What is the Difference Between Ultrasound and MRI?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 23 June 2019
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Ultrasound and magnetic resonance imaging (MRI) are two types of medical imaging which are used to give doctors a glimpse of the inside of the body. They work in different ways, and each has some distinctive advantages and disadvantages which must be considered. Sometimes, a doctor may request imaging studies using both techniques to get a more complete view of what is going on inside a patient's body.

These imaging techniques have a number of similarities, starting with the fact that either method can be used to produce both still and moving pictures of the inside of the body. Both procedures are performed without the use of radiation, which is a distinct advantage of these types of medical imaging. In the case of a patient who needs multiple imaging studies, ultrasound and MRI are preferable because the patient's body is not at risk of damage from accumulated radiation. The techniques are also noninvasive to minimally invasive, depending on the specifics of the testing. In some cases, it may be necessary to inject contrast material, or to insert the transducer into the body for the purpose of getting a better image.


In the case of an ultrasound, the image is acquired by bouncing high frequency sound waves off the body cavity. The sound waves are collected upon their return, and the changes in frequency and angle are used to generate a picture of the patient's insides. With an MRI study, the patient is inserted into a large chamber which generates a magnetic field, agitating hydrogen atoms in the patient's body. The MRI machine reads the changes in the body's magnetic field and uses these changes to build a picture.

One major disadvantage of ultrasound is that it often has difficulty with obstructions. Certain parts of the body are difficult to image because thick bones or organs are in the way. By contrast, MRI images are very clear and crisp, and they can be taken along any plane of the body. In both cases, the image quality can be very high with a good machine, and the use of a contrast agent may improve visibility even more, allowing a doctor to see specific structures in detail.

The primary issue with magnetic resonance imaging is that the MRI machine will interact with magnetic objects in the room, and these objects can cause damage to the machine. The patient must remove all magnetic objects on his or her person, but in the case of patients with implanted medical devices, the MRI machine could cause a problem. MRI studies are especially dangerous for people with devices implanted in or around their hearts. In a case where MRI is not feasible, it may be necessary to use ultrasound.


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Post 7

An ultrasound is better at determining whether the cyst is made up of simple fluid or if it contains cellular debris (such as blood. Also, an ultrasound can determine if the cyst has any internal solid components that would indicate a more serious problem.

Post 2

Sometimes an ultrasound is good for measuring the size of the cysts.

Post 1

I had an MRI and they found a cyst on my kidney. Now I have to have an ultrasound. Why do I need both?

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