Soft Tissue Infection

KEY FACTS

Terminology

• Infection of soft tissue including cellulitis, infective fasciitis, necrotizing fasciitis, pyomyositis, phlegmon, and abscess

Imaging

• US excellent for localized soft tissue infection
• Use US ± CT or MR for more generalized soft tissue infection
• US cannot distinguish infective from noninfective fluid
• Always consider secondary soft tissue infection from deeper or other primary source
• Cellulitis

Edematous hyperechoic subcutaneous fat with hyperemia

– Hyperemia helps differentiate subcutaneous inflammation from subcutaneous edema

– Subcutaneous inflammation normally, but not always, equates to infection, i.e., cellulitis

Thickened interlobular septa
• Necrotizing fasciitis

Affects subcutaneous fat, fascia, and muscle

Thickened fascia with perifascial fluid (fluid above and below investing fascia)

Severe subcutaneous and muscle edema

± fascial focal defect ± muscle necrosis ± gas locules with comet-tail or shadowing artifact
• Pyomyositis

Diffuse edematous hyperechoic muscle swelling with indistinct fibrillary pattern

Hyperemia ± gas locules with comet-tail or shadowing artifact
• Abscess

Echogenicity of fluid may vary from hypoechoic to isoechoic to hyperechoic

Posterior enhancement characteristic

Echo movement on real-time imaging

Peripheral hyperemia ± septation ± gas locules

TERMINOLOGY

Definitions

• Cellulitis: Localized or diffuse soft tissue inflammation of skin and subcutaneous tissues
• Infective fasciitis: Infection of superficial &/or deep fascia
• Necrotizing fasciitis: Rapidly progressive infection of superficial and deep fascia associating with vascular occlusion and necrosis
• Pyomyositis: Infective myositis = suppurative bacterial infection of muscle
• Phlegmon: Localized, intense inflammation prior to development of abscess or ulceration
• Abscess: Localized collection of pus

IMAGING

General Features

• Best diagnostic clue

Edema and hyperemia of affected tissues ± fluid accumulation
• Location

Any soft tissue tissue can become infected

Most common sites

– Cellulitis: Dorsum of hands and feet

– Necrotizing fasciitis: Thigh and leg

– Pyomyositis: Large muscles of pelvis and lower limb; frequently multifocal
• Size

Varies from small localized abscess to multicompartmental disseminated infection
• Morphology

Inflammation ± fluid ± abscess

Radiographic Findings

• Often noncontributory; exclude other causes of soft tissue swelling such as fractures
• Soft tissue swelling with obliteration of fascial planes

± gas locules within soft tissues ± ulcers ± radiopaque foreign body ± concomitant osteitis or osteomyelitis

CT Findings

• Increased attenuation of subcutaneous fat
• Swelling of subcutaneous tissue ± muscles
• Fascial thickening or perifascial fluid
• ± gas locules within soft tissues ± foreign body ± concomitant osteitis or osteomyelitis

Highest sensitivity for detection of soft tissue gas and foreign body
• Contrast enhancement is useful in detecting fluid collection, abscess, or necrosis

MR Findings

• T2WI FS

Cellulitis: Diffuse hyperintensity of subcutaneous tissues

– ± feathery edema of adjacent tissues

Necrotizing fasciitis: Hyperintense thickening of deep fascia with perifascial fluid

– Edema of adjacent musculature

– ± fluid areas indicative of necrosis, exudate, or abscess

– ± hypointense foci due to gas locules

Pyomyositis: Diffuse hyperintensity of muscle

– ± fluid collection

– ± edematous changes in adjacent subcutaneous tissue > fascial planes > bone and joints

Abscess: High signal intensity area with surrounding edema ± hyperemia

– ± hypointense foci due to gas locules
• T1WI C+

Useful in delineating fluid collection, sinus, abscess, or necrosis

Ultrasonographic Findings

• Cellulitis

Edema and hyperemia of subcutaneous fat

– Edematous fat is echogenic on US

– Thickened interlobular septa; characteristic cobblestone appearances

– ± periseptal fluid and fluid around investing fascia

– Need combination of edema and hyperemia to diagnose cellulitis

Hyperemia is not seen with noninflammatory edema in, for example, venous insufficiency, heart failure, or lymphedema

– ± secondary tenosynovitis with mild synovial proliferation and tendon thickening

Appearance of subcutaneous edema varies according to severity and architecture of subcutaneous fat

– Varies from amorphous swelling with indistinct lobules to cobblestone appearance with accentuated lobules

1st line to differentiate cellulitis from venous thrombosis, but both conditions may coexist
• Necrotizing fasciitis

Affects subcutaneous fat, fascia, and muscle

Thickened deep fascia (> 3 mm) with perifascial fluid (fluid above and below investing fascia)

– Extensive involvement of deep intermuscular fascia, not just area contiguous to investing fascia; 3 or more compartments involved

– ± fascial irregularity or focal defect

Severe, associated subcutaneous and muscle edema

± muscle necrosis (difficult to detect)

Gas locules with comet-tail artifacts or shadowing artifact are suggestive of necrosis

– rare finding but high specificity
• Pyomyositis

Diffuse muscle swelling with edema ± hyperemia

Edematous muscle is echogenic on US with indistinct fibrillary pattern of muscle

± focal hypoechoic areas due to serous exudate, abscess, or necrosis

Usually involves single muscle; multiple muscles are involved in 40% of cases

Only gold members can continue reading. Log In or Register to continue
Share this:
Related posts:

Hand Vessels Musculoskeletal Ultrasound Artifacts Epidermoid Cyst Shoulder Foreign Body and Injection Granuloma Baker Cyst

Stay updated, free articles. Join our Telegram channel

Tags: Diagnostic Ultrasound Musculoskeletal

Apr 9, 2020 | Posted by admin in MUSCULOSKELETAL IMAGING | Comments Off

KEY FACTS

Terminology

TERMINOLOGY

Definitions

IMAGING

General Features

Stay updated, free articles. Join our Telegram channel

Radiology Key

Fastest Radiology Insight Engine

Soft Tissue Infection

Soft Tissue Infection

Imaging

Radiographic Findings

CT Findings

MR Findings

Ultrasonographic Findings

Full access? Get Clinical Tree

Fastest Radiology Insight Engine

Soft Tissue Infection

Share this:

Related posts:

Stay updated, free articles. Join our Telegram channel

Full access? Get Clinical Tree