Magnetic resonance imaging (MRI) uses magnetic fields and radio waves to create detailed pictures of the body’s internal structures. Within neuroimaging, specific sequences generate contrast to highlight different tissues and potential abnormalities in the brain and spinal cord. The T2-weighted sequence and the Fluid-Attenuated Inversion Recovery (FLAIR) sequence are two distinct methods developed to maximize the detection of fluid accumulation and pathology. These sequences are routinely utilized together to provide complementary perspectives on neurological health.
The Basics of T2 Weighting
T2-weighted imaging relies on T2 relaxation, which measures how quickly the magnetic signal from water protons decays after excitation. Tissues high in water content, such as cerebrospinal fluid (CSF), edema, and inflammation, retain their signal longer and thus appear very bright, or hyperintense, on the image. This brightness is the signature characteristic of T2 images, often referred to as “water-white.”
Normal brain tissue appears in shades of gray, with gray matter slightly brighter than white matter. Dense structures like bone and air-filled sinuses appear dark, or hypointense.
The T2 sequence is highly effective for showing general pathology because most disease processes, including tumors and strokes, involve localized water accumulation (edema). This increased water content stands out brightly against the intermediate signal of healthy brain tissue, making T2 a general-purpose detector for neurological abnormalities.
How FLAIR Modifies the T2 Image
The FLAIR sequence (Fluid-Attenuated Inversion Recovery) is a specialized modification of the standard T2 sequence. In standard T2 images, the bright signal from the CSF surrounding the brain and filling the ventricles can obscure lesions adjacent to these spaces. FLAIR overcomes this by using Inversion Recovery to nullify the CSF signal.
This nullification is achieved by applying an initial 180-degree radiofrequency pulse, which inverts the magnetization of all tissues. The sequence waits for a precise period, the Inversion Time (TI), calculated so that the CSF magnetic signal is zero at that exact moment. When the imaging pulse is applied at this zero point, the CSF produces no signal and appears dark, while pathology remains bright. Suppressing the bright CSF background dramatically enhances the visibility of abnormalities, particularly those located near the ventricles or on the brain’s surface.
Why T2 and FLAIR are Essential Diagnostic Tools
The combined use of T2 and FLAIR sequences helps physicians differentiate between various neurological disorders. The standard T2 sequence offers a broad view of pathology by making all fluid-containing abnormalities bright, including large areas of edema or cystic structures. This is useful for identifying the overall extent of a major event, such as a large stroke or a tumor.
FLAIR, due to CSF suppression, is particularly sensitive for detecting subtle or superficial pathologies that standard T2 might miss. This includes the small, oval-shaped lesions characteristic of Multiple Sclerosis, which frequently occur in the periventricular white matter.
FLAIR is also adept at detecting subtle signs of subarachnoid hemorrhage or inflammation near the brain’s surface. The dark CSF provides a high-contrast background against the pathological bright signal, making both sequences required for a comprehensive brain examination.
Decoding Signal Appearance
Understanding the terminology used in an MRI report helps patients better grasp their diagnostic results. The terms “hyperintense” and “hypointense” describe the relative brightness of a tissue region compared to its surroundings. On both T2 and FLAIR images, a hyperintense area appears bright white and signifies an abnormality, such as inflammation, edema, or a lesion with increased water content.
Conversely, a hypointense area appears dark, representing tissues with very little water or short signal decay times, such as bone, air, or areas of chronic injury. In FLAIR, the most significant hypointensity is the CSF, which is intentionally nulled to black to enhance the contrast of adjacent brain lesions.