It is a commonly held belief that if blood flow can be shown in an ovary that ovarian torsion can be excluded. This is not entirely correct.
The most important factors for a diagnosis of ovarian torsion on ultrasound are the size and grayscale appearance and the clinical presentation. If the spectral Doppler exam is set with appropriate sensitivity and no blood flow is detected, then you can feel confident about diagnosing torsion (94% PPV in premenopausal women)... although this would be a late finding...
...however, if you detect blood flow in an ovary that otherwise looks torsed (as in this case), then torsion cannot be excluded. Blood flow in a torsed/torsing ovary may occur from intermittent torsion or from the ovary's dual blood supply.
In this situation, the original ultrasound was read as an "ovarian mass", because it demonstrated blood flow on spectral Doppler and presumably could not be torsed, thus prompting the CT and MRI studies which confirmed ovarian torsion. Of note, on the MRI, you can see a small amount of streaky contrast in the ovary, demonstrating that a tiny amount of blood is flowing into it... but the ovary is clearly twisted around its pedicle and is torsed. The patient went to the OR and the diagnosis was confirmed.
In summary: ovarian torsion can be a tricky diagnosis. It can occur with spectral Doppler flow, without spectral Doppler flow, with an enlarged ovary and even rarely with a normal-sized ovary. The grayscale appearance and the clinical presentation are the most important imaging factors.
[Case courtesy of Dr. Siva Raman]