Detailed calculations show that we can control and enhance x-ray line emission in the so-called water-window wavelength region using laser irradiated nano-porous or foam-like targets. The effects of target porosity on the non-LTE plasma ionization are studied to obtain optimum conditions for maximum narrowband line emission. Results show that for specified irradiation conditions, the population of emitting ions can be significantly improved using a target with optimum initial mass density. In such conditions, an efficient line x ray can be emitted from the created plasma making it a suitable flash point source for high-contrast x-ray imaging of living samples.