Chapter 11 Flashcards
Contrast
Arises from the areas of light, dark, and shades of gray on the x-ray image.
Contrast Resolution
is the ability to image adjacent similar tissue
X-Radiation Produced by Compton Scatter
produces noise, reducing image contrast, and contrast resolution, it makes the image less visible.
Three factors contribute to increased scatter radiation:
increased kVP, increased x-ray field, and increased patient thickness
Beam restricting devices
are designed to control and minimize scatter radiation by limiting the x-ray field size to only the anatomy of interest.
The three principle types of beam restricting devices are-
aperture diaphragm, cones or cylinders, and collimators
Scattered x-rays from remnant beam
the grid removes a major source of noise, thus improving radiographic image contrast
Two principal characteristics of any image
spatial resolution and contrast resolution
Spatial resolution and contrast resolution
together are referred to as:
image detail or visibility of detail
Spatial resolution
is determined by focal spot size and other factors that contribute to blue.
Contrast resolution
is determined by scatter radiation and other sources of image noise
Two principal tools are used to control scatter radiation:
beam restricting devices and grids
Two types of x-rays are responsible for the optical density and contrast on a radiographic image:
that that pass through the patient without interacting and those that are Comptom scattered within the patient
x-rays that exit from the patient are
remnant x-rays
x-rays that exit and interact with the image receptor are called
image-forming x-rays
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Collimation reduces patient radiation dose and improves contrast resolution
Increase of scatter radiation
radiographic image loses contrast and appears gray and dull.
three primary factors influence the relative intensity of scatter radiation that reaches the image receptor
kVp, field size, and patient thickness
X-ray energy is increased
the absolute number of Compton interactions decreases, but the number of photoelectric interactions decreases much more rapidly. Therefore, the relative number of x-rays that undergo Compton scattering increases.
low kVP
fewer x-rays reach the image receptor at low kVP, this is usually compensated for by increasing the mas. This result is higher patient dose.
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approximately 1% of x-rays incident on the patient reach the image receptor
large patients
kVp must be high to penetrate the appropriate body type
increasing mAs usually generates enough x-rays to provide a satisfactory image but may result in high radiation dose.
on the other hand, a much smaller increase in kVP is usually sufficient to provide enough x-rays, and this can be done at a much lower patient radiation dose but when kVP is increased, the level of scatter radiation also increases, leading to reduced image contrast.
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scatter radiation increases as the x-ray beam field size increases
patient thickness
imaging thick parts of the body results in more scatter radiation than imaging thin parts
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compression of anatomy improves spatial resolution and contrast resolution and lowers the patient radiation dose.
Control of scatter radiation
Effect of Scatter Radiation on Image Contrast
One of the most important characteristics of image quality is contrast, the visible difference between the light and dark areas of an image.
Optical density
contrast is the degree of difference in OD between areas of a radiographic image.
Contrast resolution
is the ability to image and distinguish soft tissues
remnant x-rays
are scattered even after the most favorable conditions
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reduced image contrast results from scattered x-rays
two types of devices reduce the amount of scatter radiation the reaches the image receptor
beam restrictors and grids
Beam restrictors
aperture diaphragm, cones or cylinders, and the variable aperture collimator
Aperture diaphragm
simplest of all beam restricting devices, it is basically lead or lead lined metal diaphragm that is attached to the x-ray tube head.
cones and cylinders
considered modifications of the aperture diaphragm.
in both, an extended metal structure restricts the useful beam to the required size.
Variable aperture collimator
The light localizing variable aperture collimator is the most commonly used beam restricting device.
