Radiology Myth-Busters: CR Myths #2

Radiographic Image Contrast is Controlled only by kVp: FALSE

While it is true that kVp still controls the attenuation of the x-ray beam through the anatomy, there are some additional things that have a greater effect on image contrast:

1. The subject contrast - how the anatomy varies to influence differential absorption.  An anatomic part with a high subject contrast (chest) will have greater image contrast than a part with low subject contrast (abdomen) if all factors remain the same.  Additionally, we can influence subject contrast with positive or negative contrast media.
2. Presence or absence of a grid - I see this often when I notice a Radiographer performing portable chest x-rays without a grid.  There are often complaints from the Radiologists that the image quality is poor, and they have trouble seeing lines through the mediastinum.  If a grid is added, the image contrast improves, and as a result of the higher kVp range applied when using a grid, the chest is more uniformly penetrated by the beam, revealing information through the mediastinum that could not be seen at a lower kVp range (click here to learn more about grid use)
3. Processing algorithm - application of the appropriate processing algorithm is crucial during the initial image acquisition (click here to learn more about processing algorithms).

We have all seen what images look like with and without grids and contrast media... but let's look at the following set of images to evaluate the effects of image contrast with a change in kVp:

As you can see, the kVp range differs drastically between exposures ranging from 70 kVp to 110 kVp.  For the most part, when viewing the image from a distance (at the QC station for example), the images do not appear drastically different compared to what you might expect on a film/screen system.  Once magnified, you can see subtle differences between the 70 kVp range exposure and the 110 kVp range exposure confirming that kVp range does affect image contrast, but not to the degree that it once did with conventional film/screen systems:

Just because there is little difference in appearance does not mean you can use any old technique and expect the software to adjust your image for you (more in tomorrow's post).  It is even more important for us to utilize our training to harness all of the advantages that CR and DR imaging systems offer while maintaining the standards of ALARA.

Back to my point about the processing algorithm... we just compared a lateral knee radiograph at 70 kVp with one at 110 kVp and saw some subtle changes, but they were processed under the same algorithm of "lateral knee".  Here is an optimum exposure of a lateral knee phantom taken at 70 kVp and 7.3 mAs (like image 1 above) table-top and small focal spot, processed as "lateral knee"... and to the right of it, a knee with identical exposure factors but processed as "PA chest":

As you can see, the image on the left appears as you would expect a knee radiograph to appear.  The image on the right used a processing algorithm designed for a body part with more subject contrast.  The software normally recognizes data collected on the image plate from a chest x-ray as having very light pixel values and very dark pixel values, and the processing algorithm is designed to reduce that black and white appearance for maximum visualization of both mediastinum and fine lung detail.  If improperly applied to a body part with a lower subject contrast, the overall image contrast will decrease on the final image.


Additional posts in this series:

Radiology Myth-Busters: CR Myths #1 (An increase in exposure creates an increase in density)

Radiology Myth-Busters: CR Myths #1

An increase in exposure creates an increase in density: False

Scenario: you are performing a knee x-ray using the table bucky and you use 75 kVp and 10 mAs, a technique that you have been using with the same equipment for the last 5 years on patients of similar size.  Upon processing the image, it appears to have low density on the monitor – more lightly shaded pixels than the appropriate darker shades.  What do you do?

If you come from the film/screen era, your first instinct might be to increase your mAs.  You go to the control panel, double your mAs and re-shoot your image only to find out that the image is lighter than the original one… what the heck is going on?

The truth is, the original image probably had the correct technical factors, but other factors come into play that may not be apparent.  Typically, when a CR image is too light, this is the computer software’s response to an over-exposure, rather than not enough technique.  The computer thinks the image is over exposed, and adjusts the entire image to be lighter in order to compensate.  The default processing algorithm does not take into account the different scenarios that may have caused more exposure on the image plate than what you intended such as:

Pathology: the patient could have a destructive pathology causing a once-good technique to be overexposed.

Image plate fog: it was a slow weekend and the knee x-ray was the first exposure made on the image plate in 3 days.  Excess background radiation has accumulated on your PSP causing what looks like overexposure to the software.

Collimation error: meaning a lack thereof.  If you don’t collimate, the software will compute an “average” pixel grayscale value and shift the brightness of your image to the lighter side.

There are a few additional errors that may cause this effect depending on manufacturer, but use the tools at hand to determine if you need to repeat, and if so, what technical factors you should be using.  The exposure indicator should be within the proposed range, and we need to make a habit of viewing it during our image QC.  Choosing the correct processing algorithm is a must.  Ensure that image plates are erased daily, or at least every other day.  And pay attention to how you collimate.

You may find your system has errors that seem unexplainable at the time… create documentation of these errors so that your QC team can evaluate more than one of them in the future.  Is it only one particular piece of anatomy and one particular view?  The processing algorithm may need adjustment.  Is it one particular cassette that does it?  Perhaps there is a flaw and it needs replacement.  Is there one particular kVp or mAs setting that the error occurs at?  Your x-ray tube and/or generator may need to be serviced.

CR and DR imaging can still be difficult to adjust to, especially if you have been a radiographer using film/screen for a majority of your career and if you have not had the appropriate education.  Just remember to utilize the tools at your disposal, and take some additional time to observe what you can before charging back into the exam room for a repeat using a method that would have worked with film/screen.

Stay tuned for more CR myths debunked!
*Related post:  The CR Image Plate

Switching to a Smaller Focal Spot: Please Give Feedback

As I reflect back over the last 5 years to the beginnings of this blog, it has grown and evolved to the point in which the amount of information and abundance of topics I would like address here can become overwhelming to someone with two jobs and a family, and there have been times when my ambitions have proved a little bit unrealistic for someone with two jobs and a time-consuming hobby. I am enjoying the increased amount of discussion and feedback that I have been getting specifically within the last year, and I would like to ask my readers for additional feedback yet again.

As I embark on exciting new coinciding projects, which you can receive updates about if you subscribe to my newsletter, I am expecting to have less time to contribute to the blog for a while.  What that is going to mean in the future is probably fewer posts.  Having learned a few things along the way, I plan to place more attention to detail and the quality of content that I am posting... a "less is more" approach to the blog.  This will keep things around here more focused and will ultimately cause you, the reader, to have less information about topics that may not interest you to sift through, and easier access to information you find valuable.

This is where you come in.  Every once in a while, I like to get a sense of who is reading my blog.  The best way for me to know what kind of information that you find valuable is to ask.  I have posted an anonymous survey here with two quick questions.  I would love to hear what you have to say.  Thank you for continuing to stop by.

Create your free online surveys with SurveyMonkey, the world's leading questionnaire tool.

How to Perform Fluoroscopy QC

Alright, the title is a little bit misleading in that I do not plan on covering the many, many, many fluoroscopic quality control tests that exist for our equipment, but I chose one that, in California, we are required to perform weekly:

Setup:  Prior to testing, you should have your log book with document records available, a test phantom, and a baseline kVp and mA set by a Physicist.  For the phantom, the old school method utilizes a gallon jug of water, but most prefer to use a lucite phantom.

Step 1: Place phantom on table top (or image intensifier if performing on c-arm).

Step 2: Expose live fluoro constantely until kVp and mA do not fluctuate - this should only take a few seconds at most.

Step 3: Record mA and kVp on your log and solve the following formula:

Ideally, your recorded mA and kVp should be exactly the same as the baseline reading.  Our acceptable variance is  <1.25 (this can vary based on preference or regional standards).

For example, if your baseline kVp was 90 and baseline mA is 1.0, this is what the equation would look like if it were exact and within :
 

 

An example of a fluoroscope that would be out of the acceptable variance may read 1.2 mA and 98 kVp:

In the event that variance is greater than 1.25, the test should be repeated to confirm that it is out of variance.  If it still fails, a qualified engineer should service the fluoroscope and a new baseline should be acquired.

There are literally dozens of quality control tests that we can perform ourselves as technologists... what tests are you performing?  Does your fluoro QC differ from this method?  I would love to hear what you're doing.

Radiograph of the Week

Wanted: Pictures of X-ray Equipment

Having worked at several hospitals and imaging centers, I always find it interesting to see the variation between the x-ray equipment that is used.  One facility I worked at had a Picker x-ray tube from 1958, and to my knowledge, it is still being used at that facility (although I'm certain the original tube inside the housing is probably long gone).  Some places have brand-spanking new equipment, and most have something in between.

I would LOVE to get a library of images of different faces of x-ray tube controls... would anyone be willing to contribute some images to this library?  If I get enough, I plan to create a dedicated page to display them all on this blog.  Just send me a quick picture of the x-ray tube controls similar to the ones above.

Bad Clinical Experiences

For the most part, the clinical experience during school can be somewhat stressful, and definitely uncomfortable, but occasionally we run into a particular person who simply makes your life difficult. They might not purposefully mean for this to happen, but for one reason or another, you clash. I'm sure we've all had those particular Instructors or Technologists in our lives and in our education, but for some reason, we rarely speak about those individuals. Let me introduce you to the elephant in the room.

When I was in school, there was one staff Technologist in particular who made my clinical experience very difficult. Public humiliation and threats of being thrown out of the clinical site happened all too often.

I remember performing a cross-table lumbar spine for localization as a student (on film/screen). I hung up my image to place a date sticker on it and write "portable" with my sharpie. The images was slightly underexposed, but there was enough density and visualization of the anatomy to clearly make out the vertebral bodies and visualize trabeculae, as well as the spinal needle that the Surgeon wanted the Radiologist to confirm that he was cutting at the correct level.

This Technologist (with about 5 other Technologists standing in the room looking at my image) saw my image on the light box and said, "Oh my God... What the hell do we pay you for? That's unacceptable!" This was toward the end of my clinical experience, so I had gradually built up enough bravery (and stupidity) to compose my response, which I don't recommend using... I replied in a calm, collected tone, "you don't pay me." I think I actually saw steam coming out of her ears... if I only had an egg to crack on her head, I'm sure it would have cooked instantly. Infuriated, she yelled, "Go repeat that right now!"

The tech I was assigned to walked me out of the room and led me straight into the reading room. He told me he thought that I didn't really need to repeat it, but we would ask the Radiologist just in case. When the Radiologist said it wasn't necessary, he also suggested I alter my technique for the rest of the procedure once the hardware was inserted.

I thought I was through the woods until about an hour later, when I found out that she had followed up to see if I repeated the image, and began ridiculing me in front of her staff again. I don't remember her exact words, but she mentioned the word "insubordination" and "do that again and you're out of here." She knew very well that we showed the image to the Radiologist as well, and I found myself on transport duty for the next two weeks.

She liked to do this to me because the hospital was always short on transporters, and she knew I transported for a different hospital on the weekends. I was a victim of my own initiative because I was usually done with my competency evaluations for the semester early, so the school couldn't necessarily say my education was being hindered, although I still disagree. I should also mention that the dress code for male Radiography Students was slacks, dress shoes, long-sleeved shirt, tie and lab coat, so it meant two straight weeks of dripping sweat in my nice clothes and lots of laundry. For some reason, the female students were allowed to wear scrubs... never understood that.

This was just one event from one day of almost two years worth of dread. If you don't know me, I usually find it easy to get along with most people, so it goes to show that no matter what kind of personality you have, you are going to encounter someone like this in your career eventually that just seems to have it out for you. I hear of other stories similar to mine that seem to make people question whether or not they could handle their clinical rotation, as it did for me. I only got through it by constant evaluation of my progress... this person, whether she had a grudge against me, men in general, or was just really unhappy, managed to motivate me to make as few mistakes as possible. I paid attention to detail and tried to learn as much as possible as quickly as possible so that I could attend a day while flying under her radar.

I questioned whether or not to write this post because I did have plenty of good experiences during my clinical rotation, and I don't want to sound negative. There is something therapeutic when you know that others have experienced what you're going through, and have persevered through the thick of it, but it was not until many years after these experiences that I have found anyone willing to open up about it.

I would love to hear similar experiences from anyone willing to post, and please DO NOT mention names - the last thing I want to do here is call anyone out directly, so I also recommend posting anonymously. I would also be interested in hearing how you think the person in your experiences could have handled themselves differently that would have created a more valuable learning experience. No matter how much experience we all build, we still have things to learn too.

Looking for tips on success through Radiography school?  Check out my book coming Summer 2012... more info HERE.