Which of the following is hypointense (dark) on T2-weighted MRI?
**Core Concept**
T2-weighted MRI is a type of magnetic resonance imaging sequence that highlights differences in proton relaxation times between tissues. In this sequence, tissues with long T2 relaxation times appear bright (hyperintense), while those with short T2 relaxation times appear dark (hypointense). This property is useful for identifying various conditions, including hemorrhage, iron deposition, and calcification.
**Why the Correct Answer is Right**
Hemorrhage, particularly old hemorrhage, is characterized by the presence of hemosiderin and ferritin, which contain iron. Iron is a paramagnetic substance that shortens the T2 relaxation time of surrounding tissues, making them appear dark or hypointense on T2-weighted MRI. This is why hemorrhage is often seen as a hypointense area on T2-weighted images.
**Why Each Wrong Option is Incorrect**
**Option A:** Fluid-filled spaces, such as the ventricles or cysts, are typically hyperintense on T2-weighted MRI due to their long T2 relaxation times.
**Option B:** Fatty tissues, like those found in the bone marrow, are also hyperintense on T2-weighted MRI, as they have long T2 relaxation times.
**Option C:** Proteinaceous or gelatinous tissues, such as those seen in abscesses or tumors, may appear hyperintense on T2-weighted MRI due to their long T2 relaxation times.
**Clinical Pearl / High-Yield Fact**
When interpreting T2-weighted MRI, remember that hemorrhage is often the only lesion that appears dark (hypointense) against the bright background of surrounding tissues. This helps to distinguish hemorrhage from other lesions that may mimic its appearance.
**Correct Answer: C. Hemorrhage**