Radiographic contrast can be defined in a number of different ways but the simplest is the best. Contrast is what allows us to see recorded details.

Transcription

1 Radiographic contrast can be defined in a number of different ways but the simplest is the best. Contrast is what allows us to see recorded details. It is essentially the different shades of density in a radiograph. The classic definition can be explained with the equation Radiographic contrast is equal to the product of the Subject times the film contrast. This equation applies to film screen radiography, but not necessarily to digital radiography. We will discuss in a later unit, how contrast is controlled in digital radiography, but first we must see how contrast can be controlled by the technologist when using x-ray film. The term subject is in reference to the patient or part that you are x-raying. The subject can be very different and depending on the physical make up of the patient, this will influence the outcome or type of contrast that will eventually result. This may be explained this way. If the patient is hypersthenic, then this will require the radiographer to increase the kilovolts peak which in turn influences the contrast directly. Kilvovolts peak is the controlling mechanism of contrast and if you are required to change the kilovolts peak from a lower level to an increased level, this will most definitely affect the contrast you are using. The second part of the equation or the film contrast has to do ith the type of film that you are using. Please remember from a previous lecture that x-ray film can be designed with a specific type of contrast. Taken together, these two factors will very definitely cause changes in the contrast. Because using radiographic film is definitely on the decline, it may be best to replace the film contrast part of the equation with the term imaging system contrast. This will permit us to describe the contrast that new digital imaging systems can provide with a click of a mouse. 1

2 Because radiographic contrast is the mechanism that permits us to see details on an image it is very important that we understand how a variety of subject characteristics or factors will influence contrast. The first factor is patient size. You will find that the larger the patient is, the greater the amount of kilovolts peak you will need to use and this can vary tremendously. To cite an example, take the average size male patient that may weight 150 pounds. For an abdominal x ray, this patient would require approximately 80 kilovolts peak. This level of kilovolts will permit the radiation to go through the patient and will contribute to the image. The different levels of absorption in the body will in turn result in a specific type of contrast on the film because of how the radiation was absorbed by this patient as the radiation went through the abdomen. If the kilovolts peak was adequate, then the results of the image should be acceptable and diagnostic. If the patient is quite large, the kilovolts peak we used in the average size patient will no longer work, because as the patient becomes larger, you will be required to use a higher level of kilovolts kilovolts peak which in turn will most definitely affect how the radiation is absorbed and the nature of the contrast will be changed. Tissue type also affects the contrast. Generally the more dense the tissue is, the harder it is to penetrate the tissue which in turn it will require you to change the kilovolts peak. Also, if a patient is muscular or if a patient has a lot of fatty tissue, this will result in the radiation being absorbed at different rates. The contrast on patients that are sthenic is easiest to control. 2

3 The health status of a patient can determine or influence the absorption of radiation. An example of this is the individual who has a lot of fluid retention or another example like such as this is a patient that may have a lot of fluid in their abdomen. In either scenario, an increase of fluid in the body will require the radiographer to increase the kilovolts peak to ensure adequate penetration which in turn will influence the type of contrast that will be seen. The opposite is also true, if a patient is emaciated or very think because of disease, this will also require the technologist to adjust the kilovolts peak in a downward fashion which will also change the contrast. As you can see, the human body can challenge the technologist by making sure he or she understands the type of attenuation properties of the anatomical area that they are x-raying. When the tissue type is very dense such as bone, the x-rays have a difficult time penetrating it and we can therefore say that this tissue has a high differential absorption properties. Any fluids in the body which are in high concentration will also influence the attenuation or absorption of the radiation. Softer tissue such as internal organs like the liver, kidneys, intestines, and stomach also have different attenuation or absorption properties. Tissues such as these have such poor absorption properties such that they are very difficult to visualize unless we use contrast media so that we can see their internal structures. When tissue is very soft like this we can say that it has very poor subject contrast and low differential absorption. In order for it to be visualized, the patients physician will need to order a special study where the patient is administered contrast media such as Barium Sulfate. 3

4 The health status of the patient is important for the technologist to understand because it will help determine what exposure factors or technique will need to be used when an x-ray examination is completed. In this example here, a chest x-ray has been completed. It definitely demonstrates that there is something clearly wrong with the patient s chest. Note the following characteristics of the right lung. It appears somewhat dark and enlongated. The darkness of the image is referred to as hyperlucency meaning that it was penetrated very easily which resulted in an image that is slightly darker than normal. In the lung, when we see this type of characteristic, it means that the patient has a lot of gas or air trapped in the lung tissue. Remember that gas or air in tissue will facilitate the transmission of x-rays through tissue. When there is a condition in the patient that causes the technologist to back off on the technique, this is referred to as a destructive condition. The left lung demonstrates that there is something in the lung that is causing the x- rays to be attenuated and therefore the image looks very opaque. Because the x-rays had difficulty penetrating the lung tissue, the pathology is know as additive meaning that the technologist must add milliampere seconds or kilovolts peak or both to penetrate and demonstrate the tissue adequately. An example of pathology which is additive is the build up of fluid in the patient. Sometimes this occurs in the lungs or in the abdomen. 4

5 This is an x-ray of a normal chest x-ray. Compare it to the previous image and you can definitely see a difference in the patient. Radiographic contrast is simply a collection of many density differences in the image. Areas that appear dark have been penetrated easily and therefore the film or image will be dark in these areas. This is what is called low differential abosorption because the structures the x-rays went through did not attenuate or absorb the x-rays and therefore permitted them to go through very easily and make the image dark. On the other hand, the areas that look opaque, light, or white are areas that attenuated or absorbed the x-rays at a high rate and this type of tissue has a high differential absorption. When there is tissue that has a high attenuation rate next to tissue that has a low attenduation rate, this will cause a very abrupt density difference in the image. When you have very abrupt dark and light regions on a film, this is known as short scale contrast. The chest xray image on this slide demonstrates high contrast or short scale contrast. 5

6 This slide demonstrates x-rays of areas of the body that have low differential absorption rates. The image on the left is a mammogram. The tissue of the human breast has a low attenuation rate and therefore requires a relatively low kilovolts peak to penetrate. It is important to enhance the contrast and with a low kilovolts Peak, the contrast can be enhanced and the anatomy can be seen better. The next image is that of the lower abdomen area. Note that you can see the vertebrae fairly clearly because the bony tissue attenuates the x-rays well and therefore it makes them very visible. If you were trying to see soft tissue structures such as blood vessels on this image, it would be impossible because the arteries and other blood vessels are relatively small and have an extremely low differential absorption. Because of this low attenuation rate, soft tissue structures such as blood vessels have a poor subject contrast. Subject contrast is inherent in the anatomical part and depends on the physical make up of the tissue. When there is little difference in the soft tissue, radiographically it is difficult to see any internal structures unless the tissue is enhanced artifically. The third and fourth images of this slide demonstrate how the addition of aqueous contrast media will make the blood vessels visible. Because the contrast media is difficult to penetrate by the x-rays, it will make visible any structure in which it is placed. As the blood vessels are filled with the contrast media, this makes the structures opaque and therefore the vessels will be visible on the x-ray. If there is anything wrong with the blood vessels such as a leak, aneurysm, or some other pathology, it can be clearly seen because of the contrast media. 6

7 It is very important to understand that the imaging system that you use can also influence the type of contrast that will result. X-ray film can be designed with different characteristics and the ability to give you different types of contrast. In the day when film was prevalent, the radiographer had a choice as to the kind of film to use when x-raying different areas of the body. For example it was common to use high contrast film when performing examinations on extremities such as hands and feet. On the other hand, x-ray film can be designed to provide low contrast or subtle contrast when evaluating areas such as lung tissue. Film is therefore designed with inherent contrast depending on the application. X-ray film must also be protected from accidental exposure prior to the actual exposure. If x-ray film has been exposed to radiation, light, or some other type of exposure, it will acquire density before you actually make an exposure. Depending on the severity of the exposure, the film will definitely show evidence of the exposure by having poor contrast and increased density. This can make the visibility of details very difficult or impossible. 7

8 This demonstrates how radiographic film can be affected when it is exposed improperly. It is difficult to differentiate what kind of exposure caused the effects you see. When film is stored improperly or in a place where the environment is allowed to get too hot, it will cause the film to become exposed prematurely and then when it is exposed by x-rays correctly, the previous exposure will prevent the image from having diagnostic levels of contrast and density. Film also has a definite shelf life. As previously discussed in an earlier unit, film that is kept on a shelf too long will acquire exposure from natural background sources and the film will have density before it is exposed as designed. A film that is dated will not be able to give you the subtle details that are necessary to render a diagnosis. Other types of environmental issues such as the ventilation of the storage area are also important. If the film is stored in an area where there are fumes that may be corrosive or that can cause the film to react, the film will also acquire density which will prevent diagnostic levels of contrast. It should also be noted that the higher the speed class of the film, the faster it will react to any type of exposure. 8

9 A critical factor to remember with film radiography is its increased sensitivity after it has a latent image. It is not uncommon for a film with a latent image to be ruined by simply permitting the film to sit around without prompt development after the exposure. Film has a unique characteristic that causes it to become six to eight times more sensitive to exposure than it had when it was fresh out of the box. The correct thing to do with film is to process it as soon as possible right after the exposure is taken. The main thing that happens with film is that it will acquire significantly more density which will of course make it impossible to see the fine details that would normally be seen. Processing is also a crucial factor to remember. It is said that radiographic film processing is a very carefully balanced process involving the concentration of processing chemistry, the time of development, and the temperature of the processing chemistry. If any of these three factors are not correct, the film will be undiagnostic because it will not have the proper level of density or contrast. 9

10 The mechanism which controls radiographic contrast is the kilovolts peak. Remember that the technique that is used must just right so that the anatomical part is penetrated adequately and so that the density is acceptable. If the kilovolts peak is excessive or insufficient, anatomical parts that would normally appear clearly and distinctly visible will either be too dark or very underpenetrated. In either case, the image will not be diagnostic. Differential absorption is essentially the amount of absorption or attenuation that anatomical structures possess. As the x-ray beam passes through the patient, anatomical structures that are very dense will absorb the radiation and not permit it to go through to the film. This structure would have a high differential absorption and will tend to provide high contrast. If the anatomy is very easy to penetrate, then it is said to have a low differential absorption rate and will tend to have lower contrast. 10

11 Kilovolts peak is used to control contrast because of its ability to affect the differential absorption of different anatomical parts. Study the three images on this slide. The image on the far right is underpenetrated because the kilovolts peak was set low. Using a lower kilovolts peak tends to increase the absorption of the radiation and therefore the differential absorption rate is increased. The image in the middle used an excessive amount of kilovolts and the image appears overpenetrated. Irregardless of the absorption rate of the part, if the kilovolts peak is too high, the attenuation properties of the anatomy will be overwhelmed by the kilovolts peak and the part will appear excessively dark. As the kilovolts peak is increased above the level it should be, the differential absorption rate decreases and the contrast becomes very low or dark. When the kilovolts peak and the milliampere seconds are just right, the image demonstrates excellent quality such as the image on the far left. It will demonstrate optimum details. Note how you can see the bones very clearly as well as the soft tissue. This indicates that the technique was just where it needed to be. An important factor to remember is that as the kilovolts are reduced, the contrast of the image does increase and the dose to the skin also increases. 11

12 There are studies that require a moderate to high level of kilovolts. When the kilovolts peak is high, this actually reduces exposure to the patient. As the kilovolts peak increases, the wavelength of the photon also decreases and this is what actually reduces the exposure. The wavelength of the x-ray photon is the mechanism of injury and as the wavelength becomes smaller, it becomes less damaging. High kilovolts techniques are useful for large anatomical parts such as chest radiography and Barium sulfate studies. Barium is the substance that is mixed with water and then is administered to the patient to be able to visualize internal organs such as the stomach, small, and large bowels. It is difficult to penetrate with x-rays and this is why it can be used to visualize these structures. 12

13 As you progress through the program, you will learn many new terms and concepts which are very important to radiology. One such term is long scale contrast. This is a description of an image which demonstrates many different and subtle shades of grey to black. When the density differences are very subtle, it is said to have low contrast or long scale contrast. Evaluate the image on this slide. This device is called a penetrometer or an aluminum step wedge. It is a small block of aluminum that is cut in a stair step fashion. The purpose of this device is to help calibrate the x-ray machine, but as you can see when you take an exposure of the device, it will demonstrate density steps on the film which correspond to the different thicknesses of the aluminum step wedge. As you can see, with the lower kilovolts exposures, the density differences are more abrupt and as the kilovolts increases, you can see more subtle density steps. The subtle steps are referred to as long scale contrast and the lower kilovolts steps are short scale which is the opposite. 13

14 This is a picture of the device that we can radiography to obtain the varying density images in a step fashion like in the previous slide. Note the stair step design. The thicker part of the wedge appears as the lightest part on the actual x-ray image. 14

15 In these images, you can definitely see the difference between short scale and long scale contrast. In the short scale image, there are few density steps and the difference in blackness is significant. In the long scale image, you can see many more subtle steps. In the image of the elbow, the contrast is relatively high so this can be called short scale contrast. One problem with short scale contrast is the fact that subtle details can be missed or not imaged. While the image is asthetically pleasing to the eye, the fine details of the bony tissue can be missed. 15

16 One important process that the technologist can use is the fifteen percent rule. The fifteen percent rule is a procedure where the technologist can make a change of kilovolts peak of fifteen percent. For example, if a technique calls for twenty milliampere seconds at fifty kilovolts, the fifteen percent rule can be applied to produce a change in contrast. Remember that as kilovolts peak increases,the contrast of the image becomes longer scale. An important facto to remember with this conversion is that whenever you increase the kilovolts peak by fifteen percent, you must decrease the milliampere seconds by half because as you increase the kilovolts by fifteen percent, this is equivalent to doubling the milliampere seconds. So to keep the density approximately the same, then the milliampere seconds must be reduced by 50 percent. See this concept on the next slide. While kilovolts do control contrast of the image, other factors that also influence contrast significantly is grid or bucky use. Remember also that film can be designed with inherent qualities that will give the image a particular contrast. 16

17 In this sample technique, the milliampere seconds are twenty at 50 kilovolts peak. To adjust the technique by fifteen percent, you must multiply the kilovolts by fifteen percent and then adjust the milliampere seconds by cutting it in half. This will ensure that the density of the image will be consistent with the original technique, however because of the increase in kilovolts, the contrast will become longer scale. 17

18 We have demonstrated one example of how the fifteen percent rule can be applied to change contrast. In the previous model, we increased the kilovolts by fifteen percent to produce longer scale contrast. The process can also be reversed by reducing the kilovolts by fifteen percent to make the contrast short scale however when you reduce the kilovolts by fifteen percent, you must double the milliampere seconds to ensure that the image will have adequate or consistent density. As you have seen, the technique was changed in this case by changing the exposure time, so if exposure time is an issue such as when a patient is moving too much, you may use the fifteen percent rule to adjust exposure time, and also use it to change the exposure to the patient. A typical maneuver that can be used to reduce exposure to the patient is to increase the kilovolts by fifteen percent and reduce the milliampere seconds by one half. 18

19 Here is another model technique where the original kilovolts is increased by fifteen percent and the milliampere seconds is reduced by one half. This technique reduces exposure to the patient by making the x-ray photon wavelength less biologically effective on human tissue. 19

20 Here is a model showing how increasing the kilovolts by increments of ten will definitely cause a change in contrast. And as the kilovolts increases in increments of ten, the radiation becomes more effective in penetrating the anatomy and changing the density of the image so that it has less contrast or longer scale contrast. 20

21 In this image, if you look at the lower kilovolts images, you can see relatively few shades of density on the model. This is because as the kilovolts is reduced, the penetrating power is reduced and the contrast of the image increases. Unfortunatley, what also happens is that the radiation becomes more biologically effective or in other words it can increase exposure to the patient s skin. 21

22 Another quality factor that radiographers must always keep in mind are the recorded details of the image. There are many factors which affect recorded details. One of the main factors which helps control the recorded details is the type of image receptor you are using. If the image is being recorded on x-ray film, it is important to remember that film can be designed with high resolution or detail capability, or it can be designed for speed. It is very important to understand that the faster the image receptor is, the worse the recorded details it will provide this is why when film was used extensively, most x-ray departments had multiple kinds of films. In example, there was a special film for chest radiography, other films for extremities, and yet other types of films for routine studies. Recorded details is best defined by how sharp the image is. This is to say that you should be able to distinguish between very minute structure details. The term acutance is in reference to how abrupt the difference is between different structures and tissues. The interface of tissues is where one tissue ends and the other begins. An example of this is the difference in going from bony tissue to soft tissue such as muscle. The sharper the difference between tissues, the more acutance you have. 22

23 AS we have mentioned before, recorded details are affected by the type of image receptor you use. However, there are multiple geometric factors that also have a significant affect on recorded details. The first is the focal spot size. In most typical x- ray tubes, the focal spot usually comes in two sizes, one millimeter square and two millimeters square. The larger the focal spot that is used, the better the heat loading it will endure, however on the down side is the fact that a large focal spot will also have significant levels of geometric unsharpness. Geometric unsharpness is an area of slight blurring along the margins of bony structures. It appears as just a very small area of blurring. The smaller the focal spot is, the less the geometric unsharpness produced. The source to image receptor distance is also a critical influential factor with details. As the distance increases between the focal spot and the image receptor, the smaller the geometric unsharpness will be which improves recorded details. The object to image receptor distance or the object to film distance is also important. The larger this distance is the greater the magnification distortion that will be seen. 23

24 The most important geometric factor that influences details is the size of the focal spot. The reason that the technologist has a choice in focal spot sizes is to select the focal spot that is best suited for the part that is being studied. For example, if a chest x ray is to be done, generally a relatively high kilovolts will need to be used and it is best to spread the heat associated with the technique over a larger focal spot area. If the technologist will be doing an anatomical area that requires high resolution or the use of a high detail image receptor, then it will be important to select the small focal spot. With older technology, it is not unusual to have the focal spot undergo some defects during operation. If we have a focal spot whose size is one millimeter by two millimeters, it is expected that the x-rays will all be produced within those dimensions, however if the focal spot is malfunctioning, it can actually exceed those dimensions during the exposure. This phenomena is called focal spot distortion. The focal spot can actually expand well beyond its designed size and it will lead to an image that can have unsharpness or distortion. The blooming effect occurs when the projectile electrons on their journey from cathode to the focal spot can stray well outside the focal spot dimensions and actually make the focal spot bigger. If the concentration of projectile electrons is along both edges of the focal spot, then it is called edge banding. It is impossible to completely eliminate this phenomena in older x-ray tubes, but there are some simple precautions that the technologist can observe to ensure that the best details are produced in the image. Using a low milliampere station such as a one hundred or two hundred milliampere station will help because what happens is that as the milliampere is increased, this tends to cause more of the blooming phenomena. Another simple thing that the technologist can observe is the use of moderate to high kilovolts. It s important to point out that not all studies can be done with a high kilovolts peak. The geometric unsharpness of an image can be calculated using the following equation on the next slide. 24

25 The equation over the years has been known as the geometric unshaprness equation or the penumbra equation. The word peneumbra is in reference to that very small zone of unsharpness that is produced around the anatomy that is being visualized. Because this zone of unsharpness takes away recorded details, the idea is to use technical factors that will minimize this phenomena. The equation is simple and requires you to multiply the width of the focal spot by the object to image receptor distance and then divide thisi produce by the source to object distance. The answer is expressed in millimeters. Example calculations follow in the next slides. 25

26 This is typical set up for problems where you will be asked to select the set of parameters that will result in the smallest geometric unsharpness and therefore the best recorded details. Note that you can also be asked to select the set of parameters that will produce the largest geometric unsharpness and therefore the poorest details. In time, the technologist will be able to select the set of parameters that will produce the best details without having to calculate. This is done by simply recognizing what parameters generally contribute to better details. 26

27 In the initial calculation, you multiply the two and a half inch object to image receptor distance by the two point five millimeter square focal spot. The source to image receptor distance is determined by subtracting the object to image receptor distance from the source to image receptor distance and this will give you the source to object distance of thirty-three point five. The results of this calculation is point one hundred eighty six millimeters. 27

28 With the second set of parameters, you will notice that the object to image receptor distance and the focal spot size is smaller while the source to object distance is significantly greater than before. When this calculation is completed, as you can see the results are significantly smaller than in the previous problem which means that this set of parameters does have the best operating conditions to produce an image with the least geometric unsharpness or the best recorded details. 28