The arrangement and method

Arrangement for defining a location within an organism, said arrangement comprising: a hollow needle (2), comprising a longitudinal channel (5) and a distal end (4); a mandrin (1), accommodated in the longitudinal channel (5) in mobile manner in relation to it and removable from said channel (5) and having a distal and (3) of closed design, comprising an electrode pattern including at least two electrodes (7a, 7b, 7c, 7d); as well as a processing unit, configured to monitor the advancement of the distal end (4) only on the basis of a measurement signal of bioimpedance received from the electrodes (7a, 7b, 7c, 7d) of the electrode pattern. The application includes also an independent claim for a method for manufacturing a mandrin to be accommodated in a needle.