Current uses of diagnostic high-frequency US in dermatology

The diagnosis of most skin diseases, both focal and diffuse, has long relied mainly on physical examination findings. The recent introduction of technologically advanced ultrasound equipment using 20 MHz probes has permitted the specific application of ultrasound to dermatology. Accordingly, we investigated whether the findings at very high frequencies can represent a valid adjunct to clinical assessment in many skin conditions, including neoplasms, inflammatory states and diseases of unknown origin.

Skin lesions are studied using high frequency probes, which very clearly detail the three layers (epidermis, dermis and subcutaneous tissues) forming the normal skin. The choice of the probes frequency should depend mainly on the lesion diameters and site. Electronic 7.5-13 MHz linear probes depict flat and regular surfaces effectively and provide a wider field of surface vision and, therefore, a wider view than sectorial probes. Water bath sectorial mechanical probes with 10-20 MHz frequency have very superficial focusing and are excellent to study irregular surfaces.

High frequency ultrasound can be usefully correlated with clinical tests to study focal skin lesions. The diagnosis of most benign skin cancers is usually made on clinical bases. Ultrasound examinations are performed preoperatively in questionable cases. Malignant neoplasms appear at ultrasound as hypoechoic focal lesions, generally with no specific features in relation to the histologic type; nevertheless, preoperative ultrasound may play an important role in that it measures the thickness of cutaneous melanoma, which is a very important prognostic factor. In particular, 20 MHz probes permit to assess the depth of melanoma invasion. The sonographic evaluation of melanoma thickness is usually in agreement with histologic findings. 'Satellite' neoplastic lesions growing near the main tumor can also be revealed. Color and power Doppler studies may be combined with gray-scale imaging: the identification of abnormal intra- or peritumoral low-resistance pulsatile flow signals suggests the malignant nature of the cutaneous lesion. High frequency ultrasound can also be used to study diffuse cutaneous conditions. Among them, ultrasound can provide a valid morphologic representation of psoriatic skin lesions and it is also a noninvasive and accurate method for evaluating the therapeutic efficacy of antipsoriatic drugs. In scleroderma, sonographic findings vary depending on disease activity and the patterns vary; therefore, 20 MHz probes may also prove useful over the other instrumental tools to monitor the disease course and treatment efficacy in focal scleroderma. Other potential applications include allergic dermatitis, nodular erythema, dermatomyosis, sarcoidosis, lymphedema of the limbs and allergologic conditions. Ultrasound can also be used in monitoring the response to or complications of topic drugs administration, and in the follow-up of focal burns.

High frequency ultrasound can provide a reliable morphologic representation of skin lesions but it is also an accurate noninvasive tool for monitoring the therapeutic efficacy of drugs administration in focal or diffuse diseases. The application of high frequency studies to dermatology is very challenging. Indeed, the very high frequency probes up to 20 MHz currently available are particularly useful for reliable studies. Contrast-enhanced color and power doppler are very promising techniques. Advancements in technology will improve the correlation of clinical with high frequency ultrasound findings in the assessment of several skin diseases.