Lassen has been called the greatest Danish brain researcher since Niels Steensen.
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 David H. Ingvar [sv] from University of Lund began the development of methods for regional measurements on the brain with krypton-85 and xenon-133 isotopes.
The Niels Lassen Award[5] is presented by the International Society for Cerebral Blood Flow and Metabolism (ISCBFM).
The Danish medical doctor, brain researcher and "science entertainer" Peter Lund Madsen was a student of Niels A. Lassen, and Peter Lund Madsen received the Niels A. Lassen Prize in 2002.
[6] Previous recipients have included : Lassen himself received The Novo Nordisk Prize in 1968, from the Novo Nordisk Foundation,[7] and was the first recipient of the inaugural International Society for Cerebral Blood Flow and Metabolism (ISCBFM) Lifetime Achievement Award in 1997.