High-Resolution Ultrasound in Alopecias: Principles, Sonoanatomy, and Imaging Patterns.

High-resolution ultrasound with Doppler (HRUD) is an emerging imaging modality that enables noninvasive visualization of hair follicles, perifollicular structures, and vascular changes. Recent studies have identified characteristic ultrasonographic findings in both non-scarring and scarring alopecias, expanding the role of ultrasound in their clinical evaluation. This narrative review summarizes HRUD principles, sonoanatomy of the hair, scanning techniques, indications, and representative imaging findings in alopecias. PubMed and Scopus were searched to identify publications describing HRUD applications in alopecias, and ultrasonographic images and illustrative cases from the authors' collections were included to demonstrate key findings. Across alopecias, HRUD demonstrates recognizable imaging patterns that may support diagnosis and clinical assessment. Reported findings include follicular miniaturization in pattern alopecia, drop-like follicular widening in alopecia areata, perifollicular inflammatory changes and increased vascularization in scarring alopecias, follicular fusion in folliculitis decalvans, and fistulous tracts in dissecting cellulitis of the scalp. HRUD may also facilitate assessment of disease activity, treatment response, and clinicopathologic correlations. In conclusion, HRUD provides clinically useful structural and vascular information in alopecias and may complement clinical examination and trichoscopy. Standardization of imaging protocols and further clinical studies may help define its role in diagnosis, monitoring, and research.