Optical topography is able to measure and display haemodynamic changes occurring in the cortex, but is far less sensitive to deeper tissues. Optical tomography, however, uses widely spaced sources and detectors to measure light which has passed through central regions of the brain, which can be affected by disruption to the supply of blood and oxygen around birth. Light travelling through these regions is heavily attenuated and therefore more intense light sources and more sensitive detectors are required. Even so, the acquisition time is typically a few minutes compared to less than a second in the case of optical topography.