CT

Question 1

What is the usual HU for water/urine/CSF?

Answer 1

Around 0

Question 2

What is the usual HU for Bone?

Answer 2

500-1000

Question 3

What is the usual HU for fat?

Answer 3

-20 to -100

Question 4

What is the usual HU for metal?

Answer 4

> 2000

Question 5

What is the typical tube voltage of CT in adults?

Answer 5

120 kVp, mean energy of 60 keV

Question 6

What is the typical tube voltage of CT in paediatrics?

Answer 6

80 kVp, mean energy of 40 keV

Question 7

What are the focal spot sizes in CT and X-ray? (Same)

Answer 7

0.6mm, 1.2mm

Question 8

What is the predominant mode of interaction in CT?

Answer 8

Compton scatter

Question 9

What determines the slice thickness in single-slice CT?

Answer 9

Collimator width

Question 10

What determines the slice thickness in multi-slice CT?

Answer 10

Detector size

Question 11

What level of filtration and of what material are used in CT?

Answer 11

Up to 6mm Al/ Cu

Question 12

What is the least significant type of noise in CT?

Answer 12

Electronic noise

Question 13

What happens to image contrast with increased window width?

Answer 13

Reduced contrast

Question 14

CT: What happens to spatial resolution with increased window width?

Answer 14

Window width does not affect spatial resolution

Question 15

How to reduce partial volume effect? (3)

Answer 15

• decrease slice thickness
• decrease field of view
• decrease pixel size/increase matrix size

Question 16

The CT number represents the average…

Answer 16

Average LAC of tissues in the voxel

Question 17

What effect does thick slices have in noise?

Answer 17

Less noise

Question 18

CT detector arrays use what type of scintillators in their detectors?

Answer 18

Ceramic scintillation detectors (gadolinium oxysulphide)

Question 19

How do CT detectors work?

Answer 19

Scintillation layer detects attenuated X-rays and then emits light, converted to electricity by photodiodes

Question 20

How do we eliminate multi slice CT overbeaming?

Answer 20

We need to increase the width of the Xray beam to allow the penumbras to fall to the side of the active detectors

Question 21

CT pitch = what?

Answer 21

Table feed per rotation/width of acquired slices
It’s a Measure of overlaps

Question 22

CT pitch - what does it mean when it’s <1, 1 and >1?

Answer 22

Pitch < 1 = overlapping slices
Pitch 1 = complete sampling no overlap
Pitch > 1 = gaps in data acquired

Question 23

What is CT overranging ?

Answer 23

Getting additional data on either end of the scanned area to be able to reconstruct the image

Question 24

CT filtered back projection: choosing filters is a trade off between…?

Answer 24

Noise and resolution, which are proportional

Question 25

Drawbacks of filtered back projection?

Answer 25

Streak artefact Susceptible to Poisson noise

Question 26

Quantum noise in CT is reduced by...? (3)

Answer 26

Use thicker slices Increase kVs Increase mAs

Question 27

How do we decrease structural noise? (in CT)

Answer 27

increase number of projections used to create image

Question 28

When does beam hardening artefact commonly occur?

Answer 28

At bone:soft tissue junctions

Question 29

CT Number is dependent on which 2 factors?

Answer 29

CT Number is the average LAC of the tissues. It depends on kVP and filtration

Question 30

CT Spatial resolution depends on what...? (5) 1. … focal spot 2. … number of smaller detectors 3. … scan time 4. … pixels 5. Lack of …

Answer 30

1. Small focal spot 2. Increased number of smaller detectors 3. Increasing scan time 4. Small pixels, for finer sampling 5. No motion/artefacts

Question 31

CT Noise is generally increased by...? (2)

Answer 31

1. less x-rays hitting detector 2. increased matrix size

Question 32

CTDI measurements units?

Answer 32

mGy CTdI= Area under axial dose profile/slice width

Question 33

What is CTDI?

Answer 33

Average dose delivered to a standard plastic phantom

Question 34

what is the relationship between CTDI and current (mA)?

Answer 34

Proportional to each other.

Question 35

What effect does pitch have on CTDI?

Answer 35

High pitch = faster scan = Low CTDI

Question 36

CT imaging dose is proportional to...? (1)

Answer 36

1. Tube current Dose increased by anything that causes more x-rays to be detected

Question 37

Typical CT spatial resolution is…? Mins Pairs per cm?

Answer 37

10mins pairs per cm. (1lp/mm therfor less SR than plain film)

Question 38

CT gantry speeds can travel up to…

Answer 38

5 revs per second

Question 39

When is CT overbeaming more of a problem?

Answer 39

When the X-ray beam is narrow, so the penumbras make up relatively more of the dose the patient is receiving

Question 40

Iterative reconstruction advantages?

Answer 40

Very accurate, fewer artefacts

Question 41

How does Hybrid iterative reconstruction work?

Answer 41

Initial image generated with FBP Iterative methods are then used to optimise image characteristics eg noise

Question 42

Advantages of Hybrid Ir? (4)

Answer 42

Less patient dose Less noise Better than FBP alone faster than IR alone

Question 43

What is DLP in terms of Ct What is the equation?

Answer 43

Scan length x average CTDI volume A measure of the total dose in a CT study

Question 44

Will thin Ct slices increase or decrease contrast?

Answer 44

Increase

Question 45

How to decrease Noise in CT? (5)

Answer 45

1. Bigger pixels 2. Thicker slices 3. Thinner patients 4. More X-rays (kvP, mA, filtration) but will affect dose 5. Soft reconstruction filters

Question 46

How to reduce beam hardening artefact in Ct?

Answer 46

Use filtration or correction software

Question 47

CT: What does Cone beam artefact do

Answer 47

Can cause streaking in images, especially in outer detector rows