What are the dose quantities in CT ? IAEA/RCA Kampala Kampala

CT Principles Kampala

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 Kampala

CT Slice Kampala

CT Slice Kampala

CT Slice Kampala

Dose Profile Kampala

Dose profile for a series of slices Kampala

Dose Profiles in Phantoms / Patients Effects of scatter Broadened profile Peak kerma dependent on beam width Long scatter tails for wide beams Kampala

Which Dosemeter ? Kampala

Integrated Profile Kampala

Integrated Profile Kampala

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) Kampala kerm a T CTDI (mGy) Z-axis (mm) CTDI = height of rectangle if area under profile was all contained within nominal slice width

CTDI 100 – What is it ? Analagous to Multiple Scan Average Dose. Originally defined for single slice scanners. Kampala

In air : CTDI 100,Air Sometimes normalise to 1 mAs: n CTDI 100,Air Just like in radiography! Kampala

In Perspex / PMMA Kampala

CT dosimetry in Perspex The computed tomography air kerma index is also measured inside PMMA head and body phantoms The quantity CTDI 100,c is measured at the centre of the standard CT dosimetry phantom and CTDI 100,p is the average of values measured at four positions around the periphery of the same phantom. Kampala

CTDI 100 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’ Kampala

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. Kampala

Phantoms for CT dosimetry Cylindrical PMMA phantoms with holes for pencil chamber 32 cm body phantom 16 cm head phantom Kampala

Centre Periphery – 1cm depth (mean of 4 positions) C P1P1 P2P2 P3P3 P4P4 32 cm ‘Head’ ‘Body’ 16 cm 140 mm CTDI Phantoms Kampala

40 ‘Head’ ‘Body’ Periphery : centre  1:1 Periphery : centre  2:1 Typical CTDI 100 Values CTDI 100,p values quoted in mGy Kampala

CTDI W Weighted average CTDI: represents the average dose for contiguous irradiation CTDI w = 1/3 CTDI centre + 2/3 CTDI periphery n CTDI w = CTDI w /mAs CTDI C CTDI P Kampala

CTDI vol Pitch = 1 CTDI vol = CTDI w Pitch = 2 CTDI vol = CTDI w 2 Pitch = 0.5 CTDI vol = 2 x CTDI w Kampala

CTDI vol CTDI W is the average dose in the scan plane Volume CTDI considers non-contiguous scanning The CTDI vol can be considered to be the average CTDI W in the scan volume CTDI vol = CTDI W Pitch Kampala

Dose Length Product CTDI vol 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 = CTDI vol x scan length Units are mGy cm Kampala

Dose Quantities in CT Kampala

Dose Summary Sheet - Commercial CT Scanner Huda W, Mettler F A Radiology 2011;258: Kampala

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 Kampala

Equipment for CT measurements CT ionization chamber and electrometer calibrated for CT beam qualities Chamber support stand CT head & body phantoms Kampala

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 Kampala

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) Kampala