anterior scalene muscle

labeled movie:

Explanation

The area outlined in red represents the anterior scalene muscle, which extends from the cervical spine to the upper ribs.  It is an important CT landmark for the structures at the thoracic apex.  The brachial plexus emerges posterior to it, and the subclavian vein (outlined in blue) passes anterior to it, and the subclavian artery (outlined in yellow) passes posterior to it.

Image A below shows an abnormality on coronal T1-weighted MR in the region of the anterior scalenes.  Image B shows the same CT with structures outlined.

This patient has a lung cancer that has metastasized to C7 and is growing out to the right, invading the region of the brachial plexus and causing arm pain and weakness.  The tumor is shown in red.  The middle and posterior scalene muscles, which pass posterior to the brachial plexus, are outlined in yellow.  The bracheal plexus is outlined in green.  The anterior scalenes are outlined in blue.