CT Review
CT assigned number for LDPE
+-90 HU
CT assigned number for air
-1000
CT assigned number for water
0
CT assigned number for bone
1000
CT assigned number for acrylic
120HU
What generation of CT is Single ray pencil beam Single detector
1st
360 linear interpolation requires how many rotations to reconstruct a slice
2
What generation of CT is Linear scan and rotate (6 scans, 30° rotations)
2nd
What generation of CT is Single projection fan-shaped beam (permitting whole body scanning) Up to 30 detectors 10-90 second scan time
2nd and 3rd
What generation of CT is 360° rotation of x-ray tube and detectors Crude dynamic capabilities
3rd
What generation of CT is Single projection fan-shaped beam 360° ring of stationary detectors
3rd
What generation of CT is 2-10 second scan time 360° Rotation of x-ray tube Dynamic capabilities
4th
What generation of CT is 50 msec imaging
5th
What generation of CT is Annular targets
5th
What generation of CT is Cardiac Imaging
5th
What generation of CT is Electron beam scanners Stationary tube
5th
What generation of CT is Computer controlled table movement through gantry bore
6th
What generation of CT is Continuous scanning throughout entire examination
6th
What generation of CT is Helical/Spiral Imaging 360° ring of stationary detectors 360° rotation of x-ray tube
6th
What generation of CT is Multiple projection fan-shaped beam
6th
0.5 mm section thickness
7th
>10k detector elements
7th
>20 simultaneous contiguous detectors
7th
Multidetector CT (MDCT)
7th
≈12-15 sec procedure time (body region scan time)
7th
CT assigned number for teflon
990
What is a two-dimensional square of data called?
A Pixel
When slice thickness is decreased, image noise: a) Increases b) Decreases c) Does neither
A inc
What effect does slice thickness have on the radiation dose?
A thinner slice will deliver a higher dose
Which of these factors will increase the radiation dose to the patient? A) decreasing pitch B) increasing slice thickness C) decreasing the display field
A) decreasing pitch
What generation of CT is 4.5-5 minute scan time
A. First Generation
What generation of CT is Linear scan and rotate: 180 scans, 1°rotations
A. First Generation
_____ is the ability of an imaging system to demonstrate small changes in tissue contrast
Contrast resolution or tissue resolution to image objects 2 to 3 mm in size that vary slightly in density from the environment in which they are located
Factors to increase spatial resolution ___ focal spot size ____ slice thickness (assuming mAs is increased to compensate for loss of radiation reaching the detectors) ____ the number of detectors ____ matrix size ____ pixel size
Decreasing focal spot size Decreasing slice thickness (assuming mAs is increased to compensate for loss of radiation reaching the detectors) Increasing the number of detectors Increasing matrix size Decreasing pixel size
What generation of CT is Head Scan Only
First Generation
All the measurements obtained from the detector array are called
Scan Data or Raw Data
Creator of CT
Sir Godfrey Newbold Hounsfield
The ability to represent small objects and differentiate between closely spaced objects is called
Spatial resolution
FOV (field of view)
The diameter of image reconstruction
With what type of scanner are ring artifacts more commom? y?
Third generation scanners each detector views an anulus (ring) of anatomy during the examination
Voxel
Tissue volume; determined by multiply the pixel size by the thickness of the CT image slice Voxel size (mm3)=pixel size (mm2) x slice thickness (mm)
Plane that correlates to the slice thickness, or depth, of a CT slice?
Z Axis
True or False: Today, helical scanner possess a function of pitch; pitch is a ration of slice thickness to interval (the rate at which the table advances. a) True b) False
b) False
Which of the following accurately describes the relationship between patient dose and pitch? a) if pitch increases, patient dose increases b) if pitch increases, patient dose decreases c) if pitch decreases, patient dose decreases d) if pitch decreases, patient dose remains unchanged
b) if pitch increases, patient dose decreases
window level
brightness
Which of the following is defined as the ability to distinguish details in objects of different densities a small distance apart? a) contrast resolution b) cross field uniformity c) spatial resolution d) interpolation
c) spatial resolution
window width
contrast
When the FOV is increased for a fixed matrix size the size of each pixel is ______ When matrix size is increased for a fixed FOV pixel size is
increased proportionately; smaller Pixel size = FOV/Matrix Size
A grid formed from rows and columns of pixels is called
matrix
Spatial resolution is measured using
modulation transfer function (MTF)
what type of artifact occurs when an off-centre obj projects partly into scan plane A. partial volume artifact B. photon starvation
partial volume artifact
caused by low signal reaching detectors for particular projections use of thin slices make this more likely to occur
photon starvation
Linear attenuation coefficient
reflects the degree to which the x-ray intensity is reduced by a material
due to erros in individual detector calibration
ring artifacts
due to helical interpolation
shading artifacts
The mechanism in some CT scanners that allows the x-ray tube to rotate continually in the same direction is called
slip-ring
To magnify in CT you must Choose a ___ display field of view (DFOV)
smaller
due to inconsistency in a small group of readings (partial vol, photon starvation, metal artifacts, patient movement)
streaking artifact
pitch equation
table travel per rotation/ nominal slice width
DFOV (displayed field of view)
the actual viewing of the image on the monitor.
Density and contrast can be manipulated by the computer using
window level and width control
Scanned patient information is stored as
"raw" data