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Questions and Answers
What is the purpose of assigning a Hounsfield Unit value to each pixel?
What is the purpose of assigning a Hounsfield Unit value to each pixel?
What determines the number of Hounsfield units assigned to each level of gray in a CT image?
What determines the number of Hounsfield units assigned to each level of gray in a CT image?
What is the term for the path of the X-ray beam from the tube to the detector?
What is the term for the path of the X-ray beam from the tube to the detector?
What is the result of compiling the information from all the attenuation profiles?
What is the result of compiling the information from all the attenuation profiles?
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What is the purpose of the reconstruction algorithm in CT scanning?
What is the purpose of the reconstruction algorithm in CT scanning?
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What is the effect of decreasing the display field of view on the displayed image?
What is the effect of decreasing the display field of view on the displayed image?
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What is the term for an area on the image defined by the operator?
What is the term for an area on the image defined by the operator?
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What is the factor controlled by the CT operator that determines the quantity of X-ray beam used?
What is the factor controlled by the CT operator that determines the quantity of X-ray beam used?
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What is the process of correlating the attenuation properties of each ray sum to its position?
What is the process of correlating the attenuation properties of each ray sum to its position?
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What is the term for a complete set of ray sums?
What is the term for a complete set of ray sums?
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Study Notes
Image Data
- Image data is the result of the computer processing raw data, with each pixel assigned a Hounsfield Unit (HU) value.
- Hounsfield Unit is also known as density number or CT number, and is the average of all attenuation measurements of a pixel.
Grayscale Display
- CT images are displayed in grayscale, with higher HU values corresponding to lighter shades of gray and lower values corresponding to darker shades.
- Window width determines the number of HU values represented as shades of gray on a specific image.
Image Reconstruction
- Image reconstruction involves the following steps:
- Ray: the path of the X-ray beam from the tube to the detector.
- Ray Sum: measurement of beam attenuation.
- View: a complete set of ray sums.
- Attenuation Profile: the system accounts for the attenuation properties of each ray sum and correlates to the position of the ray.
- Back Projection: compiles the information from all attenuation profiles to create an image.
Scanning Parameters
- Scanning parameters controlled by the CT operator include:
- Milliamperage (mA) level
- Scan time
- Slice thickness
- Field of view
- Reconstruction algorithm
- Kilovoltpeak (kVp)
- Pitch (for helical scan)
Region of Interest (ROI)
- ROI is an area on the image defined by the operator, often circular, but can be elliptical, square, rectangular, or custom drawn.
Image Magnification
- Image magnification is a decrease in the display field of view, increasing the size of the displayed image.
- It uses only image data and does not improve the resolution, but makes the existing image larger.
CT Image Quality
- CT image quality is determined by:
- Spatial resolution
- Low-contrast resolution
- Temporal resolution
Multiple Image Display
- Multiple image display is a CT function allowing more than one image to be displayed in a single frame.
- It is often used to save film, particularly when copies are requested by the referring physician.
- The format may vary with the manufacturer.
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Description
Learn about image data in Computed Tomography (CT) scans, including Hounsfield Units, grayscale display, and window width. Understand how HU values affect image appearance and how window width determines the range of HU values.
