Computed Tomography Flashcards
Attenuation, Linear coefficients, windowing
What is Attenuation
It is the radiodensity of a material measured by how much stopping power it has on radiation by absorbing or reflecting?
Why is attenuation important in CT
The components in our body have different radiodensities so their attenuation values are different. The detector receives these values and the computer transmits these numbers into CT numbers to produce a greyscale image.
What is Attenuation measured in?
Hounsfield units
What is the Hounsfield scale ?
It is the method of measuring attenuation values and expressing them as ct numbers
What are linear coefficients and how are they used in CT
Linear coefficients are used to describe how much of the x-ray beam is attenuated by a material. These values are converted to Hounsfield units and used to produce a greyscale image
What equation is used to convert attenuation to Hounsfield units ?
Hounsfield units are based on x-ray attenuation values compared to water
tissue m - water m / water m x 1000
What is windowing in CT image display?
Windowing is the process of changing CT numbers to manipulate the CT image greyscale by adjusting the window level and window width.
What is window width?
Window width is the range of ct numbers displayed as shades of grey and these control the image contrast highlighting structures
What is window level?
Window level is what determines the CT number in the center of the window width controlling the image density which is the blackness on the image.
Why is windowing important ?
Windowing gives us the ability to manipulate the grey scale so we can see more details. The higher the window level and width the blacker the greyscale allowing us to see more dense structures such as bone. The lower the window level and width the whiter the greyscale allowing us to see less dense structures such as soft tissue.
