Imagent provides a balance between temporal and spatial resolution for the study of superficially located areas of human brain. Imagent detects variations in the oxygenation levels of activated brain areas and provides a map of the areas where the changes occur. The technique is called Diffuse Optical Tomography (DOT).
Brain imaging techniques can be broadly classified in two groups. One group includes the techniques that have a good spatial resolution (up to 1-2 millimeters) but a poor temporal resolution, such as functional Magnetic Resonance Imaging (fMRI) and Positron Emission Tomography (PET). The second group includes techniques featuring an excellent temporal resolution (of the order of milliseconds) but providing a limited spatial information. This group includes the Event Related Brain Potential (ERP) and the Magneto-encephalography (MEG).
Imagent is an instrument capable of detecting changes in areas of the brain activated by an external stimulus; the detected signal is processed and produces a map of the activated area.
The instrument working principle is based upon the use of infrared light to probe changes in the brain hemodynamic response. Infrared light at a wavelength in the range from 670 nm to about 850 nm penetrates fairly freely in tissues; it can go through the bone of the skull, traverse the dura matter and the arachnoid matter. Infrared photons can reach about 2 cm below the skull's surface all the way to the cortex area where most of the grey matter is located; from there, some of them are scattered back throughout the tissue all the way to the surface and escape out. Whenever a change in the relative concentration of oxy- and deoxy-hemoglobin occurs in the cortex, a change in the number of photons escaping the brain is detected.
Imagent delivers photons by using fiber optics that are positioned on the head of the patient (emitters); other fiber cables (collectors) are positioned at proper distances from the emitters to collect the photons that escape the tissue. Up to 64 fiber sources and up to 8 fiber collectors can be positioned on the head.
Imagent utilizes the frequency domain technology whereas the light sources are modulated at high frequency and three parameters of the detected signal are measured: the intensity, the modulation depth and the phase delay. Any two combination of the three measured quantities can be utilized to provide changes in the physiological parameters, the choice being dictated by the specific parameter to be measured, by the need to reduce physiological noise and by the time scale of the event to be measured.
Functional information, versus structural, derives from the slow (> 100 ms) and fast (< 100 ms) optical signals observed during brain stimulation. Functional measurements have been reported on the motor cortex during motor stimulation; on the visual cortex during visual stimulation; on the frontal region during mental work; and on the monitoring of cerebral hemodynamic during sleep. These are some of the techniques utilized:
- For fast signals: EEG, MEG, NIRS measurements: scattering, neuronal membrane
- For slow signals: fMRI, PET, NIRS measurements: Hemoglobin concentration, changes in blood flow, Neurovascular coupling
Imagent captures both the slow signals (hemodynamic changes) and the fast signals (the event related optical signal, or EROS).
How Imagent Works
Imagent’s working principle is based on the use of near infrared light for probing the cortical surface. The main tissue absorbers in the wavelength region spanning from 700 nm to 900 nm are oxy-hemoglobin (HbO2) and deoxy-hemoglobin (Hb). On a smaller scale, water, fat and cytochrome oxidase contribute to the partial absorption of the light. The penetration depth of light in tissues is quite significant in this wavelength range. For typical head tissue (skin/scalp, skull and cortical layer), with an absorption coefficient of μa = 0.1 cm-1 and a scattering coefficient μs' = 8 cm-1, the maximum optical penetration can be estimated to be about 1.5 cm when a detector is placed at 4 cm from the source. The penetration depth can be increased by increasing the distance between the source and the detector, although, eventually, the signal-to-noise ratio of the measurement deteriorates.
Imagent utilizes laser diodes emitting at 690 nm and 830 nm. The light is delivered by fiber optics positioned on the skull. Upon entering the tissue, the near infrared light, albeit weakly absorbed, is highly scattered by the tissue inhomogeneities. A fraction of the light leaves the tissue and it is collected by the collecting fiber that carries it back to the light detectors housed in the unit for data processing.
Comparison with fMRI
A magnetic resonance imaging (MRI) examination is not an easy undertaking for every person. Let us look at the external conditions: the patient is enclosed by the narrow tunnel while the instrument is generating a rattling noise and the patient is instructed not to move while the machine is taking measurements, which may take up to twenty minutes. In a word, the examination is not conducive to relaxation. These limitations of fMRI are severe when the subjects to examine are children or infants, adolescents with attention deficit hyperactive disorder and when the patient is confused and feels claustrophobic.
Imagent does not have these limitations. Fiber optics sensors are placed on the head of the patient who can be sitting on a chair in a friendly environment and move freely; no noise is generated by the instrument. The instrument also frees the scientist to conduct extended neural analysis studies of brain activity during movement - research that is impossible or impractical with fMRI. The operator of the instrument can be sitting at a distance from the subject or close to the subject. Fibers can be as long as 10 meters, which opens several interesting options for investigational purposes.
In some applications the researcher wants to monitor the brain activity for an extended time period; when using Imagent there is no time limitation as the electrodes can be left in place even for several hours.
Comparison of the signals measured by Imagent and by fMRI
Functional MRI (fMRI) is a powerful technique for the study of cerebral activation. It does not have any penetration limits, provides high spatial resolution and allows event related measurements. Most studies of cerebral hemodynamics are based on the use of blood oxygen level dependent (BOLD) signal. The increase of the BOLD signal is typically interpreted as a decrease in the concentration of deoxy-hemoglobin (washout) due to the increase supply of oxy-hemoglobin.
Although the technique is widely used, it also suffers some severe limitations. The BOLD signal depends not only on the deoxy-hemoglobin concentration changes but also on changes in the total blood volume. In addition, a positive BOLD signal can be due to an increase in the water fraction in the measured volume. As more than one variable contributes to the signal, fMRI does not provide the biochemical specificity needed to distinguish physiological parameters.
Imagent measures changes in both oxy- and deoxy-hemoglobin concentration; therefore providing the researcher with two simultaneous parameters unequivocally related to the brain hemodynamics. The instrument is also capable of directly detecting the neuronal activity that manifests through the fast signal (event related optical signal - EROS). NIRS techniques have the inherent capability to distinguish physiological parameters.
|Operations:||• Method: frequency domain
• Modulation frequency 110 MHz
• Sampling time: minimum xx ms
• Number of channels: 4; upgradeable to 16
|Measured Parameters:||• Changes in [O2Hb] oxygenated hemoglobin
• Changes in [HHb] deoxygenated hemoglobin
• Changes in [Hb] total hemoglobin
|Light Sources:||• Fiber coupled laser diodes|
• Wavelengths: 690 nm and 830 nm
• Laser power: 10 mW average
|Light Detectors:||Photomultiplier tubes|
|Sensors:||• Selectedside-on photomultiplier tubes in room-temperature or cooled housing|
• Optional: CCD camera
|Interface:||Interfaceable to FastTrack by Polemus for Talairach registration technique of brain coordinates|
|Pre-Amplifier Discriminators:||100 MHz bandwidth, TTL output|
|Computer and Operating System:||Intel-type CPU, Windows XP operating system|
|Power Requirements:||Universal power input: 110-240 V, 250 W|
Selected publications featuring Imagent
Below is a list of selected publications featuring Imagent. Click here to browse our complete library of publications.