Medical optical imaging

Medical Optical imaging describes various imaging techniques using visible, ultraviolet, and infrared light used in imaging.

Because light is an electromagnetic wave, similar phenomena occur in X-rays, microwaves, radio waves. Chemical imaging or molecular imaging involves inference from the deflection of light emitted from (e.g. laser, infrared) source to structure, texture, anatomic and chemical properties of material (e.g. crystal, cell tissue). Optical imaging systems may be divided into diffusive and ballistic imaging systems.

Diffusive optical imaging in neuroscience

Diffusive Optical Imaging (DOI) also known as Near-Infrared Optical tomography (NIROT), Diffuse Optical Tomography (DOT) or Optical Diffusion Tomography (ODT) is a technique that gives neuroscientists the ability to simultaneously obtain information about the source of neural activity as well as its time course, allowing to "see" neural activity and study the functioning of the brain.

In this method, a near-infrared laser is positioned on the scalp. Detectors composed of optical fiber bundles are located a few centimeters away from the light source. These detectors sense how the path of light is altered, either through absorption or scattering, as it traverses brain tissue.

This method can provide two types of information. First, it can be used to measure the absorption of light, which is related to concentration of chemicals in the brain. Second, it can measure the scattering of light, which is related to physiological characteristics such as the swelling of glia and neurons that are associated with neuronal firing.

Typical applications include rapid 2D optical topographic imaging of the event-related optical signal (EROS) or Near-infrared spectroscopy (NIRS) signal following brain activity and tomographic reconstruction of an entire 3D volume of tissue to diagnose breast cancer or neonatal brain haemorrhage. The spatial resolution of Diffuse Optical Tomography (DOT) techniques is several millimeters, comparable to the lower end of functional magnetic resonance imaging (fMRI). The temporal resolution of EROS is very good, comparable to electroencephalography, and magnetoencephalography (~milliseconds), while that of NIRS, which measures hemodynamic changes rather than neuronal activity, is comparable to fMRI (~seconds). DOT instruments are relatively low cost ($150,000), portable and immune to electrical interference. The signal-to-noise ratio of NIRS is quite good, enabling detection of responses to single events in many cases. EROS signals are much weaker, typically requiring averaging of many responses.

Important chemicals that this method can detect include hemoglobin and cytochromes.

Ballistic optical imaging

Ballistic optical imaging systems ignore the diffused photons and rely only on the ballistic photons to create high-resolution (near-diffraction limited) images through scattering media.

See also

• Photon diffusion
• Ballistic imaging
• Photon diffusion equation
• Diffuse Optical Imaging
• Optical Tomography
• Optical coherence tomography

External links

• Medical Optics Group at ICFO, Barcelona, Spain
• Understanding Near-Infrared Imaging – Resource to better understand the benefits of Near-Infrared imaging.
• Diffuse Optics Lab at University of Pennsylvania, Philadelphia
• DOI at Massachusetts General Hospital, Boston
• Biomedical Imaging Group at Dartmouth
• DOS/I Lab at the Beckman Laser Institute, University of California, Irvine
• A review article in the field by A.P. Gibson et al.
• An article on optical breast imaging
• Illinois ECE 460 Principles of Optical Imaging Course lecture notes
• MRRA Inc. fNIRS Systems [1]