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35 Cards in this Set

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What are the 5 characteristics that determine radiographic quality?
1. Spatial Resolution
2. Contrast Resolution
3. Noise
4. Distortion
5. Artifacts
Several
What is Radiographic Quality infulenced by?
Several factors related to processing, geometry, motion, image receptors, subject contrast, focal spot size, view conditions, observer performance.
Wat are some factors that infulence Image Quality?
Xray beam characteristics, dose, transmissivity of subject, slice thickness, scatter, efficency of analog /digital converter, pixel size, recontruction algorithm and display resolution.
How can one improve image quality?
One can change the dose and pixel size since tranmissivity CANNOT be changed.
What are some methods used to measure image quality?
- Point Spread Function (PSF)
- Line Spread Function (LSF)
- Contrast Transmission Function (CTF)
- Modulation Transmission Function (MTF)
- Edge Response Function (ERF)
- Fourier Transformation
What is Modulation Transmission Function (MTF)?
Modulation Transmission Function (MTF)is the most commonly used description of spatial resolution in CT and Convention Radiography. It can be derived from the LSF, PSF, ERF.
It measures the resolution capabilities of an image system by breaking down an object into its frequency componants MTF of 1 = object reproduce exactly.
What is the Point Spread Function (PSF)
The Point Spread Function (PSF)describes the lack of sharpness or blurring effect
What is the Full Width at Half Maximum (FWHM)?
The is Full Width at Half Maximum (FWHM) is the measure of the spatial resolutions distance between points on the curve at which the the point spread function reaches half its maximum value.
What is Line Spread Function (LSF)?
Line Spread Function (LSF)describes the unsharpness of an imaging system where a line is misrepresented.
What is Contrast Transmission Function (CTF)
Contrast Transmission Function (CTF)measures the contrast response of a image system.
What is Edge Response Function (ERF)?
Edge Response Function (ERF)describes the response of a image system to adjacent regions of high and low densities.
How is noise measured?
Noise of an image can be measured by the Noise Power Spectrum or Weiner Spectrum, obtained from the Fourier Tansformation Technique by breaking down the image noise into frequency components to get the contrast resolution.
What is spatial resolution?
The degree of an object blurring, the measure of the ability to descriminate objects of varying densities against a uniform background.
Represented by the PSF, LSF, MTF.
What are the Geometric Factors?
Geometric Factors play a role in the data acquisition process.
focal spot size, detector aperature width, slice thickness, distance between the focus, isocenter, detector and sampling distancee.
What is effective Focal Spot Size (FSS)? And what happens when FSS increases?
The effective focal spot size is the size of the projected focal spot.
When the effective FSS is increased the details are spread over several detectors, decreasing spatial resolution.
What is aperature size?
Aperature size refers to the width of the aperature at the detector. Smaller aperature size = higher spatial resolution.
What is the Partial Volume Effect?
Partial volume effects occur when, at any point in the slice, the object has axial variations in attenuation. This nonlinearity causes inconsistencies in the data set which in turn can cause streaks in the image.
For Ex. if th object is 4mm, and a 10mm slice thickness is used it creates a incorrect CT number and atifact.
What are the two mathematical procedures involved in image reconstruction?
Convolution and Back Projection
What is Display Resolution?
Display Resolution is the number of pixels per horizontal and vertical dimension of the matrix.
What is HRCT?
High Resolution CT (HRCT) was introduced in the 1980s, to evaluate lung disease. It "optimizes the spatial resolution of conventional scanners"
How is HRCT obtained?
Through narrow beam collimation and the use of thin slices 1-2mm. Thin slices reduce artifact caused by the partial volume averaging effect.
It applies different reconstruction algorithms and requires the reduction of the pixel size to further increase spatial resolution.
What does Targeting refer to?
Targeting is when the FOV and pixel size is reduced and reconstructed, increasing the spatial resolution.
What is Low-Contrast Resolution? What is it affected by?
The ability of the imaging system to demonstrate small changes in tissue contrast.
-Low-Contrast Resolution is affected by: photon flux, slice thickness, paitent size, sensitivity of the detector, reconstruction algorithm, image display, recording and noise.
What is Contrast Resolution?
The ability of the CT unit to image objects 2-3mm in size that vary slightly in density.
What is Photon Flux?
Photo Flux is determined by the kVp, mAs, patient size and beam filtration, which affect both the quality and quantity of the photons.
What is the Contrast-Detail Diagram (CDD)?
The CDD is a graph from which the information can be obtained for both the low-contrast and high contrast resoltion of a CT scanner.
What is Noise in regards to CT?
Noise in the fluctuation of CT numbers with in a same density area.
The noise levels for CT scaners can vary and depend on several factors- mAs, scan time, kVp, slice thickness, object size and the algorithm.
What is Linearity?
Linearity refers to the relationship of CT numbers to the linear attenuation coefficients of the object imaged.
What is Cross-Field Uniformity?
The uniformity of CT numbers throughout the entire scan field. It can be verified with a phantom, 5 ROIs equal to 5% of the total phantom area.
What are the 4 main types of artifacts?
1. Streaks
2. Shadings
3. Rings
4. Bands
What are beam hardening artifacts?
Beam hardening refers to the increase in the mean energy of the xray beam as it passes through the patient. (blurring, noise around perifery of brain)
What are partial volume artifacts?
Partial volume artifacts occur when the very different CT numbers are averaged together (ex. bone and air)
They show up as bands or streaks.
What is a Ring Artifact, what is it caused by and what scanners are it seen on more often?
Ring artifacts are a circular rings that show up in the middle of the image.
They are caused by detectors that are offset from the neighboring detectors, indicating that the machine needs to be calibrated.
The Ring artifacts are seen more often on 3rd generation scanners.
What are Aliasing Artifacts? What can be used to reduce the Aliasing Artifacts?
Streaks, that are seen when structures and spaces cannot be distinguished apart.
Many different methods can be used to reduce streaking: increased rays / number of views, a convolusion filter can be used to smooth the image.
What are effect does Spiral/Helical CT have in regards to artifacts?
-Motion artifacts are minimized
-Corrected table motion artifacts
-Partial volume artifacts are exaggerated because of slice sensitivity
-Strips in 3D MPRs due to uneven slices
-New "miscellaneous artifacts" like basket weave & Moire pattern.