What is spectrophotometric analysis?
Spectrophotometric or spectral analysis of skin lesions refers to the use of a skin imaging device to help evaluate pigmented skin lesions (moles) and make it easier to identify and diagnose early stage malignant melanomas (skin cancers).
Spectrophotometric analysis is an advanced form of dermoscopy using a computer software program that calculates and extracts information about the cells and structures of the skin. Currently, most dermoscopic devices work by using a powerful lighting system and a high quality magnifying lens. Light penetrates the skin 20 microns deep and magnified digital photographic images are taken. Spectrophotometric analysis takes dermoscopy a step further by using a light beam that penetrates to a depth of 2-2.5 mm (1000x deeper) beneath the skin surface. Light images taken with a digital camera or hand-held scanner are then fed into a computer.
Spectrophotometric analysis works on the principle that light energy is absorbed and remitted by particular target cells with colour (chromophores) in the skin.
What is spectrophotometric analysis used for?
Spectrophotometric analysis is used to aid in the early detection and diagnosis of malignant melanoma. In expert hands, it has been shown to increase diagnostic accuracy (sensitivity) from around 70% by clinical examination alone, to 95%.
Devices discussed here are the SIAscope® and MelaFind®.
The SIAscope is a type of spectrophotometer. SIAscope stands for Spectrophotometric Intracutaneous Analysis and is a trademark of Astron Clinica Limited.
- The device emits visible and infrared light
- Various chromophores in the layers of skin respond to the light differently and send back remitted light
- Melanin absorbs ultra-violet light
- Haemoglobin (red blood cells) absorb infrared light
- Collagen absorbs and remits light across the spectrum corresponding to the size and amount of collagen cells in the deeper layers of skin (papillary dermis).
- Reflected light received by the spectrophotometer is analysed by a computer software program that calculates the quantity of light absorbed at various wavelengths.
- The images created from computer analysis show the presence of melanin, blood or collagen changes in the area examined. For example, melanocytes may be shown in the deeper layers of skin, which may be an early indication of melanoma.
The SIAscope procedure
Spectrophotometric analysis is performed using a specialised digital camera to examine the skin.
|Contact spectrophotometric analysis
|Non-contact spectrophotometric analysis
MelaFind® is an automated multispectral device that has FDA approval for melanoma detection. In contrast to the SIAscope, MelaFind uses 75 features to evaluate the degree of 3-dimensional disorganisation in the tumour using 10 distinct wavelengths from blue light to near infrared. Highly disorganised lesions should be excised. Lesions with low disorganisation can be left alone.
It was designed to be used by dermatologists to help decide whether to remove suspicious pigmented lesions.
The MelaFind procedure
Patient details including history of the lesion are recorded. The lesion is identified and its location is carefully marked on a computer mannikin.
The pigmented lesion is first carefully sprayed with alcohol to provide a wet surface without bubbles for refractive index matching. The scanning head is placed on the skin centering the lesion of concern, and imaging commences. Six seconds later, ten images are displayed on the computer screen with the results of the analysis: biopsy or monitor.
The images are discarded if they are poor quality and the procedure is repeated.
Automatic analysis is in five parts.
- Calibration to reduce artifacts and measure skin reflectance
- Assessment of image quality
- Lesion segmentation i.e. the edge of the lesion is identified
- Feature extraction algorithms
- Lesion classification as positive (biopsy) or negative (monitor)
Which skin lesions are suitable for MelaFind examination?
MelaFind® is not used to confirm a typical melanoma, nor is it necessary for typical benign lesions. Skin lesions that are ideal for MelaFind® examination have the following characteristics:
- Pigmented – tan to dark brown
- 2-22 mm in diameter
- On flat skin surface
- With one or more of the ABCD characteristics (asymmetry, border irregularity, colour variation or diameter >6 mm) or other clinician concern
- Without ulceration or bleeding or scar
- Located greater than 1 cm away from the eye
- Not located on palmar, plantar or mucosal surfaces or nails
- There should be no tattoo ink, marker pen ink, hair or splinter.
Please note, this information was provided by the manufacturer of the devices. DermNet's editors consider there is insufficient data to determine the role of these devices in the overall management of patients at risk of melanoma.
Patients concerned about melanoma should undergo full skin examination from a suitably trained health professional. Lesions of concern are best evaluated using derm(at)oscopy.