mAs
mAs, which determines the QUANTITY of x-rays being produced, is calculated by multiplying the mA x time (in seconds). When you see a question that has mA and time separated, you must calculate mAs before going any further in the question. For instance:
- A hand x-ray uses a technique of 60 kVp, 100 mA, and .04 seconds. So what is the mAs? You multiply 100 x .04 = 4 mAs. You won't always see the time listed conviently seconds.
- What if the time in the question was listed as 40 milliseconds? You would first have to convert it to seconds by moving the decimal place 3 spots to the LEFT (or divide the time by 1000), which would give you .04 seconds, then caculate the mAs from there.
- What if the time was given as a fraction of 1/25 of a second? You would first have to convert the fraction to whole numbers by dividing the numerator (top number) by the denominator (bottom number). (1 divided by 25 = .04). From there you can calculate your mAs.
- More examples:
- 200 mA @ 0.3 sec = 200 x 0.3 = 60 mAs
- 200 mA @ 70 ms = 200 x 0.07 (move the decimal place 3 spots to the left to convert milliseconds to seconds) = 14 mAs
- 200 mA @ 1/2 sec = 200 x 0.5 sec (divide top number by the bottom number) = 100 mAs
Density Maintenance
(Old mAs/New mAs) = (Old Distance Squared/New Distance Squared)
The most common mistake we tend to make is confusing this with the Inverse Square Law and having the numbers on the distance side of the equation backwards. For density maintenance, make sure you have (old/new)=(old/new). The second most common mistake is forgetting to SQUARE the distances.
For example: If a radiograph was taken using 400 mA, 200ms at 72", what would your new mAs have to be in order to maintain radiographic density at a distance of 40"?
- First you have to calculate mAs. In order to do that, you must convert 200ms to seconds by moving the decimal 3 places to the left (or divide by 1000). If you divide 200ms by 1000, you get 0.2 seconds. Multiply the mA and time (400 x 0.2 sec = 80 mAs)
- Assign the numbers to your formula - Don't forget to square the distances
- Old mAs = 80
- New mAs = X
- Old distance = 72"
- New distance = 40"
- Plus those numbers into your formula
- (80 mAs / X) = (722/402)
- Cross multiply and you get: (X * 722) = (80 mAs * 402) . If you need to reference cross multiplication, go to http://en.wikipedia.org/wiki/Cross-multiplication
- Calculate the squared distances and you get: (X * 5184) = (80 * 1600)
- Multiply and you get: 5184X = 128,000
- In order to solve for "X", you must divide each side by 5184: (5184X / 5184) = (128,000 / 5184)
- x = 24.69 (can be rounded up to 24.7 or 25)