Radiography calculations

Question 1

penetrating power of the XRAYS

Answer 1

kV

Question 2

mAs

Answer 2

Number of xrays present controlled by the number of electrons and the exposure lengths

Question 3

If the kV is too low

Answer 3

xrays cannot penetrate the paitent

Question 4

If the kV is too high

Answer 4

The film will be over exposed as all tissues will have been penetrated

Question 5

If the mAs changes

Answer 5

The image will be darker/lighter due to amount of xrays produced

Question 6

what are the units for exposure time?

Answer 6

A: mAs

A Machine is set at the following:

60kV x 25mA, and 0.32seconds

Calculate the mAs

mAs = mA x s

mAs = 25mA x 0.32s

8 mAs

(mAs = mA x s

mA = mAs/s

s = mAs/mA)

Question 7

10: 2 rule

kV : mAs rule

Answer 7

Inverse relationship between kV and mAs

an increase of 10kV means the mAs must be halved

+10:/2

a reduction of 10kV means that the mA must be doubled

-10:*2

Question 8

10 kV rule:

machine set to 50kV and 20mAs - increase kV to 60. mAs = ___

machine set to 50kV and 20mAs - decrease kV to 40. mAs = ___

Answer 8

mAs = 10mAs

mAs = 40mAs

Question 9

-Grid factor (printed on the grids if used)

new mAs = grid factor x old mAs

Answer 9

when using a grid, the mAs must be increased to maintain the degree of film blackening.

Question 10

Grid factor example

radiograph taken at 50kV and 6mAs.

then a grid is uesd, grid factor of 2. What is the new mAs needed?

Answer 10

New mAs = GF x Old mAs

New mAs = 2gf x 6mAs

New = 12mAs

Question 11

Xraying plaster casts

WET

DRY

Answer 11

Wet = mAs x 4

Dry = mAs x 2

Question 12

Film focal distance (FFD)

If the focal distance varies, a new mAs will need to be calculated

Answer 12

new mAs = Old mAs x new FFD²
old FFD²

If there is an increased in the FFD the mAs will be increased

If there is a desrease in the FFD the mAs will be decreased