Читать книгу Salivary Gland Pathology - Группа авторов - Страница 62

On T1 weighted images, a short repetition time (tr) and short echo time (te) are applied resulting in an image commonly used for anatomic depiction. Water signal is very low and is displayed as dark gray to black pixels on the gray scale. Fat is very bright, allowing tissue planes to be delineated. Fast flowing blood is devoid of signal and is therefore very black. Muscle tissue is an intermediate gray. Bone which has few free protons is also largely devoid of signal. Bone marrow, however, will vary depending on the relative percentage of red versus yellow marrow. Red marrow will have a signal slightly lower than muscle, whereas yellow marrow (fat replaced) will be bright. In the brain, cerebrospinal fluid (CSF) is dark, and flowing blood is black. Gray matter is dark relative to white matter (contains fatty myelin) but both are higher than cerebrospinal fluid (CSF) but less than fat. Cysts (simple) are dark in signal unless they are complicated by hemorrhage or infection or have elevated protein concentration, which results in an increased signal and slightly brighter display (Figure 2.8) (Table 2.2).

Figure 2.8. Axial MRI T1 weighted image at level of the skull base and brainstem without contrast demonstrating high signal in the subcutaneous fat, intermediate signal of the brain, and low signal of the CSF and mucosa. Note dilated right parotid duct (arrow).