Please read pages 41-42 of your textbook before continuing.
When x-rays penetrate the body, one of 4 things happen. Then can be completely absorbed, they can be partially absorbed yet still make its way to the image receptor, or they can pass completely through to the image receptor. These 3 things are what need to happen in order to create the image on the image receptor. Unfortuantly, there is a 4th "thing" that will occur. Some of the x-rays will scatter. When x-rays scatter, they randomly head in a new direction than the primary beam. Some will scatter almost straight back at the x-ray source. Some will head off in a 90 degree angle and there will be scattering x-rays everywhere in between. Some of this scatter will make its way to the image receptor. This causes the image to take on a "hazy" appearance by lowering the contrast (makes things more gray).
This is where grids come in. By placing a grid between the patient and the image receptor, you can intercept this scatter radiation before it gets to the image receptor. Grids increase the contrast by absorbing the scatter and eliminating the haziness of the image...but this comes at a price. Scatter may not contribute to image quality, but it does contribute to the density of an image. If you remove the scatter without chaning anything, the image will appear light. In order to maintain proper radiographic density, mAs must be increased. Increased mAs means increased dose. So there is that constant battle between patient dose and image quality again. In order to increase image quality, patient dose suffers. Grids lead to increased patient dose.
Not all grids are equivalent. The grid ratio is the number of lead strips per distance within the grid (lead is used to absorb the scatter). A higher grid ratio means more lead strips. The higher the grid ratio, the more you need to increase mAs.
By using a grid, what happens to the following?
Below is a good example of the differences between using a grid or not. You can clearly see the difference in contrast.