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!
Awesome post. My professor would throw a fit if i brought up this as a debate. He is very stubborn to the old ways of doing things and doesn't like to admit that technology can possibly change the way we take radiographs
ReplyDeleteThat's unfortunate, but understandable in a way. I guess when what we have known to be true for years is challenged, it takes a lot of evidence (and time) for people who have been doing this a long time to become convinced that the change may be plausible. I remember when we first switched to PACS... we had an older Radiologist who was an amazing doc, but wanted us to print a hard copy of every image on the dry laser so he could look at it with a magnifying glass and hold the film at an angle to the light... I'm guessing because of the dual-emulsion of traditional film/screen systems we used to use. He eventually got used to PACS, but would grumble about it from time to time.
ReplyDeleteI'm still confused by the use of the word "contrast" in this article. While kV does control the tissue contrast in the raw (projection) data, the window width/center controls the contrast displayed on the monitor. It's not at all clear that the author is adequately distinguishing between these two "contrast" effects. If anything, more care must be taken in digital imaging to ensure that the tissue contrast is optimized by the proper use of kV and NOT by kludging the visual contrast by applying the width/center lookup table after the data has been acquired with a sub-optimal kV.
ReplyDeleteHi Lance. By "contrast" I meant image contrast unless otherwise stated. Tissue (subject) contrast is inherent and cannot be changed unless we add contrast media. To avoid confusion, I left contrast media examinations out of this post and am strictly describing image appearance.
ReplyDeleteI do not recommend "kludging" the visual contrast by manipulation of the image at all... in fact, many places in my blog advise against any image manipulation prior to sending it to PACS. The purpose of this post was to show that with digital imaging systems (compared to film/screen), you may use a higher kVp range, thus lower mAs reducing radiation dose, and still achieve optimum radiographic contrast - without any other image adjustments... go radiation safety!
Hi Lance. By "contrast" I meant image contrast unless otherwise stated. Tissue (subject) contrast is inherent and cannot be changed unless we add contrast media. To avoid confusion, I left contrast media examinations out of this post and am strictly describing image appearance.
ReplyDeleteI do not recommend "kludging" the visual contrast by manipulation of the image at all... in fact, many places in my blog advise against any image manipulation prior to sending it to PACS. The purpose of this post was to show that with digital imaging systems (compared to film/screen), you may use a higher kVp range, thus lower mAs reducing radiation dose, and still achieve optimum radiographic contrast - without any other image adjustments... go radiation safety!
I'm a longtime RT(R), but also a longtime radiography educator, and I have to say that, while I understand (& agree with, to a point) the idea that the ability to control the window & level in digital imaging (assuming one is seeing a "soft copy", on a good monitor, and not viewing a "hard copy"/film) allows us to use higher kVp's than we have been accustomed to with film/screen radiography, I still contend that, because it is one of the controls of "subject contrast"--See Selman, Christensen et al., Cahoon, & other physics/technique books; tissue density, thickness & effective atomic number, and the presence & amount (if any) of contrast media, are not the only controls of this--kVp is still a major factor in the control of subject, radiologic (exit beam) & radiographic contrast, and that the visibility of detail is still dependent (albeit perhaps to a lesser degree when digital imaging is involved) on the kVp used. In addition to affecting the differential absorption within a part, kVp also affects the amount, energy & direction of scattered radiation, which will also impact the image. In fact, I'd say that, even were it not for the increased quantum mottle seen in the 110 kVp image, there would most likely be--are?--areas that, due to lower contrast, would not be seen as well, regardless of how much one manipulates the brightness & grayscale...but I guess we don't miss what we can't see, right? LOL
ReplyDeleteThanks for your comment tnxrayman... I absolutely agree with you. While it is going to be up to the individual radiology department with the radiologists' input on how high the kvp ranges are and what is acceptable quality vs. dose, I understand that there is an increasing amount of image degradation due to mottle as we increase beyond the "optimum" range further and further. This experiment is supposed to be somewhat extreme and I would never use 110 kVp on a knee myself. I agree with you also when you say that the visibility of detail is dependent on range of kVp used and there are additional variables that may need to be considered to determine what "optimum" really means in our respective imaging departments. Different vendors are using different materials for their image receptors - some will have a larger dynamic range depending on the k-edge absorption values of the image plate material. This may constitute only a subtle difference (if even detectable on a QC monitor), but it should definitely be taken into consideration in acquiring the most visibility of detail (not to be confused with recorded detail).
ReplyDelete