CT physics 2

Question 1

What two things does dose describe?

Answer 1

1) Energy desposited in patient from attenuation and scatter
2) Stochastic risk

Question 2

Units for dose regarding stochastic risk?

Answer 2

mSv

Question 3

Units for dose regarding energy deposition in patient?

Answer 3

mGy

Question 4

Is it the photoelectron or initial ionisation of the atom in tissue that gives most of dose?

Answer 4

Photoelectron

Question 5

What is the process of energy deposition called as the photoelectron travels through tissue?

Answer 5

Linear Energy Transfer

Question 6

What is photon fluence?

Answer 6

Number and average energy of the xrays

Question 7

Emission vs exposure?

Answer 7

Emission is just photons being released
Exposure is those photons then interacting

Question 8

How to measure exposure with ionisation chamber?

Answer 8

1) Electrodes at either end of gas chamber and charge difference made between electrodes, connected to voltimeter.
2) Send xrays through the chamber
3) Electrons are liberated and head to anode
4) Number of electrons released by ionising radiation is proportional to volts.

Question 9

What is exposure in ionisation chamber measured in?

Answer 9

Coulombs per kilogram (C/kg), the preferred SI unit, or the legacy unit, roentgens (R).

Question 10

How to convert charge deposited into tissue (C) into energy deposited into tissue? What is the unit for this?

Answer 10

Use KERMA (Kinetic Energy Released per Unit Mass) and unit is Gray (Gy (J/Kg)

Question 11

Equation for exposure (X)?

Answer 11

X = Q/m (Q is charge released, m is mass of tissue or chamber

Question 12

Absorbed dose (mGy) is in what unit?

Answer 12

mGy

Question 13

Does absorbed dose account for risk, energy deposition or both?

Answer 13

Energy deposition only

Question 14

If a patient is smaller, will their absorbed dose be higher or lower?

Answer 14

Higher

Question 15

Equivalent dose accounts for what?

Answer 15

Type of radiation

Question 16

What is the unit of equivalent dose?

Answer 16

mSv

Question 17

How to calculate equivalent dose (Ht)?

Answer 17

Ht = Dt x Wr
Ht = equivalent dose
Dt = absorbed dose (mGy)
Wr = weighting facto (mSv/Gy)

Question 18

What is the weighting factor of electrons in CT

Answer 18

1

Question 19

What is the weighting factor if electrons in alpha particles used in nuclear medicine?

Answer 19

20

Question 20

Do electrons in CT or alpha particles in nuclear medicine have a lower LET

Answer 20

Electrons in CT

Question 21

What is the weighting factor telling you?

Answer 21

For the same absorbed dose (Dt), between xrays and alpha particles, same J/kg deposited in tissue, that patient is 20 times more likely to get stochastic effect from an alpha particle than an xray of same absorbed dose.

Question 22

Does equivalent dose account for patient sensitivity?

Answer 22

No

Question 23

What does effective dose tell you?

Answer 23

Tissue type sensitivity

Question 24

What is the unit for effective dose?

Answer 24

mSv

Question 25

How is effective dose calculated?

Answer 25

Using tissue weighting factor (Wt) table from International Commission on Radiological Protection. Add up all organs being examined and calculate their absorbed dose (mGy). Then multiply by Wt on table.

Question 26

What unit is CTDI measure in?

Answer 26

mGy

Question 27

What does CTDI tell you?

Answer 27

An index you look up, specific to machine allowing for comparison of energy being released between different machines towards a phantom in a single gantry rotation

Question 28

What is phantom made of?

Answer 28

Polymethyl methacrylate (PMMA)

Question 29

What is the size of abdomen phantom?

Answer 29

32 cm

Question 30

What is the size of head phantom?

Answer 30

16 cm

Question 31

How are the phantoms used in CTDI calculation?

Answer 31

Two gas ionisation chambers of 10cm are placed at centre and periphery of phantom. PMMA attenuates the xrays then CTDI100, CDTIw, CDTIvol and DLP are measured.

Question 32

What is CTFI100? What is it's unit?

Answer 32

Calculate dose for peripheral then central dosimeter during one rotation (100 bc dosimetres are 10cm).

Question 33

How to measure CDT1w? Unit?

Answer 33

CTDIw = 1/3 CTDIcentre + 2/3 CTDIperiphery Unit is mGy

Question 34

Is there more dose at the periphery or the centre and why? How is this accounted for?

Answer 34

Periphery because source rotates. Account for by calculating CDTIw

Question 35

Difference between dose at aquisition and dose during CTDIw calculation?

Answer 35

Patient is moving during acquisition so dose spread along Z axis but CTDIw calculated in one rotation only.

Question 36

If you double pitch, what will happen to dose?

Answer 36

Dose will halve Dose = 1/pitch

Question 37

What does CTDIvol tell you? Units?

Answer 37

CTDIw in the volume being scanned as the patient moves through.

Question 38

What is CTDIw proxy for?

Answer 38

Absorbed dose in a single slice.

Question 39

If the pitch is lower is the CTDIvol higher or lower and why?

Answer 39

Higher because dose less spread

Question 40

What does DLP tell you? Unit?

Answer 40

Tells you absorbed dose across scan length Unit is mGy-cm

Question 41

How to calculate DLP?

Answer 41

DLP = CTDIvol x scan length

Question 42

Four ways to estimate effective dose?

Answer 42

1) DLP conversion looking up K factor for different body parts and multiply by DLP 2) Size specific correction (measure AP diameter and look up table for effective dose of patient of that diameter) 3) Mathematical modelling of each voxel eg) montecarlo model 4) Lifetime risk estimation eg) Beir VII

Question 43

What four factors influence CT dose?

Answer 43

kVP, filtration, pitch and tube current modulation (mA)

Question 44

If kVp is increased, what happens to pitch and why?

Answer 44

If kVp increased, number and electrons and their average energy is increased

Question 45

What is dose roughly proportional to what in kVP?

Answer 45

Dose roughly proportional to kVPn where n is approximately 2.5

Question 46

How does filtration infleunce dose?

Answer 46

Filtration removes lower energy photons that would otherwise only contribute to dose

Question 47

Does filtration increase or lower or no change to average enregy?

Answer 47

Increases average energy

Question 48

Does filtration increase or decrease or not change quality?

Answer 48

Improve quality

Question 49

What is the effect of pitch on dose?

Answer 49

The higher the pitch, the lower the dose

Question 50

What three things can you do to change the pitch in order to reduce dose

Answer 50

Increase table speed Smaller cone angle (smaller isocentre beam width) Increase collimation

Question 51

What does angular current modulation do?

Answer 51

Increases or decreases tube current depending on thickness and density of patient part. Works this out on scout image.

Question 52

Does lower tube current increase or lower dose

Answer 52

Lower

Question 53

What is tube current?

Answer 53

Current going through the tungsten filament in cathode so thermionic emission can happen

Question 54

What does z axis mAa modulation do?

Answer 54

Increases or decreases tube current depending on density

Question 55

What is shielding made of to reduce dose?

Answer 55

Bismuth

Question 56

Does increased coverage increase or decrease dose?

Answer 56

Increased

Question 57

If patient is placed closer to the source than isocentre, what happens to dose and noise?

Answer 57

Noise and dose increased

Question 58

If a patient is placed closer to the source, what happens to the scout image?

Answer 58

It's larger

Question 59

True or false dose is proportional to kVp2

Answer 59

True