Introduction to Ultrasound in Dermatology

Ultrasound (US) was first introduced in clinical dermatology around 1980 when two independent groups used US to measure skin thickness in normal and diseased skin (1). This non-invasive, non-expensive and easy-to-interpret tool allows physicians to assess skin findings in real-time. While frequencies of 7.5 MHz have a penetration depth of > 40 mm, allowing the visualization of deeper structures, frequencies above 10 MHz enable better spatial resolution imaging of superficial structures (Table1). (2)

According to the DERMUS (Dermatologic Ultrasound) group guidelines, the optimal frequencies for assessing dermatological conditions range from 15 to 22 MHz. Variable-frequency ultrasound can define lesions in the submillimeter range (down to 0.1 mm) and reach depths of 60 mm. However, it has limited sensitivity for detecting epidermal or extremely thin (<0.1 mm) lesions, such as pigment deposits.

Table 1 - Visualized structures according to sonographic frequency (MHz) and penetration depth (mm). (2)

* High frequency US.

Ultrasound Imaging of Skin Layers

• Epidermis: Generally seen as a thin hyperechoic line (except in the palmar and plantar areas where it is thicker and bilaminar).

• Dermis: Seen as a hyperechoic band of variable thickness, being predominantly thin in the forearm and thick in the lumbar region due to high collagen content.

• Subcutaneous Tissue: Appears hypoechoic due to fat lobules surrounded by hyperechoic fibrous septa. Subcutaneous blood vessels appear as thin ducts (venous or arterial) with low resistance and distinct peak systolic flow velocity.

Eyal Taleb, Ari Safir, Tel Aviv Sourasky Medical Center.

The main applications of US in dermatology include benign and malignant skin tumors, vascular anomalies, cosmetic fillers, nail disorders, and inflammatory diseases (Hidradenitis suppurative, morphea, abscesses). A key limitation is that ultrasound is operator-dependent, requiring extensive training and expertise.

In Hidradenitis suppurativa (HS) US aids in accurately assessing the severity and staging of HS and it can aid in surgical management. One study found that ultrasonographic evaluation led to a modification in the management of 82% of cases, with 24% transitioning from medical to surgical treatment. (5)

Rule of Ultrasound in skin tumors

US complementing clinical examination and dermoscopy. It plays a significant role in the initial differential diagnosis, surgical planning, locoregional staging, and follow-up of skin malignancies. Currently, US is underutilized by dermatologists and surgeons for assessing skin tumors. Increased efforts are needed to encourage the broader adoption of US in this field. Although most skin tumors can be treated without additional imaging studies, large or aggressive high-risk cancers, or those involving critical anatomical structures, require additional information to optimize management. It does not involve the use of ionizing radiation and can thus be conducted during a prolonged period.

A retrospective study of 4,338 skin ultrasound examinations, primarily involving localized skin lesions, found that referring diagnoses were correct in 73% of cases. The addition of ultrasound improved diagnostic accuracy to 97%. (3) This demonstrates that ultrasound is a reliable adjunct for the accurate and precise diagnosis of skin lesions.

A study of 83 BCCs showed ultrasound had a sensitivity of 96%, specificity of 84%, and accuracy of 91% in determining tumor depth (4). In SCC US is indicated in tumors larger than 2 cm in diameter or deeper than 2 mm, Poorly differentiated histology, Presence of perineural or lymphovascular invasion, and tumors located in specific anatomic sites (e.g., ears, lips, anogenital regions). In melanoma US can identify satellite and in-transit metastases both preoperatively and during postoperative follow-up, helping confirm stage III disease and guiding treatment decisions.

Tumor Assessment and loco-regional Staging: Provides rapid and precise measurement of tumor diameters and thickness, evaluates the extent of tumor involvement in deeper tissues, and detects non-palpable, satellite lesions.

Pre-Surgical Planning: Detects infiltration of surrounding anatomical structures (e.g., fascia, muscles, cartilage), and relevant neurovascular structures. In addition, Preoperative sonography of lymph node basins in high-risk skin cancers can detect subclinical involvement.

Post surgical planning: early detection of local or nodal recurrences through regular assessments of surgical scars and regional lymph nodes.

monitoring non-invasive therapies in skin cancer patients.

Nail Unit Assessment:

• Provides high specificity and sensitivity for diagnosing malignant and benign nail lesions.

• Helps in optimizing the surgical approach and discussing potential complications related to nail matrix damage.

References:

(1). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8058126/

(2). https://onlinelibrary.wiley.com/doi/10.1111/ddg.14781#:~:text=While%20frequencies%20of%207.5%20MHz,superficial%20blood%20vessels%2C%20and%20other

(3). https://pubmed.ncbi.nlm.nih.gov/19962214/

(4). https://pubmed.ncbi.nlm.nih.gov/33404160/

(5).https://pubmed.ncbi.nlm.nih.gov/30519375/