CT

Question 1

what is CT used for

Answer 1

localisation NOT verification

Question 2

higher HU units have a … density

Answer 2

higher Z/density

Question 3

what value is HU for soft tissues

Answer 3

close to 0

Question 4

what is the HU value for bone

Answer 4

middle hundreds - low thousands

Question 5

what is the HU value for air

Answer 5

• middle hundreds - low thousands

Question 6

what type of anode does CT use

Answer 6

rotating

Question 7

why is CT used

Answer 7

• identification of structures (anatomical transverse slicing)
• identification of extent and surrounding tissues
• visualisation of targets with respect to other 3D structures
• outlines tumours
• BEV
• symmetrical and asymmetrical images
  follow a radiation path and anatomical position
• beam shaping
• adding beams
• dose volume histogram (DVH)
• DICOM: transfers info between computers
• monitor unit calculations
• export plan to linac
• export DRR for online assessment or treatment accuracy
• information of radiation dose and how the tissue absorbs it

Question 8

what are the advantages of HU

Answer 8

• contrast resolution
• windowing
• planar images and localisation
• 3D fast image acquisition
• reconstruction of non standard planes

Question 9

what is HU

Answer 9

a quantitive measure of radio sensitivity in evaluating CT scans

Question 10

what are the advantages in diagnosis

Answer 10

• appropriate management
• CT-sim
• reduction in hospital times
• determining treatment
• reducing exploration surgery
• improvement in diagnosis and treatment
• follow up treatment

Question 11

what are the disadvantages in diagnosis

Answer 11

• cost
• access
• radiation dose

Question 12

why is MRI better than CT

Answer 12

with soft tissue contrast, so can differentiate between tumour boundaries and normal tissue

Question 13

CT scanners:

Answer 13

• only treatment area is imaged
• X-ray tube is in the gantry as well as detectors, mechanical supports and scanner housing
• xray tube converts electrical input into x-rays, the accelerated electrons hit the tungsten target, emitting x-rays. if it has insufficient energy, majority will turn into heat
• long continuous exposure, which needs to be energised throughout

Question 14

why is CT-MRI used?

Answer 14

• excellent spatial resolution
• geometrical reproducibility
• shorter imaging times
• accurate electron density

Question 15

why is a metal ceramic tube used

Answer 15

ceramic is used due to the electrical arcing from the tungsten increasing the tubes lifespan. It insulates the high voltage parts for a more compact design. Enabling higher currents reduces off focus radiation. As well an alloy of chromium and iron is used.

Question 16

describe the max rotalic ceramic x-ray tube

Answer 16

the design contains spiral grooves with a metal alloy which acts as a lubricant. It has a long life and the high output with the rotating anode being cooled directly. The continuous rotation prevents waiting time during and between examination

Question 17

describe the aquiline X-ray tube

Answer 17

it is a high capacity multi-slice tube which has a high heat storage capacity/ cooling rate. Which has a built in grounded hood

Question 18

what are detector properties

Answer 18

• high efficiency
• small physical size
• cheap
• narrow gaps between active elements
• high dynamic range
• fast response

Question 19

describe the Stratton x-ray time

Answer 19

• zero heat capacity storage
• the entire tube rotates around the anode axis
• bearing is locates outside the tube - cooling faster
• uses magnetic deflection coil which shapes and controls the beam

Question 20

what is the role of a photon diode

Answer 20

it collects the flash of light which is converted to an electrical impulse which is then turned into an image. The solid scintillator coverts x-rays into visible light photons.
- high detection efficiency
- high geometrical efficiency
- small size

Question 21

what is dose determined by

Answer 21

exposure factors and volume
dose = total mAs x volume irradiated

Question 22

what is back projection

Answer 22

collects data which combines it and plays it back

Question 23

what is a filter

Answer 23

a maths process which adds, subtracts and multiplies
a basic filter adds data (density) to one side of the boundary and subtracts it from the other

Question 24

what is the attenuation coefficient

Answer 24

reduction in intensity by the material

Question 25

what does edge enhancement do?

Answer 25

maximises small details

Question 26

describe the 3rd gen scanner

Answer 26

x-ray hits a row of detectors, which rotate together to image at a different angle. This is repeated until a single slice is taken, which continuously rotates. It has an arc of detectors opposite the tube

Question 27

describe the 4th gen scanner

Answer 27

it has a fixed rotation, with a complete ring of detectors. The source rotates to capture a slice

Question 28

describe the spiral CT

Answer 28

it has a spiral movement which is achieved simultaneously by the tube rotation and the linear movement of the patient

Question 29

describe the multi-slice CT

Answer 29

it has more than one bank of detectors, which produces two or more slices in one rotation, however this has a negative impact.

Question 30

what is window level

Answer 30

tissue which occupies the midpoint of the available grey levels

Question 31

what is window width

Answer 31

the range of tissues which occupies a range of grey levels

Question 32

describe windowing

Answer 32

- it changes the density relationship between tissues - short range = high contrast - changes tissue types

Question 33

what is the conventional bore size and what is the bore size used

Answer 33

conventional = 70cm (to minimse scatter) 85cm is used reducing the quality due to scatter

Question 34

what is the role of collimators

Answer 34

to produce a narrow beam, with no divergence and little scatter

Question 35

what can cause a change in view

Answer 35

metal artefacts

Question 36

what is the role of PET

Answer 36

to visualise the spatial distribution between the metabolic and biochem activity