Fundamentals of CT

Question 1

What are the main principles of CT?

Answer 1

X-ray tube
Patient
Fan of x-rays
X-ray detectors

Question 2

What are the effects of a large focus spot?

Answer 2

Blurring

Geometric unsharpness

Question 3

What focal spot should an X-ray have?

Answer 3

Small focal spot-high load/heat

Good cooling system

Question 4

What does pre collimation determine?

Answer 4

Slice thickness

Question 5

What does post collimation limit?

Answer 5

Amount of scatter “noise” reaching the detector.

Question 6

What does filtration do?

Answer 6

Filters excess intensity at edges of projection

Question 7

What are the essential components of CT?

Answer 7

X-ray tube
Collimation
Filtration

Question 8

What do gas detectors do?

Answer 8

Ionise Xenon and electrons are attracted to the conductor producing an electrical signal.

Question 9

What do solid state detectors do?

Answer 9

Crystals absorb radiation

Connected to a photomultiplier tube

Light is proportional to radiation

Question 10

What are the advantages of SSD?

Answer 10

Fast
Stable
Small

Question 11

How many detectors did a 1st Gen CT have?

Answer 11

Single

rotated round to fixed angles to acquire data

Question 12

How many detectors dis the 2nd gen have?

Answer 12

Multiple (up to 30)

several projections at once

Question 13

How many detectors dis the 3rd gen have?

Answer 13

Large bank (100S)
Continously rotating fan
Bowtie filter

Question 14

What did the bowtie filter control?

Answer 14

Excessive variations in signal strength.

reduce skin dose/beam hardening

Question 15

How many detectors dis the 4th gen have?

Answer 15

Ring of fixed detectors
X-ray tube rotates 360 degrees

Restrained to two rotations due to power cords

Question 16

What does spiral/helical scanning allow?

Answer 16

Allows continuous rotation

Move patient in z direction allows large quantities of data.

Question 17

What do multi-slice scanners allow

Answer 17

3rd or 4th gen
Multiple banks of detectors
Several slices of acquisiton per gantry rotation

Question 18

What is the process of image acquisition?

Answer 18

Measure average LAC between X-ray and detector (generates a projection)

Question 19

What is a projection?

Answer 19

Tube and detector are rotated a small amount and LAC is calculated (Voxel)
650-900 detectors
1-2000 projections

Question 20

What is spiral CT?

Answer 20

Gantry continuously rotating in 1 direction on slip ring

Question 21

What is the benefit of faster CT rotations?

Answer 21

Less patient motion artefacts.

Question 22

What is spiral pitch?

Answer 22

Ratio of patient movement through the gantry relative to beam collimation (slice thickness)

Question 23

Spiral pitch (equation)

Answer 23

Table movement per rotation/collimation

or slice thickness/collimation

Question 24

What is an increment?

Answer 24

Distance between slices

Question 25

What is the process of reconstructing images?

Answer 25

filtered back projection and adding information

Question 26

What is back projection?

Answer 26

overlaying multiple images over each other at different angles to improve quality

Question 27

What are some common CT artefacts?

Answer 27

Streaks Shading Rings Distortion

Question 28

What causes streak artefacts?

Answer 28

-Patient motion -High density materials Noise -Mechanical failure

Question 29

What causes ring artefacts?

Answer 29

Bad detector channels

Question 30

What is the partial volume effect?

Answer 30

Averaging a variety of CT densities causing blurring.

Question 31

What affect PVE?

Answer 31

Slice thickness | and spiral pitch

Question 32

What are some CT imaging Limitations?

Answer 32

2D representation of a 3D mobile object | Resolution limited to finite slice thickness and scatter processes

Question 33

What are the CT numbers for heart, brain, blood, liver, muscle and spleen?

Answer 33

``` Heart 10-60 brain 20-40 blood 20 liver 20-80 Muscle 50 Spleen 40-60 CSF 15 white matter 45 Grey matter 40 ```

Question 34

What are the CT numbers for Bone and metal?

Answer 34

Bone 150- 500 Bone 350-1000 dense Metal >2000

Question 35

What is the CT number for water?

Answer 35

0

Question 36

What is the CT number for fat lung and Air?

Answer 36

fat -100 lung -200 Air -1000

Question 37

CT apertures for diagnostic and therapy?

Answer 37

70-90cm diagnostic | 90-105cm therapy

Question 38

What is a scout image

Answer 38

Standard area projection image with relevant anatomical structures and localised relative to the couch

Question 39

What are the advantages of a scout?

Answer 39

Provides reference for slice data acquisition and display Initial planning is performed on the scout Scanning of desired anatomical section

Question 40

Production of scout?

Answer 40

Position patient Set limits Zero slice

Question 41

What are some characteristics of a scout image?

Answer 41

Tube and detectors are fixed or stationary Couch moves at low speed Low dose exam Low spatial and contrast resolution

Question 42

What are two common image reconstruction algorithms?

Answer 42

Filtered back projection | Interpolation

Question 43

What are two types of image reconstruction?

Answer 43

Prospective and retrospective reconstruction

Question 44

Is lower pitch better for resolution in helical scanning?

Answer 44

Yes

Question 45

What are some types of image manipulation?

Answer 45

Multi-planar reformation 3D reconstruction Volume rendered 3D 3D reconstruction

Question 46

What does PET provide?

Answer 46

Functional and metabolic information