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25 Cards in this Set

  • Front
  • Back
What are some limitations of CT?
Superimposition, inability to differentiate between slight differences in subject contrast.
What is the goal of CT?
1. Produce high quality images with minimal radiation dose and physical discomfort to patient.
2. Minimal superimposition
3. Improved image contrast
4. Recording of very small differences of tissue contrast
2 principles of CT
1. Physical principles - involve physics and mathematics concepts to understand the way the image is produced
2- technological considerations involve practical implementation of scientific and engineering principles
What are stored as raw data?
The relative transmission values, which are gathered from the radiation transmitted through the patient to the detectors.
What is attenuation?
Attenuation is the reduction of the intensity of a beam of radiation as it passes through an object.
What is a homogeneous beam? And what are other names for them?
In a homogeneous beam, all the photons have the same energy. The quality of the beam does not change. If the starting beam has 125 keV, all the transmitted photons will have an energy of 125keV.
Momochromatic or monoenergenic beam.
What is a heterogeneous beam? What is another name of it?
Heterogeneous beams consist of a range of energies. During attenuation of a hetergeneous beam, as the beam passes through equal thicknesses of material the quality and quanitiy of photons change.
Polychromatic beam.
What are the Data Aquisition Geometries of CT?
The Data Aquisition Geometries are the way that the xray tube and detectors are arranged to collect transmission or penetration measurements.
What are the 3 steps in Date Processing?
1. Raw data undergoes preprocessing
2. Image reconstruction - attentuation readings are converted into a digital image / CT number
3. Image storage - memory / disc
What are CT numbers? and how are they determined?
The computer calculates the CT number. CT numbers are related to the linear attenuation coefficients of the tissues that comprise the slice. Each pixel in the image is assigned a CT number.
What is Windowing?
Windowing is the relationship between the CT numbers and the variable shades of gray.
Why is high-kV techniques used in CT?
1. Reduce the dependancy of attenuation coefficients on photon energy
2. reduce the contrast of bone realtive to soft tissues
3. to produce a high radiation flux at the detector
What is FOV?
Field of view, the reconstruction circle. A circular region from which the transmission measurements are recorded.
What is the formula to determine pixel size?
Pixel size, d = FOV / Matrix size.
Ex. FOV of 25 , matrix 512
25 X 10 / 512
= 250mm / 512
What does the voxel size depend on?
Thickness of the slice, matrix size and FOV
the brightness of the image at the pixel position
What is a bit depth?
Each pixel can have a range of greys, represented by bits, a CT image can be characterized by the number of bits per pixel (8,9,10,11,12), therefore consisting of a series of bit planes = bit depth
What does the transmission beam do?
records the xrays that are transmitted through the patient.
What does the reference beam do?
Measures the intensity of radiation from the xray tube
What happens in the preprocessor?
the digital data is corrected and reformated, including; subtraction, obtaining averages of detectors = reformated raw data; to be further corrected by computer software
What is convolusion and what is preformed on and by what?
Convolusion is a digital image processing technique to modify images through a filter function performed on the data by the array prcessors
What is back projection?
"summation method" "linear superposition method" where the (back) projections from multiple beams are summed and averaged together to create an image
Why is excellent low-contrast resolution possible?
Highly collimated beam, special detectors are used to measure the radiation transmitted through the slice
What are some advantages of spiral data acquisition?
Volume data acquisition, single breath holds, improved 3Ds, multiplanar imaging, continuous imaging, Ct angiograhy.
Different CT Techniques
Xenon CT- uses inhaled, stable xenon gas to study blood flow
Quantitative CT- Study bone mineral content (Bone Density)(QCT)
Dynamic and Perfusion CT
CT Limitations
decreased spatial resolution, higher dose, limited to trasverse and axial images, artifacts due to metal