Researchers at Duke University in Durham, N.C., have developed an imaging method based on optical coherence tomography (OCT) to monitor proteins that cause cancer. They used the technique to observe as they gently heated a solution of gold nanoparticles that they inserted into cells and into artificial material meant to represent tissue. The investigators showed that they could perform this imaging technique without harming cells or tissue. The intensity of the laser beam did not exceed the American National Standards Institute (ANSI) safety standard used with human patients.Figure 1. Scientists added gold nanoparticles attached to an antibody for the epidermal growth factor receptor, a protein well known to be involved in cancer. They imaged the cells with a variation of OCT called photothermal OCT because in the future, when they do experiments that build off this work, the method will allow them to see deeper than multiphoton or confocal microscopy. Reprinted with permission of Nano Letters.In the experiments with cells, they used gold nanoparticles attached to antibodies for the epidermal growth factor receptor, a protein well known to be involved in cancer. The researchers believe that measurement of the receptor represents the first time anyone has imaged a molecular target with OCT. They heated the gold nanoparticles with a 532-nm Coherent Verdi laser system. When gold nanoparticles are heated, the refractive index of the surrounding medium changes, causing the optical path length to change as well. The researchers measured the change in optical path length with their OCT system. In this way, they monitored the heating process without directly measuring temperature. They call this technique “photothermal OCT.”Dr. Melissa Skala, the postdoctoral researcher who led the study, and her colleagues used a special fan with the heating laser beam. The fan blades stopped the laser beam, while the spaces between the blades allowed the beam to pass. This way, they knew exactly when they were heating the nanoparticles and could measure the change in the refractive index at those time points.They integrated the heating laser and OCT setup with a microscope. Both the cells and the artificial material meant to represent tissue were placed on microscope slides.A closer lookSkala and others used a very bright LED to perform OCT. This type of light source is used in most OCT systems.However, in contrast to classical OCT systems that have separate arms that direct the light to and from the sample and reference mirror, the reference and sample light travel a common path (Figure 2). The reflection from the coverslip on the microscope slide serves as the reference.Figure 2. The 532-nm laser heats the gold nanoparticles in the cells or in the artificial material meant to represent tissue. A special fan called an optical chopper ensures that the samples are heated at known time points. A superluminescent LED is used as the light source for OCT, and a spectrometer with a line-array CCD is used as the detector. The OCT system and laser are connected to a microscope, and the samples are viewed on microscope slides. Reprinted with permission of Nano Letters. The researchers used an established variation of OCT called spectral-domain OCT. This type of system uses a diffraction grating and a spectrometer with a line-array CCD.As opposed to the movable mirror in first-generation time-domain OCT systems, which encodes the image information in time, the diffraction grating disperses the collected light onto the detector strip and thus encodes the image information in space. The information is converted into an image with a special mathematical equation called a Fourier transform.The researchers detailed their results in the October 2008 issue of Nano Letters. Their system successfully detected the epidermal growth factor in cells, and it measured the gold nanoparticles down to 14 parts per million in the artificial material.A test with a blue dye after laser treatment confirmed the viability of the cells. The dye passes through the broken membrane of dead cells, whereas cells that have an intact cell membrane can expel the blue dye.Although the researchers wanted to show that this imaging method can heat cells and tissue gently without causing harm, “A future application would be to use photothermal therapy to heat up cells that are cancerous,” according to Skala. Within the next year, the investigators plan to collaborate with Dr. Mark Dewhirst, an expert in cancer treatment at Duke. “We usually look at oral cancer, esophageal cancer, cervical cancer because they’re on the surface, and they’re fairly prevalent,” Skala said.However, she cautioned that medical applications “might be a ways off” because FDA approval takes a long time to earn, and the agency requires approval for each new application. Even so, gold nanoparticles are promising for medical treatment. Currently, doctors use a solution of gold nanoparticles to treat arthritis, and a lot of the research has shown that animals and cells tolerate gold nanoparticles very well.