Magnetic resonance generates images using particular properties of hydrogen ions, which are ubiquitous in the body. In view of the fact that the majority of pathological processes alter the content, distribution and the surrounding environment of hydrogen ions, magnetic resonance is an extremely effective technique for detecting these alterations.
Magnetic resonance imaging (MRI) studies involve the acquisition of images obtained in different sequences which are processed to provide differentiated information on the structures of interest, exploiting the different behaviour of hydrogen ions as the magnetic field changes. The magnetic resonance sequences that are commonly used in the examination of intracranial nervous structures are pre- and post-contrast T1-weighted sequences, T2-weighted sequences, and fluid-attenuated inversion recovery (FLAIR) sequences.
T1-weighted sequence sare those that provide the greatest anatomical detail. In these sequences adipose tissue appears hyperintense (bright) and fluids appear hypointense (dark). Soft tissues have a characteristic, intermediate intensity. Following the administration of a paramagnetic contrast agent some normal tissues (e.g. pituitary gland) or pathological tissues (e.g. some tumours) appear more hyperintense.
T2-weighted sequences provide less anatomical detail but are more sensitive at showing pathological alterations in soft tissues. In T2-weighted images fluids and tissues which are rich in liquids (due to the presence of oedema, inflammation, neoplasm) appear markedly hyperintense. The hyperintensity of adipose tissue is variable, and more modest when compared to that in the T1-W images (Fig. 1.)
FLAIR sequences are used in comparison to T2-weighted images in order to assess the characteristics of lesions that are hyperintense in T2 images, because with FLAIR the intensity of pure fluids (such as cerebrospinal fluid) is suppressed (thus becoming hypointense) while tissue lesions remain hyperintense. This is extremely relevant clinically as it allows the type of lesions detected to be defined in greater detail (Fig. 2).
With regards to meningoencephalomyelitis, at MRI encephalitis may appear as totally normal, or be characterized by the presence of multifocal/diffuse lesions that are hyperintense in T2-weighted and FLAIR sequences, while they are isointense or hypointense in T1-weighted images. Meningeal inflammation may not be visible until a paramagnetic contrast agent is administered.