Magnetic Resonance Imaging (MRI) is an invaluable tool in the visualization and assessment of brain anatomy and pathology. The soft tissue contrast provided by MRI is superior to that of other imaging modalities, making it a cornerstone in neurological diagnosis. This article will discuss the MRI anatomy of the brain and the appearance of various tissues in T1-weighted, T2-weighted, STIR, FLAIR, and DWI sequences.

MRI Brain Anatomy

The human brain, enveloped by the skull, consists of the cerebrum, cerebellum, and brainstem. These structures are distinguishable on MRI due to their unique tissue properties and the use of different pulse sequences that highlight anatomical details.

In T1-weighted images, the normal brain anatomy reveals a high-intensity signal from white matter, contrasting with the lower signal intensity of gray matter. This inversion is because white matter contains more myelin, which shortens the T1 relaxation time. The cerebral cortex, basal ganglia, thalami, and internal capsule are clearly distinguishable, allowing for detailed anatomical study.

T2-weighted images provide a reversed contrast to T1, with cerebrospinal fluid (CSF) appearing very bright, and brain parenchyma appearing darker. This sequence is particularly useful for identifying edema, demyelination, and gliosis.

T1, T2, STIR, FLAIR, and DWI Appearance

T1-weighted Imaging (T1WI): Ideal for evaluating the anatomy of the brain, it highlights fat as high-intensity and water as low-intensity. It can delineate the gray matter (hypointense) from white matter (hyperintense) and is beneficial for detecting acute hemorrhage, fat-containing structures, and contrast enhancement after gadolinium administration.

T2-weighted Imaging (T2WI): Water and other fluids are high signal intensity, making them appear bright. T2WI is sensitive to pathology such as edema, inflammation, and demyelination. The high contrast between CSF and surrounding tissue aids in evaluating ventricular system and sulcal patterns.

Short Tau Inversion Recovery (STIR): This sequence is used to suppress fat signal, making it beneficial in the detection of lesions adjacent to bone or within the bone marrow. It's particularly sensitive to changes in water content, thus highlighting edemas and inflammations.

Fluid Attenuated Inversion Recovery (FLAIR): FLAIR imaging is similar to T2WI but with the suppression of free fluid signal, notably CSF. This modification allows for the detection of periventricular lesions, which are otherwise obscured by the brightness of CSF on standard T2WI.

Diffusion-Weighted Imaging (DWI): DWI assesses the molecular diffusion of water. In the brain, it is particularly sensitive to acute ischemia, as diffusion is restricted in cytotoxic edema. High-intensity signals in DWI can indicate recent infarcts or other pathology restricting water movement.

Clinical Applications and Pathological Appearances

Each MRI sequence has its role in diagnosing and characterizing brain pathologies:

In ischemic stroke, DWI reveals cytotoxic edema as hyperintense areas within minutes of onset, while T2WI may take hours to days to show changes.

Multiple sclerosis plaques appear hyperintense on T2WI and FLAIR but can be hypointense on T1WI (black holes), indicating severe damage.

High-grade gliomas often show a ring-enhancing pattern on T1WI post-contrast and are hyperintense on T2WI and FLAIR due to surrounding edema.

Metastases can have a varied appearance but often show marked contrast enhancement on T1WI after gadolinium administration and edema on T2WI.

Future Directions and Advanced Techniques

Advanced MRI techniques such as functional MRI (fMRI), Magnetic Resonance Spectroscopy (MRS), and perfusion MRI provide additional functional and metabolic information. Machine learning and AI applications are being developed to enhance the detection and quantification of brain pathologies.

References

Lee, S. K., & Kim, D. I. (2005). Perfusion MR imaging: A neurovascular application. AJNR. American Journal of Neuroradiology, 26(3), 427-438.

Westbrook, C., Roth, C. K., & Talbot, J. (2011). MRI in Practice (4th ed.). Wiley-Blackwell.

Elster, A. D., & Burdette, J. H. (1998). Questions and answers in magnetic resonance imaging. Mosby-Year Book.

Hatabu, H., Gaa, J., Tadamura, E., Edinburgh, K. J., Stock, U. A., & Westra, S. J. (1999). MR Anatomy of the Thorax. Magnetic Resonance Imaging Clinics, 7(3), 441-457.

Barkovich, A. J. (2005). Pediatric Neuroimaging (4th ed.). Lippincott Williams & Wilkins.

Rowley, H. A. (2005). The four "A's" of MRI: Anatomy, architecture, abnormality, and artifact. Journal of Neuropsychiatry and Clinical Neurosciences, 17(4), 484-489.

Schaefer, P. W., Grant, P. E., & Gonzalez, R. G. (2000). Diffusion-weighted MR imaging of the brain. Radiology, 217(2), 331-345.

Atlas, S. W., Thulborn, K. R. (1998). MR detection of hyperacute parenchymal hemorrhage of the brain. AJNR. American Journal of Neuroradiology, 19(8), 1471-1477.

Huisman, T. A. (2003). Diffusion-weighted imaging: Basic concepts and application in cerebral stroke and head trauma. European Radiology, 13(11), 2283-2297.

Deo, A. A., Choudhary, A. K., Sharma, R., & Pande, S. (2014). Role of MRI in brain imaging for epilepsy. Journal of Pediatric Neurosciences, 9(3), 291-297.