Imaging tests can help pinpoint the source of musculoskeletal pain. Ultrasound has some advantages.
When aches, pains and other symptoms stem from a problem with the musculoskeletal system (meaning the bones, muscles, cartilage, tendons, ligaments and joints) your doctor may order an imaging test to help determine the underlying cause.
There are several ways of looking inside the body, including X-rays, computed tomography (CT) scans and magnetic resonance imaging (MRI). Another one, high-resolution ultrasound, offers some advantages over the others for musculoskeletal problems.
"The most common exam we do is a shoulder ultrasound to evaluate a person for rotator cuff problems, bursitis or tendinosis," says Cleveland Clinic radiologist Patricia Delzell, MD. Ultrasound also is commonly done for problems with tendons, ligaments and bursae in other parts of the body, as well as for arthritis, muscle injury, nerve entrapment and other conditions.
Ultrasound vs. MRI
"A common question people ask us is, why are you doing an ultrasound instead of an MRI," says Dr. Delzell. MRI and ultrasound look at the body in different ways.
MRI, which uses powerful magnets to produce three-dimensional anatomic images, is a high-contrast resolution modality that can determine changes in the tissue quality. With a muscle injury, for example, MRI images often show a bright signal indicating that there is more water in the muscle, which is a sign of injury.
Ultrasound, which uses high-frequency sound waves to look at organs and structures, is a high-spatial resolution modality that provides greater internal detail of a structure. With the same muscle injury, "we can see the individual fibers of the muscle and early changes of the injury," says Dr. Delzell. Both MRI and ultrasound can detect large tears in tissue. "But microtears and chronic injury are better seen on ultrasound," she says.
"When we look at the musculoskeletal system, we're looking at many different structures in one area," says Dr. Delzell. There are nerves, muscles, tendons, joints, ligaments and bone, as well as layers of connective tissue (such as fascia) that surround these structures. Any of those structures can get injured. Often there are injuries to or abnormalities of several structures.
"Ultrasound is able to pinpoint the location of a patient's pain and correlate it to a problem with a particular structure in that location," says Dr. Delzell. Ultrasound also allows the clinician to observe what happens during movement, which is not usually possible with other imaging tests. For example, a dynamic evaluation can be used to see whether bursae (fluid-filled sacs that provide cushioning) are being squeezed between two structures in a joint, indicating impingement.
"Certain structures that are not well evaluated on other imaging modalities are very well evaluated on ultrasound," says Dr. Delzell. These include the nerves, the fascia, and chronically injured muscles and tendons. "People can have significant disease with those three things and have normal results on MRI or CT," she says.
Ultrasound is not just a diagnostic tool. When some type of injection treatment is needed, such as a corticosteroid shot into a joint, ultrasound can be used to visualize the structure to ensure that the needle is placed in the correct location.
Image-guided joint injections also are done with X-rays or CT scans. But with ultrasound, there's no radiation and no need to inject a contrast dye.
When to Get an MRI
Ultrasound does have some limitations, and there are situations when MRI is needed instead. "Ultrasound does not show the structures inside joints," says Dr. Delzell. "We can only see the soft tissues outside the joint."
To evaluate damage to cartilage, bone or other structures inside and around a joint, MRI is the better choice. MRI is also preferred for conditions that impact deep or large areas. The ultrasound probe can evaluate only a small area at a time.
"If a patient needs a detailed look at the whole hand and wrist or the whole ankle, an MRI would be the best choice," says Dr. Delzell. "But if a patient has a focal complaint, such as chronic heel pain, and the doctor suspects an issue with the plantar fascia, the patient would come for an ultrasound."
What to Expect
With ultrasound, the technician moves a handheld probe over the area to be studied. Performing a diagnostic musculoskeletal ultrasound can take up to an hour, which is longer than other imaging tests. "When we do ultrasound, we have to look at each structure individually and then do the dynamic study, so it takes longer," says Dr. Delzell.