Imaging for Suspected PCOS
Transvaginal ultrasound using transducers with frequency bandwidth ≥8 MHz is the preferred imaging modality for evaluating polycystic ovarian morphology in reproductive-age women with suspected PCOS. 1, 2
Primary Imaging Approach
Use transvaginal ultrasound as first-line imaging if the patient is sexually active and finds this approach acceptable. 1, 2, 3 This modality provides superior visualization for counting follicles and measuring ovarian volume compared to other approaches.
Diagnostic Thresholds for Polycystic Ovarian Morphology
The ultrasound must demonstrate on either ovary:
- ≥20 follicles measuring 2-9 mm in diameter per ovary, AND/OR 1, 2, 3
- Ovarian volume ≥10 mL 1, 2, 3
- Ensure no corpus luteum, cysts, or dominant follicles ≥10 mm are present when making measurements 1, 3
The follicle number per ovary (FNPO) is the most accurate sonographic marker, demonstrating 87.6% sensitivity and 93.7% specificity. 3
Alternative Imaging Modalities
Transabdominal Ultrasound
Use transabdominal ultrasound when transvaginal approach is not feasible (non-sexually active patients or patient preference). 2, 3 However, focus reporting on ovarian volume with threshold ≥10 mL, as reliably assessing follicle number is difficult with this approach. 1, 2
MRI Pelvis
Consider MRI pelvis without contrast when ovaries cannot be adequately visualized by ultrasound, particularly in obese patients where ultrasound is limited. 2, 3 MRI provides reproducible ovarian volume assessment and can identify follicle patterns, though it is not routinely necessary. 4, 5
Critical Diagnostic Caveats
When Ultrasound Should NOT Be Used
Do not use ultrasound for PCOS diagnosis in patients with gynecological age <8 years (less than 8 years after menarche), as multifollicular ovaries are physiologically normal at this life stage and have high incidence. 1, 3 This is a common pitfall that leads to overdiagnosis.
When Ultrasound Is Not Required
In patients with both irregular menstrual cycles AND hyperandrogenism, ovarian ultrasound is not necessary for PCOS diagnosis, though it will identify the complete PCOS phenotype. 1, 3 These two criteria alone are sufficient for diagnosis per Rotterdam criteria.
Essential Reporting Standards
When ultrasound is performed, the report must include: 1
- Last menstrual period
- Transducer bandwidth frequency
- Approach/route used (transvaginal vs transabdominal)
- Total follicle number per ovary measuring 2-9 mm
- Three dimensions and volume of each ovary
- Endometrial thickness and appearance
- Presence of ovarian cysts, corpus luteum, or dominant follicles ≥10 mm
- Other ovarian and uterine pathology
Important Limitations
Ovarian morphology overlaps significantly between PCOS patients and controls—follicle number and ovarian volume were not concordant with clinical/biochemical PCOS status in 23% and 34% of ovaries, respectively, in one study. 4 This emphasizes that imaging findings must always be interpreted alongside clinical and biochemical presentation, never in isolation. 3, 6
Anti-Müllerian hormone (AMH) levels should NOT be used as an alternative for detecting polycystic ovarian morphology or as a single test for PCOS diagnosis, as standardization and validated cut-offs are still lacking. 1, 3
Technology Considerations
Newer ultrasound technology with higher probe frequencies has improved diagnostic accuracy, and some studies suggest a higher threshold of ≥26 follicles may better discriminate PCOS from normal ovaries when using advanced imaging. 7 However, current international guidelines maintain the ≥20 follicle threshold for standardization. 1, 2
Three-dimensional ultrasound and AI-assisted analysis show promise for reducing operator variability and improving follicle counting accuracy, but these remain investigational and are not yet standard of care. 8