Test 2 Flashcards
(84 cards)
1
Q
CT History
A
I THINK MOST IMPORTANT:
1968-70: Hounsfield constructs a CT prototype at EMI
1971: CT brain scanner installed at Atkinson-Morley’s Hospital in London
1972: CT debuts at Radiologic Society of North America
1974: DR. Robert Ledley made 1st Whole body scanner at Georgetown
2
Q
1st generation CT
A
single detector
translate-rotate mode
pencil beam
180 translations separated by one degree
3
Q
2nd generation CT
A
Mechanical gantry motion
multiple detector assembly
fan shaped beam
translate-rotate mode
18 translations separated by 10 degree increments
scan time reduced to 20 seconds per slice
4
Q
3rd generation CT
A
Curved detector assembly
fan shaped beam
rotate mode
scan times reduced to one second
5
Q
4th generation
A
fixed circular detector array
rotate mode
scan times reduced to one second
6
Q
x axis
A
sagittal plane left to right
width
7
Q
y axis
A
coronal plane, anterior to posterior
height
8
Q
z axis
A
axial head to feet
thickness of the slice (depth)
9
Q
Image Matrix
A
Layout of cells in rows and columns
10
Q
Pixel
A
2d cell
Pixel size = Field of view/Image matrix
11
Q
Voxel
A
3D cell
12
Q
Spatial Resolution
A
Image matrix increases
Resolution increases
The ability to differentiate small forms that are close together as separate objects
AKA: image detail, line pairs per centimeter
13
Q
FOV
A
FOV determines area within gantry from which raw data is acquired
As FOV decreases, Resolution INCREAES
14
Q
Contrast Resolution
A
The ability to distinguish between small differences in densities
As slice thickness decreases, resolution INCREASES
15
Q
Absorption Profile
A
The degree to which a beam is reduced by an object is attenuation
beam attenuation is quantified by attenuation profile and absorption profile
16
Q
Hounsfield Unit
A
CT numbers
attenuation value of water
17
Q
CT Components
A
Gantry: x ray tube, detector array, collimator assembly
Computer: two types of software, operating system and applications
18
Q
Window width
A
determines the range of HU
Values higher will be white and values lower will be black
19
Q
Window Level
A
Selects the CENTER CT VALUE of window width
AKA window center
20
Q
Window width and level for bone
A
Window level: 350
Window width: 2500
21
Q
Window width and level for Lung
A
Window level: -500
Window width: 1500
22
Q
Window width and level for Soft tissue
A
Window level: 40
Window width: 400
23
Q
Isotropic
A
Window width, length and depth is the same
24
Q
Slip rings
A
permit gantry frame to rotate continuously
Make helical scan modes possible
25
Generator
Mounted on the slip rings
External to the gantry
26
Filtration
Reduces pt dose
shapes the beam
creates beam uniformity
27
Collimation
Pre patient: restricts the xray beam to a specific area and decreases patient dose
Post patient: reduces scatter radiation and improves contrast resolution
28
Algorithms
a finite set of unambiguous steps performed in a prescribed sequence to solve a problem
29
Fourier transform
an important image processing tool that decomposes an image into components
30
Interpolation
A mathematical method of estimating the value of an unknown function using the known value on either side of the function
Adding two densities into one
31
Image reconstruction
the process of using raw data to create an image
32
Prospective reconstruction
What is automatically produced during scanning
33
Retrospective reconstruction
using the raw data later to create a new image
34
Back projection
The process of converting the data from the attenuation profile to a matrix
35
Smooth Filter
Reducing the difference between adjacent pixels
reduces artifacts but also reduces spatial resolution
Improves contrast resolution
We want this for soft tissue
36
Sharp Filter
Enhances contrast by accentuating the difference between neighboring pixels
Improves spatial resolution and reduces low contrast resolution
37
Reconstruction Algorithm vs Window setting
Changing the algorithm changes the way the raw data are manipulated to reconstruct the image
This contrasts with changing the window setting, which merely changes the way the image is viewed
38
Adaptive Statistical Iterative Reconstruction
Newer method of construction
Complex method that computes projections from the image, compares it with the original projection data, and updates the image based on the difference
can reduce image noise
shown to reduce the radiation dose to the patient by as much as 50%
39
LIST TWO ADVANTAGES OF ADAPTIVE STATISTICAL ITERATIVE RECONSTRUCTION
REDUCES IMAGE NOISE
REDUCES RADIATION DOSE TO PATIENT
40
Scan field of view
determines the area, within the gantry, from which the raw data is acquired
in the isocenter of the gantry
SFOV selection determines the number of detector cells collecting data
41
Display field of view
determines how much of the collected raw data is used to create an image
cannot be larger than SFOV
AKA ZOOM
Changing DFOV changes the pixel size
42
Spatial Resolution
Pixel size = Field of view/Image matrix
43
2 reasons why we do AP localizer
Start-Stop
| Right left centering
44
Lateral Localizer
Start-stop
Anterior posterior centering
45
!!!!! Disadvantages of slice by slice scanning?!!!!!!
The cumulative effect of the pauses between each data acquisition adds to the total examination time
Inefficient use of contrast
Slice misregistration
Poor 3D an/or MPR
46
!!!!Requirements of Helical scanning
Continually rotating x ray tube/detector assembly
high heat capability/dissipation x ray tube
Continuous table movement
Increased data processing system
47
!!!!!!!Advantages of helical scanning!!
Eliminates the interscan delay
Ability to optimize iodinated contrast agent administration
Reduces respiratory misregistration
Reduces motion artifacts from organs
Improve 3D and MPR
48
Pitch
Ratio between table speed and slice thickness
Calculates amount of anatomy examined during a particular multislice scan
Pitch= bed increment x rotation
Divided by Slice thickness
Contiguous= 1 and all next to each other. table and beam are identical
Non contiguous is above 1 and there are gaps. Table is more than beam collimation
Over lapping is below 1 and there is overlap. table is less than beam colimation
49
!!!MDCT
Thin slices (0.625) can be added together to create thick (2.5) reconstructed slice for viewing
50
!!!!!!!! Two main features are used to measure image quality!!!!!!!!!
Spatial resolution: ability to resolve (as separate objects) small, high contrast objects
Contrast resolution: The ability to differentiate between objects with very similar densities as their background
51
!!!!!!!Factors affecting spatial resolution!!!!!!
```
matrix size
display field of view
pixel size
slice thickness
reconstruction algorithm
Pitch
Patient motion
lp/mm
```
52
Contrast Resolution
AKA low contrast detectability or system sensitivity
CT is superior to all other modalities in its contrast resolution
objects with a 0.5% contrast variation can be distinguished
53
Factors affecting contrast resolution
```
mAs/dose
pixel size
slice thickness
reconstruction algorithm
patient size
```
54
Temporal resolution
how rapidly data is acquired
Controlled by: gantry rotation speed, number of detector channels in the system, speed with which the system can record changing signals
Reported in milliseconds
55
Image noise
Noise is measured by obtaining the standard deviation of the CT numbers within an ROI
56
Image Uniformity
Ability of CT scanner to yield the same CT number regardless of location in ROI
Center CT number must be between -7 and +7 HU (+/-5 preferred)
57
Quality Control
Window width and level is 100
5 lp/mm bar clearly resolved for adult abdomen protocol (smooth filter)
6 lp/mm bar must be resolved for high resolution adult chest protocol (Sharp filter)
58
Linearity
Relationship between CT numbers and linear attenuation values
Requires phantom with standard materials
59
Image artifacts
Classifications: physics based, patient based and equipment induced
60
Beam hardening artifacts
Caused by the polychromatic nature of the xray beam
As an xray beam passes through an object, lower energy photons are preferentially absorbed creating a "harder" beam that can't be adjusted for by the system
Systems minimize this in 3 ways: filtration, calibration correction, beam hardening correction software
looks like dark bands or streaks between dense objects in the image
61
Partial volume artifacts
occurs when more than one type of tissue is contained within a voxel
minimized by thinner slices
62
Edge Gradient Effect artifacts
results in streak artifact or shading arising from irregularly shaped objects that have a pronounced difference in density from surrounding structures- bone and soft tissue (skull and brain) or soft tissue and lung (chest and abdomen)
largely unavoidable but can be reduced by thinner slices or using a low HU oral contrast instead of barium
63
Motion artifacts
artifacts from patient motion appear as shading, streaking, blurring or ghosting
64
Metallic artifacts
metal objects in SFOV creates streaks
best reduced by removing metal
non removable objects avoided by angling the gantry (dental fillings)
65
Out of field artifacts
caused by anatomy that extends outside the selected SFOV
appear as streaks and shading
66
Ring artifacts
Caused by imperfect detector elements
appear on the image as a ring or concentric rings
can sometimes be eliminated by recalibrating the scanner
67
Reconstructions
When RAW data are manipulated to create pixels that are then used to create an image
68
Reformation
when IMAGE data are assembled to produce images in different planes or to produce 3D images
69
3D reformation
represents the entire scan volume in only one image
manipulate/combine CT values to display an image
3 types: surface rendering shaded surface display, maximum intensity projection and volume rendering
70
Surface rendering or Shaded surface display
Only voxels on the surface of the structure are used
Threshold CT value is selected
71
Maximum Intensity Projection
Selects voxels with the highest value to display
Displays 3D data on a 2D image
72
Volume Rendering
3D semitransparent representation of the imaged structure
favored 3D image technique
an advantage is that all voxels contribute to the image
Allows image to display multiple tissues and their relationship to one another
73
Endoluminal Imaging
Virtual endoscopy
74
Unit of x ray exposure in air
Roentgen (R)
| SI unit is C/kg
75
Unit of absorbed dose
Radiation absorbed dose (rad)
Gray (Gy) is the SI unit
100 rad=1Gy
1 rad= 1cGy
76
Equivalent dose
absorbed dose x radiation weighting factor
77
Effective dose
Absorbed dose x radiation weighting factor x tissue weighting factor
78
CT dose parameters
CT dose index
Dose length product
multiple scan average dose
effective dose
79
CTDI
| CT Dose Index
Measure of dose (mGy) per slice
One revolution around
obtained using standard head and body CTDI phantoms and a pencil ionization chamber
80
Dose Length Product
DLP (mGy)= CTDI x scan length
contiguous slice by slice
81
Multiple Scan Average Dose
MSAD (mGy) = DLP + overlap
helical/spiral
82
!!!!!!! Effective Dose Efd
EfD (mSv)= MSAD x tissue weighting factor
83
Dose Comparisons
Avg background radiation for Americans is about 3.6 mSv
exposure from a chest xray is baout 0.1 mSv
Exposure from a chest, abdomen or pelvis CT is about 10 mSv
84
Strategies for Reducing dose
automatic bolus tracking
Decrease mA to suit individual pt
Automatic tube current modulation
increase pitch (table speed)
Limit use of thin slices
prevent repeat exams
shielding
