Question

a radiograph is to be made with a 40 inch source - to - image distance (sid) using 12 miliampere - seconds (mas). if the sid is increased to 72 inches, how could the mas be changed to maintain the same receptor exposure?
24 mas
6 mas
39 mas
3.7 mas

Explanation:

Step1: Recall the inverse - square law formula

The relationship between mAs and SID for maintaining the same receptor exposure is $\frac{mAs_1}{SID_1^{2}}=\frac{mAs_2}{SID_2^{2}}$.

Step2: Identify the given values

We have $mAs_1 = 12$, $SID_1=40$ inches, and $SID_2 = 72$ inches.

Step3: Solve for $mAs_2$

Cross - multiply the formula: $mAs_2=\frac{mAs_1\times SID_2^{2}}{SID_1^{2}}$. Substitute the values: $mAs_2=\frac{12\times72^{2}}{40^{2}}=\frac{12\times5184}{1600}=\frac{62208}{1600}=38.88\approx39$ mAs.

Answer:

39 mAs