Creating a Veterinary Radiographic Technique Chart

A technique chart is the backbone of any veterinary radiology department. Without one, you are destined to take crummy radiographs or radiate your staff with needless retakes. Fortunately, creating a technique chart is easy. Follow these 6 steps and you should be on your way.

This webpage describes how to make a variable kVp technique chart. Remember, you will need a different technique chart for each of the different studies you perform. That means you will need at least four different technique charts (thorax, abdomen, extremity, bone). You will need to understand a little physics to understand this guide. If you need a refresher I suggest you read my lecture series on the physics of diagnostic radiology found elsewhere on this website.

STEP 1 Get your radiology house in order: A little preparation will make all the difference:

• Have your processor serviced: This will ensure that development is accurate. Remember 90% of all processing errors happen in the dark room!
• Be sure that you are using screens that are all the same age, speed, and manufacturer: Remember intensifying screens are not like a wardrobe... you cannot mix and match them. Every technique chart will be specific to the particular film screen combination you are using.
• To create the technique chart use a dog that weighs about 40 pounds but is not overweight. It would be easiest if the dog is anesthetized.

STEP 2 Select your mAs: In veterinary medicine patient motion is a great concern so we like to use a high mAs and low kVp technique. This allows us to increase contrast (because of the low kV) and increase speed (because of the high mAs).

The following are recommended mAs based on a par (medium, 200) speed intensifying screen****

• Table top (no grid) extremity: 2.5mAs
• Thorax: 5mAs
• Abdomen: 7.5mAs
• Spine: 10mAs

To set your machine for the mAs listed above remember you must set the mA and the time separately.

• If your x-ray machine only has one mA station you just select your mA and pick the speed to give you the desired mAs. For example, if you have a 300mA station and you wanted to use 5mAs for a thoracic technique chart you would pick
  • 300mA x 1/120sec = 2.5mAs (extremity)
  • 300mA x 1/60sec = 5mAs (thorax)
  • 300mA x 1/40sec = 7.5mAs (abdomen)
  • 300mA x 1/30sec = 10mAs (spine)

   

• If your x-ray machine has two mA stations I suggest that for extremity techniques you use the smaller of the mA stations (usually 100mA) and for all other applications you use the larger mA station (usually 300mA). In this case your settings would be as follows:
  • 100mA x 1/40sec = 2.5mAs (extremity)
  • 300mA x 1/60sec = 5mAs (thorax)
  • 300mA x 1/40sec = 7.5mAs (abdomen)
  • 300mA x 1/30sec = 10mAs (spine)

****The previous suggestions were for par (medium, 200) speed intensifying screens. If you use high (fast, 400) speed screens you would decrease the mAs by half. Similarly if you are using slow (detail, 100) speed screens you would double the mAs.

STEP 3 Select your initial kVp: The next step is to create a "perfect" radiograph. To do that we need to find a kVp to go along with our suggested mAs setting. To pick a starting kVp use Sante's Rule:

• kVp=(2 x tissue thickness) + FFD + grid factor
  • FFD is the film focus distance and is usually set to 40 inches in veterinary medicine.
  • The grid factor is how much extra kVp you will need to account for the grid you are using. Remember, grids increase contrast but require more radiation to achieve the same exposure. If you are using a...
    • 5:1 grid add 6-8kVp
    • 8:1 grid add 8-10kVp
    • 12:1 grid add 10-15kVp

Most veterinary applications use an 8:1 grid. If you know you are using a grid but don't know what kind of grid is under your table assume it is an 8:1 grid.

Remember, grids are not used in table top techniques. If you are making a technique chart for a table top study use 0 (zero) for the grid factor.

To demonstrate Sante's rule lets say you have a dog that measures 15cm thick at the widest point in the abdomen, an FFD of 40 inches, and you are using an 8:1 grid.. you would use 80kVp ((2 x 15) + 40 + 10).

STEP 4 Expose the Perfect Film: The goal in this step is to obtain a radiograph with a "perfect" exposure. The technique you use to generate this radiograph will be used to create the rest of the technique chart. Therefore, take your time and make this radiograph as good as it can be. It may take 5 or 6 tries to get it right but the time you spend on this step will be worth it in the end.

To get started take a radiograph with the mAs I recommend above for each particular study and set your kVp according to Sante's rule. For example….if I were creating an abdominal technique, using a 40 inch FFD, an 8:1 grid, and my dog measures 15cm then I would expose a radiograph at 7.5mAs and 80kVp. Hopefully, that exposure will be in the ballpark and the radiograph will be fairly well exposed. Unfortunately this is not always the case...

• …if the radiograph is too dark then decrease the kVp by 15%
• ...if the radiograph is too light then increase the kVp by 15%

After you use the 15% rule to get you closer, keep increasing or decreasing the kVp in small increments (5% or as needed) until you get the exposure just right.

STEP 5 Make the Technique Chart: Now that you have a properly exposed radiograph, you are over the hump and the rest is easy. To create the technique chart simply start with your "perfect exposure" and interpolate to find the values of kVp for other measurements according to the following rules:

• Subtract 2 kVp from the original kVp for each cm decrease from the original measurement.
• Add 2 kVp to the original kVp for each cm increase from the original measurement up to 80 kVp.
• Add 3 kVp for each cm increase that places the kVp above 80 up to 100.
• Add 4 kVp for each cm increase that places the kVp above 100.

For example…let's say that we started with a perfect exposure of 7.5mAs at 80kVp and our patient measured 15cm we would make an abdominal technique chart as follows:

Step 6 Create a technique chart for each different study such as abdomen, thorax, extremity, spine. Repeat the above process to create these additional technique charts.