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18 Cards in this Set
- Front
- Back
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Who was an early pioneer in the development of volume CT scanning?
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Dr. Willi A Kalender
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Why was the term Spiral Ct choose over Helical?
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The definition of "Spiral" - cylindric and conic configurations. whereas helical- only refers to cylindric configurations
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What is required for Volume Data Acquisition?
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-Contiuous rotating scanner with slip-ring technology
-Contiuous couch movement -Increased loadability of xray tube -Increased cooling capacity -Spiral/Helical weighting algoithm -Mass memory buffer |
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What are some problems that occur from spiral geometry data acquisition?
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-No defined slice, thus localization of single slice is difficult
-Nonplanar geometry, where the patient and tube are constantly moving, data is collected from different regions of volume -Effective slice thickness increases -Projection data is inconsistant due to abscence of defined slice -Streak artifacts from inconsistant projection data |
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How can the problems that occur from spiral geometry data acquisition be improved?
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-Dedicated reconstruction algorithms
-Interpolarization |
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What are two ways to eliminate motion artifacts in spiral scanning?
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1.Calculation (interpolarization)
2.Reconstruction of images using filtered back projection algorithm |
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What is 360-degree Linear Interpolation Algorithm?
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360-degree LI Algorithm was used during the initial developement of sprial/helical CT. The data slice is interpolarated using data measurments 360-degrees apart. Image quality is poor, due to the broaden Slice sensitivity profile but there is less noise that the 180.
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What is the 180-degree LI algorithm?
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The 180-degree LI algorithm has better image quailty than 360, because the points measured are closer together. There is also a 2nd spiral/slice is calculated from the data set, offset by 180 degress.
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What are the major differences in spiral/helical CT from Conventional CT?
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1- Slip rings
2- Rotating system 3- Increased storage for high volume scanning 4- Fast solid state, magnetic disk 5- Increase heat capacity and faster cooling rates |
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What are the differences in scan parameters for Conventional and Spiral/Helical CT?
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Conventional CT
-100-400 mA -1-10mm slice thickness -0.75-2.0 sec. @360-degree rotation scan time Spiral/Helical CT -10-700 mA -0.625-20 slice thickness -0.5-2.0 speed |
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What is pitch?
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Pitch is associated with a fastener. It is the distance between that the CT table moves during one revolution of the xray tube.
A pitch of 1 results in the best image quality in Spiral/helical CT. |
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What is volume coverage?
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volume coverage = pitch X slice thickness
or beam collimation X scan time or pitch X collimation X scan time DIVIDED by gantry rotation time |
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What is collimation?
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Collimation determines slice thickness and is usually the same as table increment, it depends on they type tissue being imaged.
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What is table increment?
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Table increment/ table feed/ table speed, the mm per sec the table moves
Increase table increment + increased pitch resulting in loss of image quality |
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What is the reconstruction increment (RI)?
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Reconstruction increment/interval/spacing (RI) is (unique to Spiral/Helical CT) determines the degree of sectional overlap. As RI decreases image quality increases, processing time increases.
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What is the Slice Sensitivity Profile (SSP)?
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The SSP describes how thick a section is imaged and to what extent details within the section contribute to the signal.
As SSP widens image quality decreases. |
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Patient dose in Conventional CT and Spiral/Helical CT is usually about the same, what are the several reasons that dose is less in Spiral/Helical CT.
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1. Tube currents are set lower
2. Decreased need for repeats 3. Overlapping images with one scan rotation. 4. Increase pitch values (greater than 1) |
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What are two limitations of Spiral/Helical CT?
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1. The need for higher power xray tubes, improved cooling times.
2. Partial volume averaging. |
