QA for MSCT - beyond IPEM 91 CTUG Nov 06
IPEM 91 • IPEM Report 91 (2005) – Recommended Standards for Routine Performance Testing of Diagnostic X-Ray Systems • Chapter 12 CT – Image quality – CT number calibration – Radiation dose – Mechanical tests – Axial, helical images – Inner and outer detector rows CTUG Nov 06
Beyond IPEM 91 • All the slices? • MPRs • AEC • Cone beam artefacts • Other issues ? CTUG Nov 06
All or some of the slices ? • Four slice, outer slices noise ~ 5% high 0.40 0.35 Image Noise (%) 0.30 0.25 0.20 0.15 0.10 0.05 0.00 1 2 3 4 Detector Bank CTUG Nov 06
All or some of the slices ? • Noise 16 slice – not so predictable 1.60 noise % 1.20 0.80 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Std Head Slice # StdBody CTUG Nov 06
All or some of the slices ? • Z-sensitivity (slice thickness) 16 slice 0.62 0.6 0.58 fwhm (mm) 0.56 0.54 0.52 0.5 0.48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Slice number • QC is about change – does this graph matter ? • Test some or all ? CTUG Nov 06
Volume Imaging • Volume imaging – MPRs, 3-D • Should it be tested? – directly ? – indirectly? CTUG Nov 06
MPR Direct Testing • Noise CTUG Nov 06
MPR Direct Testing • Resolution – bead or wire orientated appropriately (PSF -> MTF) – Visual repeating pattern 100 90 80 70 MTF (%) 60 50 40 30 20 10 0 0 2 4 6 8 10 Frequency (lp/cm) CTUG Nov 06
MPR Direct Testing • Advantage – What you see is what you get – Takes into account any special reconstruction or interpolation algorithms • Disadvantage – MTF analysis, but fwhm PSF or visual can be fine 100 90 80 MTF (%) 70 60 50 40 30 20 10 0 0 2 4 6 8 10 Frequency (lp/cm) CTUG Nov 06
MPR Indirect Testing • Noise from helical slice Recon position 1 CTUG Nov 06
MPR Indirect Testing • 3-D resolution – z-axis (helical z-sensitivity) 2.5mm 5mm 0.05 mm perspex 200 tungsten rod thin disk FWHM 150 C T N um bers FWTM 100 Helical Z-Sensitivity tool 50 0 -10 -8 -6 -4 -2 0 2 4 6 8 10 CTUG Nov 06 mm
MPR Indirect Testing • 3-D resolution – x-y (scan plane) 100 90 80 MTF (%) 70 60 50 40 30 20 10 0 0 2 4 6 8 10 Frequency (lp/cm) CTUG Nov 06
MPR Indirect Testing • Advantage – Doing these tests anyway • Disadvantage – Is it ok ? Recon is from raw data not from slices – But looking for change… – Doesn’t take into account special interpolation algorithms in the 3-D CTUG Nov 06
MPR Testing • Should it be tested ? – Directly or indirectly ? CTUG Nov 06
Testing the AEC • Tube current modulation – Patient size, z-axis, rotational – Axial and helical modes From patient to patient Along patient length Around the patient mA mA High mA Low mA -180 +180 angle CTUG Nov 06
Testing the AEC • Test object to vary in z-axis and rotationally – eg Perspex phantom, conical with elliptical cross section Catphan carrying case CT scanner couch End view Side view • Based on ‘Apollo’ phantom developed by Muramatsu, National Cancer Centre, Tokyo • Nick Keat – now at GSK CTUG Nov 06
Testing the AEC • Image along length of phantom – AEC off, on – Monitor image noise, mA, CTDI vol , AEC off, Constant mA CTUG Nov 06
Testing the AEC • Circular, elliptical phantoms of various sizes – Scan short lengths over each section – Monitor image noise, mA, CTDI vol , E. Castallano – RMH, London CTUG Nov 06
Testing the AEC 28 automA off 24 Noise Index 12 20 Noise (%) 16 12 Increased Decreased 8 mA mA 4 0 -150 -100 -50 0 50 100 150 Decreased Increased Z-position (mm) mA mA GE LightSpeed 16 www.impactscan.org/bluecover.htm CTUG Nov 06
Testing the AEC – Viewing with MPR Sagittal view Coronal view z-axis AEC off Noise increases z-axis AEC on Constant noise CTUG Nov 06
Testing the AEC • Should this be a described test ? • In a specified phantom ? • How often? CTUG Nov 06
Cone beam artefact • Teflon (PTFE) rod in water, to simulate rib at an angle to scan plane David Platten – now at Kings CTUG Nov 06
AMPR Algorithm Standard reconstruction AMPR CTUG Nov 06
Windmill artefact in consecutive images • Teflon (PTFE) rod in water, to simulate rib at an angle to scan plane ( 60 ° , Pitch x = 1.5, 16 x 1.5 mm acquisition, 5 mm image) CTUG Nov 06
Testing the cone beam artefact • Should this be a described test ? • In a specified phantom ? • How often? CTUG Nov 06
Beyond IPEM 91 • All the slices ? • MPRs (+3-D), AEC, cone beam artefacts – Should these be included in routine testing ? • Should there be anything else ? Catphan 0.62 carrying 0.6 case 0.58 fwhm (mm) 0.56 CT 0.54 scanner 0.52 couch 0.5 0.48 End view Side view 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Slice number CTUG Nov 06
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