Ultrasound is an invaluable diagnostic tool in gynecology, as it allows visualization of the reproductive organs without x-ray radiation exposure.
This test used to be done by placing an external transducer (which emits sound waves) over the abdomen. Ultrasound examination using a vaginal transducer is now used for evaluation of many gynecological conditions, as it allows closer visualization of the structures of interest.
The transducer is about the size of a large tampon. It is inserted into the vagina only, and thus causes no opening of the uterus. Performance of this exam should not render a woman niddah. There is no obligation to check the transducer for blood or to ask the doctor if blood was found.
If blood is noted on the transducer she should ask the healthcare provider performing the test to explain the source of the bleeding. If they attribute the bleeding to the procedure or to vaginal irritation, then it is considered dam makkah, (blood from a lesion, which does not make a woman niddah). Otherwise, she should consult a halachic authority.