USG in Dermatology

Definition

• High-frequency ultrasound (HFUS): ≥15 MHz
• Ultra-high-frequency ultrasound (UHFUS): approximately 30-70 MHz
• Higher frequency = better resolution but lower penetration.

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Principle

Modes

• B-mode / gray-scale USG: morphology, size, depth, margins, internal architecture.
• Color Doppler / power Doppler: assesses presence and pattern of vascularity.
• Spectral Doppler: quantifies flow velocity and resistance index when required.
• Elastography: assesses tissue stiffness, used mainly as an adjunct in tumors and fibrosis.

Important rule

• Frequency ↑ → resolution ↑, penetration ↓
• Frequency ↓ → penetration ↑, resolution ↓

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Technique

• Use a high-frequency linear probe, preferably ≥15 MHz.
• Generous gel, minimal pressure to avoid compressing superficial lesions and blood flow.
• Examine lesion in two perpendicular planes.
• Record:
  • Site and dimensions: length × width × depth
  • Layer of origin
  • Shape and margins
  • Echogenicity and homogeneity
  • Posterior acoustic features
  • Vascularity on color/power Doppler
  • Relation to fascia, muscle, vessels, nerves, and bone
  • Regional lymph nodes where indicated
• Compare with contralateral normal skin if needed.

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Normal USG Anatomy of Skin

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Doppler in Dermatology

• Active inflammation
• Infection
• Neoangiogenesis in tumors
• Hypervascular scars
• Vascular malformations

• Active vs inactive inflammatory disease
• Cyst/abscess vs solid vascular lesion
• Benign vs suspicious tumor, as an adjunct only
• Viable tumor vs scar/fibrosis during follow-up

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Clinical Applications

1. Inflammatory Dermatoses

Psoriasis

• Dermal thickening
• Reduced echogenicity of upper dermis: subepidermal low-echogenic band (SLEB)
• Increased Doppler flow in active plaques
• Useful for assessing severity and response to treatment

Atopic dermatitis

• Increased skin thickness
• SLEB due to edema/inflammation
• Increased vascularity in active lesions
• May help assess subclinical inflammation and treatment response.

Lichen planus, lupus erythematosus, dermatomyositis

• Variable dermal edema, altered echogenicity, thickening/atrophy, and increased vascularity in active disease.
• Useful for objective monitoring, especially in autoimmune connective-tissue disease. Autoimmune skin disease review

Localized scleroderma / systemic sclerosis

• Inflammatory phase: dermal thickening, reduced echogenicity, increased Doppler flow.
• Sclerotic phase: increased dermal thickness and echogenicity.
• Atrophic phase: thinning of dermis/subcutis with reduced vascularity.
• Useful for activity assessment, staging, and serial follow-up.

Panniculitis

• Determines whether inflammation is:
  • Predominantly septal or lobular
  • Confined to subcutis or extending to fascia/muscle
• Helps guide biopsy from an active representative lesion.

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2. Skin and Soft-Tissue Infections

Cellulitis

• Dermal and subcutaneous thickening
• Increased echogenicity of fat lobules
• Edema produces a cobblestone appearance
• Increased Doppler vascularity

Abscess

• Hypoechoic/anechoic fluid collection
• Internal debris/septations may be present
• Peripheral hyperemia on Doppler with absent central flow
• Helps distinguish abscess from cellulitis and guides drainage.

Hidradenitis suppurativa

• Dilated hair follicles
• Pseudocysts/abscesses
• Fluid collections
• Dermal tunnels/sinus tracts
• Increased vascularity
• Assists staging, choice of medical versus surgical therapy, and monitoring.

Scabies

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3. Tumors and Tumor-like Lesions

General role

• Tumor thickness and depth
• Margins and local extension
• Cystic versus solid nature
• Vascularity
• Involvement of fascia, muscle, cartilage, or bone
• Satellite/in-transit metastases
• Regional lymph nodes
• Preoperative planning and follow-up.

Melanoma

• Usually hypoechoic, often spindle/fusiform dermal lesion
• Doppler vascularity may be increased
• Can estimate tumor thickness, detect satellites/in-transit metastases, and evaluate regional nodes.
• Histologic Breslow thickness remains the reference standard; USG is complementary.

Basal cell carcinoma

• Hypoechoic lesion in dermis
• Well-defined or irregular margins
• Hyperechoic foci may correspond to calcification, keratin, or tumor nests
• Doppler vascularity may be present
• Useful for estimating depth before surgery/Mohs surgery.

Squamous cell carcinoma

• Hypoechoic, heterogeneous lesion
• Irregular/infiltrative margins
• Increased Doppler flow
• Useful for local extent and nodal examination.

Dermatofibroma

• Usually a well-defined hypoechoic dermal lesion, sometimes extending into subcutis.
• USG helps distinguish it from cysts and assess atypical or deep lesions.

Dermatofibrosarcoma protuberans

• Hypoechoic dermal-subcutaneous infiltrative mass
• Often hypervascular
• Defines deep extension before surgery and helps detect recurrence.

Epidermoid cyst

• Well-defined oval dermal/subcutaneous lesion
• Hypoechoic or heterogeneous with internal echoes
• Posterior acoustic enhancement
• Usually absent internal Doppler flow, unless inflamed.

Lipoma

• Compressible, well-circumscribed lesion in subcutis
• Variable echogenicity, commonly echogenic with linear striations
• Minimal vascularity.

Pilomatricoma

• Well-defined heterogeneous hypoechoic lesion
• Internal hyperechoic foci due to calcification
• Posterior acoustic shadowing.

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4. Vascular Lesions

• Depth and extent
• Flow characteristics
• Arterial versus venous component
• Thrombosis or phleboliths
• Relation to deeper structures
• Response to laser, sclerotherapy, or systemic treatment.

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5. Hair Disorders

• Alopecia areata: changes around follicles and perifollicular vascularity.
• Androgenetic alopecia: reduced follicular diameter and altered follicular density.
• Cicatricial alopecia: loss/destruction of follicular structures, fibrosis, and activity-related Doppler changes.
• Guides biopsy to an active edge in cicatricial alopecia.

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6. Nail Disorders

• Nail plate thickness and integrity
• Nail matrix and nail bed
• Distal interphalangeal joint
• Vascularity of nail bed/matrix

Uses

• Nail psoriasis: thickened nail bed, increased Doppler flow, enthesopathy may coexist.
• Onychomycosis: altered nail-plate morphology and subungual hyperkeratosis.
• Glomus tumor: small hypoechoic, hypervascular subungual mass.
• Subungual exostosis: bony continuity with distal phalanx.
• Onychomatricoma / other nail tumors: defines site and extent before surgery.

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7. Aesthetic Dermatology and Procedures

• Measurement of skin thickness in photoaging.
• Assessment of dermal collagen and edema.
• Identification, location, depth, and volume of fillers.
• Detection of filler complications:
  • Nodules/granulomas
  • Abscess
  • Migration
  • Fat necrosis
• USG-guided hyaluronidase injection or filler removal in selected cases.
• Mapping vessels before high-risk filler procedures can improve procedural safety.
• Monitoring scars, keloids, and response to intralesional therapy.

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8. Wounds, Ulcers, and Scars

• Measures wound depth and volume.
• Detects fluid collections, abscess, sinus tracts, foreign body, and undermining.
• Assesses perfusion with Doppler.
• Distinguishes active inflammatory scar from mature fibrotic scar.
• Useful in pressure ulcers, diabetic foot lesions, and chronic wounds.

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9. Foreign Bodies

• Wood
• Thorns
• Plastic
• Glass fragments
• Retained sutures

• Echogenic focus
• Posterior acoustic shadowing or reverberation
• Surrounding hypoechoic inflammatory halo
• Guides removal.

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10. Lymph-node Assessment

• Melanoma
• Cutaneous SCC
• Merkel cell carcinoma
• Cutaneous lymphoma
• Suspicious chronic infection/inflammatory disease

Suspicious nodal features

• Round shape
• Cortical thickening/asymmetry
• Loss of echogenic fatty hilum
• Peripheral or chaotic vascularity
• Necrosis/cystic change

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Advantages

• Non-invasive and painless
• No ionizing radiation
• Real-time and repeatable
• Dynamic examination and compression possible
• High spatial resolution for superficial structures
• Doppler permits vascular assessment
• Helps select biopsy site and plan surgery
• Useful for serial monitoring
• Less expensive and more accessible than CT/MRI for superficial lesions.

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Limitations

• Operator- and equipment-dependent.
• Limited penetration with high-frequency probes.
• Deep lesions may require lower-frequency USG, MRI, or CT.
• Air, thick keratin, crust, calcification, and bone can cause artifacts/shadowing.
• Cannot reliably characterize all tumors as benign or malignant.
• Poor for very small/intraepidermal macular lesions, for example lentigines and café-au-lait macules.
• Does not substitute for histopathology.

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High-Yield Exam Points

1. HFUS in dermatology = ≥15 MHz.
2. It shows epidermis, dermis, subcutis, appendages, vessels, and superficial nodes.
3. Epidermis and dermis are hyperechoic; subcutis is hypoechoic.
4. Frequency and penetration are inversely related.
5. Color Doppler indicates inflammatory activity and tumor vascularity.
6. Major uses: inflammatory dermatoses, HS, skin tumors, melanoma staging adjunct, vascular lesions, nail/hair disease, fillers, abscesses, foreign bodies, and lymph nodes.
7. USG is excellent for depth, extent, vascularity, and follow-up, but biopsy remains the diagnostic gold standard for many tumors and dermatoses.