As soon as the film-screen combination was developed in our profession, there has been optimum kVs used for every body part. Even when Rare Earth screens came on the market in the early 80’s and the mAs was cut to one third (from 9 to 3 for example) the kV stayed exactly the same. This is because these kVs were perfect for the density and molecular make up of that body part. This is also why we use the same120 kV on a chest x-ray for a 100 or 300 lb. patient.
The beauty of knowing, and using, the optimum kV is you are always using the perfect kV. By using the optimum (or in other words the best) kV, it also means that any technique problem you will ever encounter is mAs related, not kV. This concept holds just as true today with digital radiography as it did with film except for 1 huge difference. The optimum kVs are now higher with most of the body parts.
Back in the early days of CR Barry Burns, an adjunct Professor of Radiologic Science, University of North Carolina School of Medicine in Chapel Hill, now retired, who was also a radiographer and physicist, immediately realized that the optimum kV for film was not the same as CR. After careful research and experimentation, he discovered that when using CR everyone can increase 15-20 kV from film/screen techniques (except Konica which is 5-10 kV). When both Gadolinium and Cesium based DR detectors were introduced, it was discovered that they too could use kVs 15-20 higher than those used with film/screen.
To see these “new” optimum kVs, please go to All Charts on the Homepage and view chart 1 and 2 (Konica). You will discover that with the exception of chest, barium work and pediatrics, all other body parts have a very noticeable increase in kV.
Why would you want to increase the kV in the first place? This would be so you could decrease the mAs. By increasing the kV 15% and cutting the mAs in half (what most people call the 15% Rule), the entrance dose to the patient is decreased by 33%. This is such a simple way to cut one third of the dose to your patient.
As always, it boils down to patient dose. How high a kV and how low a mAs can you use and still have a perfectly diagnostic image? That’s the question we all need to be asking ourselves before every exposure.
In the upcoming months I will have at least two more articles related to this subject. They will be “CR & DR Universal Technique Charts”, which is pretty self explanatory and “How Low Can You Go?” which will discuss how low can you take the mAs (dose) and still get that perfectly diagnostic image.