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Physics > CT > Flashcards

CT Flashcards

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1
Q

what is CT used for

A

localisation NOT verification

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2
Q

higher HU units have a … density

A

higher Z/density

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3
Q

what value is HU for soft tissues

A

close to 0

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4
Q

what is the HU value for bone

A

middle hundreds - low thousands

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5
Q

what is the HU value for air

A
  • middle hundreds - low thousands
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6
Q

what type of anode does CT use

A

rotating

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7
Q

why is CT used

A
  • 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
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8
Q

what are the advantages of HU

A
  • contrast resolution
  • windowing
  • planar images and localisation
  • 3D fast image acquisition
  • reconstruction of non standard planes
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9
Q

what is HU

A

a quantitive measure of radio sensitivity in evaluating CT scans

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10
Q

what are the advantages in diagnosis

A
  • appropriate management
  • CT-sim
  • reduction in hospital times
  • determining treatment
  • reducing exploration surgery
  • improvement in diagnosis and treatment
  • follow up treatment
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11
Q

what are the disadvantages in diagnosis

A
  • cost
  • access
  • radiation dose
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12
Q

why is MRI better than CT

A

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

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13
Q

CT scanners:

A
  • 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
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14
Q

why is CT-MRI used?

A
  • excellent spatial resolution
  • geometrical reproducibility
  • shorter imaging times
  • accurate electron density
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15
Q

why is a metal ceramic tube used

A

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.

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16
Q

describe the max rotalic ceramic x-ray tube

A

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

17
Q

describe the aquiline X-ray tube

A

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

18
Q

what are detector properties

A
  • high efficiency
  • small physical size
  • cheap
  • narrow gaps between active elements
  • high dynamic range
  • fast response
19
Q

describe the Stratton x-ray time

A
  • 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
20
Q

what is the role of a photon diode

A

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

21
Q

what is dose determined by

A

exposure factors and volume
dose = total mAs x volume irradiated

22
Q

what is back projection

A

collects data which combines it and plays it back

23
Q

what is a filter

A

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

24
Q

what is the attenuation coefficient

A

reduction in intensity by the material

25
Q

what does edge enhancement do?

A

maximises small details

26
Q

describe the 3rd gen scanner

A

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

27
Q

describe the 4th gen scanner

A

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

28
Q

describe the spiral CT

A

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

29
Q

describe the multi-slice CT

A

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

30
Q

what is window level

A

tissue which occupies the midpoint of the available grey levels

31
Q

what is window width

A

the range of tissues which occupies a range of grey levels

32
Q

describe windowing

A
  • it changes the density relationship between tissues
  • short range = high contrast
  • changes tissue types
33
Q

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

A

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

34
Q

what is the role of collimators

A

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

35
Q

what can cause a change in view

A

metal artefacts

36
Q

what is the role of PET

A

to visualise the spatial distribution between the metabolic and biochem activity

Physics (25 decks)

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