The junctional zone is low intensity on T2 due to decreased water content, and is formed by smooth muscle cells that are tightly packed. The extracellular matrix and water content are sparse. It usually measures less than 8mm. 8-11 mm is equivocal, and 12mm or greater is abnormal.
A T 2 weighted MRI in the sagittal plane (left) reveals a uterus with a junctional zone that measures 13mm characteristic of adenomyosis. The image on the right is an overlay showing the endometrial cavity in yellow, then a light maroon color of the abnormally thickened junctional zone or inner myometrium, and finally the normal outer myometrium (maroon).
Adenomyosis is a disease of the myometrium caused by misplaced or ectopic endometrium in the myometrium resulting in thickening of the junctional zone.