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MED 107- MEDICAL PHYSICS II > 35. CT: QUALITY ASSURANCE > Flashcards

35. CT: QUALITY ASSURANCE Flashcards

1
Q
  1. Why is the determination of doses of central importance in CT?
A
  1. CT IS THE LARGEST MEDICAL CONTRIBUTOR
    - of collective dose to the patient population
  2. IRRADIATION OF THE IMAGE RECEPTOR
    - must be sufficient to obtain a usable image
    - there have to be enough photons given to the
    receptor to obtain an image of acceptable quality
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2
Q
  1. What is difficult to do with regards to CTs?
A
  • it is difficult to achieve optimal balance as we have
    many detectors and angles
  • assessing and reporting the doses is complex
  • the doses tend to be high
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3
Q
  1. What do CTs tend to result in in every occasion?
A
  • they result in a very non-uniform irradiation of a
    variety of organs
  • each of these have different radio sensitivities
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4
Q
  1. How much do CTs contribute to the Total Radiation Exposure?
A
  • 24%
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5
Q
  1. How much do CTs contribute to Medical Radiation Exposure?
A
  • 50%
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6
Q
  1. What is meant by the Overall dose for a CT?
A
  • the closest thing we have is the Effective Dose (ED)
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7
Q
  1. What calculation can we use to estimate the overall risk from a CT exposure?
A
  • CONVERSION FACTOR: 0.05 x 1/ Sv
  • ED = Effective Dose
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8
Q
  1. What do some CT machines display?
A
  • they display a crude estimate of the Effective Dose
  • this is found using a three-part computation
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9
Q
  1. What is the first step of the 3 part computation?
A
  • we calculate the CT dose index (CTDI) for the given
    technique factors
  • these are chosen for the current patient
  • it does not take into account the characteristics of the
    patient
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10
Q
  1. What is the CTDI?
A
  • it is a measure of machine output for a given set of
    technique factors
  • it is independent of the patient
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11
Q
  1. What do we do once we have the CTDI?
A
  • we multiply the CTDI value
  • by the length of the region that is being irradiated
  • this gives us a Dose Length Product (DLP)
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12
Q
  1. What happens once we have the Dose Length Product (DLP)?
A
  • we use a conversion factor
  • to transform the DLP into the Effective Dose

THIS CONVERSION FACTOR COMES FROM:
- the Monte Carlo Calculations
- they are numerical and statistical

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13
Q
  1. Define the CTDI.
A
  • it is an index of the X-Ray output of a CT machine
  • it does not take into account the characteristics of the
    patient
  • it only takes into account the exposure factors
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14
Q
  1. What are the Exposure factors?
A
  • the mass
  • the kV
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15
Q
  1. Why was the CTDI invented?
A
  • it is meant to give an average dose for a single axial
    slice
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16
Q
  1. What is the starting point of the CTDI?
A
  • the dose profile
  • this comes from a single narrow axial study of a CTDI
    phantom
17
Q
  1. What does this image show?
A

THE HEAD PHANTOM:
- this is an acrylic cylinder
- it is 15 cm long
- it is 16 cm in diameter
- contains 5 longitudinal holes

THE BODY PHANTOM:
- the diameter is 32 cm
- contains 5 longitudinal holes

18
Q
  1. Why are there holes in the Head Phantom?
A
  • the pencil ion chambers are placed into the holes
  • these pencils are 100 mm long
  • these pencils are calibrated

THEY ARE USED TO:
- measure the dose
- this find the dose profile

19
Q
  1. What is a CT Dose profile?
A
  • it is a measure of the presentation of the X-Ray
    attenuation values
  • of the scanned object
  • at various angles
20
Q
  1. What leads to the shape of CT Dose profile?
A
  • the geometry of the scanner
  • the properties of the objects being scanned
21
Q

21.What does the beam angle determine?

A
  • the distribution dose being deposited in the phantom
  • at any instant
22
Q
  1. What does the CT Dose profile depend on?
A
  • it depends on the dose averaged
  • this is averaged along the central axis from a full
    rotation of the Gantry
23
Q
  1. Why is the CT Dose Profile Bell Shaped?
A
  1. THE FINITE SIZE OF THE FOCAL SPOT
    - this causes a small amount of shadow blurring at the
    edges of the primary beam
  2. COMPTON SCATTER
    - this happens within the phantom
    - or within the patient
24
Q
  1. What leads to the variations of the Dose along the ion chamber?

D = Dose

A
  • the variations are marked as D (z)
  • this happens when the doses are exposed to a beam
  • this is a single, narrow beam
  • it has a width of W mm
25
Q
  1. What is the CTDI₁₀₀ defined as?
A
  • it is defined as the integral dose
  • it is recorded by the camber over its entire 100 mm
    length
26
Q
  1. What does the CTDI₁₀₀ indicated?
A
  • it indicates the length of the ion chamber
  • this means the ion chamber is 100 mm
27
Q
  1. How is the CTDI₁₀₀ measured?
A
  • it is measured with the ion chamber in the central hole
  • it is measured with one or more ion chambers in the
    peripheral holes
  • this means that there is a lower dose at the centre

A WEIGHTED AVERAGE OF THESE FINDINGS:
- defines the CTDIw
- this accounts for the tomographic variations in the
human body

28
Q
  1. What can a modification to the CTDI₁₀₀ give us?
A
  • it gives us the pitch in a real clinical helical scan
29
Q
  1. What can be said if the Pitch is lower than one?
A
  • the Beam Width has some overlap at each view angle
  • from rotation to rotation
  • higher image resolution
  • more image projections are obtained
  • increased patient dose
30
Q
  1. What can be said is the Pitch is equal to one?
A
  • there is no overlap of the Beam Width at each view
    angle
  • no view angles are not covered at certain table
    positions
31
Q
  1. What can be said if the Pitch is greater than 1?
A
  • some view angles are not covered by the beam width
  • at certain table positions
  • lower image resolution
  • fewer image projections are obtained
  • decreased patient dose
32
Q
  1. What does the term “Pitch” mean in spiral CT techniques?
A
  • this is the distance (speed) of the table
  • for one X-Ray rotation in the Gantry

THIS IS COMPARED:
- to the number of detectors
- and the width of each detector
- nT

33
Q
  1. How do we calculate the CTDI volume?
A
  • we divide the weighted CTDI (CTDIw)
  • by the Pitch

MED 107- MEDICAL PHYSICS II (108 decks)

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