# 12 HOW LOW CAN YOU GO? 6/15 /13
This is how I teach ALARA (As Low As
Reasonably Achievable) to all of my students and whenever I give any kind of
talk on digital radiography techniques to students or techs. It's all about using the smallest/lowest mAs
possible so that the dose is as low as it can be. This can only be done by using the optimum kV
(see blog #7 "Optimum kV for DR & CR Equipment" from April 1),
which is a noticeably higher kV than was used with film. With a higher kV, less mAs can then be used.
This new technique is only the first step in
discovering how low you can go, although by adding
15% more kV and halving the mAs the entrance dose to the patient will be cut by
about 33%!! With such a quick and
obvious way to save a patient 1/3 of the dose, you would think everyone would
want to be doing this. The fact is
though, that instead of techniques being at an all time low (about 75% of the
country now uses digital equipment which can employ lower doses than film)
mAs/Dose Creep has occurred. To read
more about that, please see my first blog " The Problems of “CreepingmAs/Dose” in America" from January 1.
The second step is knowing how perfect the
image needs to be. Some radiologist's
want an image with absolutely no noise/mottle in it while others are absolutely
fine with a little bit of noise. In
fact, the radiologist's at my hospital (Community Hospital of the Monterey
Peninsula - CHOMP) want to see a little bit of noise. This is called acceptable mottle and they actually
dictate this term into their reading.
They know that by seeing a little bit of mottle we truly are taking the
mAs/dose as low as we can go. They also
know that they cannot miss any kind of pathology with this small amount of
noise in the image.
I tell the students and radiographers that
I would love it if they would make getting the lowest mAs a competition. It is total bragging rights for whoever can
use the lowest mAs for any given body type/size. As the software in our digital computers
allow for an inordinate amount of over radiation and still get a get a perfect
looking image (automatic rescaling), the skill of "technique-ing" is
becoming a lost art. To see this first
hand, go to the archive section in my Current Research and look up "Howmuch can you over-radiate and still get a perfect image?".
Basically anyone can use too much mAs and
still have a perfect image every time, so that doesn't really take any talent. What does take real technique-ing skill is to
make an exposure that is very close to whatever Exposure Index (EI) number
shows that the minimum amount of radiation was used. My goal is to go for the perfect EI number to
50% more than that number. If I am in
that zone I know I nailed it!!
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