A: Each technology has strengths and weaknesses and is therefore ideal for different circumstances. In general, X-rays are great at imaging hard tissues or objects. It is also a good technology to visualize anything surrounded by air. Ultrasound machines use sound waves that are blocked by solid objects and dissipated by air filled spaces.
Therefore, if your veterinarian is looking to see if your dog has a broken bone or your cat swallowed a sewing needle, X-rays would be the preferred technology. Similarly, looking for problems in air-filled structures, like the lungs or sinus passages, would best be done using an X-ray machine.
When it comes to soft tissue structures, X-rays have significant limitations. They condense all those mushy internal organs into a single two dimensional image. Fluid buildup and overlapping anatomic structures can obscure X-ray images much like bone and air obscure ultrasounds. Also, X-ray films capture only a split second in time, so cannot demonstrate motion.
Ultrasound machines allow us to look at details within soft tissue organs and see motion. Images are actually improved in the presence of fluid, like within the urinary bladder, heart, or in the presence of free fluid in the abdomen or chest. So if your veterinarian feels an abdominal tumor and wants to see if it is operable, an ultrasound is usually the better technology. If there is a problem with your pet’s heart, an ultrasound will allow us to watch the heart as it beats measuring specific chamber sizes and contractility.