Looking skin deep with dermoscopy
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Looking skin deep with dermoscopy
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(
[passage] => WP_Post Object
(
[ID] => 559831
[post_author] => 12815
[post_date] => 2025-01-09 11:21:45
[post_date_gmt] => 2025-01-09 16:21:45
[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Dermoscopes are specialized microscopes used to examine the skin. Skin has several layers, and viewing one layer is difficult if another lies above it (see Figure 1). The corneal layer (corneum) is made primarily of the protein keratin. The corneum has many randomly oriented surfaces at air-tissue boundaries formed by air pockets between keratin surfaces.
When light passes between two materials (e.g. air and keratin) with indices of refraction n1 an n2 the fraction of light reflected, R is predicted by Equation 1, and the angle of reflection equals the angle of incidence.
Equation 1. Fraction reflected from index mismatch
Non-polarizing dermoscopes (NPDs; Figure 1A) counteract this reflected “glare” by introducing a refractive index matching immersion fluid to make features in the pigmented epidermis visible. Indices of refraction for various substances are included in Table 1.
Table 1. Indices of Refraction
Polarizing dermoscopes (PDs; Figure 1B) shine linearly polarized light to display structures that lie deeper than those usually visible with NPDs. Most of the photons that enter tissue will scatter at surfaces between materials with different refractive indices. Each scattering interaction carries a probability of changing the polarization of the photon. Photons returning to the light detector with randomized polarization likely went through 10 or more scattering interactions and penetrated 50-100μm into the skin. Light returning to the detector passes through a second linear polarizing filter that is orthogonal to the incident beam’s polarization, largely absorbing photons that have not penetrated deeply into the tissue.
Figure 1. Photon paths in non-polarizing and polarizing dermoscopes
- Non-polarizing dermoscope: immersion fluid permeates corneum
- Polarizing dermoscope: heavily scattered light predominates in detector
What produces most of the “glare” that a non-polarizing dermoscope must overcome?
[value] => Array ( [answer] => 3 [description] =>Reason for the Correct Answer:
The non-polarizing dermoscope (NPD) removes glare by introducing a fluid into the corneum.
The corneum keratin’s index of refraction is closely matched to the immersion fluid, which works its way into the air pockets and fills them, reducing index of refraction mismatch and hence reflections.
The random orientation of the surfaces in the keratin-air pockets create a lot of opportunity for reflection at an interface due to index of refraction mismatch between air and keratin in the corneum. The NPD removes the interfaces by filling the pockets, where the PD simply ignores most light coming from that layer.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. Reflections from pigments found in the epidermal layer of the skin
) [1] => Array ( [each_answer] =>B. Reflection from index of refraction mismatch between air and glass lens in front of the light detector
) [2] => Array ( [each_answer] =>C. Reflections from index of refraction mismatch between air and keratin in the corneum
) [3] => Array ( [each_answer] =>D.Reflection from index of refraction mismatch between keratin in the corneum and the glass lens in front of the light detector
) ) ) [1] => Array ( [quiz_unique_key] => 3873426850 [question] =>If a non-polarizing dermoscope’s normal immersion fluid (n = 1.55) were replaced by a new fluid (n =1.65), would more or less light from the epidermis and dermis reach the detector?
[value] => Array ( [answer] => 2 [description] =>Reason for the Correct Answer:
Look at Equation 1 and Table 1: calculate Rgel and Rimmersion fluid in the corneum only as fractions, not as decimals. Note that for both gel and immersion fluid, |n–nkeratin=0.05.|
For each interaction in the corneum, Rgel < Rimmersion fluid. Because each
R has the same numerator, the gel case’s larger denominator ⇒ Rgel is smaller.
If each interaction reflects less light from the corneum, more light will reach the epidermis and dermis to ultimately be reflected back to the detector.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. More light
) [1] => Array ( [each_answer] =>B. Less light
) [2] => Array ( [each_answer] =>C. Insufficient information provided
) [3] => Array ( [each_answer] =>D. No difference in light reaching the detector
) ) ) [2] => Array ( [quiz_unique_key] => 83407773 [question] =>Which statement best describes a photon entering the detector of the polarizing dermoscope?
[value] => Array ( [answer] => 3 [description] =>Reason for the Correct Answer:
The photon may not have passed through the epidermis and dermis because it could have reflected off a single keratin fiber back into the polarizing filter and detector – that possibility is improbable, though.
The photon is not randomly polarized because the second polarizing filter ensures that only polarized light reaches the detector.
The photon may not have undergone 10 scattering events because a single scattering event carries a non-zero probability of changing the photon’s polarization and reflecting it back into the detector.
Reflections and refractions from stationary objects do not change a photon’s frequency, so it will have the same frequency as when it left the source. This is the only certain option.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. It is randomly polarized (or unpolarized).
) [1] => Array ( [each_answer] =>B. It passed through the epidermis or dermis.
) [2] => Array ( [each_answer] =>C. It has the same frequency as when it left the light source.
) [3] => Array ( [each_answer] =>D. It underwent at least 10 scattering events.
) ) ) [3] => Array ( [quiz_unique_key] => 2261298308 [question] =>At 125μm, the corneum on the palms of the hands is quite thick. Which technique would visualize a spot that a doctor suspects of being cancerous in the epidermis of the palm?
Reason for the Correct Answer:
Using a PD will visualize structures between 50μm and 100μm, which would still fall in the corneum in the palm.
Non-polarizing dermoscopes use immersion fluid to make the corneum relatively translucent.
With the glare from the corneum removed by the immersion fluid, the NPD would be able to reveal details of the suspicious area in the epidermis, but the PD would receive photons mostly from 50μm to 100μm deep in the corneum which is too shallow to see the 125μm-deep spot of interest, so the NPD would work best.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. Non-polarizing dermoscopy
) [1] => Array ( [each_answer] =>B. Polarizing dermoscopy
) [2] => Array ( [each_answer] =>C. Neither non-polarizing nor polarizing dermoscopy are well suited to this task
) [3] => Array ( [each_answer] =>D. Non-polarizing and polarizing dermoscopy would work equally well
) ) ) [4] => Array ( [quiz_unique_key] => 2377279144 [question] =>Would it be possible to construct each type of dermoscope so that the lens does not need to be in contact with the corneal layer (contact in either directly touching the skin or if both the skin and the scope are immersed in fluid)?
[value] => Array ( [answer] => 1 [description] =>Reason for the Correct Answer:
An air space between the glass and the immersion fluid would introduce a large index mismatch.
The NPD would receive a lot of glare from the large refractive index mismatch, so the NPD must be in contact through the immersion fluid.
A polarizing filter will act on any light passing through it.
Looking at Figure 1B, notice that lifting the lens off of the corneum would not introduce a new source of light for the second polarizer to filter out, so no new glare would be introduced.
Because no new light is introduced by lifting it off the skin, and there are no additional changes in the light’s polarization, a PD could be built that does not need to be in contact with the corneum.
Taken together, an NPD must be in contact and a PD can operate without direct contact.
) [answers] => Array ( [0] => Array ( [each_answer] =>A. NPD: no; PD: yes
) [1] => Array ( [each_answer] =>B. NPD: yes; PD: no
) [2] => Array ( [each_answer] =>C. NPD: no; PD: no
) [3] => Array ( [each_answer] =>D. NPD: yes; PD: yes
) ) ) ) [total_question] => 5 [correct_answers] => Array ( [559831|1] => C [559831|2] => B [559831|3] => C [559831|4] => A [559831|5] => A ) [hide_display_feedback_settings] => [hide_solutions] => )