Ultrasound should be the first imaging investigation when characterising an ovarian lesion. The International Ovarian Tumour Analysis (IOTA) group has developed several models which can be used to aid to differentiate between malignant and benign ovarian tumours.
The Simple Rules (2008) are a preoperative classification system for ovarian tumours, consisting of five features typical for benign tumours (B-features) and five features typical for malignant tumours (M-features)  .
Further details on definitions and the Simple Rules can be found on the following link:
CT is routinely used to assess the extent of disease at diagnosis (staging), monitor response to therapy and to detect recurrent disease. CT is generally of limited value for local staging .
MRI is the modality of choice for local disease stage in cervical, vaginal and vulval cancer. There is also a general consensus that all patients with histological high grade endometrial tumours should undergo MRI preoperatively .
For ovarian lesions, MRI is used to characterise indeterminate cysts or masses found on ultrasound, particularly in young patients or when CA125 is normal or only slightly elevated .
PET-CT is not usually indicated for routine preoperative staging but may be useful on occasions to define disease extent, particularly when follow-up surgery or radical radiotherapy treatment are being considered. PET-CT can also have a useful role in patients with equivocal findings on other imaging .
1- International Ovarian Tumour Analysis group. https://www.iotagroup.org/
2- The Royal College of Radiologists. Recommendations for cross-sectional imaging in cancer management; 2nd edition. 2014. https://www.rcr.ac.uk/clinical-radiology