Now that we've learned the x-ray system, its components, technique factors, beam modifiers, and other dose reducing tools, we can begin to address what each has on the impact of patient dose, image detail, density, and contrast. Most of this information is review, but there will be some new information as well.
It has been stated on more than one occasion in this course that there is a constant struggle between patient dose and image quality. This will again be demonstrated on this page. Pay attention to this relationship. If you are able to determine the impact each parameter has on patient dose, you can conclude what will happen to the image quality (and vice versa).
If we INCREASE the following without changing anything else, what happens to the above listed items?
Technique factors
- mAs - Controls the quantity of x-rays produced.
- Patient dose - Increasing mAs increases dose due to the increased quantity of x-rays produced.
- Image detail - No appreciable change in detail is noted in increasing mAs.
- Density - As mAs increases, image density increases due to the increased quantity of x-rays produced.
- Contrast - No change in contrast
- kVp - Controls the quality of the x-ray beam.
- Patient dose - Increasing kVp without changing anything else results in increased patient dose. As you increase kVp, you increase scatter.
- when you apply increased kVp with a decrease in mAs, you lower patient dose.
- Image detail - Decreased. Increasing kVp results in lower contrast (more gray). A loss of contrast results in a loss of detail in the image.
- Density - Increased. Increasing kVp allows for greater penetration of the x-ray beam. More of the x-rays make their way through the patient and onto the image receptor, which causes images to be darker.
- Contrast - Decreased. Increasing kVp makes the image appear more gray (low contrast). A high kVp and lower mAs setting will result in lower patient dose, but there is always an optimal kVp range for each exam type. You must remain in that range in order to maintain image quality.
X-ray machine and components
- filtration - material within the x-ray system used to absorb lower energy x-rays. There are 2 types of filtration, inherent and added. Inherent filtration is incorporated in the system (x-ray tube is made of leaded glass, there is insulation oil surrounding the x-ray tube that serves to absorb heat and stray x-rays). Added filtration is material, usually aluminum, put into the path of the beam to absorb lower energy x-rays.
- Patient dose - Decreases. The main purpose of filtration to decrease patient dose. When you increase filtration, you increase the absorption of the x-rays leaving the machine. A reduced number of x-rays will enter the patient as a result.
- Image detail - Decreases. The purpose of filtration is to absorb lower energy x-rays that would normally be absorbed by the patient. Most of these x-rays do no contribute to the creation of the image. Filtration will absorb some x-rays within the diagnostic range. Essentially, increasing filtration will increase the quality of the x-ray beam (average energy of that beam).
- Density - Decreases. Increased filtration results in a reduced number of x-rays entering the patient.
- If you increase filtration too much (to the point where it is absorbing x-rays in the diagnostic range), then density can certainly be decreased.
- Contrast - Decreases. Increasing filtration raises the quality of the x-ray beam. Raising quality of the beam results in lower contrast (gray).
- focal spot size - Within the x-ray tube, there are usually 2 filaments (one large, one small). These filaments are responsible for dictating the focal spot size, which is the size of the electron beam striking the target on the anode.
- Patient dose - No change
- Image detail - Decreases with increased focal spot size. This is why exams requiring greater detail, like hand x-rays, use small focal spot sizes.
- Density - No change
- Contrast - No change
- collimation - defines the size and shape of the x-ray beam through use of leaded shutters.
- Patient dose - Increases with the increase in x-ray field size. This is why it's so important to remember to collimate down to the area of interest.
- Image detail - Decreases. If you increase the x-ray field size, you increase the area of the patient being exposed. This results in increased scatter. Some of this scattered radiation will make its way to the image receptor, which will degrade image quality.
- Density - Increases. Increasing x-ray field size will result in more x-rays making their way to the image receptor, causing a darker image.
- Contrast - Decreases. Increasing x-ray field size increases scatter. Scatter, when it strikes the image receptor, will make the image appear more gray.
- grids - Grids are placed between the patient and the image receptor to absorb scatter radiation before it reaches the image receptor. They use very thin lead strips to accomplish this. The grid ratio is the concentration of these lead strips within the grid. The higher the ratio, the greater the absorbing capabilities.
- Patient dose - Increases. This is the downfall of using grids. In order to maintain radiographic density, mAs must be increased. As the grid ratio goes up, the mAs needs to go up as well to compensate.
- Image detail - Increases. By reducing the amount of scatter on the image receptor, detail is increased.
- Density - Decreases since the very nature of a grid it to absorb x-rays. An increase in mAs is required to compensate.
- Contrast - Increases. By absorbing the scattered x-rays, you eliminate the "greyness" that the scatter would produce on the image. A more black and white image = higher contrast.
- system speed - refers to a systems ability to convert x-rays into an image. The greater the efficiency, the higher the speed.
- Patient dose - The sole purpose of using an increased system speed is to reduce patient dose. The higher the system speed, the lower the dose can be.
- Image detail - When you increase system speed, you will decrease detail. For example, in film screen imaging, a slower speed cassette is used for exams that require greater detail, such as extremity imaging.
- Density - Increases. If you increase the system speed, you increase its ability to efficiency in converting x-rays to an image. The greater the efficiency, the darker the image will be.
- Contrast - No change.
Geometric Considerations
- SID - Source to Image receptor Distance.
- Patient dose - Decreases. As you increase SID, you increase the distance from the source to the patient. The Inverse Square Law states that every time you double the distance, you decrease the exposure by a factor of 4. That's why we use the Density Maintenance formula...to correct for this phenomena when a different distance must be used that what is normally required (this is a rare occurrence)
- Image detail - Increases. Increasing the SID will cause the edges of the objects being radiographed to appear sharper. This is due to a reduction in penumbra, which is a word to describe the "fuzziness" of the outline of an object.
- Density - Decreased due to the increased distance of the source. The further the x-ray tube is from the image receptor, the lighter the image (unless the density maintenance formula is used).
- Contrast - No affect.
- SOD - Source to Object Distance - parameters will always be the same as the SID
- OID - Object to Image Receptor Distance
- Patient dose - Increases. As you increase the OID, you are moving the body part further away from the image receptor, but you are also moving it closer to the x-ray tube. This will result in an increase in dose.
- Image detail - Decreases. As you move the body apart further away from the image receptor, you reintroduce penumbra (fuzziness of the outline of a structure).
- Density - Decreases. If you increase the OID, you are increasing the distance from the body part to the image receptor and bringing the body part closer to the x-ray tube. So why does this result in a decreased density? Remember that x-rays can do one of 4 things. They can pass completely through a body part, they can be partially absorbed and lose energy as it passes through the body part on to the image receptor, they can be completely absorbed, and they can be scattered. By increasing the distance between the body part and the image receptor, you actually give the scatter the opportunity to miss the image receptor all together. This will cause a decrease in the density of a radiograph.
- Contrast - Increases. For the same reason as listed above. Scatter causes an image to appear more gray. If you reduce the scatter, you will increase the contrast of your image.
Bottom line for geometric considerations: In order to reduce patient dose and increase image quality, you must use the biggest SID that's reasonable (usually that's 40" or 72" depending on the exam), and you must have the body part as close to the image receptor as possible.