mAs

Question 1

State what mA is in terms of rate and what mAs is

Answer 1

mA - rate of current. The number of electrons from cathode to anode per second OR
A measure of the quantity of current passing through the tube during the exposure.

mAs is mA/sec. The rate of current per second.

Question 2

Relationship between mAs and intensity

Radiographic Density, Patient Dose

Answer 2

mAs is directly proportional to intensity

mAs is directly proportional to RD and patient dose

Question 3

Intensity =

Answer 3

amount of energy per unit area

Question 4

Define Reciprocity and give examples of when this would be used

Answer 4

Changing either mA or time to keep mAs the same. Intensity the same, RD the same, patient dose the same. Total quantity of x-ray photons does not change.
Use Reciprocity to:
control motion (patients) - shorter time, higher mA.
Focal spot size - small focal spot size has a max mA of 100 so need to adjust time (increase).
Breathing techniques - to blur anatomy for example. Lower mA and longer time.

Question 5

Inverse Square Law

Answer 5

The further we get away from the source of radiation, the lower the intensity. Radiographic intensity is INVERSELY proportional to the square of the distance from the source:
I = 1/d^2

Question 6

If I1 - 100 mR @ 40” SID changes to 72” I2 =

Answer 6

100 x (40/72)^2 = 30.8 mR.

If SID increasing, mR DECREASING - so put lower # on top.
If SID decreasing, mR INCREASING - so put higher # on top.

Question 7

Intensity and SID

Answer 7

If you increase SID, intensity leaving the tube stays the same. What changes is what is hitting IR. If you increase SID fewer photons are hitting IR because they diverge. Have to increase mAs to increase the # of photons.
** closer to source - increase intensity, further from source, decrease intensity.