a needle comprises an elongated first shaft element with a c - shaped cross section and an elongated second shaft element with a c - shaped cross section . the first shaft element and the second shaft element are adapted to be detachably connected to each other . this is achieved by an overlap of respective end sections of the first and second shaft elements . within the channel formed by the shaft elements , an optical probe may be accommodated from which the shaft elements may be removed by separating the two shaft elements from each other .