Noninvasive Imaging Techniques in Dermatology: A Brief Overview
By Shaunt Mehdikhani, MS, Cindy Wassef, MD, MBA, FAAD, Associate Professor at Rutgers Robert Wood Johnson Medical School
Dermatology as a medical discipline has been transformed by noninvasive imaging technologies. Historically, the evaluation of skin lesions has been a clinical visual exam. However, some conditions and presentations necessitate semi-invasive or invasive techniques for confirmation of diagnoses and treatment planning, including skin biopsies and excisions for histopathologic analysis. In recent decades, noninvasive imaging techniques have provided physicians with the ability to visualize deeper skin layers without the need for invasive procedures. In this article, we will outline specifically the principles of dermoscopy, reflectance confocal microscopy, and optical coherence tomography for the evaluation of dermatologic conditions.
Dermoscopy, also known as epiluminescence microscopy and dermatoscopy, is a widely used in-vivo diagnostic tool primarily in dermatologic practice. Dermatoscopes function as handheld variable objective (10x to 100x) lenses with built-in illumination, which can be polarized or non-polarized. This differentiation allows for reduced skin surface reflection, which enhances the visibility of deeper dermal structures to the level of the reticular dermis. In certain cases, dermoscopy requires direct contact with the skin with the use of a linkage fluid, such as alcohol, to improve the clarity of the image.
In summary, noninvasive imaging techniques, including dermoscopy, reflectance confocal microscopy, and optical coherence tomography, are revolutionizing dermatologic practice.
Most commonly, dermoscopy is used to differentiate between malignant and benign skin growths. The ease-of-use of dermatoscopes and their more recent digital capabilities allow for direct image capture for longitudinal monitoring in patients with suspicious and/or multiple lesions. This has further expanded the application of dermoscopy to a wider variety of conditions, including infectious, inflammatory, and pigmentary dermatoses. Specialized techniques, including onychoscopy and trichoscopy, have been developed to assess conditions involving the nails, hair and scalp, respectively. Dermoscopy is, however, not without challenges. Artifacts from topical products and Fitzpatrick skin types may obscure images for subsequent analysis. Despite these shortcomings, dermoscopy is regularly recognized to enhance diagnostic accuracy.
The evaluation of skin lesions has advanced from visual inspection and dermoscopy to more novel tools. Reflectance confocal microscopy (RCM) provides in-vivo, cellular-resolution imaging, capturing detailed skin structure views to the depth of the superficial dermis. RCM utilizes a near-infrared low-power laser to scan single focal points of skin tissue, enabling dermatologists to examine complex lesions in detail. This imaging modality captures optical sections that provide horizontal cross-sections of skin. Wide-probe RCM is also able to create mosaic images by stitching together adjacent sections for the evaluation of larger lesions. Handheld RCM devices not requiring skin fixation offer additional flexibility for imaging of areas otherwise too difficult to biopsy. RCM is becoming increasingly used for diagnosing skin lesions with ambiguous clinical and dermoscopic findings, particularly in sun-damaged areas or on cosmetically sensitive skin regions.
When diagnosing lesions with unclear malignant features, RCM is a potent addition to dermoscopy. For example, RCM enables visualization at the cellular level, revealing features such as polygonal keratinocytes in the stratum granulosum and spinosum, or the distinctive “cobblestone” pattern in the stratum basale. These differentiations allow dermatologists to distinguish between benign and malignant skin lesions, which often present with unique imaging patterns not visible through standard clinical or dermoscopic examination. Additionally, RCM may be used to monitor the effectiveness of noninvasive therapies by providing a detailed view of changes in tumor structure at a cellular level. RCM does present with limitations, including a relatively shallow imaging depth, which may miss deeper lesions such as nodular melanoma, and challenges in imaging hyperkeratotic or ulcerated areas that may distort the quality of images. Interpreting RCM images is not part of standard dermatology training and is a skill that must be learned independently.
Optical coherence tomography (OCT) is an emergent noninvasive technique that provides real-time cross-sectional images. OCT has found significant applications in dermatology, particularly in the diagnosis of skin cancers, by visualizing both cross-sectional and horizontal en face imaging of the skin, allowing for easy comparison to histological sections. This makes it particularly useful in diagnosing non-melanoma skin cancers, such as basal cell carcinoma (BCC), though it has not been as helpful with pigmented lesions like melanoma due to inferior image resolution.
Line-field confocal optical coherence tomography (LC-OCT) is a more recent addition to the noninvasive imaging technique toolkit, combining OCT with RCM to offer improved resolution and deeper penetration compared to traditional OCT. LC-OCT captures both vertical and horizontal sections in real-time with the added benefit of producing three-dimensional images. The technique offers near-cellular resolution, making it ideal for detailed imaging of skin structures and helping to detect and assess skin cancers. LC-OCT is effective in diagnosing BCC by visualizing the characteristic dermal lobules and features such as the “millefeuille” pattern and surrounding clefting, which help differentiate BCC from other lesions. Additionally, LC-OCT assists in subtyping BCC into categories such as superficial, nodular, and infiltrative forms.
Beyond BCC, LC-OCT also shows promise for diagnosing actinic keratosis and squamous cell carcinoma. It can reveal features like hyperkeratosis, nuclear pleomorphism, and disruptions in the dermal-epidermal junction, which are key in distinguishing these conditions from benign lesions. The ability to generate three-dimensional reconstructions further enhances its diagnostic power by providing a deeper understanding of the morphological changes in skin tumors.
In summary, noninvasive imaging techniques, including dermoscopy, reflectance confocal microscopy, and optical coherence tomography, are revolutionizing dermatologic practice. While each imaging modality offers specific advantages, the success of future diagnostic practice lies in combining these multiple modalities with gross inspection and histologic analysis.
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