Primary scarring alopecias (PSAs)—including lichen planopilaris (LPP), frontal fibrosing alopecia (FFA), discoid lupus erythematosus (DLE), folliculitis decalvans (FD), and dissecting cellulitis (DC)—lead to irreversible hair loss through inflammatory destruction of the follicular epithelium. While scalp biopsy remains the diagnostic gold standard, it is invasive, may lead to additional scarring, evaluates only a limited focal area, and may miss active disease (1). High-frequency ultrasound (HFUS) and ultra-high-frequency ultrasound (UHFUS) have emerged as valuable non-invasive imaging techniques capable of visualizing the entire hair follicle, perifollicular structures, and the dermo-hypodermal junction.
Ultrasound is a widely used non-invasive imaging modality that employs high-frequency sound waves to generate real-time images of internal body structures. In trichology, HFUS operates at 15–30 MHz, achieving a penetration depth of approximately 10 mm and enabling assessment of the superficial hypodermis, dermis, and superficial structures such as hair follicles. UHFUS, which operates at 50–70 MHz, provides superior axial resolution (<30 μm), allowing highly detailed visualization of hair follicles and perifollicular tissue (2,3). Unlike trichoscopy, which is limited to surface evaluation, ultrasound enables assessment of the entire follicle, surrounding tissue, and deeper structures, thereby facilitating detection of key morphological changes in PSAs (2,3). These capabilities make ultrasound a powerful tool for gaining comprehensive insights into disease activity and progression.
Understanding the normal sonographic appearance of hair follicles and hair shafts provides the foundation for identifying abnormalities in PSAs. Table 1 summarizes the sonographic findings described in three recent studies.
Ultrasound offers several advantages that make it particularly useful for assessing PSAs. One of its major clinical applications is monitoring disease activity. Ultrasound allows evaluation of the full extent of active inflammation rather than a single focal area. Color Doppler imaging can reveal changes in perifollicular vascularity that correlate with inflammatory activity, helping clinicians distinguish active from inactive disease. This distinction is essential for guiding therapeutic decisions and preventing overtreatment in patients with inactive disease (1).
Although histopathology remains the definitive diagnostic method, ultrasound is increasingly recognized as a valuable complementary tool for evaluating PSAs. It provides a non-invasive, reproducible, and dynamic process for assessing disease progression and activity. However, ultrasound also has limitations: it is highly operator-dependent, requiring training and experience to ensure consistent image acquisition and interpretation. Furthermore, certain sonographic features may overlap across different types of PSAs, limiting diagnostic specificity and reinforcing the need for histopathological confirmation in uncertain cases.
Valentina Ross Massanes, MS – Pontificia Universidad Católica de Chile
Reviewed by Jorge Larrondo, MD, MSc – Clínica Alemana de Santiago
Table 1: US findings in different PSAs
|
Argalia et al |
Ortiz-lazo et al |
Rodríguez‐Cuadrado et al |
|
|
LPP |
- Active LPP: Cigar-like shaped follicular structures, perifollicular hypoechogenicity in the mid-dermis and distal ambiguity of hair follicles - Inactive LPP: irregular and saw-like dermal-subepidermal boundary. |
- Active LPP: Cigar-like shaped follicular structures - Inactive LPP: Decrease in follicular structures with saw-like dermal or subdermal border. |
Irregular follicular widening (75%) and increased vascularization on doppler (87.5%), together with the absence of follicles in the cicatricial areas. |
|
FFA |
- Active FFA: Cigar-like shaped follicular structures, perifollicular hypoechogenicity in the mid-dermis and distal ambiguity of hair follicles - Inactive FFA: irregular and saw-like dermal-subepidermal boundary. |
- Active FFA: Cigar-like shaped follicular structures - Inactive FFA: Unique follicular structures, saw-like dermal or subdermal border. |
Irregular thickening of the follicles (83.3%), variable increase in vascularization on doppler (75%) and elongated hyperechoic structures extending vertically from dermis to hypodermis with sawtooth appearance (41.7%). |
|
DLE |
Active: Widened follicular structures with hypoechogenic bands. |
- Active DLE: Focal duplication of input echoes, widened follicular structures, unstructured hypoechoic bands in the dermis - Inactive DLE: Decrease in follicular structures with no specific pattern, blurred dermal or subdermal border. |
Irregular follicular widening (83.3%) and increased vascularization on Doppler (83.3%). Diffuse dermal hypoechogenicity. Presence of foci of subtle epidermal thickening (50%), in relation to keratotic plugs. |
|
FD |
-Active: Fusion of hair follicles, decreased dermal echogenicity, and increased subcutaneous echogenicity. |
(-) |
Moderate to severe follicular widening (100%), fusion of follicles (75%), large increase in vascularization in the dermis and even hypodermis (87.5%). |
|
CD |
Echogenicity with a clear border, dilation of hair follicles, anechoic or hypoechoic pseudocysts, fluid collections, tunnels. |
Dilated hair follicles, pseudocysts (<1 cm), hypoechoic collections (≥1 cm), tunnels running through the dermis and subcutaneous tissue. Color doppler with varying degrees of dermohypodermic hypervascularization at the periphery of the lesions. |
Pseudocysts (83.3%), fluid collections (33.3%) and fistulas (83.3%) with a significant intralesional vascularization on Doppler. |
References
(1) Rodríguez-Cuadrado FJ, Martínez-Mera C, Roustan-Gullón G, Alfageme-Roldán F. Trichosonography: ultrasound of alopecias and hair disorders. J Ultrasound. 2025;28:137–50. doi:10.1007/s40477-024-00966-w.
(2) Argalia G, Reginelli A, Molinelli E, Russo A, Michelucci A, Sechi A, et al. High-frequency and ultra-high-frequency ultrasound in dermatologic diseases and aesthetic medicine. Medicina. 2025;61(2):220. doi:10.3390/medicina61020220.
(3) Ortiz-Lazo E, Velasco-Tamariz V, Saceda-Corralo D, Wortsman X. Técnicas de imagen en tricología: ecografía, microscopia confocal de reflectancia y tomografía de coherencia óptica. Actas Dermosifiliogr. 2025;116(9):755–63. doi:10.1016/j.ad.2024.12.020.
(4) Mikiel D, Polańska A, Żaba R, Adamski Z, Dańczak-Pazdrowska A. High-frequency ultrasonography of the scalp - Comparison of ultrasound and trichoscopic images in healthy individuals. Skin Res Technol. 2021;27(2):201-207. doi:10.1111/srt.12930