Imaging Characteristics and Measurement Parameters of Axillary Apocrine Glands in Osmidrosis Using 33 MHz Ultra-High-Frequency Ultrasound

Objective: To evaluate the application value of 33 MHz ultra-high-frequency ultrasound (UHFUS) in imaging axillary apocrine glands in patients with axillary osmidrosis and to systematically measure their anatomical parameters. Methods: This retrospective single-center study included 30 patients with axillary osmidrosis (14 males, 16 females; mean age 19.3 ± 6.8 years). A Mindray Resona AR ultrasound system with a 33 MHz linear-array transducer (L33-8U) was used. Using the axillary skin crease as the midline, each axilla was divided into a 3× 3 grid. Continuous scanning was performed in each region to count all sonographically identifiable glands, and the largest gland in each axilla was selected for size measurement. Long diameter (LD), anteroposterior diameter (APD), minimum depth from the skin surface to the shallowest gland margin (MinD), and maximum depth to the deepest gland margin (MaxD) were recorded. Area was calculated as π/4 × LD × APD. Paired t-tests were used for bilateral comparison, and Pearson correlation was used to assess the relationship between glandular area and vertical distribution range. Results: Apocrine glands appeared as elliptical or oval hypoechoic nodules at the dermal–subcutaneous fat junction, with loose internal echogenicity exhibiting a "honeycomb” or "spongiform” pattern. Sixty axillae were evaluated, and 1543 sonographically identifiable glands were detected (mean 25.7 ± 9.0 glands per axilla; range 12– 55). All measured glands exhibited a honeycomb or spongiform internal echogenicity. Mean LD, APD, and area of the largest gland were 2.90 ± 0.55 mm, 1.32 ± 0.32 mm, and 3.05 ± 1.05 mm 2 , respectively. Mean MinD and MaxD were 0.76 ± 0.14 mm and 2.62 ± 0.52 mm, with a vertical distribution range of 1.86 ± 0.49 mm. Size parameters showed no significant bilateral differences, whereas depth parameters showed mild left–right differences. Microvascular signals were observed within the larger glands in some cases. Glandular area correlated moderately with vertical distribution range (r = 0.52, P < 0.01). Conclusion: 33 MHz UHFUS can non-invasively visualize the morphology and distribution of axillary apocrine glands, with sonographic features consistent with previous reports. This study reports the depth range and size parameters of axillary apocrine glands, providing quantitative anatomical data for clinical injection therapy that may inform the selection of injection depth. However, no ultrasound-guided injections were performed and there were no efficacy or safety endpoints; the clinical value of these data requires prospective validation. Keywords: axillary osmidrosis, apocrine glands, ultra-high-frequency ultrasound, 33 MHz, ultrasound measurement