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Fig. 10  Coronal (a) and sagittal (b) T2 weighted. MRI images show a bilateral tubo-ovarian abscess (TOA— long arrows on image a) with a fistulous communication (long arrow on image b) with the sigmoid colon (S). Cervix (C)

background. MRI is more useful than CT in differentiating a hydrosalpinx from a cystic ovarian tumor (Forstner et al. 2017).

2.5\ Ovarian Torsion

Ovarian torsion is most commonly associated with tubal torsion. The age groups which tend to be affected are children, young women in their first three decades, and postmenopausal women.

Presentation is usually with acute severe pelvic or lower abdominal pain and vomiting; the patient may have clinical signs of acute surgical abdomen.

Ovarian torsion is caused by partial or complete rotation of the ovarian vascular pedicle. While venous flow is initially compromised, causing swelling and edema, arterial flow is usually maintained until late in the course, a phenomenon that is attributed to the dual blood supply of the ovary (Lee et al. 1998).

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Finally, hemorrhagic infarction leads to irreversible loss of the ovary. Predisposing factors for ovarian torsion include an underlying unilateral ovarian tumor (50–60%), most likely teratomas and cystic ovarian tumors including para-tubal cysts. Lesions larger than 6 cm have a greater risk for torsion (Sherard et al. 2003). However, torsion may also be encountered in normal-sized ovaries, particularly in children (Graif and Itzchak 1988). Furthermore, hypermobile adnexa or elongated fallopian tubes and increased abdominal pressure have been reported to be responsible for ovarian torsion. Women in their first three decades have the highest incidence of ovarian torsion, which is related to the higher frequency of physiological cysts and benign cystic tumors, infertility therapy and pregnancy. Approximately 20% of torsions occur during pregnancy, typically during the first and second trimesters. In postmenopausal women, torsion typically affects a benign adnexal tumor, most commonly serous cystadenomas, whereas malignant tumors tend not to undergo torsion (Koonings and Grimes 1989).

Benign massive edema of the ovary is a rare disorder found in the second and third decades of life and may be a variant of ovarian torsion. It results from partial or intermittent torsion and is characterized by an excessively enlarged edematous ovary (Machairiotis et al. 2016). There may be an acute or progressive clinical presentation with pain. Approximately 43% of cases are found to have signs of torsion at surgery (Praveen et al. 2013). The right ovary is more likely to twist than the left, suggesting that the sigmoid colon may help to prevent torsion.

2.5.1\ Imaging Findings

The imaging findings depend on the degree and duration of torsion. Thickening of the fallopian tube with hemorrhage is suggestive of torsion, especially when associated with an enlarged ovary or an adnexal cystic mass; torsed adnexal masses are often located mid-

line, cranial to the uterine fundus and there is often uterine deviation (Moribata et al. 2015). A twisted edematous pedicle can be seen connecting the lesion to the uterus with mixed signal intensity on all sequences on MRI (Haque et al. 2000). Sometimes when tracking down the ovarian vascular pedicle, a coiled vascular pedicle may produce the whirlpool sign (Fig. 11) (Lee et al. 1993). In a recent study using multivariate analysis, the whirlpool sign and a thickened fallopian tube (>10 mm) were associated with torsion, with substantial interreader agreement (Beranger-Gibert et al. 2016). In prepubertal and pubertal girls where torsion of a normal ovary occurs in 50%, a unilateral solid mass with peripheral small cysts is indicative of a torsed ovary (Fig. 12). In cases of hemorrhagic infarction, the enlarged ovary may show low signal intensity on T2WI due to interstitial hemorrhage, without wall enhancement of the displaced follicles (Haque et al. 2000). The presence of hemorrhage has been found to be associated with nonviable ovary in 70% of cases and viable ovary in 27% of cases (Beranger-Gibert et al. 2016).

The most common appearance in adults is of a mass with areas of hyperintensity on T1WI due to hemorrhage and hyperintensity on T2WI due to ovarian edema (Kimura et al. 1994). Smooth wall thickening of the twisted adnexal cystic mass and a thin hyperintense rim at the periphery of the lesion on T1WI are further signs of ovarian torsion.

A tubular or comma-like structure partially covering the ovary represents the fallopian tube and may also display hemorrhagic contents. CT studies have reported a diameter of the fallopian tube of 2–4 cm (Ghossain et al. 1994).

Contrast enhancement on CT and MRI depends on the degree of viability (Kimura et al. 1994). MR findings in hemorrhagic infarction include lack of enhancement, engorged vessels surrounding the lesion, and signal intensity of hematoma (Rha et al. 2002).

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Fig. 11  CT and MRI images of a right ovarian torsion. The CT (a) shows a hyperdense right adnexal lesion (open arrow) in a lady presenting with acute abdominal pain. A transvaginal ultrasound could not be tolerated. On T2-weighted MRI (b) there is a whirlpool sign (long

arrow) anterior to the enlarged edematous right ovary which is intermediate signal intensity on T2WI (star). Follicles are seen at the periphery of the ovary. The T1 fat saturation MRI (c) demonstrates blood at the periphery of the torsed ovary (short arrow)

Nonspecific findings include deviation of the uterus to the twisted side, ascites, and obliteration of the pelvic fat.

2.5.2\ Differential Diagnosis

Clinically, ruptured ovarian cysts may resemble ovarian torsion. However, in the case of ruptured ovarian cyst, the ovary is usually normal in size and free fluid or blood may be seen in the pelvis;

there is neither edema of an adnexal mass nor engorged adnexal vessels or dilatation of the fallopian­ tube. Tubo-ovarian abscess and hydrosalpinx­ may resemble advanced adnexal torsion. Lack of enhancement supports the diagnosis of ovarian torsion. In children, sonography usually allows the diagnosis of appendicitis as a cause of acute pelvic pain. In the case of a suspected­ abscess or an ovarian mass, MRI may