Road to Radiography

I did a few experiments in clinical today.  In my quest to learn and understand manual exposure techniques, I have learned a few important items.  The biggest thing I have learned recently is a small tidbit of information that is invaluable to me.  The digital processors (CR) will tell you if your image is exposed properly or not.  With this bit of information, a tech can easily correct techniques and update technique charts that aren’t producing quality images.  In my current clinical assignment, I’m seeing some fairly frequent underexposures with the techniques that are being used.  I now have the tools to understand why these exposures are bad and how they can be corrected.

The problem begins with the fact that my clinical site has not been using digital for very long.  It’s been less than a year since they converted from film.  The tech I’m working with told me that they didn’t have to modify their technique charts much at all after the switch.  This immediately created a red flag for me.  We have been taught that digital takes more exposure than film to produce good images.  I was curious if the unchanged techniques were affecting the exposures, and I think they are.  The digital processor reports an “S” number for each image to indicate its exposure level.  I found the following list of proper S numbers for various exposures posted in the room where the processor is located, and I can only assume that the list is accurate and that my ‘median’ target number is also accurate:

• Skull - 100 to 400 (250 median)
• Chest - 200 to 600 (400 median)
• Abdomen - 100 to 400 (250 median)
• Extremities - 75 to 200 (140 median)
• Spine - 100 to 400 (250 median)

The median number just represents the middle of the ‘acceptable range’ of S values.  I suppose the median is the true target though.  S numbers above the median represent underexposure and numbers below represent overexposure of the x-ray image on the digital receptor.  With this knowledge in hand, I started making a few notes on some of the exposures that came up next.

L-Spine (AP) - Large Patient - 80 kVp / 75 mAs - S=1005
L-Spine (LAT) - Large Patient - 80 kVp / 225 mAs - S=333
L-Spine (AP) - Average Patient - 80 kVp / 75 mAs - S=318
L-Spine (LAT) - Average Patient - 80 kVp / 225 mAs - S=459
L-Spine (AP) - Large Patient - 80 kVp / 75 mAs - S=515
L-Spine (LAT) - Large Patient - 80 kVp / 225 mAs - S=340

As you can see, the technique chart in use for these two exams is 80 kVp with 75 mAs for the AP view and 225 mAs for the lateral view.  I didn’t see any technique adjustments for the varied sizes of the patients coming through.  This explains why the S values for the average size patient were closer to the median (250) than they were on the large patients, but in any case, all of the images were relatively underexposed, with the first AP view being extremely underexposed.  My assumption is that if the technique used is correct for the patient, the S number should fall as close to the 250 median on these L-Spine exams as possible.  The digital system can correct for improper exposures, but corrections introduce ‘noise’ or ‘grain’ into the image, and more severe corrections increase that amount of noise in the image. 

My current objective is to determine how to correct the problem so I can experiment with a solution at clinical.  These exposures that are being made are obviously not a concern at my clinical site, because the doctors are satisfied with what they are getting.  The problem is that, in my stage of learning, I want to learn how to do it properly.  So, I’m using these examples for my own benefit to learn what should be done to hit the target exposure after seeing quite a few underexposures.  There are a few unknowns here, so any comments or suggestions would be appreciated…

Question:  Is the S number in a direct relationship with the exposure level?  For instance, would doubling the mAs on that first exposure from 75 to 150 bring that S number down from 1005 to approximately 500?  This, I do not know. 

Observation:  The technique chart in use does not adjust the kVp for patient part thickness changes. 

Next Step:  My next step will be to figure out what needs to be done to correct these exposures so they come out closer to the median (if the median is the true target number).  I will be asking some questions about the proper procedures and see what I can come up with.  I’ll follow up here when I find those solutions…

On a good note…

I had another very productive day at clinical.  I logged another 35 procedures and completed four more master comps (wrist, pelvis, calcaneus, and ankle). 

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Tags: 2nd Semester, Clinical //