Radiograph of the Week

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Radiology Myth-Busters: CR Myths #5

This CR system only responds to certain kVp ranges:  FALSE

We know that digital imaging systems no longer utilize the traditional H&D curve, but instead have a wider “Dynamic Range” of exposures that can be considered useful for the final image. 

For this reason, processing algorithm (See post #2 in this series) has greater impact on image contrast than kVp selection

With CR systems, kVp does not have the same impact as with film/screen imaging.  We know that the fundamentals of x-ray interaction with matter remain constant, but as we have always known, even with film/screen imaging, is that the interaction of the phosphor material will vary based on composition and kVp range.  Consider the variation in k-edge absorption values.  The k-shell electron binding energies of the phosphors that are used in traditional film/screen imaging, as well as the BaSrSO4 in the CR phosphor plate are compared below:

Y2O2S:Tb      = 17 keV

BaFBr:Eu or BaFI:Eu     = 37 keV

LaOBr:Tm       = 38 keV

Gd2O2S:Tb      = 50 keV

CaWO4       = 69 keV

In order to produce the most efficient beam, the average energy of the beam should be enough to free the k-shell electron from its orbit using one of these phosphor compounds.  You could say that yttrium has an “edge” over the rest because it has the lowest binding energy of all the compounds… but is it going to be the most efficient for the diagnostic range that we need for plain radiography.  The PSP compound is extremely low, meaning that you can have just as much screen efficiency at lower kVp values compared to most of the other screens.  Anything above that range should not affect efficiency when comparing one screen to another, so the CR image plates should not be considered to require different kVp ranges, and certainly not one specific kVp range for that particular screen.

This doesn’t mean that we CAN’T use certain kVp ranges, it just means that there is an optimum efficiency that we should be taking advantage of.  It also means that we have the capability with CR imaging to use higher kVp ranges.  If the processing algorithm is primarily responsible for image contrast, there is a great bit of latitude with the range of kVp that we can use... as long as we maintain the appropriate exposure to the image receptor.  The added benefits of that include more uniform part penetration with better visualization of anatomy (see Reducing Radiation Dose in Diagnostic Radiography), and that we can reduce the amount of mAs required at higher kVp ranges, thus extremely lowering patient dose.  These can be accomplished without affecting image contrast to the degree in which it would be affected with film/screen systems.

The main take away with all of this should be that what the CR system needs in order to perform its job as designed is exposure to the plate.  We still control that as the Technologist, and we still have to be educated properly to determine what the BEST method of exposure should be taking all things into account; patient dose, image quality, and variations with each patient and all of the different types of equipment we are using.  It is far more important to have the proper beam/part/film alignment, along with exposure factors that produce the least exposure necessary for the highest image quality when using CR imaging systems.

I hope you have enjoyed this series on Myths about CR Imaging... here are the other posts in the series:

Myth #1:  An Increase in Exposure Creates an Increase in Density

Myth #2:  Radiographic Image Contrast is Controlled Only by kVp

Myth #3:  Room Light will Fog the Phosphor Plate

Myth #4: You Don't Really Need to Collimate with CR - You Can Simply Crop Your Images

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Radiology Myth-Busters: CR Myths #4

You don't really need to collimate with CR... you can simply crop your images.

Unfortunately, this is a practice seen all too often.  Most Technologists will not admit to it, but I know it happens.  Just ask any Radiography Student.  There are several reasons this is a poor habit to get into.

Radiation Protection - it should be a priority of every Technologist to maintain radiation dose ALARA.  If you are knowingly including anatomy within your field that is not necessary for the sake of the exam you are performing, it is unethical to expose your patients in this manner.  Pure laziness!

Reduction in image quality - the same basic principles of radiation physics apply during image production whether you are talking about film/screen, CR or DR imaging.  If you are exposing a larger area of tissue than you need to for the body part you are imaging, you will be creating unnecessary scatter radiation.  Knowing that digital systems are more sensitive to scatter and background radiation should influence the Technologist to be even more conservative on the collimation, and not the other way around.  It's true that the image can be adjusted post-processing to make changes to image contrast, but not recommended.  The raw data from the initial exposure will contain information from scatter radiation that degrades image quality even if you manipulate it later... garbage in, garbage out.

Increased risk of processing errors - we know that the initial data will be evaluated by the software to find the "values of interest" for each radiographic exposure.  The idea is that the anatomical regions that you are attempting to obtain a diagnostic image of should be manipulated during the initial processing algorithm to be displayed to appear appropriately at the display after processing - the best way to accomplish this is to use the correct exposure factors and collimate.  The more data included in your initial data set that does not need to be included in the image, the greater the risk that the "values of interest" will not be properly detected (see "Anatomy of a Histogram" for more detail).  If a large number of dark (exposed) pixels are included in the VOI by mistake, the software assumes that there is an overexposure due to the average pixel value being on the darker side.  Performing its function, the software will make the entire image display lighter than desired.

Legal concerns - I have not been able to reference any legal cases in which cropping anatomy that was recorded has ever caused a patient harm, but that doesn't mean the potential for such a situation does not exist.  We will be required to report dose information in the near future for all exams.  It's already starting with CT, and general radiology will be soon to follow.  If the total dose administered for every type of exam that you perform is 30% greater compared to another technologist, you can see how your employer may consider you to be a liability.  

I'm not sure that it's appropriate to say that this post is about a myth, but it does seem like a general assumption that many Technologists make.  Perhaps we all need to be reminded that just because technology exists that help with sloppy practices, it doesn't mean we should become complacent about our imaging techniques... and I'm off my soap box.

Other posts in this series:

Myth #1: An increase in exposure creates an increase in density

Myth #2: Radiographic image contrast is controlled only by kVp

Myth #3: Room light will fog the phosphor plate

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Radiograph of the Week "Fish-hooked"

Radiology Myth-Busters: CR Myths #3

Room light will fog the phosphor plate:  FALSE

By now, I'm sure that we have all used a CR cassette that has worn-out latches and springs.  Due to the way some manufacturers construct their photostimulable phosphor plates, they can slide right out of the cassette and onto the floor, leaving the device holding the latent image susceptible to room light and/or dirt and scratches.  You pick up the PSP from the floor, and what do you do?

Let me reassure you by saying that your image will not be fogged at all like it would have been with film/screen.  In fact, a mistake like that with film/screen would completely ruin your image no questions asked; no need to run it through the processor to check... it would go straight to the repeat bin.  You can freely process it without fear of image fog, although you're going to want to clean your screen before re-inserting it into the cassette.

So why isn't the PSP affected by room light?  When compared to film/screen, the plate does not respond to the lower frequency of room light like film does.  In fact, in order to see any radiographic density at all, the plate has to be ionized, which releases electrons from the valence band and sends them to the electron traps within the PSP (click here for more information on how the CR Image Plate works).  Room light is simply not going to cause ionization of the plate.

I can hear what you are thinking already... "but why is it called 'photo' stimulable if it doesn't respond to light?"  When a high frequency light source, like that of a laser seen in the previously mentioned post, this frees the electron traps of their charge and in turn, releases energy - some of it in the form of light.  But this has the opposite effect of fog... here's how the relationships mesh together:

The higher the x-radiation exposure to the plate, the more electrons are released from the valence band and collected into the electron traps.  The more electrons collected in the trap, the higher signal received during processing for that particular region of the image.  The higher signal received, the greater the density displayed (darker pixel values). 

The release of the electrons from the image plate by the laser scanner will actually reduce the amount of charge collected in the trap, therefore reducing density.  If you read the whole post I referred to above, there is a very cool lab experiment that can be performed to display this concept with a laser-pen for the instructors out there that are reading this.

So a few minutes of exposure to room light will not fog your radiograph, and exposure to high-intensity laser or fluorescent light will actually decrease density on your final image.  The most common ways you will see fog on your image receptor is if you have your image plate in the x-ray room during an exposure, leaving it exposed to scatter radiation (click here to learn about CR Image Plate Response to Scatter), or if you neglect to erase your image plates over long periods of not being used - most manufacturers recommend erasing the plates daily, or at least every 48 hours.

Additional posts in this series:

CR Myths #1 - An Increase in Exposure Creates an Increase in Density
CR Myths #2 - Radiographic Image Contrast is Controlled by kVp

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.