Abstract: The present invention discloses indicator fluids, reference indicator fluid, and usage thereof, and systems and methods for assessing movement of molecular substances within, to or from a cerebrospinal fluid, brain or spinal cord compartment of a human cranio-spinal cavity. Indicator fluid moving from the cerebrospinal fluid compartment enables measurements of levels of indicator fluid in blood or urine and assessment of the cranio-spinal cavity's ability to remove molecular substances. The indicator fluids may be contrast agents used for imaging, such as by computed tomography imaging, and magnetic resonance imaging, or imaging utilizing radioactive substances by positron emission tomography, single-photon emission computed tomography or scintigraphy.