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Questions and Answers
What is the purpose of windowing in CT imaging?
What is the purpose of windowing in CT imaging?
To enable the shades of grey to represent a limited range of HU values, allowing different structures to be imaged.
What is the effect of decreasing the window level in a CT image?
What is the effect of decreasing the window level in a CT image?
The CT image will be brighter.
What is the purpose of adjusting the window width in a CT image?
What is the purpose of adjusting the window width in a CT image?
To control the contrast in the image.
What type of tissue would be classified as hypodense?
What type of tissue would be classified as hypodense?
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What is the significance of spatial resolution in CT imaging?
What is the significance of spatial resolution in CT imaging?
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What is the effect of increasing the window width on the image?
What is the effect of increasing the window width on the image?
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What is the effect of increasing the field of view on pixel size and spatial resolution?
What is the effect of increasing the field of view on pixel size and spatial resolution?
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How does the size of the focal spot affect the spatial resolution of a CT scanner?
How does the size of the focal spot affect the spatial resolution of a CT scanner?
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What is the effect of increasing the pitch on the spatial resolution of a CT scanner?
What is the effect of increasing the pitch on the spatial resolution of a CT scanner?
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What is the primary limitation of CT image quality?
What is the primary limitation of CT image quality?
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What is the effect of decreasing the pixel size on the spatial resolution of a CT scanner?
What is the effect of decreasing the pixel size on the spatial resolution of a CT scanner?
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What is the effect of increasing the slice thickness on the spatial resolution of a CT scanner?
What is the effect of increasing the slice thickness on the spatial resolution of a CT scanner?
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What is the primary type of noise that results from too few photons reaching the detector?
What is the primary type of noise that results from too few photons reaching the detector?
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What is the purpose of calibrating a CT scanner so that water is consistently represented as zero HU?
What is the purpose of calibrating a CT scanner so that water is consistently represented as zero HU?
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Study Notes
Windowing
- Windowing is an electronic manipulation of data to represent a limited range of HU values, enabling different structures to be imaged.
- Window level represents the central HU of all numbers within the window width, controlling density.
- When the window level is decreased, the CT image will be brighter, and vice versa.
- Increasing the window level helps view lungs and other airways, while decreasing it helps view denser bones.
Window Width
- Window width converts the HU of all tissues of interest and displays them as various shades of gray, controlling contrast.
- When the window width is decreased, the CT image will be brighter, and vice versa.
- Increasing the window width increases contrast in the image, making it good for looking at differences in soft tissue.
Tissue Densities
- Hyperdense: areas with the highest density (bone/white)
- Isodense: areas between hyperdense and hypodense
- Hypodense: areas with the least density (air/black)
Image Quality Characteristics
Spatial Resolution
- Refers to the degree of blurring, ability to distinguish between objects or structures that differ in density.
- Factors affecting spatial resolution:
- Field of view: increases in FOV decrease spatial resolution
- Pixel size: smaller pixel size increases spatial resolution
- Focal spot size: larger focal spot decreases resolution
- Magnification: increasing magnification decreases resolution
- Motion of the patient: affects spatial resolution
- Pitch: higher pitch decreases resolution
- Kernels: edge enhancement kernels have higher resolution than soft tissue
- Slice thickness: larger slice thickness decreases resolution
- Detector size: increasing detector size decreases resolution
Contrast Resolution
- Ability to distinguish materials of one composition from another without regard to size and shape.
- Refers to the ability of the scanner to demonstrate small differences in tissue contrast.
- One of the major advantages in CT scanning.
Noise
- Defined as the percent standard deviation of a large number of final images as graininess.
- Refers to the fluctuation of CT number between points in the image for a scan of uniform matter such as water.
- Characterized by a grainy appearance of the image.
- Factors influencing system noise:
- KVP and filtration
- Pixel size
- Slice thickness
- Detector deficiency
- Patient dose
Major Types of Noise
Quantum Mottle or Quantum Noise
- Result of too few photons reaching a detector after being attenuated by the body.
- Factors that result in quantum noise:
- Anatomical structure size
- Reduction of slice thickness without increasing technical factors
- Decreasing pixel size
- Scatter radiation
Electronic Noise
- Noise contained within the image caused by vibrations of any physical components, especially rotational components or power fluctuations.
Computational Noise
- Primarily caused by statistical fluctuations that occur from reconstruction mathematics essential to produce CT images.
- Factors influencing computational noise:
- Voxel size, slice thickness, matrix, FOV
- Filter algorithm, mAs
Linearity
- The CT scanner must be calibrated so that water is consistently represented as zero HU, and other tissues are represented by their appropriate CT value.
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Description
Understanding windowing and leveling in CT scans, controlling density and HU values to visualize different structures, such as lungs and bones. Learn how to adjust window levels and widths for optimal imaging.
