Noninvasive subcutaneous skin imaging is a tool sought for use by the medical, pharmaceutical and personal care industries, but techniques have been lacking due to resolution and speed constraints. However, these obstacles may have been overcome by Jannick Rolland, PhD, and her team at the University of Rochester who developed a probe with a custom liquid lens microscope that can noninvasively image skin up to 2 mm deep with lateral resolution.
Skin Imaging Challenges
Lateral resolution is an important aspect of skin imaging, according to Rolland. “Previous capabilities have allowed the imaging of tissue under the skin, but that imaging showed only the layers and not the cells as there was no lateral resolution across the depths,” she explained. Rolland furthered that lateral resolution could be obtained by moving the slide on a stage and measuring point by point, e.g., in time-domain optical coherance tomography, but that this technique could not be applied in an in vivo clinical setting.
Rolland noted that in vivo skin imaging must be conducted quickly to ensure that the subjects’ breathing and motion do not affect the results. She added that while the recently developed Fourier domain optical coherence tomography technique allows for a quick, entire depth scan with one measurement followed by a lateral scan to obtain a 3-D image, the lateral resolution remains unclear.
“The idea was that either speed or resolution could be achieved, but not both,” Rolland said. Her team thus sought to develop a noninvasive technique to image the layers of skin with depth and lateral resolution.
Resolving Lateral Resolution
The key to obtaining lateral resolution, according to Rolland, is to open the numerical aperture of the lens to make it larger, referred to as Fourier domain optical coherance microscopy (FDOCM). “The problem is that lateral resolution can only be obtained when the lens is focused in one spot,” Rolland explained. She furthered that depending on the angle of the lens light, a wider cone would provide more lateral resolution but result in less clarity around that point, whereas a narrower cone would provide clearer depth but with little resolution.
Drawing from the liquid lens technology used to auto-focus cell phone cameras, Rolland and her team created a probe lens to image the skin quickly with lateral resolution, as well as a custom microscope designed around the technology. “This lens can refocus within 30 ms with the possibility to go faster,” Rolland said.
The device consists of two major components, the scan lens and the microscope head, which hosts the liquid lens. The liquid lens* is comprised of a droplet of water and oil and electrodes. Electrical current is then incorporated so that each time voltage passes through the electrodes in the liquid lens, it refocuses to take another image—i.e., electrowetting. Rolland explained, “As you apply voltage, the electrode becomes less hydrophobic and the junction of the oil/water interface changes shape when voltage is applied.”
The final element of the probe is the spectrometer, which utilizes infrared light to capture measurements. “To achieve a high measuring speed, we developed our own spectrometer with a broadband source to measure the spectrum of light in addition to its intensity.” The team chose infrared light to penetrate more deeply into skin since visible light limits imaging depth by 200 μ due to scattering.
The probe is capable of imaging 8 mm3 of tissue, 2 mm long x 2 mm wide x 2 mm deep, with a 2 μ resolution. Rolland explained, “We obtain multiple cubes of data, each being at a different focus of the lens. [Then] we use an algorithm to section out the focused region within those cubes and merge them together. Rolland’s team calls this technique Gabor domain optical coherance microscopy, which combines the high speed component of FDOCM with lateral resolution. Thus far, in human skin in vivo, the team has imaged 1 mm deep with a 2 μ resolution, although the probe could measure deeper based on variables such as the transparency of the skin, the wavelengths used, the type of tissue, etc.
According to Rolland, this device is currently used in the medical field to determine if skin lesions should be biopsied by identifying intact skin. In the future, it may be used to conduct optical biopsies of lesions.
The team also is working on taking its device into the fields of dermatology, ophthalmology and neuroscience. For instance, the device has been applied in modified form to imaging the cornea in ophthalmology, and to imaging the brain in neuroscience.
In relation to personal care, Rolland noted the probe device could possibly be used to determine the penetration of cosmetic ingredients and observe their effects, or to image the nail fold. In addition, through collaborations with dermatologists, the probe may be used to observe the skin’s barrier to pathogens and the penetration of nanodots into the skin of mice.