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Ct Physics Module 3 > Powerpoint > Flashcards

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1
Q

Ct detectors are called?

Does what?

A

Solid state detectors

- turns photons into lights

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2
Q

Voxel profile

A

Contents in the profile

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3
Q

Any Ct acquisition can be reconstructed with one or more reconstruction filters( bone, soft tissue)?

A

True

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4
Q

Data can be classified as(4)

A

Measurement data

Raw data

Convoluted data

Image data

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5
Q

Measurement data

A

Arise from detectors and are subject to preprocessing to correct the measurement data before the image algorithmn is applied

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6
Q

Raw data

A

The result of pre- processed scan data subject to the reconstruction algorithmn

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7
Q

Convolved data

A

Image reconstruction used by current Ct scanners is filtered back projection

Raw data is first filtered with mathematical filter or kernel. Process is called convolution technique which improves image quality by removing blur

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8
Q

Most popular reconstruction algorithmn

A

Filtered back projection

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9
Q

Kernel

A

Mathematical filter

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10
Q

Image data

A

Reconstructed data are convolved data that have been back projected into the image matrix to create Ct images displayed on a monitor

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11
Q

Image quality Ct

A

General image quality can be described by several,key performance parameters. High contrast, low contrast, spatial resolution, temporal resolution

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12
Q

Determining factors image quality

A

Artifacts

Contrast resolution

Linearity

Noise

Spatial resolution

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13
Q

Artifacts

  • are?
  • arise (4)?
A

Distortion or error in an image that is unrelated to the subject being scanned. It can degrade image quality , affect the perceptibility of detail.

Arise from number sources :

  • inappropriate selection of protocol
  • reconstruction process
  • problems relating to equipment( malfunctions)
  • fundamental limitations to physics
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14
Q

Ct # 40

A

Blood

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15
Q

Ct # 1000

A

Air

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16
Q

Streak artifacts

A

Caused by errors of isolated Channels or views and enhanced by the convolution process and manifested into lines during the back projection

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17
Q

Why streaks?

A 
Improper sampling data
Partial volume averaging
Patient motion
Metal
Beam hardening
Noise
Spiral/ helical scanning
Mechanical failure
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18
Q

Shading artifact

A

Often appear near objects of high densities and can be caused by beam hardening , partial volume, averaging, spiral/ helical scanning , scatter, off focal radiation and incomplete projections

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19
Q

Why shading artifact

A 
Partial volume averaging 
Beam hardening
Spiral/ helical scanning
Scatter radiation
Off focal radiation
Incomplete projections
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20
Q

Ring and bands artifacts

A

Produced when the projection readings of single channel or group of channels consistently deviate from the truth . It can be result of a defective detector cells or das channels, deficiencies in system calibration or suboptimal image generation

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21
Q

Why ring and band artifact?

A

Bad detector channels in 3rd gen Ct scanners

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22
Q

Patient based artifacts

A

Motion artifact

Transient interruption of contrast

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23
Q

Motion artifact

A

Decrease scan time , increase interscan delay

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24
Q

Transient interruption of contrast

A

Use of filters ( algorithmns)

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25
Q

Physical based artifacts

A

Beam hardening
Metal artifact/ high density foreign material artifact
Partial volume averaging

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26
Q

Beam hardening

A

Artifact that occurs as streaks at bone edges adjacent to soft tissue. Seen most commonly in posterior fossa of brain

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27
Q

Metal artifact/ high density foreign material artifact

A

Produce streak artifacts metal artifact correction can be used

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28
Q

Partial volume averaging

A

Occurs when tissues of widely different absorption are encompassed on the same Ct voxel producing a beam attenuation proportional to the average value of these tissues

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29
Q

Physics based artifacts

A

Quantum mottle

Aliasing

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30
Q

Quantum mottle

A

Influenced partially by the number of photons that strike the detectors. Photon starvation can occur as a result of poor patient positioning in the scan fov and poor selection of exposure technique, scan speed and limitation of scanner. Photon starvation often leads to sever streak artifacts

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31
Q

More photons means less

A

Noise and stronger detector ; combined with electronic noise and logarithmic operation

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32
Q

Aliasing

A

Arises from insufficient projection sampling or insufficient view sampling( number of photons)

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33
Q

Hardware based artifacts

A

Ring/ band artifact

Helical and multichannel artifacts

Cone beam effect

Scatter

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34
Q

Ring or band artifact

A

Produced when the projection readings of a single channel or a group of channels consistently deviate from the truth. It can be result of defective detector cells or das channels, deficiencies in system calibration or suboptimal generation

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35
Q

Cone beam effect

A

Caused by incomplete or insufficient projection samples as a result of cone beam geometry of msct

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36
Q

Scatter radiation produced by

A

Patient

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37
Q

Sample

A

Data that has been taken from all over

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38
Q

Contrast resolution

A

Signal to noise ratio

Contrast to noise ratio

Scatter radiation reduction

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39
Q

Signal to noise ratio

A

How much true signal of anatomy versus how much Noise a particular image has- higher signal lower noise better

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40
Q

Contrast to noise ratio

A

Measure to determine image quality

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41
Q

Scatter radiation reduction

A

Corrected with algorithmn by carefully measuring or estimating the scatter distribution in the projection. The estimated scatter can then be removed from the measured intensities to arrive to the true signals representing scanned object

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42
Q

Linearity

A

Relationship of Ct numbers to linear attenuation coefficients of the object to be imagined. Can be checked by daily calibration

Water and other materials tested

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43
Q

Two points are defined precisely on Ct number scale, water with Ct number ______and air Ct number is _____

A

0

-1000

44
Q

Because ____ is similar to soft tissue in terms of attenuation characteristics, it’s important to establish accuracy

A

Water

45
Q

All Ct manufactures provide a phantom filled with water for such testing. When tested the Ct number should be fairly close to______?

A

0

46
Q

Das

A

Data acquisition scan

47
Q

Flux

A

Change

48
Q

Kernel

A

Average within the matrix

49
Q

Noise

A

Measured on uniform phantoms suing roi’s

50
Q

Quantum noise

A

Determined by X-ray flux or number of photons detected. Influenced by scanning techniques, scanner efficiency ( detector quantum efficiency and geometrical efficiency) and patient

51
Q

Mottle

A

Grainy appearance, also can be caused the reconstruction parameters.

Ex: high resolution construction kernel produces an increased noise level. Because kernels preserve and enhance high frequency contents and noise presents itself a high frequency signals

52
Q

Electronic noise caused from

A

Detector photodiode
Data acquisition scan
Scatter radiation

53
Q

Spatial resolution

A

Aka high contrast resolution

Scanners ability to resolve closely placed objects that are significantly different from their background. Defined and measured within the scanning plane and cross plane

54
Q

In plane resolution

A

Specified in terms of line pairs per centimeter( lp/cm) or lp/ mm

Line pairs is a pair of equal sized black-white bars

Smallest , most closely spaces set of lead lines is said be the spatial resolution of Ct scanner.

Mtf ( modulation transfer functions) can be used to compare performance of different Ct systems

  • straight line= better mtf
55
Q

Spatial resolution test is done by?

A

Tech

56
Q

Cross plane spatial resolution

A

New technology has enable radiologist to view images in 3D

Mpr, mip, vr

Where it is hard to tell if images were acquired, for example, in a coronal plane or reconstructed coronal. Shows the special resolution is isotropic

57
Q

Improved cross plane spatial resolution reduces_____

A

Artifacts

58
Q

Cross plane spatial resolution is usually described by _____. Then the _____ can be calculated.

A

Slice sensitivity profile( ssp )

Modulation transfer function

59
Q

Testing for spatial resolution should be done ______

A

Monthly

60
Q

Temporal resolution is important to which type of Ct?

A

Cardiac Ct

61
Q

Gaiting

A

Waiting for heart rate resting between beats to image using an ekg machine

62
Q

Water phantom test is done by the tech______

Aka?

A

Daily- looking for Ct number

Linearity test

63
Q

Temporal resolution

A

Indication Ct systems ability to freeze motions of scanned object

Reduces blurring effect caused by moving object( i.e. Heart, peristalsis , contrast through vessels)

Reduced by different techniques such as physiological gating device

64
Q

In cardiac cycle , time period of no motion?

A

Diastolic and systolic phases

65
Q

During periods of cardiac temporal resolution , what can take place?

A

Data acquisition

66
Q

Advantage of prospective gating?

A

Reduces patient dose

67
Q

During times when perspective gating is not optimal, what happens to the scan?

A

Can be turned off

68
Q

Disadvantage of perspective gating?

A

Reliance on regularity of heart motion

69
Q

Low contrast resolution or low contrast detectability

A

Determined using objects having a very small difference in density from the background

Referred to sensitivity of the system

Images of tissues that vary slightly in density and atomic number

Measured by phantom that contains low- contrast objects of different sizes

Lcd also implies an object visibility is highly influenced by presence of noise

70
Q

Influencing factors of image quality

A 
Focal spot size
Beam geometry
Detector type
Motion
Subject contrast
Ambient lighting conditions( ww or wl)
Selectable factors ( ma,kv,scan time , sfov, dfov, slice thickness, interscan spacing, filtering, ww,wl)
71
Q

Calibration of scanner

A

Ensures optimum image quality

Should be carried out according to manufactures instructions

Includes frequent air calibrations

Less frequent calibrations with homogenous water phantoms

Air calibrations allow for acquiring information about the small differences in the response of individual detect elements

72
Q

Ct calibration has to be accurate within

A

0.5% and air calibration allows for appropriate calibration and correction of signal recorded by each individual detector element

Calibration with phantom allows some correction of beam hardening

73
Q

Portion of primary beam interacting with single detector ?

A

Ray

74
Q

Ray

A

Portion of X-ray beam that falls upon single detector

75
Q

Ability of Ct scanner to image high density objects controlled by the____ of the scanner

A

Spatial resolution

76
Q

Spatial resolution

A

Controls ability to image small structures

77
Q

Which following are commonly used to describe a Ct scanner ability to differentiate objects with similar linear attenuation coeffcients?

A

Sensitivity

Contrast resolution

78
Q

Contrast resolution/ sensitivity

A

Ability of Ct scanner to accurately image objects with similar densities .

Ct scanner with poor contrast resolution or low sensitivity has difficulty separating tissues whose linear attention coefficients are nearly equal

79
Q

Spatial resolution of a Ct scanner is usually given in unit measure?

A

Lp/cm

80
Q

Spatial resolution quantifies is ability to visually separate small objects in the image . Unit used to describe the spatial resolution is tested by the scanning a phantom with an embedded resolution test pattern consisting of a series of small lead lines the decease in size and spacing . Smallest most closely spaces set lead lines is said be the spatial resolution of Ct scanner. A line pair _____consists of one lead line and a space immediately next to it.

A

Lp

81
Q

Which following mathematical functions may be used to quantify the spatial resolution of Ct scanner?

A

Psf- point spread function

Mtf- modulation transfer function

Lsf - line spread function

82
Q

Line spread function( lsf), point spread function(psf), modulation transfer function(mtf) are used for?

A

Quantify the spatial resolution of Ct scanner

83
Q

Line spread function (lsf)

A

Measures ability of Ct scanner to clearly image the edge or a line

84
Q

Point spread function( psf)

A

Clearly images extremely small , point like structures

85
Q

Modulation transfer function (mtf)

A

Examines the fidelity of spatial frequency as it represents tissues with varying densities. Derived from the measurement of lsf and psf

86
Q

Which following technical adjustments may be employed to improve the temporal resolution of mdct system?

A

Decreased scan time( avoid motion)

87
Q

Temporal resolution quantifies Ct systems?

A

Ability to freeeze motion and provide an image free of blur. Controlling factors of temporal resolution: gantry rotation speed and reconstruction method

88
Q

Primary advantage of an isotropic mdct data set?

A

High quality multiplanar refirmations( MPr)

89
Q

Isotropic volumetric data set

A

Yields high quality images with equal resolution in any reconstructed plane

90
Q

Reduces the step artifact

A

Reconstruction of isotropic
Overlapping
Thin section mdct

91
Q

Number, length and organization of the individual detector elements in an msct system

A

Detector configuration

92
Q

Prospectively cardiac Ct data acquisitions occurs only during the portion of the ecg?

A

T wave

93
Q

Prospective gated cardiac Ct data acquisition occurs only during the portion ecg termed

A

T wave

94
Q

T wave

A

Prospective electrocardiography monitoring, scanning only occurs during diastole, when the heart demonstrates the least motion. User defined percentage of the r- r interval is set as the trigger for data acquisition to occur during the t wave of ecg

95
Q

Which of the following statements most accurately describes the relationships between slice sensitivity profile (ssp) and dose profile?

A

The dose profile is always wider than ssp

96
Q

Slice sensitivity profile (ssp)

A

May used to describe the reconstructed Ct section. However, section of tissue exposed to ionizing radiation or dose profile, is greater in width than the ssp

97
Q

Technique that allows the user to select the range of pixel values used in 3D ct reformation ?

A

Thresholding

98
Q

Selected threshold allows the user to select the range of ____values rendered in 3D model .

Ex: higher bone threshold(>300 hu) can be chosen to build a skeletal model of the patients skull. Decreasing the threshold to include voxel with lower attenuation values results in 3D model with the patients soft tissue

A

Pixel

99
Q

Which of the following corresponds to the period of the cardiac cycle known as atrial systole ?

A

Contraction of the left and right atrial

100
Q

________ describes the period of the cardiac cycle when the left and right atria are contracting

A

Atrial systole

101
Q

Which of the following formulas maybe used to calculate the dimensions of a pixel?

A

Pixel size= dfov/ matrix size

102
Q

Ct scanner with a limited resolution of 15 lp/cm can resolve object as small as?

A

0.3mm

103
Q

Minimum object size that a Ct scanner can resolve may be calculated by taking the reciprocal of value of scanner limiting resolution. Reciprocal of 15 lp/cm is 1/15 lp/cm . This is equivalent to 10/15 lp/cm which equals_____mm/lp . Because a line pair (lp) is equivalent to to a line space adjacent to it , this value may be divided in half to provide the minimum object that is scanner resolve: 0.3

A

0.6 mm/lp

104
Q

When one is choosing a window to display Ct image, the width define?

A

Range Ct numbers( pixels) to displayed

105
Q

______of Ct window controls the range of the pixel values that are assigned to a shade of grey. Width is centered to a level that is equal to value of tissue interest

A

Width

106
Q

Ability of scanner to image a small high density object is controlled by the ____ of the scanner

A

Spatial resolution

107
Q

Spatial resolution of Ct scanner controls?

A

Ability to image small structures

Ct Physics Module 3 (1 decks)

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