Skin and Soft Tissue Ultrasound Notes

Background

• Skin and Soft Tissue Ultrasound (SSTU) leverages point-of-care ultrasound (POCUS) to evaluate the skin, subcutaneous tissue, fascia, and muscle layers. It supports assessment of SSTIs and other dermatologic conditions.
• SSTIs are common: more than 6.3 million physician office visits annually; frequent emergency department presentations. Diagnosis can be challenging due to unreliable systemic symptoms (e.g., fever, chills) and limitations of physical examination.
• POCUS in SSTIs
  • Has high diagnostic accuracy for SSTIs and can prompt rapid bedside management changes in >10% of cases.
  • Helps avoid misdiagnosis of abscesses, reducing unnecessary incision and drainage (I&D) procedures and improving outcomes.
  • Is recommended in soft tissue guidelines as a tool to improve diagnosis and treatment.
  • Is cost-efficient and safer than CT or MRI.
• Beyond SSTIs, SSTU aids in identifying neck masses, distinguishing benign soft tissue lesions from vascular malformations, and initial assessment of dermatologic complaints when appropriate.
• Key anatomic targets for SSTU: skin, subcutaneous tissue, fascia, and muscle layers.
• The skin comprises epidermis and dermis. The epidermis appears as a thin, hyperechoic layer in the near field; the dermis is also hyperechoic but less than the epidermis and thicker than the epidermis. Subcutaneous tissue (hypodermis) contains adipose and connective tissue and is hypoechoic relative to epidermis/dermis. Fascia varies in thickness by location and is often hyperechoic relative to surrounding tissues.
• The skin and soft tissue ultrasound exam is performed in two orthogonal planes, with B-mode imaging first, then color Doppler as needed. Imaging should include comparison with the contralateral side when possible.
• Patient-centered approach: minimize discomfort via positioning and gentle compression; use copious gel to improve resolution for superficial structures. Techniques to improve visualization include standoff pads and water baths. Open wounds should be managed with water-based sterile gel and proper disinfection.
• Probe selection
  • A linear probe is preferred for most SSTI POCUS exams.
  • For superficial structures (epidermis, dermis, nails), a high-frequency wide-footprint probe (18–20 MHz) improves resolution.
  • For deeper structures, use a curvilinear probe or endocavitary probe if available.
  • Wide-footprint high-frequency probes provide better superficial detail; endocavitary probes offer higher frequency and smaller footprint.
• Practical imaging aids
  • Use two orthogonal planes to evaluate the area of interest.
  • Apply color Doppler depending on location and question.
  • Use a magnitude of measurements in two orthogonal planes to account for width, length, and depth: three measurements in two planes (W, L, D).
  • When feasible, image the unaffected contralateral area for comparison.
  • For superficial structures, consider standoff pad or water bath to improve the focal zone.
• Two common imaging tactics for superficial SSTIs
  • Water bath or gel-based approaches to move the target into the focal zone and improve resolution.
  • Plastic container approach as a lateral/modified technique to image the finger or small areas.
• Safety and infection control
  • Use water-based sterile gel for open wounds; follow high-level disinfection protocols.
  • If a wound is open or a puncture exists, take precautions to avoid contamination.
• Summary of the exam: SSTU is a practical, rapid, bedside tool that enhances diagnosis, guides management (e.g., I&D planning), and can reduce unnecessary procedures and imaging.

Indications and Diagnoses

• Indications for SSTU in soft tissue assessment
  • Soft tissue mass
  • Pain
  • Erythema
  • Swelling
• Diagnoses and broad utility
  • Skin and soft tissue infections (SSTIs) are common in acute and outpatient settings; POCUS aids in distinguishing cellulitis, abscess, necrotizing fasciitis, foreign bodies, lymph nodes, lipomas, cysts, and nail pathology.
• Clinical context and decision-making
  • SSTIs often rely on clinical exam, but ultrasound improves diagnostic certainty and helps tailor management.

Core Views and Normal Anatomy (Ultrasound Appearance)

• Skin and soft tissue layers
  • Epidermis: thin, hyperechoic layer in near field.
  • Dermis: hyperechoic, thicker than epidermis, but less echogenic than epidermis.
  • Subcutaneous tissue (hypodermis): adipose and connective tissue; appears hypoechoic relative to dermis/epidermis.
  • Fascia: varies in thickness; often hyperechoic relative to surrounding tissues.
• Normal imaging planes
  • Images obtained in two orthogonal planes for comprehensive assessment.
• Figures and normal anatomy references (descriptive visuals): illustration of normal superficial anatomy and longitudinal/transverse views with high-frequency linear probes.

SSTU Technique and Practical Considerations

• Scanning approach
  • Evaluate area in two orthogonal planes.
  • After B-mode, apply color Doppler as indicated.
  • Take three measurements in two orthogonal planes to document width (W), length (L), and depth (D).
  • When possible, image contralateral side for comparison.
• Patient positioning and comfort
  • Position for comfort and to minimize pain.
  • Apply minimal pressure to reduce patient discomfort.
• Image optimization
  • Use copious gel for superficial structures.
  • Use a standoff pad or water bath to optimize visualization for surface structures.
  • Plastic container can be used for a lateral/modified approach.
• Probe and frequency notes
  • Linear probes for most SSTIs.
  • High-frequency (18–20 MHz) for superficial detail (epidermis/dermis/nail pathology).
  • Curvilinear probe for deeper structures when needed.
  • Endocavitary probe as an option when available for higher frequency and smaller footprint.
• Practical imaging aids (figures from text)
  • Standoff pad example: finger imaging with ~1 cm depth.
  • Water bath technique: improves visualization of superficial structures.
• Safety and infection control specifics
  • Water-based sterile gel for open wounds.
  • Adhere to high-level disinfection protocols.

Clinical Applications

Cellulitis

• SSTI incidence and diagnostic challenge
  • Annual incidence in the 22–50 per 1000 persons range; >14 million cases/year in the US.
  • Cellulitis is often a clinical diagnosis but can overlap with deeper infections like abscess or necrotizing fasciitis.
• POCUS findings in cellulitis
  • Early: dermal thickening and increased vascularity on color Doppler.
  • Later: cobblestoning due to edema (fluid between hyperechoic fat lobules).
  • Cobblestoning is nonspecific and can occur with edema from other conditions (e.g., heart failure, venous stasis, lymphedema, DVT).
• Diagnostic context
  • Use clinical history, exam, and comparison views; interpret findings in context.

Abscess

• Diagnostic utility
  • Physical examination alone has limited reliability for abscess identification.
  • POCUS improves accuracy; sensitivity is about ext{sensitivity}
    ightarrow [0.90, 0.95] for adults and children.
  • POCUS can change management in roughly ext{10 ext{%}} of cases.
• Ultrasound appearance
  • Abscess: irregularly bordered soft tissue collection; may be hypoechoic or anechoic with internal debris; may show posterior acoustic enhancement.
  • Internal debris may be echogenic; swirling debris may be observed with graded compression (the swirl or squish sign).
  • Gas within abscess: may cause dirty shadowing with reverberation and speckled artifacts.
• Distinction from other findings
  • Surrounding hyperemia on color Doppler but no internal flow within the collection helps differentiate abscess from vascular structures.
• Procedural guidance and post-procedure assessment
  • Ulrasound helps define margins, depth, and optimal incision location for I&D or loop drainage.
  • Evaluate surrounding vasculature and nerves to avoid injury during drainage.
  • Post-drainage: ultrasound can assess adequacy of drainage and monitor resolution.
• Diagnostic performance
  • POCUS sensitivity for abscess: $0.90 ext{ to } 0.95$; specificity ranges with context but contributes to correct management in about 10% of cases.
• Abscess imaging characteristics
  • Abscess appears as an irregular, sometimes hypoechoic/anechoic collection with internal debris; posterior enhancement seen.

Necrotizing Fasciitis

• Importance of timely diagnosis
  • Delays worsen morbidity and mortality.
• POCUS role and performance
  • Sensitivity reported as $[0.85, 1.00]$ (85%–100%), specificity $[0.45, 0.98]$ (45%–98%).
• Sonographic signs suggestive of necrotizing fasciitis
  • Deep fascia thickening; diffuse fatty tissue thickening; fluid along deep fascia with thickness ≥ $4~ ext{mm}$; fascial fluid increases with each $1~ ext{mm}$ increment in diameter increasing specificity.
  • Gas-producing infection: hyperechoic small gas pockets with posterior “dirty shadowing.”
  • STAFF mnemonic for findings: Subcutaneous Thickening, Air, Fascial Fluid.
• Mimics and clinical context
  • Wounds, recent I&D procedures may introduce air and mimic necrotizing findings; use clinical correlation and consider CT or surgical consultation if suspicion remains.

Foreign Bodies

• Imaging role
  • Ultrasound is highly effective for soft tissue foreign bodies and often superior to X-ray, especially for radiolucent materials.
  • X-ray misses organic materials (wood) and some glass; metals are usually detected; radiopacity varies by material.
• Diagnostic performance
  • Ultrasound sensitivity: $[0.57, 0.83]$; specificity: $[0.88, 0.95]$; performance depends on material and location.
• Imaging strategy
  • Scan in two planes (orthogonal) to maximize detection in two dimensions.
  • Use Color Doppler to differentiate nearby vascular structures and inflammatory changes.
  • Superficial foreign bodies can be hard to detect due to probe limitations; widen focal length with water bath or standoff pad.
• Sonographic appearance
  • Foreign bodies: hyperechoic objects with varying echogenicity depending on material; internal debris if partially embedded; may show reverberation artifacts.
  • Metallic objects may show twinkling artifacts on Doppler.
• Halo sign
  • A halo sign (fluid pocket around the object) may appear if the foreign body has been present >24 hours.
• Common mimics
  • Sesamoid bones, scar tissue, soft tissue calcifications can mimic foreign bodies; location, appearance, and contralateral comparison help differentiation.
• Treatment guidance and technique
  • CT is not required if ultrasound clearly localizes the object and can guide removal.
  • When removing, ultrasound-guided technique can mark entry depth and guide incision, followed by open removal.
  • For removal, consider ultrasound-assisted approach with needle guidance to contact the foreign body at both ends, then perform a calibrated skin incision.
• Practical considerations
  • Wide-field sonography is helpful in gapping wounds; subcutaneous air from gaping wounds can obscure anatomy, so use copious gel or water bath.
• Common foreign body materials and imaging features (Table 18.1 reference)
  • Glass: hyperechoic with reverberation artifacts; possible posterior shadowing; sometimes radiodense on X-ray.
  • Organic material (wood, stinger): hyperechoic; reverberation often present; posterior shadowing variable.
  • Metal: hyperechoic; often with comet tail or edge shadowing; radiopaque on X-ray.
  • Medical devices (e.g., catheter, contraceptive implants): radiodense; variable reverberation.
  • Cosmetic filler: variable echogenicity; radiopaque depending on type; usually may lack a consistent shadowing pattern.
• Post-procedure and follow-up
  • After removal, re-scan to confirm absence of residual fragments and assess for residual inflammation.

Lymphadenopathy

• Lymphatic distribution and POCUS basics
  • Lymph nodes follow lymphatic drainage patterns and can be identified by POCUS.
• Lymph node categories (Table 18.2)
  • Normal: oval shape; central hyperechoic hilum; cortex hypoechoic; size usually < $1~ ext{cm}$ in most regions; inguinal nodes may be up to $2~ ext{cm}$.
  • Reactive: variable size with mild enlargement; central tree-like vascularity; hilum may be preserved but can be variably hyperechoic; may show loss of hilum; follow-up recommended to ensure resolution (e.g., 2–3 months post-vaccination reactive adenopathy).
  • Malignant: disorganized or peripheral vascularity (ring of fire); loss of hyperechoic hilum; enlarged; more round appearance.
• Interpretation cautions
  • Correlate with clinical history, infection risk, and imaging of contralateral nodes when possible.
• Figures (descriptive descriptions of nodal appearance)
  • Normal nodes: hyperechoic hilum, width greater than height, small size (<1 cm).
  • Malignant nodes: increased peripheral vascularity, loss of hilum, more round shape.

Lipoma

• Characteristics
  • Most common benign soft tissue tumor; composed of mature adipose tissue.
  • Clinically: palpable, generally painless; may be symptomatic if adjacent to vessels, muscles, or nerves.
  • Ultrasound: first-line modality due to high sensitivity (≈ 87 ext{ ext{%}}) and specificity (≈ 96 ext{ ext{%}}). The echogenicity is variable; most lipomas are isoechoic but can be hypoechoic or hyperechoic.
  • Size: typically < $5~ ext{cm}$ in diameter.
  • Ultrasound features: curved echogenic lines parallel to the skin; may have a thin fibrous pseudocapsule surrounding the lesion.
• Vascularity
  • Generally avascular; notable vascular flow on color Doppler should raise suspicion for atypical features.
• Imaging and management implications
  • Lipomas with atypical features or symptoms may require excision; most have low malignant potential.

Ganglion Cysts and Synovial Cysts

• Ganglion cysts (GCs)
  • Near joints or tendon sheaths; fluid-filled, hypoechoic superficial masses; may have internal septations.
  • Often connected to adjacent joint space.
  • Compressibility: limited compared with synovial cysts.
  • Location tendencies: GCs more common in hand and wrist; SCs more common in knee (e.g., Baker’s cyst).
  • Doppler: generally minimal or no flow.
• Synovial cysts (SCs)
  • Differ from GC by location and underlying joint/tendon pathology.
  • Treatment considerations differ between GC and SC; correct diagnosis guides management (e.g., aspiration, steroids for GC; treating underlying joint pathology for SC).

Sebaceous Cysts and Other Dermal Cysts

• Sebaceous cysts
  • Obstructed hair follicles; ultrasound often shows oval, hypoechoic, homogeneous lesions (pseudo-testis appearance).
  • May display a target or laminar pattern; a visible cutaneous pore may connect to the epidermis, which helps diagnosis.
• Pilar (Trichilemmal) Cysts
  • Common in women, often on scalp, then face/neck/arms/legs.
  • Ultrasound: hypoechoic with internal hyperechogenicity (calcifications or cholesterol crystals); typically no color flow; posterior acoustic enhancement.
• Epidermal Inclusion Cysts (EICs)
  • Typically round or oval with well-defined capsule; may resemble sebaceous cysts.
  • Some EICs have an onion-like pattern (layered echogenicity) and may show hyperemia if inflamed or infected.
• Infected or inflamed cysts
  • Hyperemia and internal particulate movement on pressure application; avoid rupturing intact capsule during I&D; consider staged removal when infection subsides.

Acne Vulgaris and Hidradenitis Suppurativa (HS)

• Ultrasound utility
  • Ultrasound detects disease burden not evident on physical exam, including pseuodocysts, nodules, calcinosis, and fistulae.
  • Wide-band/high- and ultrahigh-frequency probes expand dermatologic ultrasound applications.
• Severity scoring systems
  • SOS-Acne: grades acne severity as mild, moderate, or severe based on the number of pseudocysts and presence of fistulas.
  • Mild: < 5 pseudocysts without fistulas
  • Moderate: 5–9 pseudocysts without fistulas
  • Severe: > 10 pseudocysts with or without fistulas
  • SOS-HS (Hidradenitis Suppurativa): Stage 1–3b; staging criteria based on number of fluid collections, dermal changes, and presence/number of fistulous tracts.
  • Stage 1: Single fluid collection and dermal changes; one body area; no fistulous tracts
  • Stage 2: 2–4 fluid collections or a fistulous tract; dermal changes in one or two areas
  • Stage 3: ≥5 fluid collections, 2–4 noncommunicating tunnels, or <3 communicating tunnels; in advanced stages, more extensive disease
• Practical notes
  • Ultrasound helps in disease burden assessment and monitoring treatment response.

Nails and Nail Pathology

• Indications for nail ultrasound
  • Evaluation of undifferentiated nail pathology; nail bed injuries; subungual issues.
• Technique and equipment
  • Use a high-frequency linear probe of at least 15 MHz; apply ample gel, use a water bath or standoff pad for resolution due to superficial location.
  • Nail ultrasound protocol includes two-plane imaging of the nail plate and nail bed.
• Normal anatomy on ultrasound
  • Two nail plates (dorsal and ventral) appear as separated but parallel, smooth, linear, hyperechoic structures; minimal vascularity in the nail bed.
• Pathologies and ultrasound features
  • Nail bed injuries: high diagnostic utility (ED) with sensitivities around 93 ext{ ext{%}} for detecting nail bed injuries and 100 ext{ ext{%}} for distal phalanx fractures in certain studies.
  • Psoriasis of nails: often involves the nail bed with increased vascularity, thickened nail plate, and loss of the ventral plate; can mimic fungal infections on physical exam.
  • Onychomycosis: thickened nail plate with loss of ventral plate; ultrasound helps differentiate from psoriasis and monitor treatment.
  • Paronychia and distal digit infections
  • Blisters: simple, hypoechoic areas without internal echoes, vascularity, or mixed echogenicity.
  • Paronychia: increased vascularity and surrounding tissue thickening.
  • Felons (infections around the nail bed): similar to abscess with possible squish sign and increased vascularity.
  • Glomus tumors: typically well-circumscribed, hypoechoic cystic structures with varying degrees of vascularity.
• Clinical implications
  • Nail ultrasound can guide management and follow-up of inflammatory or infectious nail conditions.

Pearls and Pitfalls

• Imaging technique tips
  • For superficial structures, use copious gel, water bath, or standoff pad to improve resolution.
  • Use ultrasound to identify the abscess, surrounding vasculature, and the optimal I&D location.
  • Ultrasound has superior detection of foreign bodies compared with X-ray, particularly nonmetallic materials, which often appear as hyperechoic structures with reverberation artifacts.
  • Be aware artifacts can mimic pathology; image soft tissue in two planes.
  • When possible, obtain comparison views of the unaffected contralateral side.
• Necrotizing fasciitis caveat
  • Absence of sonographic findings does not rule out necrotizing fasciitis; if clinical suspicion remains, pursue surgical consultation.
• Malignancy suspicion
  • Masses that are taller than wide (i.e., cross-sectional height greater than width) should raise suspicion for malignancy.
• Practical workflow notes
  • Use wide-band/high-frequency probes for superficial lesions and plan imaging to map margins and depth before procedures.
  • Consider staged management for infected cysts to avoid rupturing a capsule during I&D.

Summary of Common Sonographic Findings (Pathology at a Glance)

• Cellulitis
  • Subcutaneous thickening; cobblestoning; thickened epidermis.
• Abscess
  • Irregular border; fluid/echogenic debris; posterior enhancement; no internal vascular flow; swirl sign with compression; possible gas with dirty shadowing.
• Necrotizing fasciitis
  • Deep fascia thickening; fascial fluid; subcutaneous thickening; gas possible; STAFF signs.
• Foreign body
  • Hyperechoic object with variable internal echoes; reverberation; possible halo; may show twinkling with Doppler; halo sign if longstanding.
• Lymph nodes
  • Normal: hyperechoic hilum, wider-than-tall shape, small size (<1 cm; inguinal up to 2 cm).
  • Reactive: mild enlargement; central/ tree-like vascularity; reduced hilum visualization.
  • Malignant: peripheral disorganized vascularity; hilum loss; round shape.
• Lipoma
  • Iso/hyperechoic to surrounding tissue; typically avascular; thin pseudocapsule; curved echogenic lines; usually <5 cm.
• Ganglion cyst / Synovial cyst
  • Anechoic or hypoechoic; limited compressibility; GC often near joints; SCs near knee; minimal to no flow.
• Sebaceous, Pilar, EICs
  • Sebaceous: oval, hypoechoic, homogeneous; may show a pore; pseudo-testis pattern.
  • Pilar: hypoechoic with internal hyperechogenicity; no flow; posterior enhancement.
  • EICs: well-circumscribed capsule; onion pattern sometimes; hyperemia when inflamed.
• Acne vulgaris / HS
  • Detect pseudocysts, fistulas, dermal changes; SOS-Acne and SOS-HS scoring schemes assist in staging and treatment planning.
• Nails
  • Nail bed injury detection; psoriasis shows increased vascularity and thickened plates; onychomycosis shows thickened plate and ventral plate loss; glomus tumors are hypoechoic and may be vascular.

Key Formulas and Quantitative References

• Lymph node size limits (demonstrative):
  • Normal nodes: typically < $1~\mathrm{cm}$ in diameter; inguinal nodes can be up to $2~\mathrm{cm}$.
• Absence/presence thresholds (diagnostic performance):
  • POCUS sensitivity for abscess: $0.90\leq \text{sensitivity} \leq 0.95$
  • POCUS-guided management changes: approximately $10\%$ of cases show a change in management.
• Deep fascial fluid threshold in necrotizing fasciitis: fascial fluid depth > $4~\text{mm}$ increases specificity per mm increments.
• Severity scoring references (SOS-Acne and SOS-HS):
  • SOS-Acne: Mild:
  • SOS-HS stages: Stage 1 (single fluid collection in one area, no fistula); Stage 2 (2–4 collections or a fistula in one or two areas); Stage 3 (≥5 collections or extensive fistulous networks).

Connections and Practical Implications

• Clinical integration
  • POCUS augments clinical examination for SSTIs, enabling rapid differentiation of cellulitis from abscess, guiding I&D planning, and reducing unnecessary procedures.
  • In suspected necrotizing fasciitis, ultrasound is a valuable adjunct but should not replace clinical judgment; negative ultrasound does not exclude the disease.
• Practical workflow
  • Use a systematic, two-plane approach with color Doppler as indicated.
  • Compare to contralateral structures when feasible to improve diagnostic confidence.
  • Apply sterile technique and appropriate infection control for wounds and open lesions when performing ultrasound-guided management.
• Ethical and practical considerations
  • Be mindful of artifact and operator dependence; correlate with clinical context.
  • Recognize limitations of ultrasound in differentiating certain conditions, and refer for advanced imaging or surgical consultation when indicated.

References and Context (abbreviated)

• Foundational POCUS principles for SSTIs, including diagnostic accuracy, management impact, and safety considerations.
• Specific condition-focused findings for cellulitis, abscess, necrotizing fasciitis, foreign bodies, lymph nodes, lipomas, cysts, and nail pathology.
• SOS-Acne and SOS-HS scoring systems for dermatologic disease burden assessment and treatment planning.
• Practical imaging aids (standoff pads, water bath, two-plane technique) to optimize superficial SSTU imaging.