The Relationship Between Intensity and Distance in Radiography

What intensity (mGya) would result at 400cm SID if a radiograph is taken with 120 mAs and a 200 cm SID producing 300 mR exposure?

A. 150 mGya

B. 75 mGya

C. 100 mGya

Answer:

The intensity resulting at 400 cm SID, with a constant mAs of 120 is 75 mGya.

According to the inverse square law in radiography, the intensity of radiation is inversely proportional to the square of the distance. In this case, we are given that a radiograph is taken with 120 mAs and a 200 cm Source-to-Image Distance (SID) producing 300 milliroentgens (mR) exposure. We are asked to determine the intensity (mGya) at a distance of 400 cm SID while keeping the mAs constant at 120.

The formula to calculate the intensity using the inverse square law is:

Intensity2 = Intensity1 * (Distance1 / Distance2)^2

Given:

Initial Intensity (Intensity1) = 300 mR

Initial Distance (Distance1) = 200 cm

Final Distance (Distance2) = 400 cm

Substitute the values into the formula:

Intensity2 = 300 mR * (200 cm / 400 cm)^2 = 300 mR * (1/2)^2 = 300 mR * 1/4 = 75 mR

Since 1 Gray (Gy) is equal to 1000 milligrays (mGy), the intensity at 400 cm SID is 75 mR, which is equivalent to 0.075 mGya.