MR images are said to have contrast if there are differing areasof signal intensity. The contrast can be controlled to a certainextent by the parameters used and can produce images which containdiffering levels of contrast depending on tissue content. Forexample, fat and water produce different image contrast. Imagescan be weighted towards particular contrast mechanisms. In T1weighted images, fat is bright and cerebrospinal fluid (CSF)is dark; with T2 weighting, fat is dark and CSF is bright. Protondensity (PD) weighting produces image contrast on the basisof the PD of the tissue. Such weighting of images has been exploitedto identify inflammatory demyelinating lesions at differentstages in their evolution.¹

T2 and PD weighted imaging

T2 weighted imaging identifies MS lesions as high signal fociagainst the low signal background of white matter. However,periventricular lesions are often indistinguishable from theadjacent CSF which is also of high signal with T2 weighting.Contrast from the lesion can be improved here by using PD weightingbecause of the lower CSF signal with this sequence. Fortunately,T2 and PD weighted images can be acquired together in a singlespin-echo sequence providing complimentary information (fig1).

View larger version (176K):   Figure 1  Axial magnetic resonance imaging (MRI) of a 30 year old man with relapsing remitting multiple sclerosis (MS) showing multiple periventricular lesions: (A) T2 weighted image; (B) proton density (PD) weighted image; (C) fluid attenuated inversion recovery (FLAIR) image; (D) T1 weighted image following administration of gadolinium (Gd) demonstrating enhancing lesions.