CT

Question 1

Doses

Answer 1

• CT Head - 2mSv
• CT Chest - 7mSv
• CT Abdo - 8mSv
• CTPA - 10mSv
• CT Abdo/Pelvis - 15mSv

Question 2

Partial Volume

Answer 2

Makes high contrast objects appear larger

Makes low contrast objecto appear smaller

Question 3

Beam Hardening

Answer 3

Causes a reduction in houndsfield units towards the centre of the image

Question 4

Anode-cathode axis

Answer 4

Parallel with centre of scanner

In Z axis

Question 5

Fan Beam

Answer 5

Is perpendicular to the anode-cathode axis

Question 6

What is resolution in CT?

Answer 6

Measured in line pair/cm

7lp/cm in the transaxial plane

When directly compared with radiography:

1lp/mm vs 3lp/mm for radiography

Question 7

Is contrast resolution better in CT than plain film?

Answer 7

Yes contrast resolution better in CT

(Spatial resolution better in plain film)

Question 8

Does a narrow window increase or decrease noise?

Answer 8

Increases noise

Smaller FOV = less signal and more noise

Question 9

What is anode cathode axis in CT?

Answer 9

Parallel to the axis of the scanner to minimise heel effect

Question 10

What filtration is used in CT?

Answer 10

6mm of aluminium

or

0.5mm Copper which is equilivent

In newer scanners:

• 3mm aluminium
• 0.1mm copper

Total filtration 6mm equivilent

Question 11

What collimation distance is used in CT?

Answer 11

50cm usually sufficient for most patients

Question 12

Are detectors separated in CT?

Answer 12

Yes, this is to reduce light crossover

Reduces detection efficiency however (to 80%)

Question 13

What is cone beam effect due to?

Answer 13

Due to divergence of beam in Z axis

Usually problematic when there are more than 4 slices

Increased number of slices = worse cone beam effect

Affects outer detectors rows more

Question 14

For same mA and kV, does slice thickness affect dose?

Answer 14

No

Dose is independent of slice thickness

Question 15

In an obese patient, is kV or mA increased? or both?

Answer 15

Just mA is increased

Question 16

Which part of patient receives higher dose?

Answer 16

Periphery

Question 17

Which scanner produces more scatter? 1st or 3rd gen

Answer 17

Third generation

Question 18

Are detector crystal embedded in a matrix in CT?

Answer 18

NO

Question 19

What is pixel size formula?

Answer 19

Pixel size = FOV /matrix size

Question 20

In single slice scanners, does pitch affect noise?

Answer 20

NO

Question 21

What is CTDIw

Answer 21

CTDI_w = ^1/3CTDI centre + ^2/3CTDI periphery

Units = mGy

Question 22

What is CTDI vol?

Answer 22

CTDI_w / Pitch

Units = mGy

Question 23

What is DLP?

Answer 23

CTDI_vol x Total Scan length

Units mGy/cm

Question 24

What is effective dose?

Answer 24

DLP x fiddle factor (dependent on body part)

Units are mSv

Question 25

How to calculate noise in CT?

Answer 25

1/ square root of number of photon Inversely proportional to the square root of number of photons

Question 26

What effect will doubling of mA have on the noise?

Answer 26

Will decrease noise by a factor of: **square root of 2** Halving rotation time also decreases noise by the square root of 2

Question 27

What is filter interpolation and 2 point interpolation used in?

Answer 27

Filter interpolation = Multislice scanners Two point interpolation = single slice scanners

Question 28

What is normal CT matrix size?

Answer 28

**512 x 512** For high resolution = 1024 x 1024 Fluoro CT uses 256 x 256 for reconstruction

Question 29

How to calculate CT number?

Answer 29

Question 30

What measures can be taken to improve cupping artefact?

Answer 30

Cupping artefact is otherwise known as beam hardening Made better by: * **Bow tie filter** * **Algorithms**

Question 31

Which has more afterglow? Sodium iodide vs Bismuth germinate

Answer 31

Sodium iodidie has more afterglow Bismuth germinante is more preferable

Question 32

Which CTDI most closely corresponds with absorbed CT dose?

Answer 32

**CTDI_vol** ## Footnote **CT dose = mA/pitch**

Question 33

Is effective dose measured on every CT scan?

Answer 33

**No**

Question 34

What is pixel size in CT?

Answer 34

**0.5mm usually**

Question 35

In CT does magnification increase spatial resolution?

Answer 35

Yes

Question 36

Can CT gantry tilt in the vertical axis?

Answer 36

Yes It can tilt up to 30 degrees in the vertical axis

Question 37

What is heat capacity in CT?

Answer 37

4MJ

Question 38

What is CT fluoroscopy used for?

Answer 38

Needle guidance in IR procedures * 180 degree rotation * low dose technique * higher skin dose than normal CT * lower effective dose

Question 39

When is post patient collimation used?

Answer 39

Used in **single slice scanners** Used to reduce scatter Results in increased dose

Question 40

Do 4th gen scanners result in a higher dose?

Answer 40

Yes The patient to detector distance is greater They have 6x the amount of detectors than other scanners Calibration can be adjusted through each scanning cycle