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 / PatientsEffects of scatter

Broadened profilePeak kerma dependent on beam widthLong 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)

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kerma

T

CTDI(mGy

)

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 PerspexThe computed tomography air kerma index is also measured inside PMMA head and body phantoms

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.• 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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Centre Periphery – 1cm depth (mean of 4 positions)

C

P1

P2

P3

P4

32 cm

‘Head’ ‘Body’

16 cm

140 mm

CTDI Phantoms

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‘Head’

‘Body’

40

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Periphery : centre 1:1 Periphery : centre 2:1

Typical CTDI100 ValuesCTDI100,p values quoted in mGy

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CTDIW• Weighted average CTDI: represents the

average dose for contiguous irradiation • CTDIw = 1/3 CTDIcentre + 2/3 CTDIperiphery • nCTDIw = CTDIw/mAs

CTDIC CTDIP

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CTDIvol

Pitch = 1 CTDIvol = CTDIw

Pitch = 2 CTDIvol = CTDIw

2

Pitch = 0.5 CTDIvol = 2 x CTDIw

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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.Kampala

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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