High-resolution ultrasound in alopecia: 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 nonscarring and scarring alopecia, expanding ultrasound's role in clinical evaluation. This narrative review summarizes HRUD principles, sonoanatomy of the hair, scanning techniques, indications, and representative imaging findings in alopecia. PubMed and Scopus were searched to identify publications describing HRUD applications in alopecia, and ultrasonographic images and illustrative cases from the authors' collections were included to demonstrate key findings. Across alopecias, HRUD shows 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 alopecia, 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 on alopecia and may complement clinical examination and trichoscopy. Standardizing imaging protocols and conducting further clinical studies may help define its role in diagnosis, monitoring, and research.