USG in Dermatology

Introduction

Principle

• Ultrasound waves are emitted by a piezoelectric transducer.
• Echoes reflected from tissues of differing acoustic impedance are converted into a grayscale image.
• Higher frequency gives better resolution but lower tissue penetration.
• B-mode provides structural imaging.
• Colour/power Doppler demonstrates vascularity, helping assess inflammatory activity and tumour blood flow.

Frequency and penetration

Normal sonographic appearance of skin

• Epidermis: thin, echogenic entry echo.
• Dermis: relatively echogenic band due to collagen.
• Subcutaneous fat: hypoechoic lobules separated by echogenic fibrous septa.
• Fascia: bright echogenic linear structure.
• In photoaged skin, a subepidermal low-echogenic band (SLEB) may be seen.

Technique

1. Use a high-frequency linear transducer with abundant gel.
2. Examine lesion in two perpendicular planes.
3. Avoid excess transducer pressure, as it may compress vessels and alter lesion dimensions.
4. Assess:
   • Site and depth
   • Size in three dimensions
   • Margins and echogenicity
   • Relation to dermis, subcutis, fascia, muscle, or bone
   • Internal vascularity using colour/power Doppler
   • Regional lymph nodes when indicated
5. Store images for serial comparison.

Indications / Applications

1. Skin tumours

• Defines tumour thickness, depth, margins, and invasion into subcutis or deeper structures.
• Assists in preoperative planning and selection of biopsy site.
• Helps evaluate regional lymph nodes.
• Useful in:
  • Melanoma
  • Basal cell carcinoma
  • Squamous cell carcinoma
  • Dermatofibrosarcoma protuberans (DFSP)
  • Cutaneous metastases
  • Cysts and benign soft-tissue tumours

2. Inflammatory dermatoses

• Psoriasis: dermal thickening, hypoechoic dermis, increased Doppler flow in active plaques.
• Atopic dermatitis: increased skin thickness and SLEB; useful for monitoring.
• Morphea/systemic sclerosis: assesses dermal thickness and distinguishes active inflammatory disease from late fibrosis.
• Lupus panniculitis and other panniculitides: evaluates depth and activity of inflammation.
• Hidradenitis suppurativa: detects subclinical nodules, abscesses, fluid collections, and sinus tracts; useful for staging and treatment planning.
• Cellulitis/abscess: differentiates diffuse inflammatory oedema from a drainable fluid collection.

3. Vascular lesions

• Differentiates haemangioma, vascular malformation, and nonvascular masses.
• Doppler assesses flow pattern, velocity, and vascularity.
• Useful in planning treatment of haemangiomas and vascular malformations.

4. Hair and nail disorders

• Assesses scalp thickness, follicles, perifollicular inflammation, and subcutaneous lesions.
• Useful in evaluating nail-unit tumours, subungual exostosis, glomus tumour, cysts, and inflammatory nail disease.
• Can detect nail-plate changes and increased vascularity in nail psoriasis.

5. Cosmetic and procedural dermatology

• Determines depth and position of dermal fillers.
• Detects filler-related nodules, granulomas, or vascular complications.
• Assists in localization of foreign material and guides dissolution or intervention.
• Evaluates skin thickness and subcutaneous fat in body-contouring procedures.

6. Procedure guidance

• Guides aspiration/drainage of abscesses or cysts.
• Helps choose an appropriate biopsy site in deep lesions.
• Guides intralesional injections and localisation of foreign bodies.

Advantages

• Non-invasive, painless, and radiation-free.
• Portable and relatively inexpensive.
• Real-time dynamic assessment.
• Can be repeated for monitoring.
• Shows lesion dimensions and depth before surgery.
• Colour Doppler assesses vascularity and inflammatory activity.
• Useful adjunct to clinical examination, dermoscopy, biopsy, and histopathology.

Limitations

• Strongly operator-dependent.
• High-frequency probes have limited penetration, hence are less useful for deep lesions.
• Sonographic features may be nonspecific.
• Cannot replace histopathology for definitive diagnosis of most tumours.
• Air, thick crust, ulceration, calcification, and irregular surfaces can produce artefacts.
• Limited standardisation of terminology, protocols, and diagnostic cut-offs.

Conclusion