1. A method for measuring long bone density of small animals, comprising: providing a phosphor screen or plate of a type that transduces incident ionizing radiation to emitted light; positioning a small animal before the phosphor screen or plate; exposing the animal to X-radiation having an energy level in the range of 10 to 16 Kev; capturing light emitted by the phosphor screen or plate using a digital camera and preparing a digital X-ray image of a long bone of the animal; and using a computer to perform the steps of: transforming the X-ray image into an X-ray density image; defining a region of interest of the long bone in the X-ray density image; scanning at least one row of pixels within the region of interest of the X-ray density image; and conducting a nonlinear least squares analysis of data obtained from the scanning step using a virtual, cylindrical model for the long bone within the region of interest and adjusting values of parameters of the model so as to minimize the chi square of a residual between the data and the model.