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18 Cards in this Set

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Who was an early pioneer in the development of volume CT scanning?
Dr. Willi A Kalender
Why was the term Spiral Ct choose over Helical?
The definition of "Spiral" - cylindric and conic configurations. whereas helical- only refers to cylindric configurations
What is required for Volume Data Acquisition?
-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
What are some problems that occur from spiral geometry data acquisition?
-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
How can the problems that occur from spiral geometry data acquisition be improved?
-Dedicated reconstruction algorithms
-Interpolarization
What are two ways to eliminate motion artifacts in spiral scanning?
1.Calculation (interpolarization)
2.Reconstruction of images using filtered back projection algorithm
What is 360-degree Linear Interpolation Algorithm?
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.
What is the 180-degree LI algorithm?
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.
What are the major differences in spiral/helical CT from Conventional CT?
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
What are the differences in scan parameters for Conventional and Spiral/Helical CT?
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
What is pitch?
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.
What is volume coverage?
volume coverage = pitch X slice thickness
or
beam collimation X scan time
or
pitch X collimation X scan time DIVIDED by gantry rotation time
What is collimation?
Collimation determines slice thickness and is usually the same as table increment, it depends on they type tissue being imaged.
What is table increment?
Table increment/ table feed/ table speed, the mm per sec the table moves
Increase table increment + increased pitch resulting in loss of image quality
What is the reconstruction increment (RI)?
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.
What is the Slice Sensitivity Profile (SSP)?
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.
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.
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)
What are two limitations of Spiral/Helical CT?
1. The need for higher power xray tubes, improved cooling times.
2. Partial volume averaging.