Helpful X-Ray Tips

Technique

When choosing a technique to make an x-ray, while you have three adjustments, you're really only dealing with two factors. Those factors are radiation wavelength and radiation quantity.

Radiation wavelength is determined by kVp. kVp stands for "kilo volts peak". This is the voltage applied to the x-ray tube. 80 kVp = 80,000 volts peak. Yes, that's eighty thousand volts. The lower the kVp is, the longer the wavelength of the resulting radiation. The higher the kVp is, the shorter the wavelength of the resulting radiation. Long wavelength radiation produced at low kVp has less energy and lower penetrating power than short wavelength radiation produced at high kVp. This is why low kVp shots such as mammography (usually shot at around 27 kVp) are generally high mAs shots. A mammogram shot at 27 kVp on a heavy breast that's compressed to only a few centimeters in thickness may require 200 to 300 mAs while a chest shot (looking at lung tissue) shot at 110 kVp may only require 10 mAs while penetrating the same breast as well as the entire rib cage and everything within. Larger quantities of low energy, low kVp radiation are required to match the penetration of high energy, high kVp radiation. A good analogy to kVp is water pressure, as voltage is quite literally electrical pressure. A gentle flowing stream will simply get your socks wet. A water stream at 100 psi from a fire hose will knock you off of your feet. A tiny water jet at thousands of psi will literally cut through several inches of solid steel.

Radiation quantity is determined by how much current (mA) you put through the tube, and how long that current flows.
mA = milliAmperes of current.
mAs = milliAmpere seconds.
mAs is calculated by multiplying mA by the time of the exposure in seconds.
Example 1: 100 mA X 1/10 second = 10 mAs.
Example 2: 50 mA X 1/5 second = 10 mAs.
If an x-ray machine is correctly calibrated, a shot made at 80 kVp, 100 mA, and 1/10 second will be identical to a shot made at 80 kVp, 50 mA, and 1/5 second.
Again, a good analogy here is water flow, with mA equaling gallons per minute. You can fill a 10 gallon barrel in 2 minutes with a hose that's flowing 5 gallons per minute, or you can fill it in one minute with a hose that's flowing 10 gallons per minute.

Changing SID
(tube distance)

To shoot a technique designed for 40 inches at 72 inches, multiply the mAs by 3.
Example: An exposure made at 40 inches, 80 kVp, 10 mAs may be shot at 72 inches, 80 kVp, 30 mAs.

To shoot a technique designed for 72 inches at 40 inches, divide the mAs by 3.
Example: An exposure made at 72 inches, 80 kVp, 10 mAs may be shot at 40 inches, 80 kVp, 3.3 mAs.

Keep in mind that changing SID will change the magnification of the image. If a shot is normally made at 72 inches and you shoot it at 40 inches, the anatomical image on the film will be larger, and may confuse the reading physician. This could lead to a misdiagnosis, such as "enlarged heart", when no such condition exists. Be sure to note any change in a procedures routine SID on the film.

Changing Image Contrast

It's often advantageous to modify a technique to produce more (or less) contrast between bone and soft tissue on the image.

To increase contrast (show less soft tissue and more bone detail), lower the kVp and raise the mAs to compensate for the loss in film density. As a general rule, double the mAs for every 10 kVp reduction.
Example: An exposure made at 80 kVp, 10 mAs may be shot at 70 kVp, 20 mAs.

To decrease contrast (show less bone and more soft tissue detail), raise the kVp and lower the mAs to compensate for the increase in film density. As a general rule, half the mAs for every 10 kVp increase.
Example: An exposure made at 80 kVp, 10 mAs may be shot at 90 kVp, 5 mAs.