Topic 20: CT lecture slides

Question 1

What kind of attenuation dominates in CT?

Answer 1

Compton scatter, which relates to the electron density

Question 2

How does CT work?

Answer 2

1. images are taken all around a patient 2. Data is reconstructed 3. A tomographic slice through the patient is formed

Question 3

Why does the X-ray source need high loading?

Answer 3

operate at high energies (12-140kV

Question 4

Detectors need to be what?

Answer 4

Fast- because gantry rotates High absorption efficiency Small and compact High stability Large dynamic range

Question 5

How does a Xenon ionisation chamber detect?

Answer 5

• X-rays ionise gas - Electric field attracts ions - Charge collected proportional to x-ray intensity

Question 6

Problem with xenon ionisation chamber detector?

Answer 6

Quantum efficiency of gas low. Uses high Z gas Gas at high pressures Make detector long Final efficiency 60%-70%

Question 7

What is used instead of Xenon ionisation chamber detectors?

Answer 7

Solid state detectors, which consists of a scintillator and photodiode which has a higher absorption efficiency however may have dead space between pixels.

Question 8

Weigh up the first generation CT scanners.

Answer 8

Con: Slow and therefore motion artifacts Pros: easy calibration low cost high scatter rejection due to beam collimation true parallel-beam image geometry

Question 9

why did people like the first CT scanner?

Answer 9

1. it had loads of contrast resolution! 2. It has decreased structural noise. you know what feature and tissue type is where! high diagnostic value

Question 10

what did they do to make CT faster? and what was the problem that arose in the third generation?

Answer 10

made the aperture wider and rotated, but ring artifacts started to appear, because different pixel recieve responses (errors) were back-projected and rotate around.

Question 11

how did they solve the problem with the third generation?

Answer 11

source was rotated and pixels were remained still.

Question 12

how can you characterise the spiral CT?

Answer 12

pitch = table travel per rotation/slice width

Question 13

what is the trade off when deciding the pitch?

Answer 13

higher pitch means lower radiation dose but at the expense of partial volume effect (loss of detail)

Question 14

No single axial slice is entirely irradiated so you are missing information so you need to solve this with……

Answer 14

interpolation

Question 15

What is a multi-slice CT?

Answer 15

Replacing array with a matrix which means you can aquire data faster

Question 16

How much faster is a multi-slice ct?

Answer 16

by the number of rows in the array

Question 17

what is the problem with multislice CT?

Answer 17

Apart from the central one, slices are seen from a angle, which causes artifacts when these are reconstructed

Question 18

thick vs thin slices?

Answer 18

thick slices have lower noise, and thin slices reduce partial volume effects and allow off axis image creation with isotropic resolution.

Question 19

mu for each pixel is converted to a CT number. how do you calculate the ct numbers?

Answer 19

if k = 1000 it is in hounsfield units.

Question 20

what is the hounsfield unit for air and water?

Answer 20

air = -1000

water = 0

Question 21

how does the window and level affect the image?

Answer 21

level - is like your base white 0

window is your max white and min

Question 22

What affects image quality

Answer 22

Image constrast

Spatial resolution

Noise - which is affected by tube current (how many photons), scan time and slice thickness?

Artifacts

Question 23

What do we look at in quality assurance? (not necessary to remember all)

Answer 23

noise,

spatial res

contrast scale

sensitivity

dose

alignment

slice thickness

Question 24

Types of artifacts?

Answer 24

1. Ring
2. Motion artifacts
3. Spectral
4. Streak
5. Partial volume
6. Cone beam
7. Noise.

Question 25

How do you remove a ring artifact?

Answer 25

removed by averaging or by appropriate filters

Question 26

Spectral artifacts

Answer 26

Because beam is polychromatic

beam is hardened across patient

average energy higher in the centre and attenuation is higher for higher energies

CT numbers higher at edge

Cupping edge

Question 27

Correction of spectral artifacts ?

Answer 27

Use bow tie filter

Question 28

Streaking

Answer 28

High density objects

cause streaks

bullet, surgical pin, hip replacement

Question 29

Partial volume effects

Answer 29

brings about blurring

3-d voxel

occupied by more than one tissue type

Question 30

Cone beam artifacts

Answer 30

When the cone beam artifact is not angled right,

problem in multislice CT

Question 31

Noise as an artifact …what type of noise dominates?

Answer 31

statistical noise dominates

Question 32

how can you reduce noise in CT?

Answer 32

increase current

increase rotation time

increase slice thickness

(more photons)

Question 33

Why is the dose in CT higher than DRadiography?

Answer 33

CT reconstructing image from huge number of angular projections.

Question 34

CT dose index

Answer 34

Dose to a depth in a scanned volume for a complete series of slices

measure with a long ionisation chamber inserted into a phantom

large contribution from scatter

Question 35

Dose reduction

Answer 35

• Expose less (reduce mA or shorten scan time)
• > but this lowers constrast-to-noise ratio
• To keep CNR constant - e.g. expose less and increase slice thickness –>may affect spatial resolution
• More sophisticated: adaptively change exposure as patient anatomy changes