Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool in modern medicine. This advanced imaging technique generates detailed pictures of organs, soft tissues, bone, and other internal body structures. An MRI system operates by combining a strong magnetic field with radio waves, which interact with the body’s water molecules to produce signals. These signals are processed by a computer to construct precise cross-sectional images, allowing differentiation between various tissue types.
Principles of T2 Imaging
T2-weighted imaging is a fundamental MRI sequence that highlights tissues based on their transverse relaxation time (T2 decay). Tissues with longer T2 relaxation times, such as fluids, edema, or inflammation, appear bright (hyperintense) because their signal persists longer during image acquisition. Conversely, tissues with shorter T2 relaxation times, like solid structures or bone, appear dark (hypointense).
The bright fluid signal on T2 images makes them highly sensitive to pathological changes involving increased water content. For instance, areas of swelling (edema) around a tumor or inflammation within a joint show up as bright regions. This contrast allows radiologists to identify and characterize lesions involving fluid accumulation or tissue damage. The sequence is useful for assessing conditions where distinguishing fluid from surrounding tissue is important.
Principles of FLAIR Imaging
FLAIR (Fluid-Attenuated Inversion Recovery) is a specialized MRI sequence that modifies the T2-weighted principle. While it also renders most fluid-rich tissues with a bright signal, FLAIR uniquely suppresses the signal from free-flowing cerebrospinal fluid (CSF). This makes CSF appear dark (hypointense), unlike its bright appearance on a standard T2 image. The technique works by applying an initial 180-degree radiofrequency pulse, followed by a specific delay (inversion time, or TI) chosen to null the signal from CSF, before a 90-degree excitation pulse and subsequent signal acquisition.
The deliberate darkening of CSF is particularly advantageous in brain imaging. Lesions located adjacent to the ventricles or within the sulci, which would normally be obscured by the bright CSF signal on conventional T2 images, become much more conspicuous. This allows for improved detection of subtle abnormalities that might otherwise be missed, as the bright signal from the lesion now stands out against the dark CSF background. FLAIR’s ability to suppress the CSF signal makes it a valuable tool for specific diagnostic challenges in the central nervous system.
Comparing T2 and FLAIR
T2 and FLAIR sequences both rely on the T2 relaxation properties of tissues, meaning that most pathologies, which often involve increased water content, will appear bright on both. Their primary distinction lies in how they manage the signal from free water. T2 sequences show all fluids, including CSF, as bright, providing good general contrast for various tissues and identifying areas of increased water. This makes T2 effective for initial assessments of many conditions.
FLAIR, by actively suppressing the CSF signal, offers a unique advantage for detecting lesions near or within CSF spaces. While a standard T2 image might show a bright lesion indistinguishable from bright CSF, FLAIR makes the lesion stand out clearly against the dark CSF. Radiologists often use both sequences in conjunction. T2 provides a broad overview of tissue pathology, while FLAIR offers enhanced sensitivity for specific types of lesions within the brain and spinal cord, especially those masked by bright CSF. This complementary information creates a more comprehensive diagnostic picture.
Common Medical Applications
T2 and FLAIR sequences are routinely employed in neuroradiology for diagnosing and monitoring neurological conditions. For instance, in suspected multiple sclerosis (MS), FLAIR imaging is highly sensitive for detecting demyelinating plaques within the brain and spinal cord. These lesions, representing inflammation and damage to the myelin sheath, appear as bright white matter abnormalities against the suppressed dark CSF, making them particularly visible for diagnosis and tracking disease progression.
In acute stroke assessment, T2-weighted imaging reveals brain edema as bright signals, indicating tissue swelling due to lack of blood flow. FLAIR also helps differentiate acute stroke from older lesions; an acute ischemic lesion appears bright on FLAIR within hours of onset, while older, chronic lesions do not show the same signal characteristics.
Both sequences are valuable for evaluating brain tumors. T2 highlights the tumor’s extent and associated edema as bright areas. FLAIR can delineate peritumoral edema more clearly from the tumor itself, aiding surgical planning and treatment monitoring. These sequences also contribute to the diagnosis of inflammatory conditions and infections affecting the brain, where increased water content associated with inflammation or pus appears bright.