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Optimum kVp

I've been getting a lot of questions on the side lately about "optimum kVp" and "how to choose what kV to use." Even though there are so many variations in x-ray producing equipment and image receptors, there are two main categories that I like to separate my answers in:

1. Optimum kVp using film/screen systems

2. Optimum kVp using digital systems

Film/Screen:

A radiographer's "old-school" medium, film/screen imaging systems require precise kVp based upon part thickness, pathology, atomic number of the part, and presence or absence of air. I would say that an "average" kVp range could be acquired for most body parts (with the exception of Chest imaging) by using calipers. We have to be precise because with film/screen systems, kVp is our primary controlling factor for contrast.

One method an x-ray tech can use to formulate a starting kV is by measuring a body part with calipers (in centimeters), then multiplying that measurement by 2, and then adding 40 to that number. Here's an example: My elbow measures about 10cm. 10 x 2 = 20. 20 + 40 = 60. Using this method, I could use 60 kVp to provide adequate part penetration and contrast using film/screen systems. My optimum mAs is another post... as you know, generator phasing and film/screen combinations will need to be addressed for proper mAs calculations.

There are also many charts available in textbooks that will display optimum kVp ranges by body part. You will need to produce some test images in the beginning phases of what kVp ranges you would like to use, and consult your Radiologists to find out what kind of "image appearance" they prefer before setting your ranges in stone. If they prefer films that are more gray (less contrast), simply add 50 instead of 40 in the example above. If they prefer higher contrast, perhaps only add 35 (recommended for extremities in an orthopedic clinic). I would refrain from using any less than 70 kVp for spine exams.

Digital Systems:

The rules change a bit here... the primary controlling factor for contrast ceases to be kVp in CR and DR systems. With these, we use the processing algorithm as the primary controller. The fundamental principles of radiation physics still apply, like the more kVp we use, the more potential difference, therefore more part penetration we will cause. But since the kVp range does not need to be as precise as film/screen systems, we have some room to play with technique a bit.

The next logical step in our reasoning is to determine if an increase or decrease in kVp will provide better images. Because the beam physics haven't changed, we can't really lower our kVp compared to optimum film ranges because no matter how magical our computed radiography and digital radiography algorithms are, they are still dependent upon adequate exposure to the image receptor. We can, however, increase our kVp.

How much should we increase? In my personal experience, I have used about 10 kVp higher than film/screen systems for small extremities, and up to 20 or 25 kVp higher than film/screen systems for thick body parts like spine. I could never use 90 kVp on a lateral c-spine with plain film because it would be too gray/washed out. Since digital radiography adjusts the scale of contrast to be ideal for a "lateral c-spine" I have two distinct advantages when using higher kVp with these systems: More uniform part penetration and less radiation dose to the patient.

For part penetration, you will be able to visualize the lower cervical and upper thoracic vertebrae at 90 kVp than at the traditional 70 kVp range. Using the 15% rule, you know that using 90 kVp will require far less mAs (about 25% than at 70 kV), which will significantly reduce the exposure to the patient. Talk about a win-win situation!

I would encourage anyone looking to make changes to their technical factors to consider a higher kV range with dose reduction. You may be tempted to use ridiculously low mAs values like 0.5 mAs on various body parts, but remember the physics... you will get quantum mottle or image noise when there are not enough x-ray photons reaching your image receptor. I would love to hear what you currently use at your facility, and if you try some of these changes, I would also love to hear what you think about them. Whatever changes you make, please keep ALARA principles in mind, and remember to consult your Radiologists.