Final answer:
An X-ray of the spinal canal with radiopaque substance is a myelogram. The safest imaging study during pregnancy is an ultrasound, which is non-invasive and radiation-free.
Step-by-step explanation:
An X-ray of the spinal canal after the introduction of a radiopaque substance is known as a myelogram. When it comes to choosing imaging studies that can be used safely during pregnancy, ultrasounds are the preferred method. Ultrasound technology is less invasive and uses no electromagnetic radiation, making it the safest option for both the mother and fetus, unlike CT scans, PET scans, and X-rays which involve exposure to some form of radiation.