Term
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Definition
1. overcome superimposition of structures.
2. Improve contrast fo the image.
3. Record very small differences in tissue contras. |
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Term
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Definition
1. Poorer spatial resolution
2. Generally higher radiation
3. Usually axial images only
4. Artifacts from metallic objects and bone surrounding soft tissue. |
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Term
| 3 major parts of CT System |
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Definition
1. gantry assembly and room components
2. Computer
3. Console
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Term
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Definition
1. Stationary Anode(early scanners)
oil cooled, large focal spot
2. Rotating Anode-
Can accomodate continuous or pulsed beam. Air cooled. 3600-10,000 RPM's.
small focal spot(increased resolution) reduced noise. made of rhenium, tungsten, molybdenum.
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Term
| 4 Qualities of a Detector: |
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Definition
1. Efficiency-how well detector is able to detect photons.
2. Capture Efficiency-how well detectors obtain photons.
3. Conversion Efficiency-% of photons incident on the detector that result into a detected signal.
4. Stability-detector steadiness, in not stable will require frequent calibrations. |
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Term
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Definition
1. Solid State(Scintillation)-Crystals coupled to a photo multiplier tube that converts light to electrons. Excellent efficiency because it can detect photons from all angles. Made of Calcium fluoride and Bismuth germanate now.
2. Gas Detectors-ionization chambers that use xenon gas. Generate electrical impulse in response to a collision between a photon and a xenon atom.
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Term
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Definition
1. Pre-Patient-design influenced by focal spot size. mounted on tube housing. creates more parallel beam. reduces patient dose.
2. Pre-Detector-restricts field viewed by detector(shapes beam). reduces scatter to detector. aperture width helps determine slice thickness. |
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Term
| 2 Purposes for Filtration |
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Definition
1. Absorb low-energy x-rays(soft radiation) that do not contribute to the acquisition of CT image. Pt dose reduced and beam quality increases.
2. Shape the energy distribution across the radiation beam to produce uniform beam hardening. |
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Term
| Methods of Image Reconstruction |
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Definition
1. Back Projection-simplest and oldest.
2. Iterative Method
3. Analytic Reconstruction Alogorithms
a. Filtered Back Projection
b. Fourier Reconstruction |
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Term
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Definition
known as the Summation or linear method.does not produce a sharp image so not used in clinical CT.
produces a "classical star pattern" artifact |
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Term
| Iterative Method Reconstruction |
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Definition
| Assumes all points in a matrix have the same value. Patient motion and quantum noise make it difficult to obtain accurately measured ray sums. Not used in commercial CT because procedure takes too long to generate an image and needs many projections. |
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Term
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Definition
An Analytic Recon Method.
Uses filters to remove star pattern artifact |
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Term
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Definition
| Analytic Recon Alogorithms- Uses the Fourier Transfromation to convert a signal in the spacial domain into the frequency domain. while in frequency domain the image can be manipulated by changing the amplitudes. transformed back into the spatial domain by inverse fourier transormation. |
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Term
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Definition
1. receives signal in analog form to be converted to a binary digit.
2. done by an analog to digital convertor.
3. stores digital signal during scan and reconstructs the image after scan.
4. data can be manipulated to reconstruct in various plans.
5. each picture is displayed on a single matrix. |
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Term
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Definition
| Permits a problem area to be outlined then deleted. |
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Term
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Definition
| Bar graphs to show Hounsfield numbers between 2 points. requires special software. |
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Term
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Definition
| # of detectors to be used. determined by pt size and part to be scanned. smaller gives better resolution and is faster. |
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Term
| Display Field of View(DCOV) |
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Definition
| true magnification is obtained by having it smaller. Less tissue per voxel. Higher resolution. larger image of a smaller area. |
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Term
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Definition
Brightness of image. The mid-point or Center of the range of CT #'s displayed. Used to change overall density(brightness).
set to correspond to the part involved. 35-70 for abdomen. -700 for lung. higher-darker image
lower-lighter image |
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Term
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Definition
| Gray Scale. Top is white. all numbers above this will be white. Bottom is Black. all numbers below this will be black. Small gives more contrast(less gray) large gives less contrast(more gray) |
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Term
| Formula For Range of CT Numbers Displayed |
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Definition
R= L (+ and -) W/2
R= Range
L= Level
W = Window |
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Term
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Definition
| Pixel Size= FOV/Matrix Size |
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Term
| 5 Factors that Affect Image Quality |
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Definition
1. Spatial Resolution
2. Contrast Resolution
3. Noise and Spacial Uniformity
4. Linearity
5. Image Artifacts |
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Term
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Definition
| Affects image quality. Used to exprss degree of blurring present. Ability to see a small dense object in a field of a different density. Influenced by collimation, detector size, pixel size, and focal spot size. |
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Term
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Definition
| Ability to demonstrate small changes in tissue contrast of large objects. Better in CT than conventional x-ray. able to show as small a difference as 4% in CT only 10% in x-ray. limited by noise |
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Term
| Noise and Spatial Uniformity |
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Definition
| Variations in CT #'s about an aberage value in uniform tissue. A mottled appearance or textrued look. Caused by lack of photons passing through tissue. |
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Term
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Definition
| Results from limited photons contributing to image |
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Term
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Definition
| Occurs due to electrical interference w/i the many electronic components of the System |
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Term
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Definition
| approximations used in measurments in the averaging process |
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Term
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Definition
| Caused by scattered radiation |
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Term
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Definition
1. Motion
2. Metal
3. Beam Hardening
4. Partial Volume Effect
5. Ring Artifacts |
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Term
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Definition
| Caused by the increase of the mean energy as it passes through an object. Appears as broad dark bands or streaks. Known as cupping artifacts. Affected by variation of kVp, tissue density, tissue thickness. |
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Term
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Definition
| affects detail because of the effect on volume averaging. Changing from 10mm to 5 mm improves resolution. |
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Term
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Definition
Disadvantages-requires twice as many scans, twice the reon time, twice the filming time, more radiation to produce same quality image.
Advantages- provides superior detail for complex anatomy, decreases the volume averaging effect. |
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