2.1.2\ Differential Diagnosis

Ectopic pregnancy is the most critical differential diagnosis and exclusion of this diagnosis is essential. The differential diagnosis of the underlying cyst type can include physiological, endometriotic, benign cystic teratoma or neoplastic ovarian cyst. The finding of free fluid or blood in the pelvic cavity with no visible cyst or a collapsing cyst and an appropriate history is highly reassuring for physiological cyst rupture. Other causes of acute pelvic pain and free fluid include inflammatory processes such as pelvic inflammatory disease, appendicitis, or diverticulitis.

2.1.3\ Value of Imaging

Imaging is used to confirm the findings of free pelvic fluid, identify the underlying cyst type if a cyst remains visible, and rule out alternative causes of the acute severe pelvic pain.

2.2\ Pelvic Inflammatory

Pelvic inflammatory disease (PID) refers to an ascending infection of the upper genital tract in women who are typically of reproductive age. Infection can involve the uterus, fallopian tubes, and ovaries. Per definition, PID should be distinguished­ from pelvic infections caused by medical procedures, pregnancy, and other primary abdominal processes. PID usually results from sexually transmitted ascending infections typically by Neisseria gonorrhoeae or Chlamydia trachomatis, although 30–40% of cases are polymicrobial. Actinomycosis and tuberculosis account for rare causes of PID and may cause tubo-ovarian abscesses (Kim et al. 2004). Actinomycosis should be considered if there is a history of intrauterine contraceptive device (IUCD) and has also been reported following in vitro fertilization, as well as in those with no history of instrumentation (Atay et al. 2005). If PID is untreated or incompletely treated, there is a sixfold risk of ectopic pregnancy. Twenty percent of patients may complain of pelvic pain, and infertility is seen in 25–60% of women with more than one episode of PID (Ghiatas 2004). Occasionally patients with PID may develop

Fritz-Hugh–Curtis syndrome due to peritonitis of the right upper quadrant surfaces and of the right lobe of the liver caused by bacterial spread along the paracolic gutters (Sam et al. 2002).

2.2.1\ Imaging Findings

Imaging findings in early PID are typically subtle and their interpretation is based on the clinical findings. Findings on CT and MRI may include mild pelvic edema that results in thickening of the uterosacral ligaments and haziness and stranding of the pelvic fat, reactive lymphadenopathy, and free fluid (Revzin et al. 2016). Contrast enhancement and thickening of the fallopian tubes may be signs of salpingitis. Enlarged and abnormally enhancing ovaries may demonstrate a polycystic appearance and inflammatory changes (Fig. 4). Peri-ovarian stranding and enhancement of the adjacent peritoneum are common associated findings. In cases of endometritis, abnormal endometrial enhancement is seen as well as fluid in the endocervical canal which has similar imaging characteristics to that in the pouch of Douglas (Fig. 4). The uterine cervix may be enlarged with an abnormally enhancing endocervical canal if there is associated cervicitis. The uterine changes are better assessed on MRI than on CT (Sam et al. 2002).

Fig. 4  CT findings in a 29-year-old woman with PID caused by Chlamydia trachomatis infection. Haziness and weblike fatty infiltration of pelvic fat (arrow), free fluid (A), marked swelling of the left ovary, and mild dilatation of the uterine cavity (U) are demonstrated. The ovaries (asterisk) are difficult to discriminate from ascites due to their polycystic appearance in oophoritis