A method for determining discrete localized blood flow in a large volume of tissue, such as local cerebral emission flow in a discrete radiation emission detection zone of a patient's brain. The brain is subdivided into a plurality of detection zones, such as 144, by means of a headgear apparatus which defines these detection zones by means of a multiplanar array of an equal number of detectors, each detector having an associated collimator to limit the field of view of the detector. A radioactive gas, such as one having two gamma energy levels, such as Xe.sup.133, or a gaseous mixture of two different gamma energy level isotopes, is inhaled by the patient, such as in a closed inhalation-exhalation breathing system, over a predetermined data collection period during which the concentration of the gas in the blood is periodically increased. Determinations of arterial blood radiation concentration and tissue radiation concentration are made during this period, and the local cerebral blood flow is subsequently determined from these determinations. The arterial blood radiation concentration is preferably determined by determining the radiation concentration of the gas in the last part of an exhalation, while the tissue radiation concentration is preferably determined from the radiation emissions quantified by the detector array such as by detecting both gamma energy levels, the field of view of the detector depending on the gamma energy level being detected.