Multidisciplinary view on uterine junctional zone in uteri affected by adenomyosis: explaining discrepancies between MRI and transvaginal ultrasound images on a microscopic level.

The uterine junctional zone is the subendometrial area in the myometrium that contributes to peristalsis and aids in spermatozoa and blastocyst transport. Alterations in appearance of the junctional zone on transvaginal ultrasound (TVUS) or magnetic resonance imaging (MRI) are associated with adenomyosis. Lack of uniform description of its appearance and ill-defined boundaries in both histology and imaging hamper understanding of the junctional zone's entity and limit its role in the diagnosis of adenomyosis. The objective of this state-of-the art review was to investigate the accordance on the definition of the junctional zone across different diagnostic approaches and examine how the imaging findings can be linked to histological findings in the context of adenomyosis diagnosis. A comprehensive literature review was conducted for articles describing the imaging appearance and histological structure of the junctional zone within the uterus. Our review suggests that the junctional zone is distinguished from the middle and outer myometrium by gradual changes in smooth muscle cell density, extracellular space, connective tissue, water content, and vascular properties. However, while the signal intensity from junctional zone to middle myometrium changes abruptly on MRI, the histopathological changes are gradual and its border may be difficult or impossible to distinguish on 2D TVUS. Moreover, the thickness of the junctional zone on MRI was larger than on TVUS. Thus, these two imaging modalities do not reflect exactly the same. Although a thickened junctional zone is often used to diagnose adenomyosis on MRI, the presence of adenomyosis can more accurately be described as interruptions of the junctional zone by endometrial tissue: direct features of adenomyosis, such as subendometrial lines and buds on 2D and 3D TVUS or bright foci on MRI. The histopathological criteria are based on enlarged uteri with severe adenomyosis, and might not reflect early stages. Clinicians should be aware that findings on MRI cannot readily be extrapolated to ultrasound. Understanding of this is necessary when investigating the potential relevance of the uterine junctional zone as a functional unit and the association between the visualization of direct features of adenomyosis in the junctional zone and clinical symptoms. This article is protected by copyright. All rights reserved.