L6.2. CT: Image Quality and Artefacts

Question 1

First generation CT

Answer 1

Detector: 1

Type of beam: pencil-like x-ray beam

Tube-detector movements: translate-rotate

Duration of scan (average): 25-30 mins

Question 2

2nd Gen CT

Answer 2

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

Question 3

3rd Gen CT

Answer 3

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

Question 4

4th Gen CT

Answer 4

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

Question 5

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

Answer 5

The stop-and-shoot principle

Question 6

Spiral CT scanners allow for what?

Answer 6

Continuous tube-detector rotation and fast acquisition of data

Question 7

What is the EBCT?

What was it developed for?

Answer 7

(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

Question 8

Increasing the number of detector rows means that?

Answer 8

Multiple channels of data can be acquired simultaneously

Question 9

What are Dual-source CT scanners equipped with?

Answer 9

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

Question 10

What is image quality primarily determined by?

Answer 10

Detector size

Number of angular projections

Focal spot of the x-ray

Question 11

What does higher slice thickness do?

Answer 11

Reduces radiation dose

Improves SNR at the expense of axial resolution

Question 12

CT artifacts, what do they do?

Answer 12

Common, can occur for various reasons

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

Question 13

How can CT artifacts be classified?

Answer 13

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

Question 14

Patient-based artifacts:

Answer 14

-Motion artifact

-Transient interruption of contrast

Question 15

Hardware-based artifacts:

Answer 15

-Ring artifacts

-Out of Field artifacts

-Tube arcing

Question 16

Physics-based artifacts:

Answer 16

-Beam hardening

-Partial volume averaging

-Noise

-Photon starvation

-Aliasing in CT

Question 17

Helical and multichannel artifacts:

Answer 17

-Windmill artifacts

-Cone beam effect

-Zebra artifact

-Stair step artifact

Question 18

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

Answer 18

Table remains stationary

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

Question 19

Disadvantage of sequential scanning

Answer 19

Relatively long scan time

Question 20

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

Answer 20

Slip-ring technology

Question 21

What do sequential CT scanners rely on?

Answer 21

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

Necessitates unwinding of wires after each acquisition

Question 22

How do spiral CTs rotate continuously?

Answer 22

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

Question 23

Continuous acquisition of data allows for?

Answer 23

Coverage of larger sections in same time

Question 24

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

Answer 24

More heat storage capacity is required

Question 25

What does the large amount of data being produced in a very short period lead to in spiral CT scanning?

Answer 25

Expanded storage

Expanded processing capacity

Question 26

What happens in mechanical CT

Answer 26

X-ray tube and detector array physically rotate around table

Forces created during rotation restrict rotation speed and temporal resolution of mechanical

Question 27

In the absence of rotating parts, what is the temporal resolution of EBCT?

Answer 27

100 ms

Question 28

What happens to emitted x-rays from the target ring in EBCT?

Answer 28

They are collimated

Then after passing through the patients are collected by the stationary detectors on the opposite side

Question 29

When was there an enormous improvement of the resolution of mechanical CT

Answer 29

1980’s

Question 30

When is EBCT used?

Answer 30

Can be used for non-invasive coronary angiography

Most often used for quantification of coronary calcium

Question 31

Advantage of multi-detector CT acquisition?

Answer 31

Increased longitudinal coverage that can be achieved per rotation

Particularly useful for cardiac acquisitions (have longer scans than non-moving organs)

Question 32

Current CT tech simultaneously acquires how many slices?

Answer 32

320

-> $2.5 mill

320-slice scanners are physically equipped with 320 detector rows

Question 33

What other detector systems are offered?

Answer 33

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

Question 34

Single z-Sampling vs Double z-Sampling

Answer 34

Double:
-Improved longitudinal resolution

-Longer total scan time