# Magnetic Resonance Roadmap in Detecting and Staging Endometiosis: Usual and Unusual Localizations
## 1 Introduction  
Endometriosis is defined as a benign chronic inflammation caused by the abnormal presence of endometrial glands and stroma outside the uterus. The ectopic endometrial cells on the surface of other organs, such as the ovaries (endometrioma), uterus (adenomyosis), tube, intestine, vagina, and bladder, undergo the same modifications of the physiological uterine endometrium during the menstrual cycle; this can generate an inflammatory state with symptoms and the formation of scar tissue and adhesions that, if neglected, can also cause infertility [1].  
Several theories exist concerning the origins of endometriosis, with the most widely accepted one suggesting that endometrial cells shed during menstruation make their way through the fallopian tubes into the abdominal cavity. This leads to their implantation on the peritoneum and the surface of the pelvic organs [2]. However, this hypothesis does not rule out alternative explanations, including a potential genetic factor, the spread of endometrial cells through the bloodstream and lymphatic system, or the metaplastic theory. Specifically, the latter theory proposes that both endometrial and peritoneal cells originate from a shared embryological precursor [3]. Therefore, it suggests that peritoneal cells have the potential to undergo a transformation into endometrial cells for reasons thatare currently unknown [4]. The peak incidence occurs between the ages of 25 and 35, but the disease can also occur in lower age groups. Diagnosis of the condition is not always immediate and may often involve a prolonged diagnostic journey [5].  
Endometiosis can be asymptomatic. When symptomatic, it usually manifests with pelvic pain, especially in the peri-menstrual phase, painful menstruation (dysmenorrhea), pain during sexual intercourse (dyspareunia), urinary urgency, hematuria or pain during defecation, sometimes together with the presence of blood in the faeces [1].  
The updated 2022 guidelines of the European Society for Human Reproduction and Embryology (ESHRE) recommend using both ultrasound and magnetic resonance imaging (MRI) as first-line diagnostic testing. It is important to be aware, however, that negative imaging does not exclude the disease. If strong clinical suspicion persists, patients are directed to a laparoscopic examination with diagnostic and therapeutic purposes [6].  
Several studies in the literature have confirmed the accuracy of MRI in the diagnosis of deep infiltrating endometriosis (DIE) and in the evaluation of its extension.  
A prospective study by [PERSON] et al. showed a sensitivity and specificity of the technique of 90.30% and 91%, respectively [7].  
In a further study, performed at the University of Udine, 44 women with clinical or ultrasound suspicion of pelvic endometriosis underwent MRI and then an operational laparoscopy. The results showed a 95% match between the proposed extension of the disease in MRI and the confirmed staging at laparoscopy; this shows that MRI plays a fundamental role, as a non-invasive diagnostic examination, in the assessment of the extent of the disease in patients who are candidates for surgery [8].  
The modern therapeutic management of endometriosis involves a medical and/or surgical approach, although in recent years, new treatment and management proposals have been considered. For instance, interventional radiology has played a role in the management of selected cases of endometriosis predominantly of the abdominal wall with ablative procedures. Cryoablation in these cases offers advantages over surgery, especially with regard to the reduction in intra and post-procedural pain [9,10].  
The aim of this review is to summarize the typical and atypical localizations of the disease using post-operative imaging to focus on the role of MRI as the technical protocol, and to provide a conclusion projected towards new discoveries and new applications in the field of diagnosis and treatment of endometriosis.