Applications for Optical Coherence Tomography in Dermatology

Optical Coherence Tomography Imaging of Melanoma

Malignant melanoma is the most serious form of skin cancer with the highest risk of metastasis, and its incidence is increasing dramatically. The risk of metastasis is minimized by early detection and removal. However, the diagnostic accuracy of visual inspection and dermoscopy is limited and a majority of pigmented lesions removed are found to be histologically benign. In view of this there is considerable need for the development of noninvasive imaging methodologies for the detection of melanoma. Early studies of OCT in diagnosis of pigmented lesions evaluated the correlation between the appearance of OCT and histological features.^[56] Evaluation of pigmented lesions is challenging with OCT as the technology does not have the capability to achieve cellular resolution and melanin pigment is opaque to light.

Diagnostic features of melanoma in comparison with benign naevi on OCT images include epidermal psoriasiform hyperplasia, melanocytic nests and vertical icicle-shaped structures.^[57–59] However, evaluation of OCT in a diagnostic setting has found that the levels of sensitivity and specificity are not presently sufficient for accurate diagnosis.^[58–60] For example, in a blinded study of 93 melanocytic lesions the sensitivity of OCT was 74·1% and the specificity was 92·4%.^[59] In addition to diagnosis, OCT may come to play a role in the preoperative risk stratification of patients, and OCT findings have been shown to correlate with tumour thickness.^[61]

In comparison with conventional OCT, angiographic OCT shows potential in the diagnosis of melanoma. Angiographic OCT is able to detect lesion progression via early alteration in vessel morphology from dysplastic naevus to melanoma.^[13] Those vascular changes were confirmed by angiographic OCT in a retrospective analysis of 127 histologically verified melanomas. At 150 μm (the most superficial depth), the authors identified early changes such as serpiginous and branching vessels with bulges in melanomas thicker than 2 mm. Those features were also observed at 300 μm.^[62] The role of angiographic OCT in diagnosis and staging of melanoma is likely to prove a fertile area for investigation.

Table 1. Features of different optical imaging methods for *in vivo* skin imaging
	Penetration depth (mm)	Lateral resolution (μm)	Axial resolution (μm)	FOV (mm)	Advantages	Disadvantages
RCM (VivaScope 1500)	0·2	< 1·25	< 5·0	0·5 × 0·5 (8 × 8 mosaic)	Cellular resolution	Limited penetration depth (up to papillary dermis layer)
Multiphoton microscopy (MPTflex)	0·2	< 0·5	< 2·0	0·35 × 0·35	Excellent cellular resolution and contrast	Narrow FOV and limited penetration depth (to top of papillary dermis layer)
HFUS (20–100 MHz)	0·35–15	33–250	17–80	8·0–12	Deep penetration depth and good FOV	Unsatisfactory cellular resolution
LC-OCT (Damae Medical)	0·5	1·3	1·1	1·2 × 1·2	Excellent spatial resolution for OCT	Limited penetration depth for OCT
FF-OCM (SkinTell)	1·0	3·0	3·0	1·8 × 1·5	Good resolution for OCT	Long acquisition time
FD-OCT (VivoSight)	1·0	< 7·5	< 5·0	6·0 × 6·0	Good penetration depth to dermis, good imaging speed, excellent FOV	Not cellular resolution

FD-OCT, frequency-domain optical coherence tomography; FF-OCM, full-field optical coherence microscopy; FOV, field of view; HFUS, high-frequency ultrasound; LC-OCT, line-field confocal optical coherence tomography; RCM, reflectance confocal microscopy.