What are the characteristic findings of ovarian torsion on a non‑contrast abdominal computed tomography scan?

CT Findings of Ovarian Torsion on Non-Contrast Imaging

Non-contrast CT has significant limitations for diagnosing ovarian torsion, as the key diagnostic features are best visualized with intravenous contrast; however, when present, findings include an asymmetrically enlarged ovary (>5 cm), twisted vascular pedicle, peripheral follicular pattern, and deviation of the uterus toward the affected side. 1

Primary CT Findings

Ovarian Enlargement

• The most consistent finding is a unilaterally enlarged ovary, typically >5 cm in diameter 1, 2
• All surgically proven cases of ovarian torsion in one retrospective series showed an enlarged ovary, ovarian cyst, or adnexal mass on CT 2
• Ovarian size can range from 5 to 18 cm in confirmed cases 3

Twisted Vascular Pedicle ("Whirlpool Sign")

• The twisted pedicle appears as swirling of the vascular pedicle and is a highly specific finding when visualized 1
• This finding may be difficult to appreciate on non-contrast studies, as contrast enhancement better delineates vascular structures 1
• The whirlpool sign was identified in only 1 case in a series of 111 CT examinations, highlighting its relative rarity on CT compared to ultrasound 4

Secondary Findings

• Peripheral follicular pattern: uniform peripheral cystic structures arranged around the ovarian periphery 5, 6
• Uterine deviation: the uterus deviates toward the side of the torsion 1, 5
• Fallopian tube thickening: may be present but is an inconsistent finding 4, 5
• Free pelvic fluid/ascites: present in many cases but non-specific 4, 5
• Peri-ovarian fat stranding: indicates inflammation of surrounding tissues 4, 6

Critical Limitations of Non-Contrast CT

Contrast Enhancement is Essential

• Abnormal or absent ovarian enhancement is a key diagnostic feature that cannot be assessed without intravenous contrast 1
• The American College of Radiology specifically describes findings on contrast-enhanced CT, including hypoenhancing ovary and engorged vessels, which are not visible on non-contrast studies 1
• One study found that 29 cases showed abnormal ovarian enhancement, a finding completely unavailable on non-contrast imaging 4

Diagnostic Performance

• A negative contrast-enhanced CT has 100% negative predictive value for ovarian torsion in one study, but this applies specifically to contrast-enhanced examinations 4
• CT has 74-95% sensitivity and 80-90% specificity for ovarian torsion, but these figures are based on contrast-enhanced studies 1

Clinical Pitfalls

When Non-Contrast CT is Inadequate

• If a non-contrast CT shows a normal-appearing ovary of normal size with no mass or cyst, ovarian torsion is highly unlikely 2
• However, if the ovaries are not well-visualized or any abnormality is present, further imaging with ultrasound is mandatory 2
• Ultrasound remains the first-line imaging modality and should be performed when ovarian torsion is clinically suspected, regardless of CT findings 1, 7

Mimics and Differential Considerations

• Ovarian torsion can mimic appendicitis, renal colic, or urinary tract infection on clinical presentation 1, 7
• The presence of an ovarian mass or cyst increases the likelihood of torsion, with mature cystic teratoma being the most common associated lesion 5

Recommended Imaging Algorithm

When ovarian torsion is suspected clinically:

• Proceed directly to ultrasound with Doppler evaluation as the first-line imaging study 1, 7
• If ultrasound is inconclusive but clinical suspicion remains high, obtain contrast-enhanced CT or MRI (80-85% sensitivity) 1, 7
• Non-contrast CT should not be relied upon as the primary modality for diagnosing or excluding ovarian torsion 1