Basics of fNIRS and Optical Tomography
fNIRS Tomography makes use of the fact that light penetrates up to several centimeters of biological tissue. This is demonstrated by an experiment such as depicted in the image to the right. Here, a bright white light source is directed toward a hand, causing a reddish glow in places where light is transmitted through the tissue. The red color is caused by the pigment hemoglobin in the red blood cells, which primarily absorbs wavelengths outside the red and near infrared spectrum. It is seen that no inner structures such as bones or veins are readily revealed by this simple transillumination technique.
fNIRS employs sensitive instrumentation to precisely measure the amount of light that is transmitted through a body site of interest. Many transmission measurements (hundreds or thousands) are performed in different locations on the target surface creating large data sets which are then used to compute an image of the underlying tissue composition.
The following sections are meant to provide more detailed information about the scientific and technological background of biomedical optical imaging.
fNIRS Tomography Characteristics
The following main properties characterize fNIRS tomography and distinguish it from other biomedical imaging methods such as x-ray, CT, MRI, ultrasound, or nuclear imaging:
• It is safe: No harmful radiation is employed with fNIRS.
• It is minimally invasive: No extrinsic contrast agents are required.
• It is a functional imaging modality; contrast is created by tissue function rather than by anatomic features.
• It relies on small, relatively inexpensive easy-to handle technology.
• It is capable of high temporal resolution allowing the study of dynamic physiologic features.
• fNIRS provides relatively low spatial resolution.
• The penetration range of light in tissue limits the size of the target tissue volume.
No Harmful Radiation
fNIRS employs non-ionizing near-infrared light, so no cancerogeneous or genetic effects are possible. The light intensity is kept well below safety limits to avoid the risk of thermal damages. No cumulative harmful effects are known so that measurements can be applied repeatedly or in a continuous manner for monitoring purposes.
No Extrinsic Contrast Agents Required
The principal absorber in the near-infrared region in tissue is the pigment hemoglobin, which due to its confinement in the red cells, is highly concentrated in blood and therefore provides a strong optical contrast. This allows the sensitive imaging of local blood supply without additional contrast agents.
NIRS Functional Imaging (fNIRS)
Using spectroscopic techniques, fNIRS provides both images of blood concentration and oxygen saturation in tissue, which makes it uniquely suited to assess a host of functional parameters including tissue perfusion, local oxygen supply/demand balance, and autoregulatory response. In addition, OT can be expanded to image other tissue constituents (e.g., water, fat, proteins), which essentially makes it a molecular probing technique. Besides these absorption features, the technology can also explore differences in light scattering properties of different tissue components, providing yet another contrast mechanism.
Furthermore, extrinsic contrast agents can be imaged, whose accumulation or activity is influenced by local physiologic factors, such as such tissue type, metabolism, or chemical environment.
Compact, Low-Cost Technology
The development of fNIRS has strongly gained from the advances in microelectronics, computer technology, and optical engineering that were made over the last decades. Typically, semiconductor lasers are employed as light sources because of their efficiency, beam quality, and ease of operation. Fiber optics can be used to transmit light to and from the tissue, thereby allowing for great flexibility in instrument design. Advances in electronics allow precision analog and digital signal processing at ever-decreasing footprints and power consumption. Combined, these technologies allow for compact and cost-effective fNIRS instrumentation. With the advent of further miniaturization and integration such as integrated optics and MEMS devices, wearable and even disposable fNIRS technology can be envisioned.
Temporal & Spatial Resolution
Although optical measurements can be exceedingly fast-on the order of 10-9 seconds and faster-fNIRS image capture rates are much slower than that in order to maximize signal quality. For dynamic applications, typical image frame rates are on the order of several Hz, enough to capture fast physiologic signals, such as the cardiac frequency or perfusion changes due to neuroactivation.
Because of the physics of light propagation in turbid media, the spatial resolution of fNIRS images is relatively low. Depending on the composition and size of the target tissue, the resolution is on the order of 5 mm.
Light intensity is heavily attenuated in tissue and falls off exponentially from the illumination point. The maximum achievable range over which a measurement can be performed is limited by the source strength - determined by the thermal damaging threshold - and the detection sensitivity, given by the detector noise. Imaging depth strongly depends on tissue composition and measurement geometry. For a brain measurement in backreflection mode, the probing depth is about 3 cm, for a cross-sectional limb image, it is about 8 cm, and for a full volumetric breast scan, up to 12 cm.