Magnetic Resonance Imaging (MRI) and Ultrasound for Inclusion Body Myositis
MRI can be helpful in the diagnosis of IBM. It often shows muscle shrinkage, replacement of muscle tissue by fat, and swelling within the muscle. If these changes are seen in the muscles that are most often involved in IBM, they add weight to the diagnosis.
Muscle ultrasound is another imaging tool that may be helpful in IBM. In muscles affected by the disease, the ultrasound image often looks brighter than normal because changes within the muscle, especially fatty replacement and scarring, alter the way sound waves are reflected.
Newer diagnostic criteria now include both MRI and ultrasound among the imaging tests that may help support the diagnosis of IBM.
Magnetic Resonance Imaging
Illustration of a patient undergoing an MRI scan
What MRI can show in IBM
Magnetic resonance imaging uses magnets to create detailed cross-sectional images of the muscles, rather than using x-rays like a CT scan. Several kinds of muscle change may be visible on MRI in IBM. One is muscle atrophy, meaning loss of muscle bulk, sometimes called muscle wasting. Another is replacement of muscle tissue by fat, an especially important finding in IBM that often becomes more pronounced over time. MRI can also show swelling within the muscle, called edema. Although fibrosis, or scarring, is part of IBM muscle damage, it is difficult to see and define clearly on a standard MRI scan, which is why it is not usually discussed as a separate MRI finding.
Of these findings, fatty replacement is usually the most prominent and most consistent in IBM. It generally reflects chronic damage that has built up over time. The more extensive it is, the more advanced the disease is likely to be. For that reason, fatty replacement is important not only in diagnosis, but also in research studies and clinical trials that use MRI to track progression.
Edema is also common in IBM, but it is usually less striking than fatty replacement. A useful way to think about it is that edema may reflect an active disease process at the time of the scan, while fatty replacement reflects cumulative damage. This is only a general guide, however, because both can be present at the same time in the same patient and even in the same muscle.
Thigh MRI in Inclusion Body Myositis Compared with Normal Muscle
Unlike a plain x-ray, MRI does not produce just one fixed image; it uses different pulse sequences to highlight different tissue properties—for example, T1-weighted images are especially useful for showing muscle shrinkage and fatty replacement, while STIR images highlight excess water in the muscle and make edema easier to see.
Which Muscles Look Abnormal on MRI?
In clinical practice, MRI in suspected IBM most often focuses on the thighs, since the quadriceps are commonly affected. Depending on the situation, the lower legs and sometimes the forearms may also be examined.
In the thigh, MRI often shows a characteristic pattern of quadriceps involvement in IBM. The vastus muscles are usually more affected than the rectus femoris, and changes are often more pronounced in the lower part of the quadriceps, near the knee, than in the upper thigh. This upper-to-lower difference is one of the classic MRI clues in IBM.
In the forearm, MRI often shows selective involvement of the deep finger flexor muscles, especially the flexor digitorum profundus, the muscle that bends the fingertips. This is one of the classic imaging clues in IBM. The muscle may show fatty replacement, atrophy, and sometimes edema, often more clearly than nearby forearm muscles.
MRI often confirms weakness that is already apparent on physical examination, especially in the quadriceps and finger flexors. It can also show disease involvement in muscles whose weakness is harder to detect in the office. A good example is the medial gastrocnemius, a calf muscle involved in pushing the foot downward to stand on tiptoe. In IBM, this muscle is often abnormal on MRI, but its weakness may be overlooked during an exam because the lateral gastrocnemius and soleus, which also help produce this movement, are often relatively spared and can mask the weakness. MRI abnormalities in IBM are also often asymmetric, meaning that one side may be more affected than the other.
Video Demonstrating the Gastrocnemius Muscle
The best-known MRI pattern in IBM involves the quadriceps, the deep finger flexors, and the medial gastrocnemius. However, newer studies show that other muscles are often involved as well. In the lower leg, this includes the flexor hallucis longus, which raises the big toe, and the tibialis anterior, which raises the front of the foot. In the thigh, muscles such as the semimembranosus, gracilis, adductor magnus, and sartorius may show abnormalities. More recent whole-body MRI studies have also found frequent involvement of the paraspinal muscles and some muscles of the shoulder girdle and upper back. These newer findings broaden the MRI picture of IBM, but the classic pattern remains the most established and clinically useful.
How MRI May Help with Diagnosis
MRI is not required in every patient, and many people with IBM are diagnosed without it. However, it can be useful when weakness is present but still mild and the typical pattern of weakness is not yet clear on physical examination. In that setting, MRI may show muscle abnormalities even before weakness in those muscles can be demonstrated confidently in the office, helping clarify the diagnosis.
MRI can also be helpful when the presentation is unusual. For example, IBM sometimes begins with isolated dysphagia or with muscle weakness that does not seem to follow the usual pattern. In cases like these, MRI may reveal muscle abnormalities elsewhere that raise suspicion for IBM and help guide further testing.
MRI is also useful when doctors are deciding whether IBM is the most likely diagnosis or whether another muscle disease is a better explanation. For example, MRI findings in IBM often differ from those in polymyositis and some other inflammatory muscle diseases. In IBM, MRI is more likely to show atrophy and fatty replacement, often with asymmetry, whereas in polymyositis inflammation may be more prominent and the distribution may be different. In this way, MRI can sometimes make other diagnoses less likely and increase suspicion for IBM.
How MRI May Help Guide Muscle Biopsy
When selecting which muscle to biopsy, doctors try to choose a muscle that is already showing signs of IBM, but not too severely. If a specific muscle is still fairly healthy, the biopsy may not show clear evidence of IBM. On the other hand, if a muscle has already been badly damaged by IBM, scar tissue and loss of muscle fibers may obscure the diagnosis. In some cases, MRI can help the doctor select an appropriate muscle to biopsy.
MRI in Research and Clinical Trials
MRI is useful not only in the evaluation of inclusion body myositis, but also in tracking the disease over time. Using quantitative MRI, doctors can measure changes such as the amount of fat within a muscle. These measurements appear to be reliable, reproducible, and related to muscle strength. In the future, MRI may become one of the outcome measures used in clinical trials to help determine whether a treatment is working.
For readers interested in newer research on MRI patterns in IBM, see this recent journal article.
Ultrasound
Ultrasound can also provide useful information in inclusion body myositis. Ultrasound creates images by sending sound waves into the muscle and measuring how they are reflected back. In affected muscles, the ultrasound image often looks brighter than normal because damage within the muscle, especially fatty replacement and fibrosis, increases the reflection of sound waves.
Studies suggest that ultrasound may sometimes show muscle abnormalities even before weakness in those same muscles is clearly detectable on physical examination. The muscles that have been especially helpful in ultrasound studies of IBM include the flexor digitorum profundus (deep finger flexor) in the forearm and the medial gastrocnemius in the calf.
Ultrasound has not been studied as extensively as MRI, and its usefulness depends heavily on the skill and experience of the person performing and interpreting the study. This can be a practical limitation, because IBM is rare and muscle ultrasound is not yet a routine test in most settings. Even so, ultrasound has several advantages: it is fast, easy for the patient, and can often be done in the office.
For readers interested in a more technical discussion of ultrasound in IBM, see this review article.
Revised March 22, 2026