finals 6 Flashcards
(54 cards)
1
Q
The projections acquired by each detector during CT are stored in the computers memory.
A
IMAGE RECONSTRUCTION
2
Q
- The image is reconstructed from these projections by a process called BLANK
- CT image reconstruction requires the solution of over 250,000 simultaneous
equations.
A
Filtered back projection
3
Q
The term BLANK in CT scan refers to a mathematical function rather than an
aluminum or other metal filter.
A
FILTER
4
Q
- the ability to image small objects that have high subject contrast
- The ability to discriminate between adjacent objects is a function of pixel size
A
Spatial resolution
5
Q
In CT scan the size of the matrix and FOV determines the…
A
Spatial resolution
6
Q
is the diameter of the body region area being imaged (e.g., 25 cm for a head or 40 cm for an abdomen)
A
Field of view (FOV)
7
Q
The larger the pixel size, the lower the subject contrast, the poorer the
A
Spatial resolution
8
Q
is the ability of the CT scanner to reproduce a high contrast edge with accuracy, and is expressed mathematically
A
Edge-Response Function (ERF)
9
Q
- is a mathematical procedure for measuring spatial resolution in a CT scan. It is also the ratio of the image to the object.
- The most common method of describing spatial resolution.
A
Modulation Transfer Function (MTF)
10
Q
defined as 1 bar and its equal width
A
Line Pair (lp)
11
Q
- The number of lines per unit length.
- expressed in line pairs per
centimeter (lp/cm)
A
Spatial frequency
12
Q
CT spatial resolution may also be improved by using a BLANK and by
designing systems with BLANK
A
- smaller focal spot
- smaller detector
13
Q
Typical resolution in CT scanning ranges from
A
0.5 to 1.5 lp/mm
14
Q
is a measure of the effective section thickness
A
slice-sensitivity profile
15
Q
It is the most important factor that affects the slice-sensitivity profile
A
pitch ratio
16
Q
Low Spatial Frequency represents…
A
- Represents large objects.
- Better Contrast Resolution
17
Q
High Spatial Frequency represents…
A
- Represents small objects.
- Better Spatial Resolution.
18
Q
The detector pitch is the center-to-center spacing of the
detectors along the array
A
Detector pitch
19
Q
- The use of smaller detectors improves spatial resolution.
- The most significant geometric factor that contributes to spatial resolution.
A
Detector aperture
20
Q
As in any x-ray imaging procedure, larger focal spots cause more geometric unsharpness in the detected image and reduce spatial resolution.
A
Focal spot size
21
Q
Increased magnification amplifies the blurring of the focal spot
A
Object magnification
22
Q
Large slice thickness reduce spatial resolution
A
Slice thickness
23
Q
- A greater pitch reduces spatial resolution.
- A larger pitch increases the width of the slice sensitivity profile
A
Helical pitch
24
Q
- Bone filters have the best spatial resolution
- Soft tissue filters have lower spatial resolution
A
Reconstruction kernel
25
The number of pixels used to reconstruct the CT image has a direct
influence on spatial resolution
Pixel matrix
26
It is defined as the ability to distinguish one soft tissue from another without regard for size or shape. This is an area in which CT excels.
CONTRAST RESOLUTION
27
Absorption of x-rays in tissue is characterized by the linear attenuation coefficient which is a function of
1. X-ray energy
2. Atomic number of tissue
28
* The BLANK directly influences the number of X-ray photons used to
produce the CT image.
* Doubling the BLANK increases the contrast resolution.
mAs
29
The BLANK increases linearly with mAs per scan
dose
30
Thicker slices use more photons increasing the contrast resolution
Slice thickness
31
* Bone filters produce lower contrast resolution
* Soft tissue filters improve contrast resolution.
Reconstruction filter
32
Larger patients attenuate more xrays, resulting in detection of fewer x-
rays. This reduces the contrast resolution.
Patient size
33
Most CT systems have an upper limit on mA, and for a fixed pitch and a
fixed mA, faster gantry rotations result in reduced mAs used to produce
each CT image, reducing contrast resolution
Gantry rotation speed
34
It is defined as the grainy or uneven appearance of an image caused by an
insufficient number of primary x-rays
NOISE
35
The most common cause of noise in CT is BLANK. – random variation in photon detection
quantum noise
36
Noise should be evaluated BLANK by imaging a 20 cm diameter water bath.
daily
37
The Region of Interest (ROI) must be BLANK pixels
100 pixels
38
Refers to the relationship between CT numbers and linear attenuation values
Linearity
39
The pixel should be constant in all regions of the reconstructed image
uniformity
40
* Also known as pilot scan and topogram.
* Displayed as the 1st image of an examination.
* Achieved prior to the examination by slit scanning as the table moves the patient
through a tightly collimated primary beam.
* The resolution of the image is affected by the speed of the scan and the exposure
output of the tube.
* Highest speed scan and lowest exposure output should be selected.
SCANOGRAM
41
* Also called calibration field of view.
* Determines the area within the gantry from which the raw data are acquired.
* Determines the number of detector cells collecting the data.
* Parts of the patient located outside the SFOV may cause inaccuracies in the
image, called out-of-field artifacts.
* These artifacts are manifest in the image as streaking, shading, and incorrect
Hounsfield numbers.
SCAN FIELD OF VIEW (sfov)
42
* Also called zoom or target.
* Determines how much of the collected raw data is used to create an image.
Display FIELD OF VIEW (dfov)
43
* It is the relationship between the patient couch movement and x-ray beam collimation.
* The ratio of the distance the table moves during one tube rotation to the section thickness.
* It is used to define the extension or contraction of the helix.
Pitch
* Spiral pitch ratio is simply referred to as pitch
44
Manufacturers design focal spot algorithms to predict the focal spot thermal state and adjust the mA accordingly
Focal spot design
45
x ray tube is expected to last for at least 50,000 exposures which is the appropriate x-ray tube life for CT
Heat dissipation
46
* Reconstructs an image by selecting the pixels with the highest value along any arbitrary line.
* Widely used in CT Angiography because they can be reconstructed very fast.
* It is the simplest form of 3-D imaging.
Maximum intensity projection (mip)
47
* It was initially applied to bone imaging.
* Identifies a narrow range of values as belonging to the object to be imaged
and displays that range.
Shaded surface display (SSD)/surface rendering
48
* Provides depth to an image.
* Incorporate the entire volume of data into a 3D image by summing the
contributions of each voxel along a line from the viewers eye through the data
set.
* VR has become the favored 3D imaging technique.
volume rendering
49
* is the result of averaging the linear
attenuation coefficient in a voxel that is heterogeneous in composition.
* increases with increasing pixel size and section thickness.
Partial volume artifacts
50
* are the result of the 0.5 to 2 second scan times, which allow both involuntary (cardiac) and voluntary patient motion.
* Structures therefore move from one voxel to another during data acquisition, introducing errors in the reconstruction. -Random or unpredictable motion.
Motion artifacts
51
* "Cupping" artifacts are caused by
The polychromatic nature of the X-ray beam.
* As the lower-energy photons are preferentially absorbed, the beam becomes more penetrating, causing underestimation of the attenuation coefficient (HU).
* Beam-hardening artifacts are most marked at high contrast interfaces, such as between dense bone in the skull and the brain.
Beam hardening artifacts
52
* may arise in third-generation systems if one or more detectors are faulty or miscalibrated.
* Artifacts caused by equipment defects are rare on modern CT systems.
Ring artifacts
53
Measures the radiation dose to the patient within the primary beam of the scanner
Computed Tomography Dose Index
54
Represents the average dose a patient receives during an examination
Multiple Scan Average Dose
