Infectious muscle inflammation: pyomyositis and abscesses
One of the classic examples of an infectious lesion is pyomyositis. In the early stages, before the process reaches the stage of a fully formed abscess, the disease manifests as pronounced diffuse edema of the affected muscle tissue. As the process progresses, the imaging pattern changes, and localized pus collections begin to form within the muscle.
Ultrasound and tomographic methods are actively used to assess the state of soft tissues:
- Ultrasound imaging (US): allows for detecting local changes in the echogenicity (reflectivity) of muscle tissue. Small microabscesses may be visualized within the muscle bulk. An important diagnostic criterion is the use of color Doppler imaging (CDI), which demonstrates pronounced surrounding hyperemia, indicating active inflammation.
- Magnetic resonance imaging (MRI): has high soft tissue contrast. On T2-weighted images (T2-WI), formed soft tissue abscesses are defined as hyperintense (bright) yet heterogeneous (non-uniform) structures. A zone of perifocal edema is always visualized around such a lesion.
It is important to remember that in addition to infectious agents, non-infectious inflammatory myositis and specific tissue reactions to foreign body insertion can also cause soft tissue masses.
Synovial disorders
Pathologies of synovial structures are another extremely common cause of soft tissue masses. This group includes a number of conditions requiring differential diagnosis.
The main synovial pathologies include:
- Synovial cysts of various locations.
- Ganglia (tendon ganglia).
- Inflammatory lesions of synovial bursae (bursitis).
- Tenosynovitis (inflammation of tendon synovial sheaths).
The most well-known example is the popliteal cyst (Baker's cyst). This is one of the most frequent masses detected during imaging of the knee joint region. The cyst has a characteristic, easily recognizable visual pattern. However, clinicians and diagnosticians should remember possible complications: a Baker's cyst can become involved in any pathological process affecting the knee joint itself. Infectious processes may develop in its cavity, hemorrhages may occur, and signs of inflammatory arthritis or even neoplasms may appear.
Clinical case analysis based on MRI data
The analysis of specific MRI images helps to better understand the topographical anatomy and imaging semiotics of rare and traumatic lesions.
Accessory soleus muscle (m. accessorius soleus) When examining the leg in proton density (PD) mode in sagittal and axial projections, a developmental variant—an accessory muscle—may be identified. It is located in the posterior part of the leg, occupying the space between the flexor hallucis longus muscle (m. flexor hallucis longus) anteriorly and the Achilles tendon posteriorly. As an incidental finding on axial slices, another accessory muscle is sometimes visualized—the fourth peroneal muscle (m. peroneus quartus), the presence of which is generally clinically silent.
Morel-Lavallée Lesion This specific post-traumatic condition represents a heterogeneous, lobulated mass localized in the deep layers of the subcutaneous adipose tissue. Most commonly detected along the lateral surface of the hip joint.
The MRI semiotics of a Morel-Lavallée lesion are quite specific:
- On T2-WI, a low-signal zone is noted in the posterior parts of the lesion, which morphologically corresponds to areas of chronic hemorrhage.
- In the anterior parts, a thick, irregular capsule-like wall is visualized, which also produces a low MR signal.
- On T1-weighted images (T1-WI), the contents of the lesion appear hyperintense.
- With contrast enhancement, a characteristic peripheral enhancement (ring-like) is noted.
Key diagnostic sign: specifically the localization (lateral surface of the thigh) combined with the described heterogeneous hemorrhagic appearance are considered pathognomonic for a Morel-Lavallée lesion.
Briefly: answers to topical questions
What imaging signs are characteristic of tenosynovitis on ultrasound and MRI?
In the presented materials, signs of tenosynovitis are described for ultrasound imaging:
- increase in the amount of anechoic fluid within the tendon sheath;
- change in the appearance of the tendon itself.
Ultrasound and MRI are indicated as highly informative methods for tendon visualization, however, the description of the MRI picture of tenosynovitis is absent in the sources.
Class and exam questions
What does a developing soft tissue abscess look like on MRI?
On T2-weighted images, an abscess is visualized as a heterogeneous, hyperintense (bright) mass surrounded by a zone of pronounced perifocal edema.
What ultrasound signs indicate the development of pyomyositis?
Ultrasound determines changes in echogenicity within the affected muscle, the presence of small hypoechoic microabscesses, and local hyperemia when using color Doppler imaging (CDI).
What constitutes the pathognomonic sign of a Morel-Lavallée lesion?
Pathognomonic is the combination of a specific localization (in the deep layers of adipose tissue along the lateral surface of the hip joint) and a heterogeneous hemorrhagic structure with signs of chronic hemorrhage and a thick, irregular wall.
Where topographically is the accessory soleus muscle located?
It is localized in the posterior part of the leg: anterior to it is the flexor hallucis longus muscle (m. flexor hallucis longus), and posteriorly is the Achilles tendon.
What else to study on this topic
- Pathogenesis and differential diagnosis of non-infectious inflammatory myositis
- Imaging signs of soft tissue reactions to foreign body insertion
- MRI semiotics of various types of synovial cysts and tendon ganglia
- Anatomical variants of leg muscle structure: clinical significance of m. peroneus quartus
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