Inverse Square Law

Just like the Density Maintenance formula, you must be able to set up the Inverse Square Law formula correctly.  Instead of labeling them with 1's and 2's, let's try labeling them with old and new:

Old Intensity/New Intensity = New Distance Squared/Old Distance Squared.  Just like the Density Maintenance formula, you must remember to SQUARE the distances.

When it comes to test time, how will you know when to use the Density Maintenance formula and when to use the Inverse Square Law formula?  That answer is actually quite simple.  If mAs is mentioned in the question and is asking for a solution for mAs, you use the Density Maintenance formula.  If the question deals with exposure (mR, Sv, mR/hr, etc.), you will automatically know to use the Inverse Square Law.

Example:

If the intensity of the x-ray beam is 100 mR at 80" from the source, what would the intensity of the beam be at 50" from the source. 

 

As long as you label your numbers correctly and input them in the formula (remembering to square the distance), you should be fine.  Remember that you test will be in the form of multiple choice, both the course quizzes as well as the ARRT limited scope exam.  That helps a great deal.

Remember the concept that every time you double your distance, the intensity of the x-ray beam is cut by a factor of 4.  This becomes important in the practice of radiation protection.  As occupational workers, the utilization of distance is important to protect yourself from scatter. 

*A tidbit of knowledge for you - In general, X-rays should scatter TWICE to be deemed safe for the occupational worker.  You might want to store this away as it is possible you may see it on the ARRT test.  I won't go into specifics on why, other than the fact that every time x-rays scatter, it loses energy.  By the time it scatters twice, the energy it possesses is low enough to be fairly insignificant.