In the 1950s and 1960s calcium-45 was investigated, but as a beta emitter proved difficult to image. Imaging of positron and gamma emitters such as fluorine-18 and isotopes of strontium with rectilinear scanners was more useful.[8][9] Use of technetium-99m (99mTc) labelled phosphates, diphosphonates or similar agents, as in the modern technique, was first proposed in 1971.[10][11]