Magnetic Resonance Imaging (MRI) is a sophisticated method doctors use to look inside the body without surgery. This technology relies on powerful magnets and radio waves to create detailed images of organs and tissues. To generate different types of contrast, the MRI machine uses various imaging protocols, known as “sequences.” The T2 Fluid Attenuated Inversion Recovery (T2 FLAIR) sequence is a specialized protocol that has become a necessary tool in brain imaging, providing distinct visual information that standard sequences cannot capture effectively.
The Basics of T2 Weighting in MRI
Standard MRI sequences differentiate tissues based on how quickly water molecule protons return to their resting state after being excited. T2 weighting measures transverse relaxation, where tissues high in water, such as cerebrospinal fluid (CSF) or inflammation, have a longer relaxation time. On a conventional T2-weighted image, these tissues appear bright (hyperintense), while normal brain tissue appears gray. Free-flowing water, like the CSF surrounding the brain, naturally shows up with a very bright signal. While this bright background is excellent for anatomical detail, the high intensity of the CSF can mask or obscure small lesions located close to the brain’s surface or the ventricles. This limitation led to the development of the specialized FLAIR technique.
Understanding the FLAIR Technique
The FLAIR sequence (Fluid Attenuated Inversion Recovery) is essentially a modified T2-weighted image designed to overcome the problem of bright CSF. The term “attenuated” means the signal from free fluid is intentionally reduced or suppressed. This suppression is achieved by adding an initial, powerful radiofrequency pulse called an inversion pulse. This pulse flips the magnetic orientation of the protons in the CSF.
The machine then waits for a precise period, called the inversion time, which is carefully calculated to coincide with the exact moment the CSF signal crosses zero. By acquiring the image at this moment, the signal from the free water is effectively nullified. The result is a T2-weighted image where the CSF appears dark (hypointense), instead of its usual bright appearance. Pathological tissues, such as those caused by inflammation or edema, have a different water composition than free CSF, so their signal is not suppressed. This fundamental difference makes T2 FLAIR uniquely sensitive for detecting abnormalities, particularly for visualizing lesions located next to the fluid-filled spaces of the brain.
Clinical Significance and What T2 FLAIR Reveals
The T2 FLAIR sequence has become an indispensable tool in neuroradiology because of its high sensitivity to subtle changes in tissue water content. When pathology occurs, such as inflammation or damage, it causes water to accumulate in the tissue, known as edema. This restricted water appears bright (hyperintense) on the FLAIR sequence, clearly visible against the dark surrounding CSF. This clear delineation makes T2 FLAIR useful for diagnosing and monitoring several neurological conditions.
Multiple Sclerosis (MS)
In Multiple Sclerosis (MS), the sequence is highly sensitive to the small, scattered plaques of demyelination that form in the brain’s white matter. These MS lesions appear as distinct, bright spots that are easily counted and measured for disease tracking.
Acute Stroke and Infection
T2 FLAIR is crucial in the evaluation of acute ischemic stroke, as it detects areas of brain tissue that are beginning to swell due to lack of blood flow. Furthermore, it is routinely used to identify signs of infection, such as encephalitis or meningitis, which cause inflammation and edema in the brain tissue and the spaces around it.