Have you ever wondered why your doctor chooses to order one type of imaging over another? Magnetic resonance imaging (MRI) and ultrasound are two modalities that can sometimes be used for the same purposes. Understanding the differences between them and the strengths and limitations of each can give you insight into the reasons behind your doctor’s choice.
Difference in Technologies
Both MRI and ultrasound have the advantage of allowing doctors to look inside the body without the use of ionizing radiation, which is associated with an increased risk of developing cancer. MRI accomplishes this by using a powerful magnet to cause the water molecules in the body to align in a way that the machine’s scanner detects and translates into a three-dimensional picture of what’s going on in the body. Ultrasound technology, on the other hand, transmits high-frequency sound waves through the body, and a picture is created by detecting the echoes, much like the way bats or submarines use sonar.
Imaging Capabilities and Limitations
Both ultrasound and MRI can be useful in assessing the condition of organs and soft tissues such as breasts and blood vessels. However, ultrasound waves do not transmit well through bone or gas. As a result, MRI or another modality is generally recommended when doctors need to get a look at what's happening inside or behind bony structures or air-filled organs such as the lungs or bowel.
An ultrasound exam offers the benefit of producing images quickly and in real time, so movement can easily be observed on ultrasound. MRI images can provide greater detail, but these images are time consuming to capture are typically static.
Sometimes, the reason for choosing one modality over another is that one of the options is contraindicated for some reasons. Ultrasound has the advantage of posing no known risk to human health. MRI is considered a very safe procedure, but it does carry some risks, which can make it unsuitable for certain patients.
Because MRI uses a very powerful magnet to obtain images, it can be unsafe for patients who have implanted ferromagnetic objects, such as certain medical devices or shrapnel.
Additionally, some MRI procedures call for the use of gadolinium contrast in order to obtain sufficiently clear images. This type of contrast solution can cause severe side effects in patients with impaired kidney function. As a result, a creatinine test, which measures kidney function, is ordered before a patient is given gadolinium contrast, and if the test shows impairment, then another type of imaging, such as ultrasound, may be recommended, or additional precautions may be taken to prevent side effects. Gadolinium is also generally avoided during pregnancy to avoid any unknown risks, but it is sometimes used when it can help to obtain critical information that can’t be gained through other modalities.
Due to pain, physical limitations, or psychological limitations such as claustrophobia and anxiety, some patients find MRI exams challenging to endure. The procedure requires patients to lie still for extended periods of time, and the machine itself can feel confining. Wide-bore and open-bore MRI machines provide some relief from the feeling of confinement, and the use of sedatives can help some patients tolerate the procedure. In some cases, however, a different modality may be preferable.
Cost and Availability
MRI requires more expensive and much larger machinery than ultrasound. As a result, ultrasound is available to more patients in more places throughout the country. Sometimes, ultrasound can be used as primary imaging to locate the source of a problem, and MRI or CT can be used to follow up on abnormal findings and get a more detailed view if necessary.
At Iowa Radiology, we work to provide the information patients need to make informed decisions about their health. You can learn more about MRI by clicking the image below to access our free ebook. To get regular updates on important health care topics via email, subscribe to our blog.
 "MRI: Is gadolinium safe for people with kidney problems?" MayoClinic.org. Mayo Foundation for Medical Education and Research, 16 May 2014. Web. 21 April 2017.