9.2.2 Non-invasive imaging methods in Dermatology

Although many diagnoses in dermatology are made clinically, sometimes extra help is required. Skin biopsies are still the goldstandard if by macroscopy or dermatoscopy a proper diagnoses can not be made, however, the area and lesion has to be selected carefully  be representative of the whole lesion. Various non-invasive techniques have therefore been developed to close the gap between macroscopy and biopsy to visualize the skin in certain situations. Some of these techniques will not be available in all centres.

Photography is very helpful in diagnosing intermittent skin diseases which may not be present on the day of clinical review, e.g. intermittent urticaria. Serial photographs can be very helpful in tracking pigmented lesions to assess the chance of malignant change. In store-and-forward telemedical settings, clinical pictures are pivotal for monitoring the course of diseases (i.e. wound healing).

Examining the skin, hair, follicles or nails under a magnifying, polarized light (x10-20 the size of normal) can give additional information especially for pigmented lesions and some forms of inflammatory dermatoses, as well as vascular lesions. Digital images (or serial images) may be stored from dermoscopic examination and either reviewed by a clinician, or by artificial intelligence. The original term “dermatoscopy” is still used synonymously with the internationally consented term “dermoscopy”.

Producing anemia of inflamed skin by pressure with a glass spatula may reveal the typical apple yellow colour of granulomatous infiltrates e.g. in sarcoidosis.

This machine provides input about the content of melanin, haemoglobin and collagen in the epidermis and papillary dermis . This technique may be useful especially for non-experts to calculate risk of melanoma in a clinically atypical pigmented lesion.

This technology relies on the fact that there may be a difference in impedance in skin cancers compared with normal skin and that this can be elicited by a sensitive electrode, such as in the Nevisense machine. It may be a useful adjunct to the clinician prior to a biopsy.

Ultrasound is regularly used in dermatology for measuring depth of lesion ( i.e. melanoma or other tumor thickness or mass of a lipoma before dermatosurgery), for lymph nodes or in vessel blood flow diagnostics (i.e ,Doppler-technique; Duplex-sonography for the assessment of deep thrombosis).

Sometimes X-ray of other organs may be required for the work-up of certain dermatological conditions (e.g. chest X-ray for cutaneous sarcoidosis). X-ray may be helpful to evaluate some skin problems e.g. calcinosis cutis. CT or PET-CT (see below) scanning is a mandatory investigation of advanced skin tumours which may have metastasized. It may also be useful for hunting any underlying tumours in conditions such as dermatomyositis or paraneoplastic pemphigus. 

PET scans involve a radioactive tracer being visualized in vascular or metabolic processes. It may identify neoplasms earlier than other techniques such as CT, and can be used to seek an underlying (clinically undisclosed) malignancy, i.e., sentinel lymph node metastasis in malignant melanoma. It may be combined with CT (PET-CT) to enhance sensitivity.

Similar to CT scanning, MRI can be helpful in certain situations in dermatological diseases, e.g. investigation of neurofibromatosis or Sturge-Weber syndrome.

This technique focuses light at sequential levels with a spatial pinhole and provides a horizontal image of the skin with high lateral resolution of 1µm. The maximal penetration of the skin is around 250µm, which does not allow examination of the deeper part of the dermis at all and does not deliver clear results of the papillary dermis. It is frequently used for assessment of epidermal manifestations of skin malignancies, but may also be useful in inflammatory skin conditions.

This vertical laser technique provides deep penetration in the dermis and can help to close the gap between dermatopathology and reflectance confocal microscopy.

Conventional OCT provided both horizontal and vertical sections of the skin and penetrates around 2mm, with a lateral resolution of around 7.5µm. High-definition OCT penetrates the skin deeper, but linefield-confocal OCT is the best, with a penetration of 500µm and a lateral resolution of 1µm. This can be very useful to delineate margins of tumours in vivo and confirm or exclude malignancy without the need for a skin biopsy.

For other skin test methods, refer to the chapter on Cutaneous physiology, and 9.2.4.