Z-axis geometric efficiency in CT - ImPACT CT Scanner Evaluation ...

Comparison of Definitions of GeometricEfficiency in Computed Tomography ScannersSue Edyvean, Nicholas KeatImPACT* (Imaging Performance Assessment of CT Scanners)LondonUK*An MHRA Evaluation centre for theUK Department of Health(Medicines and Healthcare productsRegulatory Agency)www.impactscan.orgWC 2003 August 2003Z-axis geometric efficiency in CT• Characterises the extent of the radiation beam thatis used for image creation (along the z-axis)z-axisWC 2003 August 2003

Geometric efficiency (g.eff.)• Single slice– Usually little wasted dose,as all of beam is used forimaging, except for narrow sliceswhere sometimes post-patientcollimation is usedWC 2003 August 2003Geometric efficiency (g.eff.)• Multi slice– Usually more unused dose, as aneven irradiation of slices is required– Penumbral region of the beamtherefore can’t be used for imagingWC 2003 August 2003

Definitions for z-axis geometric efficiency (g.eff.)• IEC 60601- 2- 44 Ed. 1 1999: Original definition– “the full width at half maximum of the sensitivity profileexpressed as percentage of the full width at halfmaximum of the dose profile.”• IEC 60601- 2- 44 Ed. 2 Am.1 2003: New definition– “the integral of the dose profile along the z-direction,integrated over the range subtended by the detectorelements used during acquisition, expressed as apercentage of the total integral of the dose profile in thez-direction”• g.eff. to be displayed on scan console when lessthan 70%WC 2003 August 2003Definitions – paraphrased• Old definition:– Ratio of the measured imaged width to the measureddose profile width (add imaged widths for multi-slice)• New definition:– Integral of dose profile over range of nominal slice widthdivided by integral of whole dose profiledoseprofilewidthmeasured add -> total imaged widthWC 2003 August 2003z-axisnominal slice widthtotal nominal acquisition width(nominal total collimation)integral ofdose profile overrange of nominalsliceintegral ofwhole doseprofile

Purpose of this study• Identify problems associated with old g.eff.• Compare two calculation approaches for new g.eff.• Compare values between old and newWC 2003 August 2003Methods of measurement – dose profiles• Kodak X-Omat V Radiotherapy film• Scanned at iso-centre• Read out with scanning micro-densitometer• Optical density profile converted to dose profile876dose profiledose54321fwhm00 5 10 15 20 25 30 35 40distance along z-axisWC 2003 August 2003

Methods of measurement – imaged slice width• Angled metal plates scanned in axial mode• Width projected into image → Z-sensitivity profileWC 2003 August 2003Problems with old g.eff.• Different measurement conditions (air vs phantom)• For multi slice scanners– Many image width combinations for one beam widthWC 2003 August 2003• Many measurements needed– e.g. Philips Mx8000 four slice scannerBeam width Image width combinations(mm)No. x slice (mm)12 x 0.5 1 x 14 4 x 1 2 x 2 1 x 410 4 x 2.5 2 x 5 1 x 10162 x 820 4 x 5 2 x 10

Problems with old g.eff. (continued)• Different focal spots– Potentially doubles the number of values• Sixteen slice scanners – narrow slices– Need accurate measurement for imaged widths ~ < 1 mm– Not many existing test objects will do thatImPACT’s ‘thin slice’ tool50 µm Titanium plates8° angle to scan planeWC 2003 August 2003Potential problems for calculation of new g.eff.• Dose profile– ..‘integrated over the range subtended by detectorelements used’ ( nominal total acquisition width)– Position of dose profile relative to detectors is not defined– Shape asymmetric assume a reference position8dose profile76dose5432100 5 10 15 20 25 30 35 40distance along z-axisWC 2003 August 2003

Two approaches for new g.eff. calculation• Could either– Centre integration range on centre of dose profile• Define as halfway between fwhm– Position of range which gives maximum geometric efficiencyA. dose profileB.dose profile887766dose543dose543221100 5 10 15 20 25 30 35 4000 5 10 15 20 25 30 35 40distance along z-axisdistance along z-axisWC 2003 August 2003Two approaches for new g.eff. calculation• Comparison of two techniques– Maximum difference = 0.04 % (ss), 0.6 % (ms)– Mean difference = 0.1% (ms)A. dose profileB.dose profile887766dose543dose543221100 5 10 15 20 25 30 35 4000 5 10 15 20 25 30 35 40distance along z-axisdistance along z-axisWC 2003 August 20031: Centred on dose profile2. Maximum g.eff.

Results• Analysed data from 13 scanners (all manufacturers)– 4 single slice– 5 four slice– 1 eight slice– 3 sixteen slice• Total of 123 different slice width combinations– Old g.eff.• Compared new and old g.eff.WC 2003 August 2003Single slice - old g.eff. (imaged width / dose width)– Most scanners have values of approximately 100%– Values higher than 100% probably due to scatter– Low for narrow slices using post patient collimation120Geometric Efficiency (%)100806040200Scanner AScanner BScanner CScanner D70%0 1 2 3 4 5 6 7 8 9 10Slice width (mm)WC 2003 August 2003

Single slice - new g.eff. (dose in nominal width/wholeprofile)– g.eff. values lower than 100%– Radiation falling outside the nominal width– Non rectangular dose profiles as widths gets thinner100WC 2003 August 2003Geometric Efficiency (%)80604020070%Scanner AScanner BScanner CScanner D0 1 2 3 4 5 6 7 8 9 10Slice width (mm)Multi slice - old g.eff. (imaged width / dose width)• Unused penumbra – values less than 100%• Many values for each scanner– e.g. Siemens Sensation 16 (small focus only)WC 2003 August 2003Slice width(mm)Imaged width1 slice 2 slice 3 slice 4 slice 6 slice 12 slice0.6 71%1 106%0.75 77%1.5 77% 88%3 88% 76%4.5 77% 87%5 95%6 87%9 77% 88%10 96%

Multi slice - old g.eff. (continued)• Same beam width– Different values9 mmbeam18 mmbeamSlice width(mm)Imaged width1 slice 2 slice 3 slice 4 slice 6 slice 12 slice0.6 71%1 106%0.75 77%1.5 77% 88%3 88% 76%4.5 77% 87%5 95%6 87%9 77% 88%10 96%WC 2003 August 2003Multi slice - new g.eff. (dose in nominal width/whole profile)• Values for each beam width only• Unused penumbra – values less than 100%dose within nominalwidth equals totaldosedose within nominalwidth less than totaldoseWC 2003 August 2003Geometric Efficiency (%)1008060402000 2 4 6 8 101214161820222426283032Collimation (mm)Four slice scanners onlyScanner AScanner BScanner CScanner D70%

Multi slice - new g.eff. versus old g.eff.• Mean ratio = 0.99, but standard deviation = 10%• Biggest differences at narrow collimations– total measured imaged width matches dose width, butboth greater than nominal• Some valuesnow below70% 802 x 1 mmWC 2003 August 2003Geometric Efficiency (new) (%)120100604020070% new70% old0 20 40 60 80 100 120Geometric efficiency (old) (%)2 x 0.63 mm2 x 0.63 mmConclusions• Old g.eff. definition– Worked well enough for single slice systems– Drawback is many slice width combinations for multi slice• New g.eff. does not use imaged width; only the doseprofile and nominal width– Two calculation techniques are comparable– Results for single slice are all generally lower• spread of dose outside of nominal width not always identified before– Multi-slice new g.eff. values similar to old values• except for some thin slices where both imaged slice width and dosewidth are > nominal Some values now below 70% thresholdWC 2003 August 2003

Comparison of Definitions of GeometricEfficiency in Computed Tomography ScannersSue Edyvean, Nicholas KeatImPACT* (Imaging Performance assessment of CT Scanners)LondonUK*An MHRA Evaluation centre for theUK Department of Health(Medicines and Healthcare productsRegulatory Agency)www.impactscan.orgWC 2003 August 2003