looking for white matter and subcortical changes.

• Magnetic resonance imaging (MRI) uses the body's natural magnetic properties to produce detailed images from any part of the body. In MRI, the hydrogen nucleus (a single proton) is used because of its abundance in water and fat.
• By varying the sequence of radio-frequency pulses applied and collected, different types of images are created:
  • Repetition Time (TR) is the amount of time between successive pulse sequences applied to the same slice.
  • Time to Echo (TE) is the time between the delivery of the RF pulse and the receipt of the echo signal.

• MRI has a clear advantage of no ionizing radiation. It also has an advantage over CT in that it can detect blood flow, occult/cryptic vascular malformations, and demyelinating diseases like multiple sclerosis. There are also no beam-hardening artifacts as seen in CT, and the posterior fossa is more easily visualized on MRI than CT.

• MRIs are more expensive than CT scans, scans take longer, and are not always available at healthcare facilities. Individuals with metal implants may also not be able to have MRIs.

• Tissue can be characterized by two different relaxation times: T1 and T2. The most common MRI sequences are T1-weighted and T2-weighted scans.

• T1 (longitudinal relaxation time) is the time constant which determines the rate at which excited protons return to equilibrium. This is a measure of the time it takes for spinning protons to realign with the external magnetic field. T1-weighted images are produced using short TE and TR times. The contrast and brightness of the image are predominately determined by T1 properties of tissue.

• T2 (transverse relaxation time) is the time constant which determines the rate at which excited protons reach equilibrium or go out of phase with each other. T2-weighted images are produced by using longer TE and TR times. In these images, the contrast and brightness are predominately determined by the T2 properties of tissue.

• The Fazekas scale is used to quantify white matter T2 hyperintense lesions due to chronic small vessel ischemia (though not all such lesions are due to this).^[1] In routine clinical practice, the Fazekas scale is generally not used. General clinical descriptor such as “mild”, “moderate” and “severe” are typically used.

• Fluid Attenuated Inversion Recovery (FLAIR) is a sequence is similar to a T2-weighted image except that the TE and TR times are even longer. Abnormalities remain bright but normal CSF fluid is attenuated and made dark. This sequence is very sensitive to pathology and makes the differentiation between CSF and an abnormality much easier. Note that FLAIR will miss strokes in the basal ganglia.

• Diffusion Weighted Imaging (DWI) looks at movement of water molecules between two radio-frequency pulses, and has the highest sensitivity for acute stroke (detects abnormalities within 3 to 30 minutes).

• Soares, J., Marques, P., Alves, V., & Sousa, N. (2013). A hitchhiker's guide to diffusion tensor imaging. Frontiers in neuroscience, 7, 31.
• Alexander, A. L., Lee, J. E., Lazar, M., & Field, A. S. (2007). Diffusion tensor imaging of the brain. Neurotherapeutics, 4(3), 316-329.
• Huisman, T. A. (2010). Diffusion-weighted and diffusion tensor imaging of the brain, made easy. Cancer Imaging, 10(1A), S163.

• Before undergoing MRI imaging, all patients with potential occupational, recreational, or educational exposure or known accidental exposure to metal fragments should have orbital radiographs to evaluate for the continued presence of intraorbital metallic foreign bodies.^[2]
  • Similarly, anyone with a history of neurosurgery (e.g., placed aneurysm clips) should have orbital radiographs.^[3]
• Pacemakers and implanted defibrillators are generally not compatible with MRI, though newer devices are now compatible with MRI.

• Sometimes, perivascular spaces can look like ”mini strokes”, but they are not. Perivascular spaces are usually more well defined, circular/oval, with smooth margins.^[4]