Ultrasound is a reliable first‐line imaging modality for the assessment of patients with gynaecological concerns. In patients with suspected endometriosis, ultrasound serves three purposes. First, it is used to evaluate the aetiology of the patient's symptoms. Second, it has the potential to map the disease location. Lastly, it can ascertain the extent of disease.
From a clinical perspective, these products of ultrasound may benefit patients by ensuring a thorough understanding of disease by both the patient, who needs to provide informed consent to treatment options, and the physician, who may adequately prepare for potentially advanced surgical procedures. In many cases, when deep endometriosis (DE) exists, physicians need to consider referral to an appropriate gynaecologic surgeon with advanced skill. The multidisciplinary input of other specialists such as colorectal or urologic surgeons or fertility specialist may also be necessary.
Recently, the International Deep Endometriosis Analysis (IDEA) group published a systematic approach to sonographically evaluate the pelvis in patients with suspected endometriosis. This consensus statement was developed to standardise anatomical landmarks, nomenclature of disease and the components of an ultrasound seeking to identify DE. A four‐step system was introduced, including routine evaluation of the uterus and adnexa, evaluation of soft markers such as site‐specific tenderness (SST), assessment of the pouch of Douglas (POD) using the ‘sliding sign’ and, finally, assessing the presence of DE nodules compartmentally throughout the pelvis
The ultrasound: uterus 
 The orientation (anteverted, retroverted or military) and dimensions in three orthogonal planes should be recorded. Patients with endometriosis have a high likelihood of concurrent adenomyosis and as such, signs of this should be sought. In addition, the ‘question mark sign’, signifying a fixed anteverted/retroflexed uterus with the fundus adhered posteriorly to the rectum and/or sigmoid colon can represent adenomyosis and/or endometriosis and should be documented

The ultrasound: adnexa

Includes evaluation of the ovaries and Fallopian tubes. The entire ovarian size should be measured in three orthogonal planes. Any abnormalities should be quantified, measured and documented. The sonographic characteristics of any ovarian abnormality should be described according to terminology published by the International Ovarian Tumor Analysis (IOTA) group
Ovarian mobility can be judged by applying pressure to the ovaries using the TV probe. Non‐mobile ovaries are considered a ‘soft marker’ potentially signifying superficial pelvic endometriosis and/or DE. The mobility of the ovaries is assessed against the pelvic side wall laterally, uterus medially, uterosacral ligaments (USLs) inferiorly and each other
‘Kissing’ ovaries, an ultrasound diagnosis of ovaries fixed to each other  indirectly indicates intra‐abdominal adhesions and possibly underlying DE of the Fallopian tubes and/or bowel.

Hydrosalpinx or hematosalpinx may be identified in endometriosis.

The ultrasound: site‐specific tenderness
SST-‘soft markers’- The key anatomic locations to assess in this component of the scan include the uterus, adnexa, USLs and POD. Currently, the IDEA group recommends a scoring system of 0 or 1; 0 for no pain and 1 for pain. However, this test is still limited in that no scoring system has been validated as yet.

The ultrasound: Sliding sign
The test is considered positive when the uterus and cervix move independently (i.e. slide) along the anterior rectum and sigmoid . Clinically and surgically, this is reassuring for a non‐obliterated POD. Conversely, if the uterus and cervix move in unison with the anterior rectum and sigmoid, the test is negative and the POD is thought to be obliterated.

Schematic Drawing Demonstrating How to Elicit the ‘Sliding Sign’ in an Anteverted Uterus (a) and Retroverted Uterus

The ultrasound: anterior and the posterior compartment
Lesions may appear as hypoechoic linear or spherical lesions, with or without regular contours . The uterovesical region should be examined for tethering to the uterus (i.e. obliteration of the space). The concept of the ‘sliding sign’ can be applied here as well. The operator should hold the TV probe in the anterior fornix with one hand and the other hand should be placed over the suprapubic region. By balloting the uterus between the probe and hand, the operator can judge whether the posterior bladder slides freely over the anterior uterine wall,
When the bladder and uterus move together, the operator should document a negative ‘sliding sign’, representing an obliterated space.  An independently moving bladder from the uterus represents a positive ‘sliding sign’.
The posterior compartment sites include USLs, posterior vaginal fornix, rectovaginal septum (RVS), anterior rectum, anterior rectosigmoid junction and sigmoid colon.  Done by gently placing the TV probe in the posterior vaginal fornix

Schematic and Ultrasound Images Demonstrating an Isolated RVS Nodule. Note the Hyperechoic Nature of the RVS (Red Star) and Adjacent Hypoechoic Layers of Vagina (Yellow Star) and Rectal Wall Muscularis (Green Circle).

Schematic and Ultrasound Image Demonstrating the Location of Deep Endometriosis (DE) in the Right Uterosacral Ligament in Transverse View (Within Green Circle)