The subject of this thesis is the development of new gamma ray imagers for nuclear medicine, in particular for scintimammography. The need for new compact detectors with high spatial resolution led to the design of the wavelength-shifting fibre (WSF) gamma camera, where the position of interaction of gamma rays inside an inorganic scintillator is read out by WSFs. The feasibility of the concept of a WSF gamma camera was assessed by simple analytical calculations and Monte Carlo simulations, based on the known characteristics of the individual components: the wavelength-shifting fibres, the scintillation crystal and the photodetectors. Studies were carried out of the light trapped inside WSFs coupled to CsI(Na) scintillation crystals irradiated by 122 keV gamma rays. The results confirm the feasibility of a WSF gamma camera despite the low light levels trapped in the fibres. Experimental tests of several position sensitive photomultiplier tubes (PSPMT) of the Hamamatsu R5900 series were performed to assess their suitability for the readout of scintillation crystals and of optical fibres in photon counting mode. The latter application is important for the readout of the WSFs signals in a WSF gamma camera. A WSF gamma camera prototype was built and tested. The R5900-M16 PSPMT was used to read out the signals from WSFs. The results confirm earlier predictions about its performance. In particular, the spatial resolution achieved is comparable to that of modern Anger cameras. It is expected that the inherently flexible design of the camera should allow better positioning around the object than conventional Anger cameras, which is important to optimise the spatial resolution of the system. Monte Carlo simulations show that the use of photodetectors with higher quantum efficiency than PSPMTs would significantly improve the intrinsic spatial resolution. A discussion on the most promising candidates for this application is presented.