IAEA/RCA Kampala
What are the
dose quantities in CT ?
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CT Principles
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Reminder : CT terminology
• Axial scanning
– 1 rotation of beam followed by stepwise
couch movement
– Increment – distance moved by couch
• Helical/spiral scanning
– Continuous table movement & beam rotation
– Pitch – table movement relative to nominal
slice width
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CT Slice
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CT Slice
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CT Slice
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Dose Profile
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Dose profile for a series of slices
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Dose Profiles in Phantoms / Patients
Effects of scatter
Broadened profile
Peak kerma dependent on
beam width
Long scatter tails for wide
beams
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Which Dosemeter ?
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Integrated Profile
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Integrated Profile
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CTDose Index
• Measure of dose for a single axial slice (mGy)
– no account of Helical/MDCT gaps or overlaps
• CTDI = area under dose profile (mGy.mm)
nominal slice width, T (mm)
kerm
a
CTDI
(mGy)
T
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Z-axis (mm)
CTDI = height of
rectangle if area
under profile was all
contained within
nominal slice width
CTDI100 – What is it ?
• Analagous to
Multiple Scan
Average Dose.
• Originally defined
for single slice
scanners.
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• In air : CTDI100,Air
• In air : CTDI100,Air
• Sometimes normalise
to 1 mAs:
• nCTDI100,Air
• Just like in
radiography!
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In Perspex / PMMA
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CT dosimetry in Perspex
The computed tomography air kerma index is also measured inside PMMA head and body phantoms
1
CTDI p,100 =
T
ò
+50
-50
K ( z) dz
The quantity CTDI100,c is measured at the centre of
the standard CT dosimetry phantom and CTDI100,p
is the average of values measured at four positions
around the periphery of the same phantom.
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CTDI100
• CTDI can be measured in
air or in phantoms
• CTDI in air can be used
as a measure of tube
output
• For dosimetry purposes
CTDI is measured in
perspex phantoms
• 16cm diameter ‘head’
• 32cm diameter ‘body’
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Patient Size.
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CTDI is a measure of scanner output.
CTDI does take patient size into account
CTDI is NOT patient dose
Important for paediatrics, small adults and
possibly bariatrics.
• You can correct to patient size
– To be mentioned later.
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Phantoms for CT dosimetry
• Cylindrical PMMA phantoms with holes for
pencil chamber
• 32 cm body phantom
• 16 cm head phantom
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CTDI Phantoms
Centre
Periphery – 1cm depth (mean of 4 positions)
‘Body’
‘Head’
P1
P4
C
P2
140 mm
16 cm
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P3
32 cm
Typical CTDI100 Values
CTDI100,p values quoted in mGy
‘Body’
20
‘Head’
40
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40
20
10
20
40
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Periphery : centre  1:1
20
Periphery : centre  2:1
CTDIW
• Weighted average CTDI: represents the
average dose for contiguous irradiation
• CTDIw = 1/3 CTDIcentre + 2/3 CTDIperiphery
• nCTDIw = CTDIw/mAs
CTDIC
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CTDIP
CTDIvol
Pitch = 1
CTDIvol = CTDIw
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Pitch = 2
CTDIvol = CTDIw
2
Pitch = 0.5
CTDIvol = 2 x CTDIw
CTDIvol
• CTDIW is the average dose in the scan plane
• Volume CTDI considers non-contiguous
scanning
• The CTDIvol can be considered to be the
average CTDIW in the scan volume
• CTDIvol = CTDIW
Pitch
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Dose Length Product
• CTDIvol is a measure of absorbed dose, energy
absorbed per unit mass
• Radiation risk is based on total energy absorbed
• Estimate using dose length product (DLP)
• DLP = CTDIvol x scan length
• Units are mGy cm
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Dose Quantities in CT
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Dose Summary Sheet - Commercial CT
Scanner
Huda W , Mettler F A Radiology 2011;258:236-242
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CTDI on the Console
• The CTDI value displayed will depend on
the phantom that it is based on.
• Body : 32 cm; Head 16 cm.
• CTDI for 16cm reference twice that of
CTDI for 32cm reference.
• Paediatric protocol MAY reference to 16
cm phantom
• Implementation manufacturer dependent.
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CTDI MEASUREMENT IN
PRACTICE
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Equipment for CT measurements
• CT ionization chamber and electrometer
calibrated for CT beam qualities
• Chamber support stand
• CT head & body phantoms
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Methodology for CT measurements in
air
• Position chamber in stand so
beyond couch
• Adjust couch position so chamber
along axis of scanner and beam
centred at central point of
chamber
• Record dose measurements for
single axial scan at selected
parameters (no couch movement)
• Record all data
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Methodology for CT measurements in
phantoms
• Position phantom on couch (use head rest if
appropriate)
• Position chamber in central position in phantom
• Adjust couch position so chamber centred as
before
• Record dose measurements for single axial scan at
selected parameters (no couch movement)
• Repeat for peripheral positions (north, south, east
& west)
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