#17
The 15% Rule and the Not So
Famous Bowman’s
7 ½% Rule 9/ 1/13
I believe the 15% rule was figured out in
the mid 50’s. Of course this was back in
the days of film/screen, but it still holds true today in the digital world.
The “Rule” stated that if you increase any
given technique by adding 15% more kV the following film would have twice the
density/opacity (which we will now just call opacity) as the original
film. And that’s all there is to the
rule.
Now what most people have done with it is
cut the mAs in half after they increased the kV 15%, but that’s now an addition
to the 15% Rule (which we will now call the 15% Rule with mAs compensation). And a great addition it is. It was a way to change the technique but
still end up in with basically the same opacity that you started with. This is because when you increase the kV 15%
the film was double the perfect opacity from what you wanted, but then by
cutting the mAs in half you also cut the opacity in half thereby ending up
right where you began but with a new technique.
What must be mentioned here is from the many experiments I performed on both the
15 and 7 ½% Rules (I'll get to that one), the Exposure Index (EI) number stayed
the same or was very close. This means
that after changing the techniques with either or both rules your EI number
will still be correct!!
There are really 2 main reasons why
someone would do the 15% Rule with mAs compensation. The first is to cut the patient dose. To read more about this, please refer to Blog
#12 from 6/15/2013 (How Low Can You Go?) where I explain that by increasing 15%
more kV and cutting the mAs in half you can save your patient almost 33% of the
Entrance Dose!!
The second reason would be to cut the time
for the exposure. This is really only
needed on portable machines that have a built in 100 mA station (which is most
portables on the market). 100 mA means
if your technique has 200 mAs then your exposure time will be 2 seconds, a 50
mAs exposure would be ½ a second and so on.
This usually happens when doing portable abdomens on a patient who is
unable to follow breathing instructions and your exposure time is over ¼ of a
second long. I’ve always taught (even
back in the film days) that an image taken out
of the optimum kV range which is possibly or even definitely longer scale (greyer)
than normal is always better than an
image with motion. In my opinion the
only thing worse than motion is actually cutting off the anatomy.
So now let’s quickly discuss the not so
famous (yet!!) Bowman’s 7 ½% Rule. As
you will see it is simple and easy to do and quite often the perfect thing to
use when changing techniques. In a
nutshell, all we are going to do is use half of the 15% Rule. Since it’s easier to see it with actual
techniques, let’s start with:
80 kV @ 40 mAs
92 kV @ 20 mAs = 15% Rule
with mAs compensation
86 kV @ 30 mAs = 7 ½% Rule
and cutting out a quarter of the mAs
All that you need to do is first figure
out what 15% of the kV is and what half the mAs would be and then just use half
of each. As you can see from the above
example; 86 is exactly between 80 and 92 and 30 is exactly between 40 and
20. With film all three of these
techniques would have had the same basic opacity. In the digital world (both CR and DR) the EI
number will be the same.
So why would use the 7 ½% Rule? If you are still using low kV/high mAs
techniques and now are willing to change, sometime doing the 15% Rule with mAs
compensation will still not increase the kV high enough. But if you did it all again the kV would now
be too high, but just doing the 7 ½% Rule will be perfect. Using the same example from before we see:
80 kV @ 40 mAs
92 kV @ 20 mAs = 15% Rule
with mAs compensation
106 kV @ 10 mAs = 15% Rule
with mAs compensation
99 kV @ 15 mAs = 7 ½% Rule
and cutting out a quarter of the mAs
So if you decide that you are unwilling to
go above 100 kV for a particular shot, then using the 7 ½% Rule would be
perfect as it takes you to 99 kV. Of
course this works with any starting kV or mAs and is applicable anytime you
don’t want to increase your kV a full15%.
If you have problems figuring out 15% of a
number, I have a chart already made that shows 15% from 50-120 (see below). To download this chart, go to: http://digitalradiographysolutions.com/files/29_kV%27s_and_the_15%25Rule.pdf There also is
a column that shows how much more opacity would be added if just 1 kV was
added. I have a whole discussion on that
column in my full day lectures, but since that would take another page, for now
I’ll just call it a day.
|
kV
|
15%
|
1 kV =
|
|
50
|
7.5
|
13.30%
|
|
60
|
9
|
11.10%
|
|
70
|
10.5
|
9.50%
|
|
80
|
12
|
8.30%
|
|
90
|
13.5
|
7.40%
|
|
100
|
15
|
6.60%
|
|
110
|
16.5
|
6.00%
|
|
120
|
18
|
5.50%
|
