Segmentation: Fourth Ventricle

General Description
The fourth ventricle is located between the brainstem and the cerebellum. Its anterior border is the brainstem. Laterally and posteriorly it is bordered by the cerebellum. Its posterior border (above the cerebellum) is the midbrain tectum (superior and inferior colliculi). The cerebral aqueduct in included as part of the fourth ventricle.

Segmentation Procedure
The histogram function is usually recommended for extraction of the fourth ventricle. Depending on which region of the fourth ventricle you are looking at, the box drawn for your histogram will contain CSF from the ventricle and either cerebellar white matter, cerebellum gray matter, brain stem, or some combination thereof. The intensity contour method and manual drawing are also employed.

Part I - cerebral aqueduct
The aqueduct first appears just under the posterior commissure. A histogram should be taken between the CSF of the aqueduct and the brainstem. However, because there is so much partial voluming in this area, the histogram will likely be modified using an intensity contour line. The dorsal border of the fourth ventricle will have to be drawn manually. Continue to use a histogram for the remainder of the aqueduct, modifying as necessary with the intensity contour function.

Part II - fourth ventricle in the brainstem
As you move posteriorly, you will begin to see the actual beginning of the fourth ventricle. The small circle that is the aqueduct will begin to elongate. Continue to use the histogram method; draw your box between the CSF of the fourth ventricle and the surrounding brainstem tissue. Modify as necessary with the intensity contour function. As the fourth ventricle continues posteriorly, it will start to widen. A histogram should be taken between the CSF of the fourth ventricle and the surrounding brainstem tissue. Often this histogram will not yield the dorsal border of the 4th

ventricle. Brightening the screen will enable you to see this border. It should be drawn in using the draw function, and then attached to the contour given by your histogram.

Part III – fourth ventricle in the brainstem and cerebellum

As the 4th ventricle is surrounded by cerebellum white matter, multiple histograms will yield the most accurate fit. Generate a histogram from a box containing CSF of the fourth ventricle and the cerebellum white matter. The only part of the contour that you want is that between the cerebellar white matter and the CSF of the fourth ventricle. Now generate the rest of the outline with the histogram method.
The box for your second histogram should contain equal amounts of CSF from the fourth ventricle and the brainstem. The generated contour will accurately define the border between the fourth ventricle and the brainstem.

Part IV - fourth ventricle in the cerebellum
When the fourth ventricle is no longer surrounded by brainstem, it appears between cerebellum gray and white matter. Two histograms should be used for this outline: one between the CSF and the cerebellum white matter, and the second between the CSF and cerebellum gray matter.
In its most posterior extent, the fourth ventricle will appear as two separate circles in each cerebellar hemisphere. The most accurate means to extract these structures is to do two separate histograms for each cerebellar hemispheres (CSF - white matter; CSF - gray matter). As with the most anterior extend of the 4th ventricle, modifying this

estimate with the contour line may be necessary.

Labeling
Both the cerebral aqueduct and fourth ventricle are labeled as "fourth ventricle.