Density Maintenance

Now that we've learned how x-rays behave during the use of the Inverse Square Law, how can we use distance to help image patients when the normal operating distance cannot be used?  There may come a time when you will have a situation arise which will require you to adjust your technique factors to maintain proper exposure to your image.  For instance, a regular chest x-ray is performed at 72".  What happens if you can only get a 36" SID?  This kind of situation arises when you cannot perform the chest x-ray with the patient upright using the wall bucky system.  What if the patient must remain recumbant (lying down)?  According to the Inverse Square Law, if you were to keep your mAs the same as the original setting intended to be used at 72", your image will be too dark.  Plus, what did you do to patient dose?  The patient would be exposed to more radiation than necessary to create the image.

Since the Inverse Square Law says that you increase your intesity of radiation by a factor of 4 when you halve your distance, you need a formula to maintain the same exposure.  You need to decrease your technique (mAs) by a factor of 4 to offset this. 

The formula for density maintenance is:

mAs1/mAs2 = D12/D22

You determine your original mAs, your original or old distance, your new distance (don't forget to square your distances!), and then plug in the numbers into the formula.

Sample question:

If you were performing a chest x-ray at 72" using 6 mAs, what would your mAs have to be in order for you to maintain radiographic density at a distance of 36"?

mAs1/mAs2 = D1/D22

6 mAs/x = 722/362

5184x = 1296 * 6

5184x = 7776

x = 1.5 mAs

 

We decreased our distance by half, so we had to decrease our mAs by a factor of 4.