What do x-rays and ultrasound show, and what can they not?

X-rays show bones, some swallowed objects and the large spaces of the chest and the belly, and soft tissue does not show well on them. Ultrasound shows the inside of soft organs and the fluid around them, and it cannot see through air or bone. Neither picture can say what a lump is made of, which is why the vet may ask for a needle sample or a biopsy as well.

In short

X-rays are good for fractures, tumors, injuries and deformities. Soft tissue does not show well on them.
Ultrasound shows soft organs as a live, moving picture. It cannot see through air or bone.
For x-rays your pet may be given a sedative so that it lies still. For an ultrasound of the belly, expect a fast beforehand and a clipped patch of hair.
The guidelines do not ask for routine imaging of a healthy pet. It is done when the exam or another test points somewhere.

What does an x-ray show?

The Merck Veterinary Manual calls x-rays the most common kind of imaging in clinics. The exposure is very brief and uses a low dose of radiation, the procedure is painless, and lead shields protect the rest of the pet's body and the people taking the picture. The picture shows bones, some swallowed objects such as rubber or metal, and the large spaces of the chest and the belly. The manual says x-rays are good for finding fractures, tumors, injuries and deformities.

The weakness is soft tissue. The manual's chapter for vets explains that an x-ray picture is made of only a few shades: gas, fat, fluid or soft tissue, bone, and metal. Fluid and soft tissue fall in the same shade, and the manual says soft tissues do not show up well. To get round that, the vet can use contrast, a dye that blocks x-rays and is given into a vein or by mouth to outline the kidneys, the heart or the gut.

One picture is not enough. For bones the manual asks for at least two views at right angles, and for the chest and the belly three. It shows a lung mass that is plain on one view and cannot be seen at all on another.

What does an ultrasound show?

Ultrasound uses sound waves and builds a moving picture from the echoes. The American College of Veterinary Radiology says it shows soft tissue structures live, with none of the radiation that x-rays use. Cornell University's imaging service uses it to look at the internal structure of organs, to search for fluid, to check blood flow and to look for infections and tumors. An ultrasound of the heart is called echocardiography.

The college names two limits. Ultrasound cannot see through air, which rules out the lungs and loops of bowel with gas in them, or through bone. And the quality of the picture depends heavily on the skill of the person doing the scan.

Compare	X-rays	Ultrasound
Shows well	Bones, some swallowed objects, the chest and the belly as a whole.	The inside of soft organs, fluid and blood flow.
Shows poorly	Soft tissue.	Lungs, bowel with gas in it, and bone.
What your pet goes through	Lying still in set positions, sometimes with a sedative.	A fast beforehand, clipped hair and a probe pressed on the skin.

How do I prepare my pet, and will it be sedated?

Follow your own clinic's instructions, because they differ. These examples from university hospitals show what to expect.

"motion is the bane of imaging!" Cornell University Hospital for Animals, Imaging service.

For x-rays, the Merck manual says pets may be sedated to lower stress and help them stay still, and that small animals can often be held in position with sandbags, tape and foam pads. Cornell says x-rays and ultrasound are often done with the animal awake, and sometimes with a sedative.
The radiology college says no human body part should be in the main x-ray beam, which is one reason a restless pet is sedated and not held by hand.
For an ultrasound of the belly, the University of Pennsylvania's veterinary hospital asks for a fast of at least eight hours, because food and gas get in the way. It tells owners of very young or diabetic animals to call for feeding instructions first.
The hair over the area is clipped. Penn and the University of Wisconsin's veterinary hospital both say so.
Penn says the scan takes about 30 minutes, and that a sedative or an anesthetic is always needed when a needle sample or a biopsy is taken under ultrasound.

What if the vet suggests a CT or an MRI?

Cornell says x-rays are often the first kind of imaging used, and that they can settle the question or point to another kind. The Merck manual describes CT as a computer-enhanced x-ray that takes pictures of the body in slices, which helps find deep tumors and infections and fractures. MRI uses a very strong magnet and radio waves, with no x-rays, and is especially useful for the brain, the spinal cord and soft tissues. CT needs the animal to stay perfectly still and MRI takes longer, so the manual says animals are usually anesthetized for CT and in most cases for MRI.

Who reads the pictures?

The pictures can be read at the clinic or sent out. The Merck manual says modern x-rays are stored digitally, which makes them easy to share with a specialist for a second opinion, and that a report from a radiologist can come back within minutes to a few hours. A board-certified radiologist is a vet who, the radiology college says, has spent at least three years after the veterinary degree working only on imaging. The college suggests asking whether one will perform or read a specialized scan.

What can the pictures not tell me?

They cannot say what a lump is. The Merck manual says what ultrasound shows is more often not specific to one disease, and that ultrasound can guide a needle to take tissue, which it calls much safer than taking a sample blind. What the needle test for a lump is can be read on Petipedia's page about a lump.
An x-ray shows some swallowed objects, such as rubber or metal. Some is not all, so tell the vet what you think was swallowed.

When is imaging not worth the money?

As a routine screen on a healthy pet. The American Animal Hospital Association's dog guideline says there is not enough support for a general recommendation, though screening may suit some breeds. Its senior guidelines ask for imaging when something points to one part of the body.
When the pictures already exist. Digital x-rays are easy to share, so ask whether pictures taken at another clinic can be sent over before new ones are taken.
When the answer would not change what happens next. Ask what the vet will do differently depending on the result.

The figures Petipedia could source for vet prices are on What does a vet bill cost? If a sedative or an anesthetic is added, the lines that come with it are explained on What every line on a vet bill means.

When should I not wait for a booked scan?

A booked scan is for a pet that is stable. Struggling to breathe and collapse are among the signs that mean go now on Dogs: is this an emergency? The page for cats is Cats: is this an emergency?

What to ask the vet

What are you looking for, and what will you do differently depending on what the pictures show?
Will my pet need a sedative or an anesthetic, and what does that add to the bill?
Should my pet fast beforehand, and for how long?
Will a board-certified radiologist read the pictures, and if you find a mass, what is the next test?

Sources

Sources read on 2 October 2026; every number is as the source gives it.