Normal Findings

There are basic MR signal patterns of normal tissues in the brain. These patterns are shown in Iablei23z3 . Types of Detectable Abnormalities

The MR signal characteristics of a lesion on T1WI and T2WI enables the differential diagnosis of any particular brain abnormality to be narrowed. A summary of signal characteristics is shown in Xable.23^:4 . There are specific and nonspecific patterns of MR signal regarding normal and abnormal tissues. The most specific patterns are seen with respect to CSF, fat, bone, and stages of intracranial hemorrhage.

CSF has decreased signal on T1WI, intermediate signal on proton density, and increased signal on T2WI. When evaluating a cystic lesion, one must determine if the lesion follows CSF signal characteristics on all pulse sequences. If the lesion does not, it is considered atypical. Lesions that follow CSF signal typically represent arachnoid cysts or dilated CSF spaces secondary to encephalomalacia. Complicated fluid may show increased signal on both T1WI and T2WI. These abnormalities suggest the presence of proteinaceous or complex hemorrhagic fluid and may harbor malignancy.

Fat has increased signal on T1WI and decreased signal on T2WI. Lesions that follow this pattern most commonly represent fat containing congenital lipomas and teratogenic tumors. Confirmation of a fatty lesion can be obtained with a fat saturation RF pulse sequence.

Each stage of hemoglobin molecular breakdown has its own peculiar imaging signature based on the MR signal. That is, the stages of hemorrhage whether acute, subacute, or chronic, generate unique changes that are uniquely shown only by MRI. These signals range from nearly isointense to hypointense signal as seen in acute hemorrhages to increased signal seen on both T1WI and T2WI observed in late subacute or chronic hemorrhages. ^ , y The signal patterns of brain hemorrhage are summarized in Table^S-lS .

MR signal patterns are recognizable with common diseases such as cerebral edema, neoplasm, abscess, infarcts, or demyelinating processes. These entities demonstrate a nonspecific pattern of decreased signal on T1WI and increased signal on proton density and T2WI. Therefore, to obtain a differential diagnosis with a high degree of accuracy, this information is linked with anatomical location, morphology, and a degree of contrast medium enhancement. These, with good clinical history, all help in arriving at a correct differential diagnosis.

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