Diagnosis of brain edema is important to manage severe head injuries and diseases, but there is no method of noninvasive bedside monitoring of brain edema. We speculate that brain edema changes the optical properties of brain tissue. To verify this speculation, we perform transcranial measurement of diffuse light reflectance from cold-injured brains in rats. To induce edema, a liquid nitrogen-cooled copper probe of 3 mm diameter was applied to the right parietal bone for 60 s under anesthesia. The rat skull was irradiated with 633-nm HeNe laser light and 532-nm Nd:YVO4 laser light, which were used to investigate changes in light scattering and blood flow, respectively. Reflectance intensity was measured as functions of time and position on the skull. We found that reflectance intensities were significantly decreased around the cold injury at both 633 and 532 nm, while reflectance was increased in the cold injury at 532 nm, indicating that cerebral blood volume was decreased in the cold injury. Under the condition of decreased cerebral blood volume, the decrease in reflectance intensity around the injury suggests that the scattering coefficient of brain tissue was reduced due to edema formation in this area.
We assumed that edema causes a decrease in the scattering coefficient of brain tissue and hence a decrease in the intensity of diffuse reflectance from the brain. On the basis of this assumption, we attempted to transcranially detect a formation of brain edema by measuring diffuse light reflectance. In rats, edema was induced by making a cold injury in the brain. The skull was irradiated with 633-nm and 532-nm laser light delivered through an optical fiber, and the diffuse light reflectance from the brain was collected with another optical fiber. We observed that reflectance intensities were significantly decreased around the cold injury both at 633 nm and 532 nm, suggesting that scattering coefficient of brain tissue was reduced due to a formation of edema in this area. In the injury, reflectance intensity was increased at 532 nm, indicating that cerebral blood volume was decreased in this region.
Brain edema causes an increase in brain tissue volume which results from an accumulation of transudate due to the increased permeability of the brain capillaries. Assuming the transudate to be the material with a low scattering coefficient, brain edema would decrease scattering coefficient of the brain tissue, and diffuse reflectance from the brain may, therefore, be decreased. We examined whether brain edema can be detected or not by measuring the diffuse light reflected from the brain. We induced brain edema by cold injury in rats. The rat skull was irradiated with 633-nm HeNe laser through an optical fiber, and the diffuse light reflected from the brain was collected through another optical fiber. Intensity of the diffuse light reflectance was measured with a polychromator as functions of the time and position on the skull. We found that intensity of the detected diffuse light reflectance decreased at 24 h after injury in the surround area of the injury, suggesting that brain edema can be detected by measuring the diffuse light reflected from the brain.