Having an accurate technique chart is key for maintaining image quality while using the lowest dose possible in any radiology department. Sure, you may already know what kind of technical factors to apply on that portable chest x-ray in the emergency room, but what about people who aren’t as familiar with your equipment as you are like students, registry/float techs, or new employees? Though some of these people I mentioned may already have an x-ray license, it can still be challenging for any of these people to catch up to the learning curve at your facility.
Not only are these individuals learning their way around, but they also need to adapt to new software, physician preferences, and unfamiliar imaging protocols. Why not make it easier for them to help you? Chances are, they want to do a good job to continue to be able to work there, but they might just need a little assistance getting their bearings. One of these people might say “help me help you.” Yes, I just quoted Jerry Maguire.
Creating a technique chart is not a difficult task, but it can be time consuming depending on the resources you have at hand. If you wait until a chart is needed, it's probably too late. I’ve always thought it was a great student task, but anyone can do it. I have made technique charts for many facilities I've worked for, and although there are quite a few methods for deriving one, I’m going to discuss the best methods that have worked for me.
Here’s a list of things you might need around during the creation of a chart for your facility:
- calipers
- calculator
- technique chart template
- list of available grid ratios in your site
- x-ray phantom
- acceptable exposure indicator ranges from your CR/DR equipment vendor
Steps for creating a technique chart:
Produce radiographs with technical factors above and below average level:
The hardest part of creating a technique chart for a lot of people is knowing where to start. I like to pick one specific body part that is medium in size. For instance, I usually start with a knee. It’s a common body part to x-ray, and it’s a part that you can sometimes perform table-top or with a grid. If you have a phantom, try taking a series of AP knees at different kVp ranges (60, 65, and 70 for example). For non-grid techniques, use 60 kVp first. Keep repeating the exposures until you have an image with an appropriate exposure indicator value. Then create some images at 65 kVp and 70 kVp that all produce exposure indicators within acceptable range. Note the thickness of the phantom with calipers. It is important to measure exactly through the central ray (I’ll discuss what to do with this later).
If you do not have a phantom, you may decide to record several AP knee views on patients, possibly even using AEC. I don’t recommend performing test exposures on actual patients, but pick a few exams that come out within acceptable exposure indicator ranges, and note their measurements with calipers. Remember to try different kVp ranges, and consider using AEC. Simply record the mAs value that AEC provides and measure the patient thickness if the exposure indicator is within diagnostic range.
Note: If you do not have calipers, simply note "small", "medium", or "large" for patient size reference. While this method is less accurate, having this information will be far more useful than no information at all to the newcomer to your facility.
Supervisors or Radiologists discard unacceptable images and select preferred images:
If you do not have a group of quality control personnel in your facility, consider showing your collection of images to a chief radiologist. Only select the exposures with acceptable exposure indicator ranges to present. You don’t need to let the doctor know all the technical details… simply ask them which ones they like best. A radiation-conscious doctor may inquire about dose between the images, and you can inform them when prompted about which images have more or less exposure to the patient. If you keep your original kVp ranges adequate to part thickness within reason, you should not have a lot of variation. Select the technical factors that the radiologist, or group of quality control personnel deem most acceptable to move onto the next step with. For an example, I’ll use the technical factors 65 kVp and 4 mAs for a table-top knee measuring 9 cm with the calipers.
Prepare the chart for preliminary trials:
Starting your technique chart with a knee allows you to have a wide range of body parts with similar composition to derive techniques for. As a table-top technique, the baseline of 60 kVp can be used from the knee all the way down to the toes. Here’s where all those lessons in school will come in handy… estimate some techniques using the 4cm rule. Remember, every 4cm of tissue thickness requires a double in mAs value to maintain density. If we are going to work from the knee down, we need to remember that if we decrease 4cm in tissue thickness, we need to ½ the mAs to maintain density.
An example would be an AP ankle. If we have an ankle measuring 5cm, we can compare the knee technique and reduce the mAs by ½ because it is 4cm less than the knee. Being similar tissue composition (muscle, bone, tendon, ligaments, etc.), 65 kVp at 2.0 mAs should provide an ideal exposure indicator. For a foot, consider lowering the kVp due to the dramatic decrease in part thickness compared to the knee. Keep your mAs at 2.0, but try 60 kVp if it measures 2 cm.
This, of course, can all be derived on paper before it is applied to a phantom or patients. Once you get to larger knees above 10cm, or the femur, you can start adding additional details like what grid ratios you are using. We shouldn’t use less than 70 kVp with a grid. Let’s think about how we would image a 13cm knee:
Our original technique was 65 kVp at 4 mAs. There are a couple of steps to consider here. First, we know that we need to double the exposure to maintain density with a 4cm increase in technique. Since I know that I’ll need to increase kVp to account for grid usage, I’m going to increase kVp 15% (about 75 kVp) for this step. This serves to double exposure to the image receptor as well as provide the adequate kVp I need for grid usage. Next, I have to account for the grid conversion. If I’m using an 8:1 grid, my conversion factor is 4. I need to multiply my mAs by 4 to maintain density to account for the grid. My new technique for a 13cm knee is 75 kVp at 16 mAs with the 8:1 grid. If I’m going to move to the table bucky, that’s a conversion factor of 5 (most buckies are a 12:1 ratio), so I would use 75 kVp at 20 mAs.
These techniques seem a little high for the site I’m currently working, but remember, they’re just an example based off an original technique that worked. Once you have a beginning technique with grids, you can continue to compensate for body thickness as you go proximally up the leg. You’ll find that shoulders and knees are similar in thickness, as well as ankles and elbows, feet and wrists, etc. For pediatric work, a 2 year-old abdomen is about the thickness of an adult knee. The key is to find a starting point so you can make a calculated guess on the next projection you will need to make.
Test the new chart on phantoms of different sizes:
Once you have some good calculated technical factors, it’s time to test on phantoms. If you don’t have phantoms, I would suggest performing these on patients. Start performing them on parts closer to the ones you started with so you can eliminate as many variables as possible like body habitus, pathology, presence/absence of air, etc.
Useful information to have on your technique charts include mA, time, kVp, SID, grid vs. no grid, focal spot size, exposure indicator values, body part thickness/habitus, or any other factors that you may change between exams. I recommend keeping a digital copy in excel, or any other software you feel comfortable with that is easily edited. A technique chart is never really "finished" as equipment is constantly replaced, wear and tear on equipment affects performance, and as calibration occurs over the years. Remember to update your chart with any changes. You can view and/or download a
Sample Technique Chart Here.
Creating a technique chart from scratch can be quite tedious, but it is a skill that should be developed for all technologists. Knowing manual techniques will improve your portable and OR imaging skills, and it can provide early indicators for quality control or equipment repair concerns. There are many circumstances where AEC can produce inadequate images such as improper bucky selection, prosthetics over the ion chamber, or general operator error. Manual techniques are great to know and provide the technologist with a lot of versatility.
