Anatomy of a Histogram

We've all seen a histogram before if we've used CR or DR imaging systems, but it is important that we all know how to interpret them. Take a look at this histogram:



The horizontal axis in my histogram represents the quantity of information in my image, or the optical density values. In this particular instance, I have densities ranging from a value of 0 (absolute white) on the left to 256 (absolute black) on the right side of my image.

*Note: Keep in mind that this histogram was taken using "Image J" so the bit depth is limited. On a traditional CR system, and depending on the bit depth your system utilizes, you may see up to 16,000 plus shades of gray. This histogram is simply for demonstration.

The vertical axis represents the number of pixels that are assigned within each density value.

In the following image, one of the great aspects of the Image J software is displayed. Depending on where my cursor is placed on the histogram, you can tell on each image what the density value is for each column, as well as how many pixels were assigned that density value. In the image on the left, the value (or degree of density on a scale of 0-256) is 73. The count (or the number of pixels exposed with this density level) is 456. In the image on the right, my cursor was moved over a higher density value of 188, and a higher count of 5883. So in this image, the crosshair placement represents a darker density assigned to a greater number of pixels.



Just as a point of reference, this is the image represented by the histogram:



The CR system has a pre-programmed algorithm for each type of exam you do... this is what you are selecting when you input "chest lateral" before image plate scanning. This tells the computer that a histogram similar to the pre-programmed shape will be scanned. This is where the CR system can produce errors. Depending on the raw data that is scanned, the computer will assign a range on your histogram termed "values of interest" or VOI. Unfortunately, the Image J software did not include this, but I have represented the VOI with red lines in the following image:



The VOI on the histogram helps to determine your Exposure Index (Kodak) or S Number (Fuji), as well as how your image will be rescaled. In first generation CR systems, the operator could slide the VOI to the left or to the right in order to visualize recorded anatomy better, but most current applications do not allow the radiographer to do so. It may be possible, however, to apply a different LUT (lookup table) to the image to make adjustments. So when the computer applies automatic rescaling (the computer's attempt to adjust the image due to over/under exposure) to your image, the process may fail if the original histogram analysis is incorrect. Believe it or not, it is STILL very important to utilize the proper exposure factors.

This brings us to an important role of the software, histogram equalization. Equalization is performed by the computer in attempt to produce a more uniform histogram to increase the level of contrast in your image. After the original histogram is derived, an "inverse" histogram can be calculated and a spreading (or commonly termed flattening) of histogram values can be applied. Compare this image before histogram equalization and after:





This is a very basic explanation of a histogram and how it is utilized in a CR system. It would be easy to go onto many tangents from here (which I have a bad habit of doing in-person). A number of processing and post-processing errors can occur that I hope to dive into in the near future, but a basic understanding of histogram analysis is required. In the meantime, I would like to encourage everyone reading this to familiarize yourselves with the histograms utilized in your own imaging departments, and maniplulate them if you have the capability on your CR system and/or PACS terminals. There's not a lot of information about this in current textbooks, but I can imagine that we will all be responsible for knowing more about these things as we continue to replace conventional automatic processor technology with digital equipment.

Image J

I recently became aware of some free downloadable software that I found incredibly useful both as a student and an educator. Thanks to the Radiography faculty at UNC, I'm in love with Image J!

It's an online PACS toolset that allows you to manipulate any image you can view on your computer in multiple formats. Now, this isn't a database of images, but you can do pretty much anything that a PACS system can to any picture that you already have stored on your computer. Here's what you'll see at their home page:



To download, go to the "download" option at the very top of the page and click... then select the operating system you are utilizing on your PC. I happen to be using Windows at home, but notice one thing before you download - you can download with or without JAVA software included. I know I have JAVA on my computer, so I saved some room and chose the "without JAVA" option. You'll notice the file size is much smaller (1.7 MB compared to 21 MB). If you noticed already, I used Image J to create these images with a screen capture tool.



If you're familiar with PACS systems, you'll have lots of fun bringing up any image (particularly radiographic images) and playing around with the software, but there are many user-friendly features similar to what you might accomplish on photoshop with this software. If you go to the "documentation" hyperlink, there are tutorials available for you.



Once downloaded, you can open an image by going to file, open



Then select a picture from a file on your computer (or download something online first). Disclaimer - you must be aware of copyright infringement laws depending on the use of the picture you are downloading. Make sure to reference where you obtained the picture and/or obtain permission to use it :-)



Once you have selected a picture, you can begin utilizing the software to your heart's delight. This is a quick-start to get you going, but I encourage you to check out the "documentation" link listed above to learn about all of Image J's features. If you are planning on using this software for a school project or a lesson plan, it's worth investing a few minutes. I hope you like it as much as I do!

Lumbar Spine Obliques

Even on a patient with normal anatomy, lumbar spine obliques can be quite a challenge no matter how many years of experience you have. If you've memorized the "scotty dog" anatomy, that's great and it will come into play during the critique of your images, but a basic understanding of how the anatomy is laid out in planes is good to know before you begin your positioning.

On most patients, the cervical spine is in the same plane as the lumbar spine, and this can prove to be a valuable positioning tool if utilized properly. In other words, if a patient is lying on their back and you had do take a tomo slice of the c-spine, the same exact tomo slice, centered over the lumbar spine would work as well. Additionally, if you look at a (normal) spine in the anatomical position, then rotate that spine 90 degrees, you could draw a straight line extending from the cervical spine down to the lumbar spine and it will be in the same plane.



So if you can go from AP to lateral with the C and L-spines in the same plane, then you should be able to go half-way (into an oblique position) and still have them in the same plane.



Of course, it's easy to demonstrate this with elaborate stick-man drawings, but it requires a bit of forethought when doing this on a patient. I like to use a radiolucent pad on the table for a couple of reasons... one, it's just mean not to when you have one available, and two, it is a valuable positioning tool when there is a sheet underneath it. It can easily be slid with a patient on it to better align or even help rotate the patient.

So, with the patient on a pad, position and shoot your AP film. I like to perform the RPO next, so I would inform the patient that I am going to roll them. If it is a small enough patient, it's easy to roll them instead of giving them instructions to roll. When they roll, they usually slide their hips over before rolling one way or the other. You can simply grab your 45 degree sponge and be ready to lift. Grab the pad by the patient's shoulders with one hand and hips with another. Slide them away from the center of the table if necessary so you don't roll them on the floor. Roll them up placing the sponge underneath the pad to the desired position. The sponge shouldn't slip if it's directly on the table.

If your patient is too heavy to do this with, some simple instructions will prevent them from shifting their hips, misaligning your C and L-spines. For RPO, have them bend their left knee. Then have them reach their left arm across their chest. Standing on their right side, place one hand on the left shoulder and the other on their left knee and just assist them while they roll. Wedge your sponge underneath and you're all set.

Once obliqued, align the L-spine and center your tube like you normally do. Now you have another way to check your positioning... go to the head of the table and see whether or not your central ray is in the same plane as your cervical spine. This only works when the patient does not slide their hips. Make minor adjustments as needed and check your results.

For conventional centering, it's always a good review to know the "finger-width" measurements that work for you. Look at an L-spine oblique film on an average sized patient that's centered well (print one out if you need to in order to get "actual size).



On the printed image, check to see how many fingers it takes you to place the horizontal crosshair at the center of L-3's vertebral body. Then see how many finger-widths it takes from the ASIS to the center of L-3. This should be a film that you performed, and it should be positioned exactly the same with each patient. If the patient is not obliqued the same amount, then the distance from the ASIS to L-3 will change. This is the most accurate way to know what works for you. You may have learned this in school, but I would bet that the person teaching you this had different sized fingers than you, and you have to measure this for yourself. The same thing goes with your spot film.

For laterals and spot films, you still need to keep the C and L-spine in the same plane, but the most common errors are in over/under-rotation - not the whole body, but in the shoulders and hips not being in the same plane. But that's another post...

On a Personal Note...

A couple of weeks ago, I sustained a knee injury which was a freak occurrence, random happenstance, and still, a very confusing injury to me. I am well aware of proper body mechanics and posture, and was practicing them to the best of my ability when this occurred (I know this because there were three students in the room - one helping me with the patient's legs - and I made a mental note to demonstrate proper lifting technique).

I was performing a two-man lift moving a patient from wheelchair to stretcher (or gurney as I learned on the left coast), lifted the patient off the chair, placed his hips on the stretcher, and while I was lowering his shoulders onto the stretcher, felt a "pop" in my knee accompanied by instant pain and an inability to bear weight. I thought for sure I had torn a ligament, and thankfully, this occurred after the patient was safely on the gurney.

I had an initial set of x-rays that day that showed lateral joint effusion (my opinion) which the doctor called normal. After protesting and noting a few other discrepancies, I decided to obtain a second opinion at the local Emergency Room the following morning. A second set of x-rays showed no fracture that I could see, or that the Radiologist's report could identify. Still, I was referred to an orthopedic surgeon for a follow-up visit. We did the range-of-motion tests, which were accomplished easily by the time I got in to see her as it was healing nicely I thought. And then she pushed down on the lateral part of my knee right at the joint, and I felt like I was going to pass out it hurt so bad. Needless to say, an MRI was ordered "just in case."

So I had my MRI yesterday, and showed my films to a Radiologist that I work with... it seems I have a tibial plateau fracture, nondisplaced, but rather large that didn't show up on the x-rays at all. He said it was definitely there, but the angle of the fracture made it appear to be a part of the cortex on the tibial plateau. I found myself looking at the x-rays in vain trying to see the original fracture, and I've been walking on it pain-free for about a week now.

What I've learned:

I already knew that some fractures might not be seen on plain films, but now have first-hand credibility to that claim.

Also, no matter how good your body mechanics and posture are, we still perform a job with lots of lifting that wears on your body over time. I wonder how bad my injuries could have been without proper body mechanics?

Finally, document everything - and document well. Seek second opinions if necessary and write as much detail as possible, not only for yourself, but for patients that you encounter when you are performing your daily examinations. You never know how much it will help them.

UNC Digital Radiography for Educators

I am very excited to be attending a conference for educators on digital radiography at the University of North Carolina at the end of this month. My supervisor and a coworker attended the last conference held in June and returned with high praise of the instructors, the information, and overall presentation given at this seminar, and I should have plenty to write about upon my return. I do believe they put this conference on every year, but there is limited room and reservations (for me) had to be made well in advance. For any educators out there teaching general radiography, you might want to check out their ITENERARY AND WEBSITE. It's a 25 unit seminar given over four days and only open to radiography educators.

The anatomy of an ERCP

A few of my students have expressed interest in learning more about the basics of an ERCP so they know what they're looking at while observing one. They are really quite simple for the technologist to perform, but there is a lot going on during them that you need to be aware of (for any other reason than to know what is expected of you during the procedure).

First, you need to know your basic anatomy.



Before you begin any fluoroscopy, the G.I. doctor will have the patient in an LAO position (usually) and under conscious sedation, will feed the endoscope into the mouth, down the esophagus, through the stomach, ultimately to visualize the duodenum. The video monitor that the GI team brings with them will display the region around the c-loop of the duodenum in hopes to visualize the ampilla - where the common bile duct empties into the duodenum.

The purpose of this exam (at least diagnostically) is to obtain an angiogram of the CBD and connecting vessels, or a cholangiogram. As we know, an angiogram cannot be done on specific vasculature without a selective catheter. The whole reason we use the endoscope is so that we don't have to surgically go in and dilate the CBD with a catheter and guidewire. We can do it with minimal invasion of the body as a same-day procedure.

So as you're watching the GI team's video monitor, they will be searching for the ampilla, and attempting to dilate it with a catheter and guidewire. In some cases, it is easy to spot because there will be bile spewing from it. You should look for a curve in the bowel with an elevated mound around the corner - this should be the ampilla.



The picture on the left is the pre-dilated ampilla, and on the right there is a catheter that is already placed in the CBD.

Now comes our part... the endoscope is too large to fit inside the CBD, so we must rely on fluoroscopy to visualize the hepatic vasculature. Once the ampilla is dilated, a guidewire and catheter are usually inserted. Fluoroscopic guidance is needed to assist in proper placement of the guidewire and catheter, and a diagnostic angiogram should be performed. The doctor will inject contrast to see if there are any stones, strictures, or any other abnormalities.





Once a diagnostic angiogram is performed, an interventional portion of the procedure can be performed based on the findings. If there are stones, they can be retrieved with a balloon or a basket, a stent can be placed or removed, or angioplasty can be done (widening of the vessel with a balloon). Keep your eyes open for new treatments on the market utilizing new technologies and selective devices.

Lordotic Much???

One of the most common errors I was guilty of as a student (and as a new technologist) was having a lordotic chest x-ray any time I did an AP view in the stretcher. There is a simple fix to this that I have yet to see in a textbook, but I have heard literally dozens of technologists use this tip: angle perpendicular to the sternum. Here's what your AP chest should look like; with emphasis on curvature of the ribs and the amount that the clavicles dip down into the apices:



And how here's what a portable chest sometimes looks like if you make your central ray perpendicular to the cassette:



I know what you're thinking... "Why is it that in a PA standing chest, this doesn't happen and I'm using a perpendicular beam?" Well, when you perform the PA standing chest, what do you do? You roll the patient's shoulders foreward so that the sternum is up against the bucky, making it parallel to the IR. Think about this... if you were to leave the patient in the same position and simply turn them around to be facing the tube, you would probably be able to make a fist and place it between their shoulders and the bucky. Let's look at this lateral projection for a point of reference:



The red line indicates the central ray passing through the sterno-clavicular joints. This patient's back is almost vertical, as it would be if placing a film behind them for the AP sitting chest, and there is not much lung field above the sterno-clavicular joint. You might have to lean this patient forward for the AP projection to demonstrate the clavicles in the lung field at all. Now if I angle perpendicular to the sternum, it will project the clavicles down into the apices as it would if I were rolling the shoulders forward for a PA projection:



Another common error that produces similar results is patient positioning in the stretcher. Ideally, you need the patient sitting upright with the stretcher at 90 degrees with your cassette behind the patient. What you want to do is have the patient scoot (or you may need to slide them before you sit them up) so that they are bending at the waist with the stretcher. You don't want a gap between their hips and the upright portion of the stretcher. This elevates the clavicles, making a lordotic position similar to the standing lordotic chest we all learned about in school and shown here:



If you combine this error with the previous error of not angling to the sternum, the resulting image could be horrid. Check out the following picture:



The first central ray would produce a lordotic image with a foreshortened lung field due to the angulation of the patient (caused by not having the hips flush against the stretcher). The second central ray would project the clavicles downward into the apices as needed, but would probably produce an elongated effect due to the patient's hips being away from the IR. The lower lung fields have increased OID, causing magnification which may cause the radiographer to be unable to include the entire lung field on the image.

For best results, keep it simple: sit the patient up as much as possible, bending at the waist where the stretcher flexes. Prevent OID on all of your film, and angle perpendicular to the sternum to avoid a lordotic projection (and don't forget to shield).