The Application of Ultrasound in the Diagnosis and Treatment of Hair Disorders

1. Introduction

Hair disorders are common conditions in dermatology, primarily including non-scarring hair loss such as androgenetic alopecia (AGA) and alopecia areata (AA), as well as scarring hair loss such as pilomatricoma (PM) and trichilemmal cysts (TCs). With the accelerating pace of life, increasing psychological stress, metabolic disorders, and environmental factors, the incidence of hair disorders has steadily increased, severely affecting patients’ quality of life [1]. There is a growing clinical need for highly effective, non-invasive, and reproducible diagnostic and therapeutic approaches.

The primary auxiliary examinations and laboratory tests used for the clinical diagnosis of hair disorders include the hair pull test, dermatoscopy, and histopathological examination. The hair pull test is simple to perform, but its results are influenced by the examiner and the patient’s recent hair loss status, resulting in a high false-positive rate [2]. Dermoscopy can only examine the scalp surface and the appearance of the hair shaft, and cannot reveal deep pathological changes within the subcutaneous hair follicles; histopathology is an invasive procedure that is prone to scarring, making it difficult to conduct dynamic follow-up assessments of treatment efficacy before and after therapy on the same skin lesion. Therefore, the search for a technology capable of non-invasively, in real-time, and dynamically assessing hair follicle structure and function has become a research focus.

Ultrasound, as a mature medical imaging technology, has been widely used in the diagnosis of diseases affecting superficial organs. High-frequency ultrasound (HFUS), operating at frequencies above 15 MHz, can clearly resolve the anatomical relationships within the scalp layers, enabling precise, non-invasive diagnosis at the hair follicle level. Focused ultrasound (FU), on the other hand, utilizes the thermal, cavitation, and mechanical effects of ultrasound waves to achieve precise ablation of lesions and targeted drug delivery, providing new technical approaches for the precise treatment of hair disorders.

This article aims to review the latest advances in ultrasound technology for the diagnosis and treatment of hair disorders, analyze its clinical value, current limitations, and future directions, and provide a reference for the standardized application of ultrasound in hair medicine.

2. Principles and Technical Characteristics of Ultrasound Diagnosis and Treatment

Ultrasound is a mechanical wave with an extremely short wavelength, typically less than 2 cm in air [3], and is widely used in industries such as manufacturing, medicine, and the military. By scanning the human body, ultrasound can create images of internal tissues using electronic data and computer image processing techniques; alternatively, by utilizing the direct effects of sound waves of different frequencies on tissues, it can serve as a basis for diagnosis and an effective treatment method [4]. Compared to other imaging technologies such as CT, MRI, and nuclear medicine, ultrasound imaging offers advantages such as real-time capability, non-invasiveness, painlessness, the absence of ionizing radiation, and low cost [5].

2.1. Principles of Ultrasound Diagnosis

HFUS refers to ultrasound probes with frequencies higher than 15 MHz. It clearly distinguishes the layers of the scalp—including the epidermis, dermis, hair follicle units, subcutaneous fat, and galea aponeurotica—and accurately displays the morphology, size, arrangement, and relationships with surrounding tissues of hair follicles. It can also be combined with color Doppler to assess blood flow perfusion in hair follicles and lesions, enabling precise diagnosis at the hair follicle level. Ren Weiwei et al. [6] established and validated a predictive model based on the clinical and ultrasound characteristics of trichilemmal cysts and epidermoid cysts (ECs). They evaluated 164 cysts in the experimental group and 69 cysts in the confirmed group, which had been histopathologically diagnosed as TCs or ECs. The model effectively distinguished between TCs and ECs, providing objective imaging evidence for clinical differential diagnosis.

2.2. Principles of Ultrasound Therapy

FU is an important clinical application of the thermal effects of ultrasound. The sound intensity of diagnostic ultrasound typically does not exceed 0.72 W/cm² [7], whereas that of focused ultrasound can reach 100–10,000 W/cm² [8]. Multiple ultrasound beams can be focused at a focal point with a diameter of 1 mm and a depth of 10 mm. The thermal effect of the ultrasound raises the temperature at the focal point to over 60 °C, causing coagulative necrosis of the tissue at the lesion site. This enables targeted ablation and minimally invasive treatment that spares surrounding healthy tissue [9]. Currently, focused ultrasound is widely used in ophthalmology [10], gynecology [11], and the treatment of gastrointestinal diseases [12], among other fields. Clinicians can select different frequencies based on the depth of the lesion; for example, high-intensity focused ultrasound (HIFU) at 8 MHz is often used for superficial urethral diseases, while low-intensity focused ultrasound (LIFU) at frequencies below 500 kHz is typically used for diseases located deeper within the body [13]. In the field of hair regrowth, low-intensity focused ultrasound can be used to stimulate hair follicle stem cells, complementing the ablative effects of high-intensity focused ultrasound.

3. The Application of Ultrasound in the Diagnosis of Hair Disorders

High-frequency ultrasound, which utilizes differences in intertissue impedance—similar to Doppler blood flow imaging—can provide information on deep scalp anatomy and pathology [14], clearly displaying hair follicle structure, the extent of inflammation, and changes in blood flow [15]. It not only avoids the trauma of biopsy but also allows for repeated testing and dynamic assessment of treatment efficacy, and has become an important adjunctive diagnostic tool for hair disorders.

Under ultrasound, normal scalp skin exhibits the following characteristics: the skin on the forehead is thinner than that on the occipital region; there is a distinct separation between the epidermis and the hyperechoic line of the dermis; the subcutaneous tissue appears as a hypoechoic band; and there are hyperechoic septa between the fat lobules. A deep, thin hypoechoic band corresponds to the galea aponeurotica, while linear hyperechoic structures represent the bony margins of the skull. Scalp inflammation manifests as follicular swelling, abnormal perifollicular echogenicity, and localized hypoechoic changes; color Doppler imaging reveals an abnormal increase in blood flow signals in the affected area.

3.1. Androgenetic Alopecia

Androgenetic alopecia, also known as seborrheic alopecia or early-onset baldness, is primarily characterized by progressive thinning and reduction of hair [16]. Its histopathological features include follicular miniaturization, an increased proportion of vellus hair, and significant heterogeneity in hair shaft diameter [17]; in cases with a prolonged course, dermal fibrosis and residual fibrous bundles may be observed [18].

The high-frequency ultrasound features of androgenetic alopecia primarily include follicular miniaturization, decreased hair follicle density, enlarged sebaceous glands, and hyperechoic changes around the hair follicles. High-frequency ultrasound can distinguish the length and morphology of hair follicles. In patients with androgenetic alopecia, hair follicles in the frontal and occipital regions are morphologically significantly shorter than those in healthy individuals. The hair shafts primarily exhibit a mixed three-layer/two-layer pattern or a simple two-layer hyperechoic pattern, which differs significantly from the normal scalp, where a uniform three-layer hyperechoic pattern is the primary characteristic [19]. Li Lin et al. [20] studied the combined use of 22 MHz ultrasound and trichoscopy to monitor patients with androgenetic alopecia one month after botulinum toxin type A injections. Ultrasound was already able to detect a significant increase in the average width and length of hair follicles in the treatment group. However, the number of hairs per unit area measured by trichoscopy during the same period did not show a statistically significant difference. This study suggests that high-frequency ultrasound may be more sensitive than a trichoscope in assessing morphological changes in hair follicles and holds promise for monitoring early treatment efficacy.

In summary, the diagnostic features of androgenetic alopecia observed under high-frequency ultrasound primarily manifest as shortened hair follicle length, abnormal hair shaft echogenicity patterns, and increased heterogeneity in hair shaft diameter; these features can aid in early diagnosis and disease assessment without relying on reduced hair follicle density.

3.2. Alopecia Areata

Alopecia areata is a common, non-scarring inflammatory hair loss disorder [21]. Its primary clinical manifestation is sudden, well-defined, circular patches of hair loss on the scalp, and its prevalence has steadily increased [22]. Histopathologically, it is characterized by perifollicular lymphocytic infiltration, a reduction in anagen follicles, and an increase in telogen and catagen follicles; miniaturized follicles and atrophic anagen follicles may also be observed [23]. Typical dermatoscopic findings include exclamation-mark hairs; during the stable phase, the “yellow dot sign” may appear, while during the active phase, the “black dot sign,” conical hairs, broken hairs, and uneven hair shaft thickness may be observed [24]. Compared to normal tissue, the ultrasonographic characteristics of the scalp in patients with alopecia areata are primarily manifested in morphological differences such as variations in hair follicle length and diameter, hair follicle density, empty hair follicles, hyperechoic changes in the perifollicular tissue, and reduced blood supply to the hair follicles.

Multiple studies have shown that the high-frequency ultrasound features of alopecia areata primarily include shortened hair follicles, the formation of empty hair follicles, hyperechoic changes around the hair follicles, and abnormal blood flow signals. High-frequency ultrasound can utilize reflection and scattering between different tissues to observe the morphology and location of hair follicles. Zhou Qiaochu et al. [25] used a Monica Paun et a ultrasound probe to examine alopecia areata lesions at different stages and healthy scalp tissue. They found that in healthy and recovering hair follicles, the follicle length extended to the subcutaneous tissue, making them significantly longer than those in the progressive and telogen phases. Furthermore, distal widening of hair follicles in the progressive and telogen phases was identified as a key feature of high-frequency ultrasound images of alopecia areata lesions. The boundaries of hair follicles in the progressive phase were generally clear, with a droplet-like appearance, whereas some follicles in the other groups exhibited blurred boundaries. Bakr Mohamed El-Zawahry et al. [26] examined the scalps of 31 patients with alopecia areata using an ultrasonic biomicroscope and performed histopathological examinations. This further confirmed that, under ultrasound, normal hair follicles appear as anechoic conical shadows extending from the epidermis into the dermis, narrow at the top and wider at the base. In alopecia areata areas, however, these anechoic conical shadows are narrower than in normal control areas, and hair follicle density is lower. In alopecia areata and total alopecia, the epidermis may appear anechoic, while the corresponding dermal region contains anechoic spherical shadows, which may correspond to dilated sebaceous glands—features that are difficult to observe under a trichoscope.

Ultrasound can not only detect abnormalities in hair follicle morphology but also provide a qualitative estimate of hair follicle count. It utilizes the principle that hair follicle lengths vary at different stages of the hair growth cycle to assess the hair growth cycle phase [27]. Monica Paun et al. [28] found that, under high-frequency ultrasound, alopecia areata exhibits three most common features: small oval-shaped hair follicles (corresponding to “yellow dots” observed in trichoscopy), hyperechoic areas surrounding the hair follicles in the subcutaneous tissue, and empty hair follicles. These correspond to “broken hairs” and “black dots” observed in trichoscopy, which aligns with the pathological mechanism of alopecia areata, in which immune attacks gradually lead to the breakdown of hair follicle structure during the natural course of the disease [29]. Misaki Kinoshita-Ise et al. [30] observed and analyzed scalp ultrasounds from 103 patients with scarring and non-scarring alopecia, as well as 40 healthy individuals. They found that the echo intensity of empty hair follicles was higher than that of normal anagen hair follicles; consequently, a clearly visible boundary exists between empty hair follicles and the subcutaneous tissue on ultrasound. Empty hair follicles and hyperechoic areas surrounding the follicles are commonly observed in the active phase of alopecia areata, while small, oval-shaped follicles are more common in the chronic phase. Lesions in the recovery phase resemble normal scalp tissue, although hair shaft diameters may be thinner in some lesions. Ultrasound can detect subcutaneous follicular inflammatory lesions and structural abnormalities that are not observable with a trichoscope, providing objective evidence for the staging, differential diagnosis, and monitoring of treatment efficacy in alopecia areata.

A synthesis of the above studies reveals that high-frequency ultrasound offers multidimensional diagnostic value in alopecia areata; however, its limitations and clinical role must be evaluated dialectically. On the one hand, by utilizing parameters such as follicle length, morphology, and echogenicity, this technology enables full-layer observation from the epidermis through the dermis to the subcutaneous tissue, offering unique advantages particularly in identifying deep pathological changes such as perifollicular inflammation and sebaceous gland dilation. On the other hand, although the technical parameters used in different studies have varying focuses, they all point to the common pathological characteristics of alopecia areata hair follicles—namely, “shortened morphology, increased echogenicity, and altered borders”—suggesting that these ultrasound features exhibit cross-platform reproducibility.

However, current research also exhibits certain limitations inherent to its current stage. Most studies employ a cross-sectional design; while this allows for a clear delineation of static ultrasound profiles at different disease stages, it makes it difficult to capture the dynamic transformation process of hair follicles from “cavitation” to “regeneration” under therapeutic intervention; Furthermore, while a preliminary correlation between ultrasound findings and histopathology has been established, the histological validation of the pathological correlates of ultrasound features—such as empty hair follicles and hyperechoic areas surrounding the follicles—remains lacking, particularly regarding lymphocytic infiltration and the degree of fibrosis. The consistency between ultrasound staging and clinical staging also requires confirmation through prospective studies.

3.3. Pilomatricoma

Pilomatricoma, also known as calcifying epithelioma of Malherbe, hair matrix tumor of the follicular infundibulum, or hair matrix cyst of the follicular infundibulum, originates from epithelial germ tissue differentiated from hair matrix cells and is a benign tumor of hair follicular origin [31]. It most commonly occurs in the head and face [32], with a malignant transformation rate of 2.6% [33]. Most tumors measure less than 15 mm in diameter and are prone to calcification or ossification [34].

On high-frequency ultrasound, PM typically appears as a target-like lesion with a hyperechoic center and hypoechoic margins; the lesion often contains hyperechoic foci corresponding to calcium deposits [27]. Early studies primarily focused on the basic diagnostic value of ultrasound. Li Xiaoying et al.[35] conducted a retrospective analysis of 156 pathologically confirmed cases of PM in children and found that the lesions mostly presented as solitary subcutaneous nodules with visible surrounding vascular distribution; they were most commonly located on the cheeks and arms. The Solivetti Type 2 ultrasound pattern was the most common, and a peripheral hypoechoic halo was observed only in this type. This study preliminarily established the ultrasonographic distribution patterns and basic morphological characteristics of PM. Xuan Weifeng et al. [36] further established a deep correlation between ultrasonographic findings and pathological mechanisms through a retrospective study of 133 patients with PM, analyzing 147 PM lesions. On high-frequency ultrasound, PM located between the deep dermis and subcutaneous tissue appear heterogeneous, with the most common features being an echogenic center, a hypoechoic rim, and a posterior acoustic shadow. Histological examination revealed three types of calcification: scattered punctate, nodular, and arcuate, consistent with the high-frequency ultrasound imaging findings. The study found that the scattered punctate, nodular, and arcuate calcifications observed on ultrasound were highly consistent with histological findings, confirming the pathological basis of high-frequency ultrasound in the auxiliary diagnosis of this disease. Wang Lihua et al. [37] examined 48 patients who underwent surgery and found that marginal or internal calcifications, a surrounding hypoechoic halo, and band-like blood flow signals at the margins were typical ultrasound features of this condition. Furthermore, the presence of calcifications significantly improved diagnostic accuracy, whereas lesions without calcifications were prone to misdiagnosis, suggesting that calcifications can serve as an important indicator for differential diagnosis. Yang Fan et al. [38], through the analysis of ultrasound images from 20 cases of PM, found that a diagnosis of PM should be considered when a well-defined mass with internal echogenic foci and a surrounding hypoechoic rim, or a completely echogenic mass accompanied by a strong posterior acoustic shadow, is observed on ultrasound. This finding expanded the spectrum of ultrasound features for this condition.

In summary, the ultrasound appearance of PM is highly characteristic and aids in preoperative diagnosis and differential diagnosis. Its typical presentation is a well-defined mass located in the deep dermis to the subcutaneous tissue. Key diagnostic features include: ① Internal hyperechoic foci: hyperechoic spots or clusters representing calcification are visible within the mass, which may be accompanied by acoustic shadowing posteriorly; ② Peripheral hypoechoic halo: The mass is often surrounded by a hypoechoic rim; ③ Blood flow characteristics: Band-like blood flow signals are visible peripherally, while internal blood flow is sparse. When ultrasound simultaneously reveals hyperechoic foci of calcification combined with a peripheral hypoechoic halo, presenting as a “target-like” or “double-track sign,” this has high diagnostic value for PM.

3.4. Other Hair-Related Disorders

For other types of hair disorders, such as hair follicle cysts, scarring alopecia, and inflammatory alopecia, high-frequency ultrasound reveals echogenic changes around the hair follicles and abnormal blood flow signals. Relevant studies indicate that these imaging features can aid in disease classification, guide treatment plans, and monitor treatment efficacy.

Hair follicle cysts clinically present as smooth, firm nodules, often accompanied by hair loss. Ultrasound reveals circular or oval structures in the dermis and subcutaneous tissue that are typically anechoic, though echoes from keratinous and dense cholesterol-containing cystic components may be detected. Zhao Mengzhu et al. [39] found that on ultrasound, trichocele appears as a hyperechoic subcutaneous mass with generally well-defined borders and a regular shape; the internal echoes are often heterogeneous, frequently containing liquid-filled areas and calcifications, and typically lacking blood flow signals. When ruptured and accompanied by infection, the morphology is often irregular, with increased blood flow signals; proliferative pilosebaceous cysts appear as hyperechoic nodules.

Scarring alopecia is more complex; ultrasound examination reveals inflammation in the dermis and subcutaneous tissue, characterized by hyperechoic dermis and hyperechoic subcutaneous tissue, usually accompanied by increased blood flow in the affected area. Nuchal keloid acne is a rare form of chronic keloid folliculitis, presenting as papules, pustules, plaques, or nodules on the back of the neck [40]. Similar to acne conglobata and hidradenitis suppurativa, keloid-like acne of the nape is usually caused by follicular obstruction. Ultrasound examination reveals skin inflammation, hyperechoic structures associated with blood vessels, and enlarged follicles.

Alopecic folliculitis is a recurrent suppurative inflammation that typically affects the vertex and occipital regions, presenting as follicular pustules without external openings and leading to scarring alopecia in advanced stages [41]. Ultrasound examination reveals multifocal inflammation, the absence of fistulas, and localized follicular thickening [27]. Suppurative, infiltrating perifolliculitis of the scalp is a rare, severe, progressive form of scalp detaching cellulitis or folliculitis, presenting as painful, pus-draining nodules and scarring alopecia [42]. Ultrasound examination reveals fragmented fluid collections and abscesses, as well as multiple interconnected hyperechoic fistulas linked to hair follicles, resulting in follicular swelling.

A comparison of typical high-frequency ultrasound features of common hair disorders is shown in Table 1.

4. The Application of Ultrasound in the Treatment of Hair Disorders

Ultrasound possesses the fundamental properties of waves, namely reflectivity and superposition [43]. The former enables the examination of deep tissues for diagnostic purposes, while the latter can be applied to superficial tissues for minimally invasive or non-invasive treatment. Compared to oral or topical medications alone, the combination of ultrasound and transdermal drug delivery offers distinct advantages in the treatment of hair disorders. Not only can various traditional topical treatments for hair loss—such as minoxidil and platelet-rich plasma—be effectively administered via local transdermal delivery, but certain oral medications, such as finasteride, can also be considered for topical application to enhance treatment efficacy [44]. There are two primary approaches to using ultrasound in the treatment of hair disorders: one involves direct action on the local skin through minimally invasive ablation therapy to improve metabolism; the other uses ultrasound as a carrier to indirectly deliver drugs to the affected area, thereby enhancing the efficiency of transdermal drug absorption.

4.1. Minimally Invasive Treatment with Focused Ultrasound

Focused ultrasound acts directly on the lesion and is based on the principles of minimally invasive medicine [45]. It is a non-invasive therapeutic technique that focuses relatively low-intensity ultrasound energy from outside the body onto specific tissue areas within the human body to create high-energy focal points; the resulting tissue heating induces coagulative necrosis in specific regions, thereby modulating their function and metabolism. This aligns with the therapeutic principles of Traditional Chinese Medicine surgery: “eliminating pathogenic factors without harming healthy tissues” and “removing decayed tissue without damaging new tissue” [46]. It is now widely used in clinical settings for tumors, skin diseases, neurological disorders, and cardiovascular diseases, with well-established applications, providing a solid foundation for expansion into targeted treatments for hair disorders [47]. The application of HIFU in hair regeneration is gradually gaining attention. By precisely focusing energy on the tissues surrounding hair follicles, it can stimulate hair growth and improve hair density. Studies have shown that HIFU can be safely and non-invasively used in patients with AGA and AA, demonstrating significant clinical potential. By precisely controlling the focal depth and energy, it can target hair follicles and affected dermal tissue, regulating local metabolism and improving the microenvironment, thereby offering a new minimally invasive treatment approach for androgenetic alopecia and scarring scalp lesions [48].

4.2. Ultrasound-Mediated Enhancement of Transdermal Drug Penetration

In addition to directly delivering focused therapy to tissues, ultrasound is also an ideal technology for promoting transdermal drug penetration [49], as it can improve drug bioavailability. Angelo et al. [50] found that applying external physical stimuli—such as ultrasound at a frequency of 5 MHz and an intensity of 1.2 W/cm²—to lipid carriers within hair follicles can enhance drug bioavailability, therapeutic efficacy, and safety. Donghee Park et al. [51] found that high-frequency ultrasound can improve the efficiency of drug delivery into pig skin by inducing cavitation. Ai-Ho Liao et al. [52] found that combined treatment using optimal ultrasound parameters (3 W/cm², 1 min) and microbubbles (2.9 × 10⁸/mL) increases skin permeability, thereby enhancing the delivery of α-arbutin to inhibit melanin production in mice without damaging the skin. Furthermore, combining ultrasound with microbubbles of different sizes can produce varying degrees of skin permeability and can even enhance the delivery of macromolecules (>500 Da) [53].

The mechanism of action of ultrasound technology involves focusing multiple low-energy ultrasound beams from outside the body to form a high-energy focal point. The sound waves enter the body at an intensity tolerable to the skin surface and, utilizing the thermal, cavitation, and mechanical effects of ultrasound in tissues, direct the focal energy to the target tissue. With the guidance and monitoring of medical imaging technologies such as B-ultrasound or MRI, the target areas of tissues or organs within the human body can be precisely treated [45]. Ultrasound generated by a unidirectional electric current in a fluid can enhance the cavitation effect through bubbles [54], Ai-Ho Liao et al. found that 1-MHz ultrasound (applied at 3 W/cm² and a sound pressure of 0.266 MPa) administered for 1 minute in combination with Mx-LyzMB significantly increased the scalp penetration efficiency of minoxidil and LyzMB fragments, promoted hair growth, and more effectively inhibited bacterial growth on oily scalps [55]. At higher microbubble concentrations, the combined effect of dual-frequency ultrasound and microbubbles enhances the cavitation effect more effectively than single-frequency ultrasound, suggesting its potential application in promoting hair growth and improving hair quality [56].

Ultrasound can also enhance protein permeation. Qiuying Mai et al.[57] developed an integrated transdermal delivery system that utilized ultrasound to improve the efficiency of multifunctional microbubbles in delivering minoxidil to hair follicle cells, thereby accelerating hair growth and improving hair quality. Furthermore, this permeation-enhancing method can be achieved in water without the need for any chemical methods to increase skin permeability. Jee-Yeon Ryu et al. [58] found that Cas9/sgRNA ribonucleoprotein complex carriers exhibit high delivery efficiency and biocompatibility and can be successfully delivered to hair follicle cells via ultrasound. Through cavitation-induced transmembrane penetration of the microbubbles, the Cas9/sgRNA construct can efficiently and specifically recognize and edit target genes, thereby restoring hair regrowth. The suppression of SRD 5A 2 protein production through CRISPR-based genome editing demonstrated the efficacy of ultrasound-activated nanoparticles for the treatment of androgenetic alopecia.

The above studies indicate that ultrasound can enhance the efficiency of drug, protein, and gene therapy vector penetration through the skin and into hair follicles. For example, minoxidil, stem cell-derived growth factors, and CRISPR/Cas9 vectors demonstrate improved local delivery when assisted by ultrasound, offering new approaches for the precision treatment of hair disorders. However, the application of ultrasound in the treatment of hair disorders is currently at the proof-of-concept or preclinical stage and still faces multiple translational barriers. Most existing literature consists of theoretical discussions or case reports, lacking validation through animal models and human safety data. Key issues include the alignment of treatment depth with the anatomical location of hair follicles, the impact of thermal damage on hair follicle stem cell function, and the absence of dose-response relationship studies. Regarding ultrasound-mediated drug delivery, animal models differ significantly from the human scalp in terms of thickness, hair follicle density, and stratum corneum structure, limiting the generalizability of results; most studies use ex vivo skin or short-term observations, lacking long-term in vivo safety data, and the potential interference of repeated ultrasound exposure on the hair follicle cycle remains unknown; issues regarding the stability, biocompatibility, and large-scale production of microbubble carriers have yet to be resolved.

5. Conclusions and Outlook

High-frequency ultrasound, with its advantages of being non-invasive, real-time, repeatable, and capable of precise layered imaging, can clearly display the morphological structure and blood flow characteristics of the scalp and hair follicles under both normal and pathological conditions. It effectively aids in the diagnosis, staging, differential diagnosis, and dynamic follow-up of treatment efficacy for AGA, AA, PM, TCs, and inflammatory hair disorders, thereby compensating for the shortcomings of traditional diagnostic methods. Focused ultrasound can not only perform minimally invasive ablation of lesions through thermal effects but also deliver drugs via the cavitation effect, facilitating the transdermal targeted delivery of drugs and gene vectors, demonstrating potential for integrated diagnosis and treatment.

However, its clinical application currently faces several limitations. First, the diagnostic accuracy of this technology is highly dependent on the operator’s experience and skill; the interpretation of hair follicle morphology, echogenic patterns, and blood flow signals involves a degree of subjectivity, which may affect the consistency and objectivity of the diagnosis. Second, while high-frequency ultrasound can provide detailed imaging of superficial tissues at the millimeter level, its ability to detect the fine structures of deep hair follicles may be limited—an inherent contradiction dictated by the physical characteristics of ultrasound imaging. Third, there is currently a lack of standardized operating procedures and guidelines for probe selection tailored to different hair disorders. For example, there is no consensus based on large-sample clinical evidence regarding which frequencies, modes, and parameters should be used for different types of hair loss, which limits the standardized application of the technology and comparability across different studies.

Future efforts should focus on conducting large-scale, multicenter, randomized controlled clinical trials to standardize ultrasound parameters and diagnostic scoring criteria, and to determine the optimal treatment frequency, energy dose, and intervention cycle for different conditions. Standardized operating procedures and diagnostic workflows should be established to further explore the value of ultrasound in the early screening, precise diagnosis, and personalized treatment of hair disorders, thereby promoting the standardized and large-scale clinical translation of ultrasound technology in the field of hair medicine.