Blog #6                                                                       3/15/2013             How much can you over-radiate and still get a perfect image?                                                                                                                                                                                                                                 

I decided to write and post this blog on the same day I changed my current research on my website.  This subject is one of the most exciting and scary things that is happening in our profession at this time.   If you go to the Current Research section, you’ll see handfuls of images proving that 5, 10, 50+ times too much mAs can be used and the images all look the same.

When I first started going to lectures on digital radiography, I heard that using 3-4 times too much mAs would cause noticeable changes in the images contrast and resolution.  This didn’t seem to match what I was seeing with our CR and DR equipment, so after a year of pleading and begging, CHOMP bought four phantoms and digital dosimeter to go with our abdomen phantom.  And with that; the “game” to finally understand what we were doing was on in earnest.

The first blog I wrote was about Creeping mAs/Dose.  I explained that one the reason’s this is occurring is because the digital computers all have the remarkable ability to automatically rescale the image.   So now an unbelievable amount of mAs (meaning way too much) can be used and a prefect image is created almost every time.

Without a body phantom to experiment with, it is impossible to see how easy it is to over-radiate a patient and still have a perfect (or at least very passable) image.  I have now been able to use my skull phantom on nine different manufacturers to determine how much mAs can be used, and over-used.  Currently I have four CR and five DR vendors.

From the experiments I’ve performed, all of the CR manufacturers except Agfa have the ability to “fix” an exposure that has been over-radiated by more than 50 times.  That is automatic rescaling at its finest.  It is also why no human being can see if an image was only over-radiated 2 or 3 times too much.  This is why it is impossible for anyone to be able to look at a monitor and tell if the patient was over-radiated, unless enough radiation was used to create burn on the image.  This is also why the EI numbers are the only way a radiographer can tell if the technique was correct.

I could go on and on (and do when I get to this section of my lectures) but I just hope you have a few minutes to go to the Current Research section of my website and see the images for yourself.  If there ever was a time where a picture is worth a thousand words, it will be there.

Link to:  Current Research

#5    Grid Cut-Off in the Digital World     03 /01 /2013

 

     After years of wondering about it, sometimes noticing it and finally researching it, I’ve come to the conclusion that grid cut-off isn’t mentioned anymore because no one even knows it’s happening.  With film it was totally obvious because grid lines covered 50-100% of the image.  That is not the case at all with digital images.  This is because the post processing has grid suppression software built in that does just that – it suppresses the grid lines so they don’t appear in the image.

     This is a little good and a lot bad.  Good because the image doesn’t have annoying grid lines in it which could superimpose anatomy or pathology.  Bad because the radiographer does not know that there is grid cut-off occurring, with all the ill effects that grid cut-off creates.  These include:  decreased contrast, increased brightness, increased Exposure Index (EI) number, more technique (mAs) needed and possible decreased sharpness of detail.

     Whenever you are using a grid and the image has any problem with contrast, brightness, detail or EI number, it is almost a given that you have grid cut-off.  It is happening all the time and almost no one knows it.  In fact, most of the grids that come with both CR and DR equipment are incorrect for use with chest x-rays.

     There are many different styles of grids, but the 2 most common are with the grid lines running parallel with the floor (called a horizontal grid) and 90 degrees to the floor (called a “decubitus” or “short axis” grid).  When using the grid for any AP chest work where the central ray (CR) is being angled caudally into the patient and grid, the grid needs to be short axis so that the grid lines are still parallel with the CR.

     I did some experiments recently with the grids that came with both our GE and Siemens DR equipment and both of them were horizontal grids.  This isn’t written anywhere on them, but I proved it by taking chest phantom shots with a 10 degree caudal tube angle and the grid both lengthwise and crosswise.  With the GE grid it needed to be put in behind the patient with the handle to the side which is very difficult and the Siemens grid needed to be put in portrait and not landscape.  With our CR grids we had to purchase separate grids that were short axis so that we could continue to put the grid behind the patient crosswise.

     When you get grid cut-off, it means you have used more mAs than needed to get a proper EI number.  From my research I’ve concluded it’s around 30%.

     I was thinking I could have you go to the Presentation Download on my home page and look at the slides that discuss grid cut-off, but I do not have time to cover it in my 2 hour lecture.  It is only in my 6 and 8 hour presentations.  Sorry about that.  Hopefully one day I will have it on my site in the Current Research section. 

      

Link to: digitalradiographysolutions.com