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Questions and Answers
The contrast resolution provided by ______________ is considerably better than that available in radiography.
The contrast resolution provided by ______________ is considerably better than that available in radiography.
CT
The ability to image low-contrast objects with CT is limited by the size and uniformity of the object and by the ______________ of the system.
The ability to image low-contrast objects with CT is limited by the size and uniformity of the object and by the ______________ of the system.
noise
If a ______________ medium such as water is imaged, each pixel should have a value of zero.
If a ______________ medium such as water is imaged, each pixel should have a value of zero.
homogeneous
This variation in CT numbers above or below the average value is the ______________ of the system.
This variation in CT numbers above or below the average value is the ______________ of the system.
The magnitude of noise is equal to the ______________ deviation of the CT number values within the region of interest.
The magnitude of noise is equal to the ______________ deviation of the CT number values within the region of interest.
Noise is the percentage ______________ deviation of a large number of pixels obtained from a water bath image.
Noise is the percentage ______________ deviation of a large number of pixels obtained from a water bath image.
The patient radiation dose, the number of x-rays used by the detector to produce the image, controls the ______________.
The patient radiation dose, the number of x-rays used by the detector to produce the image, controls the ______________.
In statistics, noise is called a ______________ deviation and is symbolized by σ.
In statistics, noise is called a ______________ deviation and is symbolized by σ.
If the CT imaging system is properly adjusted, the value of ______ should be zero.
If the CT imaging system is properly adjusted, the value of ______ should be zero.
The characteristic of having constant pixel values in all regions of the reconstructed image is called ______ uniformity.
The characteristic of having constant pixel values in all regions of the reconstructed image is called ______ uniformity.
A histogram or line graph can be plotted using an internal software package to test ______ uniformity.
A histogram or line graph can be plotted using an internal software package to test ______ uniformity.
If all values of the histogram or line graph are within two standard deviations of the mean value, the system is said to exhibit ______ spatial uniformity.
If all values of the histogram or line graph are within two standard deviations of the mean value, the system is said to exhibit ______ spatial uniformity.
The 'cupping' artifact is caused by ______ beam hardening.
The 'cupping' artifact is caused by ______ beam hardening.
The 'cupping' artifact can be demonstrated by imaging the water bath inside a ______ ring to simulate bone.
The 'cupping' artifact can be demonstrated by imaging the water bath inside a ______ ring to simulate bone.
Noise appears on the image as ______.
Noise appears on the image as ______.
Low-______ images appear very smooth to the eye, and high-______ images appear spotty or blotchy.
Low-______ images appear very smooth to the eye, and high-______ images appear spotty or blotchy.
Noise should be evaluated daily through imaging of a ______-cm-diameter water bath.
Noise should be evaluated daily through imaging of a ______-cm-diameter water bath.
The ROI must encompass at least ______ pixels when the CT technologist measures noise.
The ROI must encompass at least ______ pixels when the CT technologist measures noise.
Computed tomography imaging systems must be ______ frequently so that water is consistently represented by CT number zero.
Computed tomography imaging systems must be ______ frequently so that water is consistently represented by CT number zero.
Uniformity is defined as the ______ of the CT numbers of an image of a homogeneous material across the scan field.
Uniformity is defined as the ______ of the CT numbers of an image of a homogeneous material across the scan field.
Uniformity is calculated using the following equation: |______ - ______|, which is the difference in the mean CT number in the centre from that at the periphery of the image.
Uniformity is calculated using the following equation: |______ - ______|, which is the difference in the mean CT number in the centre from that at the periphery of the image.
When a uniform object such as a water bath is imaged, each pixel should have the same ______ because each pixel represents precisely the same object.
When a uniform object such as a water bath is imaged, each pixel should have the same ______ because each pixel represents precisely the same object.
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Study Notes
Image Quality
- CT images have better contrast resolution than radiography due to scatter radiation rejection by pre-patient and pre-detector collimators.
- CT can image low-contrast objects, but is limited by object size and uniformity, and system noise.
Noise
- Noise is the variation in CT numbers from the mean value in a defined area of a uniform phantom.
- Noise is measured by the standard deviation of CT number values within the region of interest.
- Factors affecting noise include kVp, filtration, pixel size, slice thickness, detector efficiency, and patient dose.
- Noise appears as graininess on the image, with low-noise images appearing smooth and high-noise images appearing spotty or blotchy.
- Noise is symbolized by σ (standard deviation) and should be evaluated daily through imaging of a 20-cm-diameter water bath.
Spatial Uniformity
- Spatial uniformity is the consistency of CT numbers in an image of a homogeneous material across the scan field.
- Spatial uniformity can be tested with an internal software package that plots CT numbers along any axis of the image as a histogram or line graph.
- A system exhibits acceptable spatial uniformity if all values are within two standard deviations of the mean value (±2 σ).
X-ray Beam Hardening and Cupping Artifact
- X-ray beam hardening can cause a decrease in CT numbers, making the middle of the image appear darker than the periphery (cupping artifact).
- This artifact can be demonstrated by imaging a water bath inside a Te on ring to simulate bone.
Calibration and Linearity
- CT imaging systems must be calibrated frequently to ensure water is represented by CT number zero and other tissues by appropriate CT numbers.
- A daily calibration check can be made using the five-pin performance test object of the American Association of Physicists in Medicine (AAPM).
- The plot of CT number versus linear attenuation coefficient should be a straight line passing through CT number 0 for water.
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