Please read pages 47-57 in your textbook.
CR Imaging
In CR imaging, the image receptor is a cassette. When the cassette is exposed a screen within the cassette absorbs the energy of the x-rays and holds onto it. In order to create the image, the cassette must be put into a reader. The reader scans the screen with a laser, which releases the image in the form of light. The light is detected by the system and is constructed into a digital image. Once the image is on the monitor, it can be altered if needed. The concept is very similar to altering a digital image from a digital camera.
In direct digital imaging, a detector is struck by x-rays and processes the image directly.
In both cases, software using complex algorithms reconstructs the image knowing what the exam is and what the image is supposed to look like. Because of this, you could have two exposures of the same view on the same patient, let's use a PA hand as an example. One image was exposed at 3 mAs, and the other one was exposed at 6 mAs. Because the software knows what the density of the image should be, it will automatically adjust the images so they look the same.
Here lies the problem. Digital imaging is generally lower in dose, but has the capacity to be much higher. X-ray operators have an ethical responsibility to minimize patient dose. There are indicators that can tell you if you are in the proper exposure range. Most manufacturers call it either an S-number or an exposure index. Each manufacturer is different, so these numbers are specific to the manufacturer. The goal is to keep the exposure index or S-number within a given range for a given exam.