What is the difference between ultrasound and MRI?

Ultrasound and magnetic resonance imaging (MRI) are two types of medical imaging used to give doctors a look inside the body. They work in different ways, and each has some distinctive advantages and disadvantages that must be considered. Sometimes a doctor may order imaging studies using both techniques to get a more complete view of what's going on inside a patient's body.

These imaging techniques have a number of similarities, beginning with the fact that either method can be used to produce still and moving images of the interior 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 non-invasive to minimally invasive, depending on the details of the test. In some cases, it may be necessary to inject contrast material or insert an ultrasound transducer into the body in order to obtain a better image.

In the case of ultrasound, the image is acquired by bouncing high-frequency sound waves off the body cavity. The sound waves are picked up on their way back, and the changes in frequency and angle are used to generate an image of the inside of the patient. With an MRI study, the patient is inserted into a large chamber that generates a magnetic field, agitating the hydrogen atoms in the patient's body. The MRI machine reads changes in the body's magnetic field and uses these changes to create an image.

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

The main problem with MRI 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 from the person from her, but in the case of patients with implanted medical devices, the MRI machine could cause a problem. MRI scans 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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