Approach Considerations
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.
In T2-weighted images, CSF and fluid have a higher signal intensity than tissue and therefore appear bright. [4]
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:
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FLAIR (T2 with water suppression)
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T2 with fat suppression
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T1 with contrast
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Echoplanar
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Proton density
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MR spectroscopy (MRS)
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Functional MRI (fMRI)
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Perfusion MR
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MR angiography/venography (see image below)
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Diffusion and diffusion tensor MR [9]
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Diffusion-weighted imaging (good for small strokes)
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Gradient echo (GRE)
In the MRI Machine
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]
Guidelines
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:
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Caution is urged in the use of gadolinium-based contrast agents (GBCAs). Use of GBCAs should be avoided when not necessary.
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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.
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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.
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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.
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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.
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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.
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Axial T1
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Sagittal T1
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Axial T2
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Magnetic Resonance Angiography
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Fetal Brain MRI (T2 sequence)
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MRI Magnet
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Head coil. Courtesy of Lee Ryan, PhD, McKnight Brain Institute, Department of Psychology, University of Arizona.
