Oregon Medical Laser Center NewsEtc., Feb. 1, 1998. Steven Jacques
A clinical investigation with a new video camera that uses polarized light is demonstrating the ability to visualize the true margins of skin cancer which are not clinically visible to dermatologic surgeons. Investigators at the Dept. of Dermatolgy, Oregon Health Sciences University (OHSU), are observing skin cancer lesions in patients at the Veterans Adminstration Hospital, Portland, Oregon.
In one example, a patient with sclerosing basal cell carcinoma was viewed with the polarized light camera which could detect a cancer margin of 11 mm (see Figs. 1 and 2). The clinical margins observable by eye was only 6 mm in diamter. Although the polarized camera predicted a larger margin, the doctors surgically excised only a little beyond the 6 mm visible lesion as is normal practice. The histology came back with margins positive for cancer and the doctors had to reschedule surgery for another larger excision. This story illustrates how the polarized light camera could improve the planning of a first surgical excision to reduce the likelihood for needing follow-up surgery, a very cost-effective strategy.
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Figure 1 compares a normal image (1A) versus polarized image (1B) of a sclerosing basal cell carcinoma.
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Figure 2 shows the cancer margins observable with the polarized image versus the doctor's unaided eye. The margins of the cancer observed in the polarized image were 11 mm in diameter. The clinical margins observable by eye were only 6 mm in diamter. Although the polarized camera predicted a larger margin, the docotres surgically excised only a little beyond the 6 mm visible lesion as is normal practice. The histology came back with margins positive for cancer and the doctors had to reschedule surgery for another larger excision.
The new camera was developed by Steven L. Jacques, Ph.D. and Ken Lee, M.D. at the Oregon Medical Laser Center (OMLC) at Providence St. Vincent Medical Center. The goal is to create an image based on the light scattered just from the upper couple hundred micrometers of the skin which is the superficial layer where most skin pathology arises. About 4% of reflected light from the skin is surface specular reflectance or glare from the air/tissue surface which are photons that never entered the skin. About 91% of skin reflectance is multiply scattered light which has penetrated deep into the tissue and become randomly polarized. Only about 5% of skin reflectance is polarized light which reflects from the superficial skin layer where pathology arises, and this light is used by the new camera to create images.
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|Figure 3 shows the prototype being tested clinically. The light source is incoherent white light polarized by a polarization filter which obliquely illuminates a skin site. An optical flat on the skin surface provides optical coupling and a smooth surface so that specular reflectance from the glass/skin interface is reflected obliquely and misses the camera. Light scattered by the tissue is imaged by a camera which views the site through a polarization filter that can be aligned to collect light polarized either parallel or perpendicular relative to the source. |
One image (Par) is taken with parallel orientation of the camera and one image (Per) is taken with perpendicular orientation. A new image is calculated pixel by pixel from the two acquired images: new image = (Par - Per)/(Par + Per). The numerator (Par - Per) subtracts out the 91% of randomly polarized light due to multiply scattered light from deep skin layers. Normalization by (Par + Per) causes cancelation of any common attenuation due to the skin pigmentation of melanin. Hence, the ratio clarifies pigmented lesions allowing the doctor to view the underlying tissue structure.
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Figure 4 compares a normal image versus a polarized image of a freckle. The freckle's pigmentation disappears in the polarized image. There is nothing abnormal under the freckle.
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Figure 5 compares a normal image versus polarized image of a pigmented nevus. The nevus pigmentation disappears in the polarized image to reveal an underlying structure.
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Figure 6 shows the prototype camera with Dr. Ken Lee.