Double line sign on MRI is seen in:
**Core Concept**
The double line sign on MRI is a radiological finding associated with a specific condition affecting the spine. It is characterized by two parallel lines seen on sagittal MRI images, which are indicative of a particular type of intervertebral disc degeneration.
**Why the Correct Answer is Right**
The double line sign is a result of the degeneration of the intervertebral disc, where the posterior longitudinal ligament (PLL) and the annulus fibrosus of the disc are visible as two parallel lines. This occurs due to the loss of disc height and the subsequent herniation of the nucleus pulposus, which causes the PLL to become visible. The annulus fibrosus is also visible as a separate line, resulting in the characteristic double line sign.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specifically relate to the double line sign on MRI. While disc degeneration is a related concept, it is not the same as the double line sign.
* **Option B:** This option is incorrect because it is a different radiological finding, not related to the double line sign. The vacuum phenomenon is a different condition that occurs in the intervertebral disc.
* **Option C:** This option is incorrect because it is not a specific radiological finding associated with the double line sign. While disc herniation is a related condition, it is not the same as the double line sign.
**Clinical Pearl / High-Yield Fact**
The double line sign is an important radiological finding that can help diagnose disc degeneration and other spinal conditions. It is essential for radiologists and clinicians to be aware of this sign to accurately diagnose and manage spinal disorders.
**Correct Answer:** D.