L6.2. CT: Image Quality and Artefacts Flashcards

1
Q

First generation CT

A

Detector: 1

Type of beam: pencil-like x-ray beam

Tube-detector movements: translate-rotate

Duration of scan (average): 25-30 mins

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2
Q

2nd Gen CT

A

Detector: multiple (up to 30)

Type of beam: fan-shaped x-ray beam

Tube-detector movements: translate-rotate

Duration of scan (average): less than 90s

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3
Q

3rd Gen CT

A

Detector: multiple, originally 288; newer use over 700 arranged in an arc

Type of beam: fan-shaped x-ray beam

Tube-detector movements: rotate-rotate

Duration of scan (average): approx 5 sec

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4
Q

4th Gen CT

A

Detector: >2000 arranged in an outer ring which is fixed

Type of beam: fan-shaped x-ray beam

Tube-detector movements: rotate-fixed

Duration of scan (average): few seconds

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5
Q

Originally, all CT scanners sequentially acquired axial slices according to what?

A

The stop-and-shoot principle

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6
Q

Spiral CT scanners allow for what?

A

Continuous tube-detector rotation and fast acquisition of data

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7
Q

What is the EBCT?

What was it developed for?

A

(Electron beam CT)

Developed to image the heart

Instead of physically rotating the tube-detector unit, it generates and directs electrons along a stationary tungsten ring

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8
Q

Increasing the number of detector rows means that?

A

Multiple channels of data can be acquired simultaneously

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9
Q

What are Dual-source CT scanners equipped with?

A

2 X-ray tubes rotating at a 90 degree angle to each other

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10
Q

What is image quality primarily determined by?

A

Detector size

Number of angular projections

Focal spot of the x-ray

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11
Q

What does higher slice thickness do?

A

Reduces radiation dose

Improves SNR at the expense of axial resolution

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12
Q

CT artifacts, what do they do?

A

Common, can occur for various reasons

Can mimic pathology (eg partial volume artefact) or degrade image quality to non-diagnostic levels

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13
Q

How can CT artifacts be classified?

A

According to the underlying cause of the underlying cause of the artifact:

-Patient-based artifacts

-Hardware-based artifacts

-Physics-based artifacts

-Helical and Multichannel artifacts

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14
Q

Patient-based artifacts:

A

-Motion artifact

-Transient interruption of contrast

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15
Q

Hardware-based artifacts:

A

-Ring artifacts

-Out of Field artifacts

-Tube arcing

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16
Q

Physics-based artifacts:

A

-Beam hardening

-Partial volume averaging

-Noise

-Photon starvation

-Aliasing in CT

17
Q

Helical and multichannel artifacts:

A

-Windmill artifacts

-Cone beam effect

-Zebra artifact

-Stair step artifact

18
Q

What happens in sequential scanning during the acquisition of a slice?

A

Table remains stationary

After completion of the acquisition: table moves to new position to perform the next scan

19
Q

Disadvantage of sequential scanning

A

Relatively long scan time

20
Q

How do spiral CT scanners allow for continuous tube-detector rotation and fast acquisition of data?

A

Slip-ring technology

21
Q

What do sequential CT scanners rely on?

A

Physical connection in form of cables between the rotating elements (x-ray tube, detectors, stationary base)

Necessitates unwinding of wires after each acquisition

22
Q

How do spiral CTs rotate continuously?

A

Transmit energy and data via electronically conductive brushes ans rotating rings

Table moves at a constant speed through the gantry

Path of acquisition relative to the subject resembles a spiral

23
Q

Continuous acquisition of data allows for?

A

Coverage of larger sections in same time

24
Q

What is required due to the energy generated by the X-ray tube for an extended period? (In spiral CT)

A

More heat storage capacity is required

25
What does the large amount of data being produced in a very short period lead to in spiral CT scanning?
Expanded storage Expanded processing capacity
26
What happens in mechanical CT
X-ray tube and detector array physically rotate around table Forces created during rotation restrict rotation speed and temporal resolution of mechanical
27
In the absence of rotating parts, what is the temporal resolution of EBCT?
100 ms
28
What happens to emitted x-rays from the target ring in EBCT?
They are collimated Then after passing through the patients are collected by the stationary detectors on the opposite side
29
When was there an enormous improvement of the resolution of mechanical CT
1980’s
30
When is EBCT used?
Can be used for non-invasive coronary angiography Most often used for quantification of coronary calcium
31
Advantage of multi-detector CT acquisition?
Increased longitudinal coverage that can be achieved per rotation Particularly useful for cardiac acquisitions (have longer scans than non-moving organs)
32
Current CT tech simultaneously acquires how many slices?
320 -> $2.5 mill 320-slice scanners are physically equipped with 320 detector rows
33
What other detector systems are offered?
16, 32, 64, 128, 256 Some systems allow for twice their slice acquisition by double sampling in the longitudinal direction -> rapid, longitudinal alternation of focal spot during acquisition, partially overlapping sets of projections from slightly differing positions are acquired
34
Single z-Sampling vs Double z-Sampling
Double: -Improved longitudinal resolution -Longer total scan time