Veterinary Nurse Training
Calculation Example

Part of Veterinary Nurse training involves being able to do calculations. Some of these are to do with working out drug dosages, but there are also calculations involving many other areas such as anaesthetic gas flow rates, making up solutions, fluid therapy, transfusions and radiography.

This is one of the areas that student nurses find most difficult. What we have tried to do in the books is to lay out the workings of every type of calculation in as simple a way as possible and to show all the workings of examples in a step by step form. Our student nurses certainly say that this has made a very difficult subject much easier for them to understand and to get right under exam conditions.

The following example is taken from the radiography notes. If you are new to Veterinary Nursing, this will mean absolutely nothing to you - for now! However, all the symbols will have been explained earlier in the Radiography section lectures, so that when the student comes to do the calculations she will be familiar with what all these symbols mean.

For now, if you are a student, just have a look and get the feel of what you’ll be doing one day. If you are a qualified Veterinary Nurse or college tutor, you will see how all the relevant formulae are gathered together and, when they are applied to a problem, the workings of the problem are clearly apparent. If you would like to see some other calculations, try the Part 2 Volume 1 Upgrade page, where there is a rewrite of the Fluid Balance calculations.

CALCULATING X-RAY EXPOSURES

Useful formulae.

mA x s = mAs

	new mAs	=	(new distance)2 x old mAs
			(old distance)2

new mAs = old mAs x grid factor

If you increase kV by 10, mAs is halved
If you decrease kV by 10, mAs is doubled

mAs using standard film = 2 x mAs using fast film
mAs using fast film = 2 x mAs using ultrafast film

mAs using standard screens = 2 x mAs using fast screens

1) To take a correctly exposed radiograph at 30 kV, the mAs must be 6 at a film focal distance of 75 cm.

a) If the machine has a mA of 60, what exposure time is needed?
b) If the film focal distance is reduced to 60 cm, what is the new mAs to produce a correctly exposed radiograph if the kV is kept the same?

a) mA x s = mAs

60 x s = 6

	s	=	6
			60

s = 0.1 seconds

2) A radiographer requires an exposure of 12 mAs at a film focal distance of 75 cm. What is the new mAs if the film focal distance is reduced to 66 cm?

3) The following chart shows the output of a typical x-ray machine at each setting:-

a) What exposure time is needed to take a radiograph requiring an exposure at 70 kV requiring an exposure of 15 mAs?

b) What exposure time would be needed to take the same radiograph using a grid of factor 2?

a) mA x s = mAs

Output at 70 kV = 45 mA (from the chart)

mA x s = mAs

45 x s = 15

	s	=	15
			45

= 1/3 or 0.3 seconds

b) If a grid is used:

Output at 70 kV = 45 mA (from the chart)

mA x s = mAs

45 x s = 30

	s	=	30
			45

s = 2/3 or 0.7 seconds