How Low Can You Go?

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 “Creeping mAs/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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