Principles of dermoscopy
- 1 Introduction
- 2 Clinical inspection of skin without the use of dermoscopy
- 3 Principles of non-polarized dermoscopy (NPD)
- 4 Principles of polarized dermoscopy (PD)
- 5 PD vs. NPD
- Allows to examine the subsurface morphologyThis glossary term has not yet been described. of cutaneous lesions, down to the depth of the superficialThis glossary term has not yet been described. dermis.
- Reveals colors and structures that are normally not visible to the unaided eye.
- Improves the diagnostic accuracy and confidence level of experienced users, for both pigmented and non-pigmentedThis glossary term has not yet been described. skin lesions.
Clinical inspection of skin without the use of dermoscopy
- Naked-eye examination (non-dermoscopic): Allows the observation of the morphological features on the surface layer of the skin (stratum corneum), and only minimally the colors and structures the deeper layers of the epidermis and the superficial dermis.
- Allows the clinician to appreciate the gross morphological features of the lesion, such as their size, shape, colors, contours, and surface topography.
Optical properties of light without the use of dermoscopy
- The refractive index of the stratum corneum is higher than that of air, so that much of the incident light is reflected off the surface of the skinThis glossary term has not yet been described..
- Surface glare: most of the incident light, which is reflected off the stratum corneum, overwhelms the retina, and thereby precludes the observer from visualizing the light reflected from the deeper layers of the skin.
- In body areas with a thick stratum corneum (e.g. on acralAcral melanoma is a type of skin cancer that occurs on fingers, palms, soles, and nail beds. surfaces), there is greater back-scatter of light and the skin appears more opaque, while in anatomic areas with thin stratum corneum (e.g., eyelids), the skin appear more translucent.
Principles of non-polarized dermoscopy (NPD)
- Non-polarized dermoscopes contain light-emitting diodes to provide illumination.
- Are equipped with a magnification lens.
- Necessitate direct contact of the dermoscope’s glass plate with the skin .
- Require the presence of a liquid interface with refractive index equal to or closely matching that of skin.
- Different immersion liquids can be used: 70% alcohol, gel (i.e., ultrasound gel, antibacterial gel), water or mineral oil.
- Allows visualization of subsurface structures located in the epidermis and dermal-epidermal junction (DEJ).
- NPD can hardly visualize structures deeper than the DEJ.
- Air bubbles between the dermoscope’s glass plate, the immersion liquid, and the skin should be eliminated.
Optical properties of light during the use of NPD
- Surface glare: is eliminated by the matching of refractive indices of the NPD's glass plate, immersion medium, and skin.
- SuperficialThis glossary term has not yet been described. penetrating light: is the main source of contrast when using NPD. The light that enters the skin is being absorbed (e.g. by melanin) or reflected back (e.g. by keratin in milia-like cystsThis glossary term has not yet been described.) at layer of the epidermis and DEJ. The superficial penetrating light undergoes minimal scattering events.
- Deep penetrating light: contributes only a small fraction to back-reflected light detected with NPD. This is because there is decay of light and multiple scattering events with increasing depth.
- Air bubbles create a skin–air interface, which causes back-scatter of light and precludes the observer from visualizing structures below the stratum corneum.
Principles of polarized dermoscopy (PD)
- Polarized dermoscopes, like NPD, contain light-emitting diodes to provide illumination and are equipped with a magnification lens.
- PDs use two polarized filters to achieve cross-polarization.
- Do not require direct contact with the skin, and do not require the use of immersion liquids.
- Allow visualization of subsurface structures located at the dermal-epidermal junction (DEJ) or superficial dermis.
- PDs are nearly "blind" to the skin's surface and to structures in the superficial epidermis (e.g., comedo-like openings[[Comedo like openings]]).
- Some PD devices allow the user to opt between non-contact PD and contact PD. Under contact PD, the use of an immersion fluid (e.g. 70% alcohol) can enhance image quality, probably by allowing more source light to enter through the stratum corneum.
Optical properties of light during the use of PD
- Light emitted from the dermoscopy unit (source) passes through a polarizer resulting in the generation of polarized (unidirectional) light.
- Light reflecting back toward our eye (detector) must first pass through a cross-polarized filter whose direction is perpendicular (orthogonal) to that of the source polarizer.
- Polarized lightThis glossary term has not yet been described. cannot pass through the cross-polarizing filter unless the light changes its direction by 90°, which occur if the original polarized lightThis glossary term has not yet been described. undergoes sufficient scattering events in the skin that change its direction ("randomization of polarization").
- Surface glare: maintains its original polarization, and cannot pass through the cross-polarized filter. Therefore, PD is "blind" to surface glare.
- Superficial light: does not undergo enough scattering events to result in randomization of polarization. Therefore, PD is "blind" to back-reflected light from the superficial layers of the epidermis.
- Deep penetrating light: goes more deeply, down to the DEJ and superficial dermis, undergoing multiple scattering events that result in randomization of polarization. Thus, back-reflected "deep penetrating light" can pass through the cross-polarization filter and enter our eye, allowing the visualization of dermoscopic structures from the DEJ and superficial dermis.
PD vs. NPD
- NPD is better for inspecting structures in the superficial skin layers (e.g., superficial epidermis down to the DEJ). PD better for evaluating the deeper skin layers (e.g., DEJ and superficial dermis).
- Milia-like cystsThis glossary term has not yet been described. (horn pseudocysts in the epidermis) and blue-white veil (orthokeratosis) are better seen with NPD due to superficial changes in the epidermis.
- Shiny white linesstreaks (chrysalis/crystalline) are better seen under PD, since this dermoscopic structure is associated with increased collagen at the superficial dermis. In addition, polarized light rapidly randomizes its polarization when it encounters a birefringent structure, such as collagen.
- Since PD does not require direct skin contact, blood vesselsThis glossary term has not yet been described. and pink colorColor (American English) or colour (Commonwealth English) is the characteristic of human visual perception described through color categories, with names such as red, yellow, purple, or blue. (vascular blush) are more evident under PD (due to lack of pressure effect). In addition, blood vessels and located in the dermis that is better seen with PD.
- In pigmented lesions with melanin at the DEJ (e.g.. junctional neviThis glossary term has not yet been described.) or superficial dermis (e.g. blue nevi), PD will show slightly darker shades of brown and blue and sometimes more variability in pigmentation compared with NPD.
- The differences between PD and NPD may impact the diagnostic accuracy and diagnostic confidence level. For example, PD can increase sensitivity for detecting amelanotic melanomas or structure-poor melanomas and basal cell carcinomas, because PD highlights the presence of blood vessels, vascular blush, and/or white shiny lines (chrysalis/crystalline). In contrast, NPD can increase specificity by allowing to correctly identify milia-like cysts and comedo openings in seborrheic keratosesThis glossary term has not yet been described..
- Hybrid” dermoscopes toggle between PD and NPD modes. These hybrid devices can enhance dermoscopic diagnosisThis glossary term has not yet been described. because PD and NPD provide complementary information.
- Hybrid devices should be in direct skin contact and used with a liquid interface, otherwise the user will see dermoscopic structures only in PD mode.
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