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CT > Test 2 > Flashcards

Test 2 Flashcards

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

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2
Q

1st generation CT

A

single detector
translate-rotate mode
pencil beam
180 translations separated by one degree

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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

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4
Q

3rd generation CT

A

Curved detector assembly
fan shaped beam
rotate mode
scan times reduced to one second

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5
Q

4th generation

A

fixed circular detector array
rotate mode
scan times reduced to one second

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6
Q

x axis

A

sagittal plane left to right

width

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7
Q

y axis

A

coronal plane, anterior to posterior

height

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8
Q

z axis

A

axial head to feet

thickness of the slice (depth)

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9
Q

Image Matrix

A

Layout of cells in rows and columns

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10
Q

Pixel

A

2d cell

Pixel size = Field of view/Image matrix

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11
Q

Voxel

A

3D cell

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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

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13
Q

FOV

A

FOV determines area within gantry from which raw data is acquired

As FOV decreases, Resolution INCREAES

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14
Q

Contrast Resolution

A

The ability to distinguish between small differences in densities

As slice thickness decreases, resolution INCREASES

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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

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16
Q

Hounsfield Unit

A

CT numbers

attenuation value of water

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17
Q

CT Components

A

Gantry: x ray tube, detector array, collimator assembly

Computer: two types of software, operating system and applications

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18
Q

Window width

A

determines the range of HU

Values higher will be white and values lower will be black

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19
Q

Window Level

A

Selects the CENTER CT VALUE of window width

AKA window center

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20
Q

Window width and level for bone

A

Window level: 350

Window width: 2500

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21
Q

Window width and level for Lung

A

Window level: -500

Window width: 1500

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22
Q

Window width and level for Soft tissue

A

Window level: 40

Window width: 400

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23
Q

Isotropic

A

Window width, length and depth is the same

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24
Q

Slip rings

A

permit gantry frame to rotate continuously

Make helical scan modes possible

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25
Q

Generator

A

Mounted on the slip rings

External to the gantry

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26
Q

Filtration

A

Reduces pt dose
shapes the beam
creates beam uniformity

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27
Q

Collimation

A

Pre patient: restricts the xray beam to a specific area and decreases patient dose

Post patient: reduces scatter radiation and improves contrast resolution

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28
Q

Algorithms

A

a finite set of unambiguous steps performed in a prescribed sequence to solve a problem

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29
Q

Fourier transform

A

an important image processing tool that decomposes an image into components

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30
Q

Interpolation

A

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

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31
Q

Image reconstruction

A

the process of using raw data to create an image

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32
Q

Prospective reconstruction

A

What is automatically produced during scanning

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33
Q

Retrospective reconstruction

A

using the raw data later to create a new image

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34
Q

Back projection

A

The process of converting the data from the attenuation profile to a matrix

35
Q

Smooth Filter

A

Reducing the difference between adjacent pixels

reduces artifacts but also reduces spatial resolution

Improves contrast resolution

We want this for soft tissue

36
Q

Sharp Filter

A

Enhances contrast by accentuating the difference between neighboring pixels

Improves spatial resolution and reduces low contrast resolution

37
Q

Reconstruction Algorithm vs Window setting

A

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
Q

Adaptive Statistical Iterative Reconstruction

A

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
Q

LIST TWO ADVANTAGES OF ADAPTIVE STATISTICAL ITERATIVE RECONSTRUCTION

A

REDUCES IMAGE NOISE

REDUCES RADIATION DOSE TO PATIENT

40
Q

Scan field of view

A

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
Q

Display field of view

A

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
Q

Spatial Resolution

A

Pixel size = Field of view/Image matrix

43
Q

2 reasons why we do AP localizer

A

Start-Stop

Right left centering

44
Q

Lateral Localizer

A

Start-stop

Anterior posterior centering

45
Q

!!!!! Disadvantages of slice by slice scanning?!!!!!!

A

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
Q

!!!!Requirements of Helical scanning

A

Continually rotating x ray tube/detector assembly

high heat capability/dissipation x ray tube

Continuous table movement

Increased data processing system

47
Q

!!!!!!!Advantages of helical scanning!!

A

Eliminates the interscan delay

Ability to optimize iodinated contrast agent administration

Reduces respiratory misregistration

Reduces motion artifacts from organs

Improve 3D and MPR

48
Q

Pitch

A

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
Q

!!!MDCT

A

Thin slices (0.625) can be added together to create thick (2.5) reconstructed slice for viewing

50
Q

!!!!!!!! Two main features are used to measure image quality!!!!!!!!!

A

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
Q

!!!!!!!Factors affecting spatial resolution!!!!!!

A 
matrix size
display field of view
pixel size
slice thickness
reconstruction algorithm
Pitch 
Patient motion
lp/mm
52
Q

Contrast Resolution

A

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
Q

Factors affecting contrast resolution

A 
mAs/dose
pixel size
slice thickness
reconstruction algorithm
patient size
54
Q

Temporal resolution

A

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
Q

Image noise

A

Noise is measured by obtaining the standard deviation of the CT numbers within an ROI

56
Q

Image Uniformity

A

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
Q

Quality Control

A

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
Q

Linearity

A

Relationship between CT numbers and linear attenuation values

Requires phantom with standard materials

59
Q

Image artifacts

A

Classifications: physics based, patient based and equipment induced

60
Q

Beam hardening artifacts

A

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
Q

Partial volume artifacts

A

occurs when more than one type of tissue is contained within a voxel

minimized by thinner slices

62
Q

Edge Gradient Effect artifacts

A

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
Q

Motion artifacts

A

artifacts from patient motion appear as shading, streaking, blurring or ghosting

64
Q

Metallic artifacts

A

metal objects in SFOV creates streaks

best reduced by removing metal

non removable objects avoided by angling the gantry (dental fillings)

65
Q

Out of field artifacts

A

caused by anatomy that extends outside the selected SFOV

appear as streaks and shading

66
Q

Ring artifacts

A

Caused by imperfect detector elements

appear on the image as a ring or concentric rings

can sometimes be eliminated by recalibrating the scanner

67
Q

Reconstructions

A

When RAW data are manipulated to create pixels that are then used to create an image

68
Q

Reformation

A

when IMAGE data are assembled to produce images in different planes or to produce 3D images

69
Q

3D reformation

A

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
Q

Surface rendering or Shaded surface display

A

Only voxels on the surface of the structure are used

Threshold CT value is selected

71
Q

Maximum Intensity Projection

A

Selects voxels with the highest value to display

Displays 3D data on a 2D image

72
Q

Volume Rendering

A

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
Q

Endoluminal Imaging

A

Virtual endoscopy

74
Q

Unit of x ray exposure in air

A

Roentgen (R)

SI unit is C/kg

75
Q

Unit of absorbed dose

A

Radiation absorbed dose (rad)

Gray (Gy) is the SI unit

100 rad=1Gy
1 rad= 1cGy

76
Q

Equivalent dose

A

absorbed dose x radiation weighting factor

77
Q

Effective dose

A

Absorbed dose x radiation weighting factor x tissue weighting factor

78
Q

CT dose parameters

A

CT dose index
Dose length product
multiple scan average dose
effective dose

79
Q

CTDI

CT Dose Index

A

Measure of dose (mGy) per slice

One revolution around

obtained using standard head and body CTDI phantoms and a pencil ionization chamber

80
Q

Dose Length Product

A

DLP (mGy)= CTDI x scan length

contiguous slice by slice

81
Q

Multiple Scan Average Dose

A

MSAD (mGy) = DLP + overlap

helical/spiral

82
Q

!!!!!!! Effective Dose Efd

A

EfD (mSv)= MSAD x tissue weighting factor

83
Q

Dose Comparisons

A

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
Q

Strategies for Reducing dose

A

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

CT (2 decks)

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