Magnetic Resonance Imaging (MRI) is a non-invasive diagnostic tool that produces detailed images of the brain and spinal cord. Unlike X-rays and CT scans, MRI uses powerful magnets and radio waves to visualize the internal structures of the brain. This high-resolution imaging is crucial in the diagnosis and management of various neurological conditions. In MRI Brain anatomy can be visualized through different sequences, including T1-weighted, T2-weighted, Short Tau Inversion Recovery (STIR), Fluid Attenuated Inversion Recovery (FLAIR), and Diffusion-Weighted Imaging (DWI), each offering unique information.
T1-Weighted Imaging
In T1-weighted (T1W) images, the brain's anatomy is depicted with high spatial resolution. The cerebral spinal fluid (CSF) appears dark, while white matter is brighter than gray matter. This contrast is especially useful in identifying anatomic details, including the borders of the brain, the distinction between gray and white matter, and the visualization of subcortical structures (1). Fat is also bright on T1, which is helpful in identifying lesions with lipid content or areas of hemorrhage that have undergone methemoglobin formation.
T2-Weighted Imaging
T2-weighted (T2W) images provide a different contrast mechanism. Here, CSF is bright, and the brain's white matter appears darker than the gray matter (2). This sequence is sensitive to edema and inflammation and is therefore crucial in assessing pathologies like gliomas, demyelinating diseases, and infarcts. T2W imaging is also useful in detecting chronic microvascular changes and white matter lesions indicative of diseases such as multiple sclerosis (3).
Short Tau Inversion Recovery (STIR)
The STIR sequence is an inversion recovery sequence with a short inversion time, which nullifies the signal from fat. This is particularly valuable in detecting lesions near bony structures or within the spinal canal, where fat can often obscure pathology (4). It also increases the contrast of lesions in the brain that might be isointense on other T1 or T2 sequences.
Fluid Attenuated Inversion Recovery (FLAIR)
FLAIR imaging is similar to T2W but with a special inversion pulse that suppresses the CSF signal, making the CSF appear dark. This sequence is particularly sensitive to detecting lesions adjacent to the CSF, which may otherwise be obscured due to the high signal intensity of the CSF on T2W images. FLAIR is particularly useful in identifying demyelinating lesions, gliosis, and subtle changes in the cortical and subcortical regions (5).
Diffusion-Weighted Imaging (DWI)
DWI is based on the molecular motion of water and is extremely sensitive to changes in the diffusion of water molecules within the brain tissue. In acute stroke, for instance, the diffusion of water is restricted within minutes of ischemia, which can be visualized as bright signals on DWI sequences (6). DWI can also identify abscesses, encephalitis, and demyelinating diseases.
MRI Anatomy of the Brain
The brain can be divided anatomically into the cerebrum, cerebellum, and brainstem, each with distinct functions and appearances on MRI sequences.
Cerebrum: The largest part of the brain, responsible for voluntary activities, intelligence, memory, and speech. On T1W images, the gray matter of the cerebral cortex and basal ganglia can be differentiated from the white matter. On T2W and FLAIR images, white matter abnormalities and lesions are more readily visible (7).
Cerebellum: Controls balance and coordinates movement. On MRI, the cerebellar folia are more finely detailed in T2W images, while T1W images provide a clearer distinction between the white and gray matter (8).
Brainstem: Includes the midbrain, pons, and medulla oblongata, controlling many vital functions. The brainstem's anatomy is complex, and both T1W and T2W sequences are useful for evaluating its internal structure and identifying pathologies (9).
In the clinical context, interpreting the MRI appearance of the brain requires an understanding of the normal anatomy and the expected variations in signal intensity across different sequences. Radiologists use these differences to discern healthy tissue from pathology. For example, high-grade gliomas may appear hypointense on T1W and hyperintense on T2W and FLAIR sequences, while acute ischemic stroke would be hyperintense on DWI and hypointense on apparent diffusion coefficient (ADC) mapping (10).
Conclusion
MRI has revolutionized the way the brain is imaged, with different sequences providing a suite of contrasts to explore the anatomy and pathology of the brain. Understanding the appearance of the brain across T1, T2, STIR, FLAIR, and DWI sequences is essential for clinicians to diagnose and manage neurological diseases effectively.
References
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