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CT Scan > Image Quality and Quality Assurance in Computed Tomography > Flashcards

Image Quality and Quality Assurance in Computed Tomography Flashcards

1
Q

relates to how well the image represents the object scanned.

A

Image quality

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2
Q

In CT, image quality is directly related to

A

its usefulness in providing an accurate diagnosis

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3
Q

critical to optimize radiation dose to the patient and image quality

A

appropriate selection of mAs and kVp

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4
Q

allow shorter scan times to be used

A

Higher mA settings

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5
Q

avoiding image degradation as a result of patient motion

A

short scan time

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6
Q

Dose is also reduced if

A

kVp is reduced while the mAs is held constant.

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7
Q

using digital technology, the image quality is not directly linked to the dose, so even when an mA or kVp setting that is too high is used, a good image results.

A

Uncoupling Effect

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8
Q

two main features used to measure image quality are:

A

Spatial Resolution

Contrast Resolution

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9
Q

the ability to resolve (as separate objects) small, high-contrast objects.

A

Spatial Resolution

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10
Q

the ability to differentiate between objects with very similar densities as their background.

A

Contrast Resolution

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11
Q

Spatial resolution is also known as

A

detaiil resolution

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12
Q

This is the system’s ability to resolve, as separate forms, small objects that are very close together.

A

Spatial resolution

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13
Q

Spatial resolution can be measured using two methods:

A
  1. Direct measurement using a phantom.

2. Data analysis is known as the modulation transfer function (MTF).

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14
Q

made of acrylic and has closely spaced metal strips imbedded.

A

line pairs phantom

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15
Q

The phantom is scanned, and the number of strips that are visible are counted.

A

Direct measurement using a phantom

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16
Q

number of line pairs visible per unit length.

A

spatial frequency

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17
Q

If objects are large, not many will fit in a given length

A

They are said to have low spatial frequency.

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18
Q

If the objects are smaller, many more will fit into the same length.

A

These are said to have high spatial frequency.

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19
Q

most commonly used method of describing spatial resolution ability, not only in CT, but also in conventional radiography

A

Modulation Transfer Function (MTF)

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20
Q

the ratio of the accuracy of the image compared with the actual object scanned

A

Modulation Transfer Function (MTF).

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21
Q

indicates image fidelity

A

MTF

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22
Q

The MTF scale is from

A

0 to 1

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23
Q

If the image reproduced the object exactly,

A

MTF of the system would have a value of 1.

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24
Q

If the image were blank and contained no information about the object

A

MTF would be 0.

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25
Q

actual MTF calculated from most objects is between these two extremes

A

it will have a value between 0 and 1.

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26
Q

Resolution in the xy direction is called

A

in-plane resolution

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27
Q

resolution in the z direction is called

A

longitudinal resolution.

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28
Q

The greater the total pixels present in the image

A

the smaller each individual pixel

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29
Q

determines how much raw data will be used to reconstruct the image.

A

display field of view (DFOV)

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30
Q

Increasing the DFOV

A

increases the size of each pixel in the image.

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31
Q

reflects how much patient data is contained within each square

A

pixel size

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32
Q

will include more patient data

A

large pixel

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33
Q

The relationship between pixel size, matrix size, and DFOV is apparent in the equation:

A

pixel size = DFOV/matrix size

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34
Q

thinner slices produce

A

sharper images because to create an image the system must flatten the scan thickness (a volume) into two dimensions (a flat image)

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35
Q

The thicker the slice

A

the more flattening is necessary

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36
Q

improves the images’ longitudinal resolution.

A

Narrowing the slice

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37
Q

When the imaging voxel is equal in size in all dimensions

A

there is no loss of information when data are reformatted in a different plane

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38
Q

depends on which parts of the data should be enhanced or suppressed to optimize the image for diagnosis

A

appropriate reconstruction algorithm

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39
Q

Bone algorithms produce

A

lower contrast resolution (but better spatial resolution)

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40
Q

soft tissue algorithms improves

A

improve contrast resolution at the expense of spatial resolution.

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41
Q

larger focal spots cause

A

more geometric unsharpness in the image and reduce spatial resolution

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42
Q

increasing the pitch

A

reduces resolution

43
Q

depends on the detector configuration and the CT projection data interpolation scheme used.

A

Optimal choice of pitch

44
Q

creates blurring in the image and degrades spatial resolution.

A

Motion

45
Q

may help improve spatial resolution to the extent that they may reduce the effects of both involuntary motion (e.g., heart) and overt patient motion.

A

Shortened scan times

46
Q

Contrast resolution also known as

A

low-contrast sensitivity

47
Q

This ability to distinguish an object that is nearly the same density as its background is referred to as

A

low contrast detectability

48
Q

superior to all other clinical modalities in its contrast resolution

A

CT

49
Q

the undesirable fluctuation of pixel values in an image of a homogeneous material

A

Image noise

50
Q

as the grainy appearance or “salt-and-pepper” look on an underexposed image.

A

Noise

51
Q

occurs when there are an insufficient number of photons detected.

A

Quantum mottle

52
Q

In CT, the number of x-ray photons detected per pixel is also often referred to as

A

signal-to-noise ratio (SNR)

53
Q

influences the number of x-ray photons used to produce the CT image, thereby affecting the SNR and the contrast resolution.

A

mAs selected

54
Q

will improve contrast resolution, but at the cost of a higher radiation dose to the patient.

A

increasing mAs

55
Q

Keeping all other scan parameters the same, as pixel size decreases,

A

the number of detected x-ray photons per pixel will decrease.

56
Q

Because thicker slices allow more photons to reach the detectors

A

they have a better SNR and appear less noisy

57
Q

larger patients attenuate more x-rays photons, leaving fewer to reach the detectors

A

This reduces SNR, increases noise, and results in lower contrast resolution.

58
Q

refers to how rapidly data are acquired

A

Temporal resolution

59
Q

It is controlled by gantry rotation speed, the number of detector channels in the system, and the speed with which the system can record changing signals.

A

Temporal resolution

60
Q

Temporal resolution is typically reported in

A

milliseconds (ms)

61
Q

is of particular importance when imaging moving structures (e.g., heart) and for studies dependent on the dynamic flow of iodinated contrast media (e.g., CT angiography, perfusion studies).

A

High temporal resolution

62
Q

designed to ensure that the CT system is producing the best possible image quality using the minimal radiation dose to the patient

A

Quality control programs

63
Q

Quality assurance programs should be designed around three basic concepts:

A
  1. The tests that make up the program must be performed on a regular basis
  2. The results from all tests must be recorded using a consistent format
  3. Documentation should indicate whether the tested parameter is within specified guidelines
64
Q

can be calculated from the analysis of the spread of information within the system using the MTF.

A

Spatial Resolution

65
Q

given as the maximum number of visible line pairs (lead strip and space) per millimeter

A

Spatial Resolution

66
Q

The spatial resolution of current scanners when images are reconstructed in a high-resolution algorithm is in the range of

A

10 to 20 lp/cm

67
Q

To evaluate contrast resolution a phantom is used that contains

A

objects of varying sizes

68
Q

At the minimum, contrast resolution should be such that with a density difference of

A

0.5% a 5-mm object can be displayed

69
Q

Measurements of selected slice thickness are determined using a phantom

A

that includes a ramp, spiral, or step-wedge.

70
Q

For a slice thickness of 5 mm or greater

A

the slice thickness should not vary more than ±1 mm from the intended slice thickness.

71
Q

For a slice thickness of less than 5 mm

A

the slice thickness should not vary more than ±0.5 mm.

72
Q

Slice Thickness Accuracy

test is usually performed

A

semiannually

73
Q

Contrast Resolution (Low-Contrast Resolution) test is performed

A

monthly in most programs

74
Q

are used extensively for patient positioning and alignment

A

Laser lights located both inside and outside the gantry

75
Q

The light field should coincide with the radiation field to within

A

2mm

76
Q

Laser Light Accuracy test is usually performed

A

semiannually

77
Q

Phantom used in noise and uniformity test

A

water phantom

78
Q

is measured by obtaining the standard deviation (SD) of the CT numbers within a region of interest (ROI)

A

Noise

79
Q

refers to the ability of the scanner to yield the same CT number regardless of the location of an ROI within a homogeneous object

A

Uniformity

80
Q

refers to the relationship between CT numbers and the linear attenuation values of the scanned object at a designated kVp value

A

Linearity

81
Q

Phantoms used for Radiation Dose

A

made using standard head and body CT dose index (CTDI) phantoms and a pencil ionization chamber.

82
Q

are defined as anything appearing on the image that is not present in the object scanned.

A

Artifacts

83
Q

Artifacts can be broadly classified as:

A
  • physics-based (resulting from the physical processes associated with
    data acquisition),
  • patient-based,
  • or equipment-induced
84
Q

X-ray beam passes through an object, lower-energy photons are preferentially absorbed, creating a “harder” beam.

A

Beam Hardening

85
Q

The beam is hardened more by

A

dense objects (e.g., more by bone and less by fat).

86
Q

Two types of artifact can result from this effect (beam hardening),

A

cupping artifacts (the periphery of the image is lighter) and the appearance of dark bands or streaks between dense objects in the image

87
Q

CT systems use three features to minimize beam hardening:

A

filtration, calibration correction, and beam hardening correction software

88
Q

The best strategy available to the operator to avoid beam hardening is to select the appropriate

A

SFOV to ensure the correct filtration, calibration, and beam-hardening correction software is used.

89
Q

can occur when dense objects lie to the edge of the SFOV and are only present in some of the views used to create the image

A

Partial volume artifacts

90
Q

The best method of reducing partial volume artifacts

A

Use thinner slices

91
Q

Insufficient projection data (for instance, when the helical pitch is greatly extended) is known as

A

undersampling

92
Q

Undersampling causes inaccuracies related to reproducing sharp edges and small objects and results in an artifact known as

A

aliasing

93
Q

in which fine stripes appear to be radiating from a dense structure.

A

Aliasing

94
Q

Aliasing artifacts can be combated by

A

slowing gantry rotation speed (i.e., increasing scan time) or by reducing the helical pitch

95
Q

results in streak artifact or shading (both light and dark) arising from irregularly shaped objects that have a pronounced difference in density from surrounding structures

A

The edge gradient effect

96
Q

typically appear as shading, ghosting (objects appear to have a shadow), streaking or blurring

A

Motion artifact

97
Q

Manufacturers have built features into the CT systems to reduce motion artifacts such as

A

overscan and partial scan modes, software correction, and cardiac gating

98
Q

presence of metal objects in the scan field can lead to severe streaking artifacts.

A

Metallic artifacts

99
Q

They occur because the density of the metal is beyond the normal range that can be handled by the computer, resulting in incomplete attenuation profiles.

A

Metallic artifacts

100
Q

caused by anatomy that extends outside of the selected SFOV

A

Out-of-field artifacts

101
Q

occur with third-generation scanners and appear on the image as a ring or concentric rings centered on the rotational axis.

A

Ring artifacts

102
Q

A common cause of equipment-induced artifact occurs when there is an undesired surge of electrical current (i.e., a short-circuit) within the x-ray tube.

A

Tube Arcing

103
Q

lines appear in a windmill formation

A

Cone beam effect

104
Q

Occur in helical scanning attributable to the helical interpolation and reconstruction process

A

Helical and Cone Beam Effect

CT Scan (8 decks)

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