Unlike radiographs, no radiation is used in an ultrasound study. An ultrasound machine uses sound waves. The ultrasound waves move out from the wand and either become absorbed into organs, pass through them, or are reflected (echo) back. Depending on how many sound waves are absorbed or reflected, an image of the internal organs is formed by a sophisticated integrated computer, and the image is then displayed on a monitor. Real-time moving images are displayed, and still images can be captured as well.

Ultrasound is painless and does not require anesthesia or even sedation in most cases.  For an ultrasound evaluation to be done, the pet does need to have the hair shaved from the evaluation area because it will interfere with the images. This test is typically done after blood tests, x-rays, or a physical examination indicates a possible problem. 

Ultrasound can “see” some things that can’t be visualized on radiographs. For example, if the abdomen is filled with fluid, the organs can’t be distinguished on traditional x-rays because fluid and tissue have the same density. However, they appear quite different from each other on an ultrasound image, so we can see through the fluid. It is also useful, for the same reason, for seeing inside an organ such as the heart or liver. On the other hand, it is not as good at seeing through air or bone, so it does not replace radiography but rather is complementary to the information we can get from radiographs. It is common to do both x-rays and ultrasound in order to get a good picture of what is going on.
 

Why Does my Pet Need an Ultrasound?

If your pet has been showing symptoms associated with a problem in one of those belly organs, an ultrasound may be needed. This can include diarrhea; throwing up; belly pain; abnormal urination; a mass that can be felt when pressing on the belly; or if internal injuries are suspected, such as after being hit by a car. Sometimes the veterinarian will find something specific during a pet’s physical exam, such as feeling fluid buildup in the belly, that could be seen by ultrasound. Other situations in which an ultrasound may be necessary are if lab work or x-rays show something unusual. Abdominal ultrasounds can also be used to take samples of fluid or tissues in order to get a diagnosis of the problem. This can include guiding a needle to biopsy unusual masses or removing fluid for analysis. Veterinarians can either view these samples themselves or send them to a pathologist for testing.

As with people, ultrasounds can also be used to check for pregnancy. Pregnancy is best diagnosed after roughly 20 days from the last heat cycle. Intestinal contents and gas can occasionally cause inaccurate results. Ultrasound is not particularly useful for estimating litter or individual fetus size, so x-rays are more commonly performed. X-rays also tend to be less expensive.

Information source: https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4952958
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