​Ovarian Torsion
Ovarian torsion is an infrequent though important gynecological pathology.

It is sometimes caused by mass or cyst which causes the ovary to spin on its pedicle. The larger the lesion the higher the risk.

It is usually a unilateral process, 60% on occur in right ovary.
Presentation 

Sudden onset of unilateral lower abdominal pain

Lower back pain

Nausea and vomiting present in 70% of cases

 Ovarian salvage rate

10%in adults

27%in pediatrics
Sonographic findings

Unilaterally enlarged ovary

Peripherally displaced follicles due to edema

Coexistent mass

Twisted pedicle (not always seen)

Free fluid

Doppler

little or no intra-ovarian venous flow 

absent arterial flow (poor prognostic sign)

absent or reversed diastolic flow

normal vascularity does not exclude intermittent torsion

normal Doppler flow can also occasionally be found due to dual supply from both the ovarian and uterine arteries

Source:

emedicine.com

Radiopaedia.com