Ultrasounds, aka “sonograms,” are a very common method of medical imaging. We say “imaging” instead of “x-ray,” because the ultrasound bounces sound waves off our organs to produce a picture, without the radiation of x-rays. As such, it’s 100% safe.
We can use ultrasound for many parts of the body, except where bone gets in the way. So we can’t use it for the brain (except in infants). We rarely use it for the lung, but commonly do so for the heart, when we call it an echocardiogram. For the uterus and ovaries, we perform it through the vagina for better detail (transvaginally).
However, plain x-rays are lots easier, quicker, and cheaper, so they’re used to find fractures, arthritis, and pneumonia (employing very little radiation). In terms of ultrasound compared to CT scan, the latter uses radiation, but is often able to see a lot more. For example, if we’re looking for cancers or infections in the abdomen, we obtain CT scans because they’re much more accurate.
We especially try to avoid radiation in children, due to its risk of future cancer (children have a lot longer time for it to develop than adults & certainly the elderly — see Radiation Risks). For a child with possible appendicitis, ultrasound is as accurate as a CT scan. If the ultrasound isn’t definite, it’s even safe to observe the child, & repeat the ultrasound if they’re getting sicker. However, the accuracy of an ultrasound depends on the ability of the technician performing it (as opposed to CT scans, which are done automatically in standard fashions).