35. CT: QUALITY ASSURANCE Flashcards
1
Q
- Why is the determination of doses of central importance in CT?
A
- CT IS THE LARGEST MEDICAL CONTRIBUTOR
- of collective dose to the patient population - 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
2
Q
- 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
3
Q
- 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
4
Q
- How much do CTs contribute to the Total Radiation Exposure?
A
- 24%
5
Q
- How much do CTs contribute to Medical Radiation Exposure?
A
- 50%
6
Q
- What is meant by the Overall dose for a CT?
A
- the closest thing we have is the Effective Dose (ED)
7
Q
- 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
8
Q
- What do some CT machines display?
A
- they display a crude estimate of the Effective Dose
- this is found using a three-part computation
9
Q
- 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
10
Q
- 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
11
Q
- 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)
12
Q
- 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
13
Q
- 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
14
Q
- What are the Exposure factors?
A
- the mass
- the kV
15
Q
- Why was the CTDI invented?
A
- it is meant to give an average dose for a single axial
slice
16
Q
- 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
- 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
- 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
- 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
- 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
- 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
- Why is the CT Dose Profile Bell Shaped?
A
- THE FINITE SIZE OF THE FOCAL SPOT
- this causes a small amount of shadow blurring at the
edges of the primary beam - COMPTON SCATTER
- this happens within the phantom
- or within the patient
24
Q
- 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
- 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
- What does the CTDI₁₀₀ indicated?
A
- it indicates the length of the ion chamber
- this means the ion chamber is 100 mm
27
Q
- 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
- What can a modification to the CTDI₁₀₀ give us?
A
- it gives us the pitch in a real clinical helical scan
29
Q
- 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
- 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
- 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
- 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
- How do we calculate the CTDI volume?
A
- we divide the weighted CTDI (CTDIw)
- by the Pitch
