 His father, H.C.A.Lassen, was professor at the University of Copenhagen, and Niels Lassen graduated as medical doctor from the same university in 1951.With his colleague Ole Munck he began in the 1950s to use radioactive isotopes for the measurement of the blood circulation in the brain, and in the beginning of the 1960s he together with from University of Lund began the development of methods for "regional" measurements on the brain with krypton-85 and xenon-133 isotopes.