Cavitation plays a significant role in high pressure diesel injectors. However, cavitation is difficult to measure under realistic conditions. X-ray phase contrast imaging has been used in the past to study the internal geometry of fuel injectors and the structure of diesel sprays. In this paper we extend the technique to make in-situ measurements of cavitation inside unmodified diesel injectors at pressures of up to 1200 bar through the steel nozzle wall. A cerium contrast agent was added to a diesel surrogate, and the changes in x-ray intensity caused by changes in the fluid density due to cavitation were measured. Without the need to modify the injector for optical access, realistic injection and ambient pressures can be obtained and the effects of realistic nozzle geometries can be investigated. A range of single and multi-hole injectors were studied, both sharp-edged and hydro-ground. Cavitation was observed to increase with higher rail pressures. Comparative analysis of several injectors indicates that rounding the nozzle inlet delays the onset of cavitation due to reduced separation, but does not always suppress it. Tapering the nozzle hole is an effective means of suppressing cavitation. A single-hole injector mesh was designed based on x-ray imaging, and high resolution three dimensional large eddy simulations were performed using a homogeneous relaxation model. A two-phase submerged simulation was compared against a three-phase compressible solver modeling both non-condensible ambient gas and cavitation. Projections showed good agreement between the three-phase solution and x-ray experiments.