One of the advantages of digital imaging systems is more tools to control image contrast. According to the ASRT's "Appendix for Digital Image Acquisition and Display" the look up table is "
the default gradient curve applied to the data set of your image determining initial display contrast." Remember the average gradient used to calculate the scale of contrast in sensitometry??? There are many parallels here in regards to digital image display.
What does this mean for us though? How does it help? If you are a seasoned technologist who has mastered the art of film/screen imaging techniques, you are already familiar with the concept that kVp is the primary controller of image contrast. However, this changes with digital imaging systems. While kVp selection still affects image contrast, the new primary controller is the look up table. In other words, the algorithm you select (when you select your body part and projection) instructs the computer to apply a gradient curve (via the look up table) to tell the computer what kind of image contrast to display. Don't believe me? Try processing a hand x-ray under a PA chest algorithm... the contrast will be long-scale even though you used 60 kVp.
So is the optimum kVp range that we all learned in x-ray school still considered optimum if it is no longer adjusted to simply manipulate contrast? Let the debate begin... the principles of physics remain the same with the penetrating power of the beam in regards to kVp selection. You still need enough kVp to penetrate a body part to acquire an optimum radiograph. Along with my students in imaging class, we decided to see just how much image contrast varied (or remained the same) with a CR imaging system. The following images were taken of a lateral knee phantom (note the annotated technical factors):
A film/screen imaging system would show a dramatic change in scale of contrast when comparing technical factors between images 1 and 4. On our CR system, there is not too much difference between the knee exposed at 70 kVp and the knee exposed at 110 kVp.
We all know that if we can produce images at higher kVp values, the mAs can be reduced, giving the patient far less radiation exposure. I think this is something that should be considered at all facilities operating digital x-ray equipment. Other than saving on radiation dose, there are other advantages and precautions that we need to consider.
If you look closely, there are subtle changes in the most dense regions of the bone, as well as the least dense regions, specifically the soft tissue surrounding the patella. I am an extreme advocate for magnification at the QC station... doing this routinely will give you a better idea of what the Radiologist is going to be seeing in the reading room that you might not see from the low resolution QC station monitor. I have magnified the 1st and 4th images below for further evaluation.
70 kVp 7.3 mAs
110 kVp 1 mAs
Because the dynamic range of the CR image plate is wider than that of film/screen, we are able to achieve diagnostic quality images outside the traditional technical factor parameters. This does not, however, mean that any old technique will work. As you can see in the 110 kVp image, we are beginning to see some image noise, otherwise known as quantum mottle. This is due to a lack of x-ray signal to the plate. The CR system is great at receiving exposure, with a wide range of kVp and mAs values, and transforming the remnant beam into a beautifully displayed manifest image. But... if there is not enough exposure to the plate, all bets are off. We see image noise and quantum mottle when there are not enough photons (signal) to the image plate. In order to maintain density at 110 kVp, the mAs has to be decreased severely. Eventually, we will lower mAs values so much that even though our 15% rule calculations are correct, the quantity of photons striking the image plate are simply insufficient to produce a good signal to noise ratio.
The tricky part is balancing image quality and dose to the patient. As always, the Radiologist will need to have a say in what kind of images they wish to see, but keep these things in mind the next time you are performing a lateral C-spine x-ray on the Incredible Hulk. If you have trouble visualizing the C7-T1 junction, you may want to consider increasing the penetrating force behind your beam with 80 or 90 kVp (don't forget to adjust your mAs) and feel comfortable that your scale of contrast will not be as severely altered as with a film/screen imaging system. You will have more uniform part penetration, giving you better visualization of the lower cervical anatomy, and you have reduced your patient's radiation dose by 50% - 75%. Not bad for a 10-minute exam!
