Contrast in Review

It's always a good idea to review topics like contrast that have so many of those arrows going this way and that. It reminds me of one of those street signs in Europe that you see on the movies where 10 roads come together at one intersection, and you can't find your destination because the signs are written in a foreign language. Hopefully, the concept of contrast does not remain a foreign language to you. Still, we can look at how changes in exposure factors effect contrast:

Changes in mA, time, or overall mAs should not affect contrast at all. It is important to remember that you must have adequate density (optimal or acceptable) in order to properly evaluate contrast. So it can be said that if you have excessive density - too dark, or insufficient density - too light, then you would experience a decrease in contrast.

We know that kVp is inversely related to contrast, and is our primary controlling factor of contrast. As we increase kVp, our contrast decreases.

Filtration is used to increase the average energy of our beam, or "harden" the beam. As we add filtration, the average kVp of photons getting through is higher, so the contrast will decrease. It's similar to increasing your kVp slightly without any other changes.

Field size has a dramatic effect on contrast. If we use the entire field size on a 14x17 cassette on a lateral l-spine, we would be irradiating much more tissue than we need to, causing tons of scatter and lowering contrast. So when we decrease field size (collimate), we are avoiding unnecessary tissue irradiation, thereby reducing the amount of scatter produced, which reduces fog reaching the film, and improving contrast - all while withholding the standards of ALARA - give yourself a pat on the back.

When we have increased motion on our films - the ability to distinguish a very light area from an adjacent dark area on the film becomes blurred. When there is less of an ability to "distinguish shades of gray from one another" contrast has decreased, even though technical factors have not changed.

Patient size affects contrast quite a bit as well. If I take a KUB on a 20cm abdomen and have optimal contrast, then I go to take a KUB on my next patient with a 35cm abdomen, I am increasing the thickness of tissue, which will absorb more of my photon energy, reducing image contrast.

Now we have our grid ratio - thank goodness for grids. As I increase in grid ratio, the quantity of scatter-absorbing lead increases, as well as the hight of the lead strips. As the lead strips become taller, their propensity to allow even slightly scattered photons to pass through decreases. So an increase in grid ratio means an increase in image contrast.

Focal spot size has no effect on contrast... one less thing to worry about.

SID - well, two less things to worry about - no effect on contrast.

OID has some effect on contrast (air gap technique). As you increase OID, the percentage of scattered photons reaching the film decreases due to their angle of scatter. If a photon is only slightly scattered, you can eventually increase OID enough so that the scattered photon will miss the film. This reduces scatter reaching the film, therefore increases contrast.

Developer time/temperature will affect contrast as well. This goes back to what we discussed with having the appropriate density before we can properly evaluate image contrast. If the developer temp or time are too short, then your image will be too light, therefore have decreased contrast. If the developer temp/time are too high, your image will be excessively dark, therefore image contrast is still decreased.

Last but not least, we have film/screen speed. I waited until last because there seems to be some discrepancies between textbooks on this one. One textbook says that there is no effect, while another textbook says that an increase in screen speed produces an increase in image contrast. It goes on to mention that the increase in contrast is probably not enough to visibly see, or to make much of a difference in your optimal image, but it does occur. I would actually like to see what everyone can find from their own sources and post in the comment section here. I've only looked at three books myself, but I'm all for having as much support as possible.

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Filtration

What is a filter? Well, if you're a coffee drinker, you are familiar with filters. A nice brew is created by placing a filter in the coffee maker and filling the filter with ground coffee beans, allowing the desired coffee (created when hot water soaks the grounds) to flow through to the pot, while preventing the unwanted portion (the grounds) from trickling down. This is not so different from radiographic filters at all.

An x-ray filter is composed of Aluminum equivalent material (Al - not to be confused with Pb for shielding) and is between the target and the patient for the purpose of preventing unwanted photons (the grounds) from passing through while allowing the desired photons (the coffee) to pass through toward the patient.

So which photons do we want to keep and which ones do we want to get rid of? Our duty as Radiologic Technologists is to keep radiation exposure levels of the patient at a minimum (ALARA standards). So, in any x-ray exposure, there is a portion of the beam that will be at a very low energy for the part being x-rayed. It is so low, that it does not even contribute to the useful beam, and it ends up getting absorbed in the patient contributing to radiation dose - BAD photon!

A filter allows us to remove a majority of the bad photons while allowing the good photons (higher energy) to get through to the film. The line of thinking goes something like this: "If the low energy photons are not going to contribute to our image anyways, why not remove them?" Take a look at the picture of an unfiltered beam:



Here I have randomly selected five photons with varying energy levels (we all know that just because we select 70 kV, it doesn't mean all photons produced are at their peak). To calculate the average energy of the beam using these photons, you add them all together and divide by 5, which gives you an average energy of 50 keV. The 30 and 40 keV photons are probably going to be absorbed by the body to contribute to radiation dose to the patient. Now, lets look what happens when we add filtration:



Notice, the two lower energy photons are removed, and the remaining photons have a higher average energy of 60 keV. This is also referred to as "hardening" the beam. Note that any time you add filtration without changing any other factors, you are reducing the intensity of your beam, so an increase in technique is always required when adding any absorbent material. The resulting energies are shown in the following graphic representation of exposures made at 120 kVp:



HVL - half value layer is any amount of material (or in this instance, filtration) that reduces the intensity of your beam to half its original value. Consequently the TVL (tenth value layer) is the amount of material that reduces the intensity to one tenth the original value, and so on.

Types of filtration:

Inherent filtration is any filter that is present as part of the radiographic equipment, and usually includes the glass envelope surrounding the tube, as well as any oil around it. This usually makes up about .5 - 1.0 mm Al equivalency.

Added filtration is just as it is described - anything added to what filtration already exists within your equipment. It usually resides between the tube housing and the collimator box. See the following picture from "Principles of Radiographic Imaging" Carlton/Adler 4th edition:



Total filtration = inherent filtration + added filtration. According to the National Council on Radiation Protection (NCRP), total filtration must be a minimum amount depending on the kVp range you are using:

Below 50 kV - 0.5mm Al
50 to 70 kV - 1.5mm Al
Above 70 kV - 2.5mm Al

Compensating filters are for another post... to be continued...