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Report 05071 Siemens Somatom Sensation Open CT scanner technical evaluation November 2005 www.pasa.nhs.uk/cep Health and social care working together About evaluation reports The Centre for Evidence-based Purchasing provides independent and objective evaluations of medical devices available on the UK market. Specialist centres, mainly in NHS Trusts, do the evaluations under contract to the NHS Purchasing and Supply Agency (NHS PASA). Results are available on our website (www.pasa.nhs.uk/cep). Our evaluations are usually of products supplied by the manufacturer. We expect these products to be representative of those on the market but cannot guarantee this. Prospective purchasers should satisfy themselves about any modifications that might have been made after our evaluation. The Centre for Evidence-based Purchasing (formerly the Device Evaluation Service) transferred from the Medicines and Healthcare products Regulatory Agency to NHS PASA on 1 September 2005. 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Siemens Somatom Sensation Open CT scanner technical evaluation Nicholas Keat David Platten, Maria Lewis, Sue Edyvean ImPACT Bence Jones Offices St George’s Hospital London SW17 0QT Tel: 020 8725 3366 Fax: 020 8725 3969 e-mail: [email protected] For more information on ImPACT visit www.impactscan.org © Crown Copyright 2005 Apart from any fair dealing for the purposes of research or private study, or criticism, or review, as permitted under the Copyright, Designs & Patents Act, 1998, this publication may only be reproduced, stored, or transmitted in any form or by any means with the prior permission, in writing, of the Controller of Her Majesty’s Stationery Office (HMSO). Information on reproduction outside these terms can be found on the HMSO website (www.hmso.gov.uk) or e-mail: [email protected]. Contents Contents .....................................................................................................4 Summary.....................................................................................................5 Siemens Somatom Sensation Open .............................................................5 Clinical scans .............................................................................................7 Dose and image quality.............................................................................8 Head scanning...............................................................................................9 Body scanning ...............................................................................................9 Image noise ..............................................................................................10 Variation of image noise with scan parameters....................................10 Variation of image noise with reconstruction filter .......................................11 Inter-slice noise variation.............................................................................12 CT number accuracy and uniformity .....................................................13 CT number accuracy and uniformity............................................................13 CT number and electron density linearity....................................................15 CT number of Catphan™ inserts.................................................................15 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Spatial resolution.....................................................................................16 Variation of spatial resolution with scan parameters ...................................16 Spatial resolution and image noise..............................................................17 Limiting resolution and comparison with Siemens data ..............................18 Slice width characteristics......................................................................19 Imaged slice thickness: axial .......................................................................19 Imaged slice thickness: helical ....................................................................20 Extended field of view .............................................................................22 Extended field of view..................................................................................22 Patient couch movement ........................................................................24 Z-axis couch movement accuracy under manual control ............................24 Z-axis couch movement accuracy under scanner control ...........................24 Couch deflection under load........................................................................24 Radiation dose .........................................................................................25 CTDI100 measured free in air .......................................................................25 CTDI100 and CTDIw in acrylic phantoms ......................................................26 X-ray beam width.........................................................................................27 Z-axis geometric efficiency ..........................................................................27 Extra rotations for helical scanning..............................................................28 Low contrast detectability.......................................................................29 Catphan™ low contrast detectability measurements ..................................29 Appendix 1: Scanner specifications ......................................................31 Appendix 2: Manufacturer’s comments.................................................40 Appendix 3: Image quality assessment and Q......................................43 Appendix 4: About ImPACT ....................................................................44 ImPACT .......................................................................................................44 Support to purchasers and users ................................................................44 Summary Siemens Somatom Sensation Open The Siemens Somatom Sensation Open is a third generation 20 slice (40 slice optional) helical CT scanner, with a wider than usual gantry aperture (see Figure 1). It features a 50 kW generator, a ‘Straton’ x-ray tube, and a fastest gantry rotation time of 0.5 seconds. Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Figure 1. The Siemens Somatom Sensation Open The scanner has a total z-axis detector bank length of 28.8 mm (see Figure 2). Routine imaging beam collimations produce 20 x 0.6 mm and 20 x 1.2 mm data sets per tube rotation with the 20 slice option. The 40 slice version uses a z-axis flying focal spot to produce 40 x 0.6 mm data sets per rotation, and can also acquire 24 x 1.2 mm data sets per rotation. Figure 2. Sensation Open z-axis detector layout z-axis 4 x 1.2 mm 32 x 0.6 mm 5 4 x 1.2 mm Summary The gantry aperture of 82 cm is wider than the 70 cm on other Siemens Sensation scanners. In addition to the standard scan field of view of 50 cm, the Open has an extended field of view that reconstructs images up to 82 cm. As the detector array covers only the central 50 cm standard field of view, special techniques are required to reconstruct image data outside this area, with reduced image quality. The main application where the larger gantry aperture and field of view are of benefit is in radiotherapy planning, particularly for breast cancer, where patients can be scanned in the same position as they are treated, with their arms raised above the head. Other applications include the scanning of obese patients, trauma patients who may be attached to life support equipment, and interventional procedures where easy patient access is particularly important. Another advantage for the scanning of obese patients is an optional patient table that bears a load of up to 280 kg. Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation The Sensation Open uses Siemens’ Straton x-ray tube, which has a novel design that allows direct and very fast cooling of the anode. Siemens claim this eliminates any delays due to tube heating. In addition, it has the ability to dynamically move the position of the focal spot along the z-axis to give 40 scan data sets per rotation from the scanner’s 20 x 0.6 mm detector rows. Image reconstruction uses Siemens’ AMPR technique, which is also used on their other scanners with 16 or more acquisition channels. ImPACT tested the 40 slice version of the Sensation Open, using software version VB19B. 6 Clinical scans The scan settings chosen for the six clinical scans, defined in ImPACT Report MDA/98/25, were given by Siemens for the Sensation Open, and said to be representative of protocols in clinical use. In addition, a helical protocol for head and abdomen were used. Table 1 shows the results obtained using these settings. In this report, the use of the z-flying focal spot is denoted with an asterisk (*), so 40* x 0.6 results from 20 x 0.6 m detectors double sampled to give 40 x 0.6 mm data sets per rotation. Results in italics are mean values for the following: GE LightSpeed16 and LightSpeed Pro16, Philips Mx8000 IDT, Siemens Sensation 16 and Toshiba Aquilion 16. Table 1. Clinical scan tables Helical Inner Ear 120 320 96 120 Axial Abdomen 105 120 Helical Abdomen 288 Low Noise Spine 120 High res. spine 120 180 180 1 1 1 0.5 1 1 0.55 5 (24 x 1.2) 250 6 x 4.8 (24 x 1.2) 250 0.6 (40* x 0.6) 0.8 380 6 x 4.8 (24 x 1.2) 5 (24 x 1.2) 120 1.2 380 4x3 (20 x 0.6) 120 4x3 (20 x 0.6) 120 7 MTF 10 (c/cm) 120 1 MTF 50 (c/cm) Standard Brain 176 Noise (HU) 120 250 Z-sens (mm) Helical Head 4x3 (20 x 0.6) CTDIvol (mGy) 1 Recon filter Time (s) 320 Recon FOV (mm) mA 120 Results Pitch kV Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Posterior Fossa Slice thickness (# x mm) Collimation (# x mm) Scan Parameters Scan H30 70 3.0 4.0 3.3 6.0 52 4.2 3.5 3.2 6.3 57 5.1 3.2 3.2 5.9 50 4.6 3.4 3.3 6.3 58 4.7 3.3 3.3 6.0 46 6.5 2.7 3.1 6.1 24 0.64 130 7.5 9.6 33 0.71 146 9.6 15 9.1 4.7 27 3.2 6.1 15 5.7 21 3.5 6.3 10 5.1 23 3.1 6.1 11 5.3 26 3.6 6.5 18 3.0 28 3.2 6.2 21 2.8 23 3.7 6.3 16 2.5 175 7.1 9.3 20 2.3 149 7.6 11 H30 H30 H60 B31 B31 B31 B60 Dose and image quality Dose efficiency is a term used to describe the quality of a scanner’s images relative to the radiation dose to the patient. It can be expressed in a number of ways, and ImPACT use the ‘Q value’, which combines measurements of noise, high contrast resolution, slice thickness and dose to produce an imaging figure of merit (see Appendix 1 for more details). Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation The Q2 values presented in this section are for head and body imaging (see Table 2 and Table 3). Standard acquisition and reconstruction parameters are used where possible to minimise slight variations that occur for different kV, slice thicknesses, scan times and reconstruction algorithms. These are indicated below: • Tube voltage: 120 kV or 130 kV when this is the ‘standard’ operating kV for the scanner. • Collimation: 20 mm, or the closest available setting. • Image width: 5 mm, or the closest available setting. • Scan time: as recommended by the manufacturer, sub-second for body scanning and 1 s or greater for head scanning. • Reconstruction filter: the one that most closely matches the average ‘standard’ head and body filter (MTF50 of 3.4 c/cm, MTF10 of 6.0 c/cm). • Reconstruction field of view: 250 mm (head) and 380 mm (body). • The mAs setting that would result in a CTDIvol of 50 mGy for head and 15 mGy for body scanning is listed. Z-sensitivity, image noise at 50 or 15 mGy and MTF values are also shown. Mean Q2 values are for the following 16-slice CT scanners: GE LightSpeed16 and LightSpeed Pro16, Philips Mx8000 IDT, Siemens Sensation 16 and Toshiba Aquilion 16. 8 Dose and image quality Head scanning Table 2. Q2 value for head scanning Scanner Siemens Sensation Open Filter H30 mAs for z-sens Noise 50mGy (mm) (HU) 277 Mean MTF50 MTF10 (c/cm) (c/cm) Q2 4.7 3.5 3.3 6.0 5.9 5.2 3.3 3.4 6.3 6.4 Body scanning Table 3. Q2 value for body scanning Scanner Siemens Sensation Open Filter B31 mAs for z-sens Noise 15 mGy (mm) (HU) 173 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Mean 9 MTF50 MTF10 (c/cm) (c/cm) Q2 4.7 21 3.2 6.1 1.8 5.2 20 3.4 6.2 1.9 Variation of image Image noisenoise with scan parameters Table 4 shows the effect of varying scan parameters upon image noise in uniform head and body sized water phantoms. Relative noise values are quoted, as different phantoms will result in different absolute values of noise, however the relative values should be similar. The bold figures in the relative noise columns show the standard setting to which other values have been normalised. The adjusted relative noise column shows the deviation from the expected noise value, due to the change in acquisition parameters. Standard axial scan parameters are: 120 kV, 320 mA, 1 s, 6 x 4.8 mm collimation, H30 filter for head and 120 kV, 105 mA, 1 s, 6 x 4.8 mm collimation, B31 filter for body. Table 4. Variation of image noise with scan parameters Parameter Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Tube voltage (kV) (Head) Tube voltage (kV) (Body) Tube current (mA) Scan time (s) Slice thickness (mm), Collimation (# x mm) Setting Relative noise 80 1.89 Adjusted relative noise - 100 120 1.31 1.00 - 80 2.48 - 100 1.40 - 120 1.00 - 140 0.79 - 33 2.65 0.85 80 160 1.88 1.37 0.94 0.97 320 1.00 1.00 400 0.5 0.91 1.33 1.02 0.94 1 1.00 1.00 5 (1 x 5) 1.03 1.05 10 (1 x 10) 0.75 1.09 3 (20 x 0.6) 1.22 0.97 6 (20 x 0.6) 0.88 0.99 12 (20 x 0.6) 0.64 1.01 2.4 (24 x 1.2) 1.37 0.97 4.8 (24 x 1.2) 1.00 1.00 7.2 (24 x 1.2) 0.83 1.02 9.6 (24 x 1.2) 14.4 (24 x 1.2) 0.73 0.61 1.03 1.05 10 Variation of image noise with scan parameters Variation of image noise with reconstruction filter Results shown in Table 5 were acquired using head and body sized uniform water phantoms. Scans are axial, parameters are: 120 kV, 320 mA, 1 s, 6 x 4.8 mm collimation for head and 120 kV, 105 mA, 1 s, 6 x 4.8 mm collimation for body. Table 5. Variation of image noise with reconstruction filter Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Parameter Reconstruction filter (head) Reconstruction filter (body) Setting Relative noise H10s 0.74 H20s H21s 0.88 0.94 H22s 0.93 H30s 1.00 H31s 1.10 H32s H40s 1.07 1.15 H41s H42s 1.26 1.22 H45s 1.52 H50s H60s 2.15 6.95 H70h 12.42 H80h 11.93 B10s 0.60 B20s 0.80 B25s 1.07 B30s 1.04 B31s 1.00 B35s 1.02 B40s 1.31 B41s B45s B50s B60s B70s B80s 1.22 2.01 3.99 5.38 6.59 6.78 11 Variation of image noise with scan parameters Inter-slice noise variation Axial scans were made of ImPACT’s head phantom, and the noise from each slice in a 12 x 2.4 mm collimation acquisition was compared. Results are for 120 kV, 320 mA, 1 s, H30 filter (see Table 6). Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Table 6. Variation of noise in each slice of a 12 x 2.4 mm axial acquisition Detector group Noise (HU) 1 4.5 Variation from mean (%) 1.0 2 3 4.4 4.4 0.3 0.7 4 4.4 -0.7 5 4.4 -0.8 6 4.4 -0.9 7 8 4.5 4.4 0.9 -1.3 9 10 4.4 4.5 0.2 2.4 11 4.4 -1.3 12 Mean 4.4 4.4 -0.4 - 12 CT number accuracy and uniformity CT number accuracy and uniformity CT number accuracy and uniformity was assessed in ImPACT’s standard head (185 mm diameter) and body (340 mm diameter) water filled phantoms. The head phantom has a bone equivalent shell to mimic a patient’s skull. Regions of interest were placed at the centre of the phantom, and 1 cm in from the phantom edges, at positions corresponding to north, east, south and west compass points (see Table 7 and Table 8). Acquisition parameters were: 120 kV, 176 mA, 1 s scan time, pitch 0.55 (320 mAs effective), 24 x 1.2 mm collimation and H31s filter for head, and 120 kV, 288 mA, 0.5 s scan time, pitch 1.2 (120 mAs effective), 24 x 1.2 mm collimation and B31s filter for body. Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Table 7. CT number accuracy and uniformity for head scanning Difference from centre (HU) Position CT number Centre 2.13 N E 6.08 6.11 3.95 3.98 S 6.67 4.54 W 6.77 4.64 Table 8. CT number accuracy and uniformity for body scanning Difference from centre (HU) Position CT number Centre -4.83 N E -3.83 -4.15 1.00 0.68 S -6.75 -1.92 W -4.50 0.33 13 CT number accuracy and uniformity In addition to the head and body phantoms, a uniform 480 mm diameter water phantom was scanned. This phantom is larger than a standard patient, and fills the entire standard field of view to within a 10 mm radius. Figure 3 shows the variation of CT number across the field of view for all three phantoms. Annular (ring shaped) ROIs are used with dimensions of 0-10%, 10-20% … 80-90% of the phantom’s internal diameter. Acquisition parameters for the 480 mm phantom were 120 kV, 350 mA, 1 s, 1 x 5 mm collimation, B31 filter (axial scan). Head and body images had acquisition parameters as described for Table 7 and Table 8. Figure 3. CT number in annular ROI at increasing diameter from the isocentre for head, body and 480 mm phantoms 15 10 Head 0 CT number (HU) Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation 5 -5 Body -10 -15 -20 480 mm -25 -30 0 50 100 150 200 250 ROI Diameter (mm) 14 300 350 400 CT number accuracy and uniformity CT number and electron density linearity An RMI 467 electron density phantom was scanned at 250 mA, 1 s, 3 x 9.6 mm collimation, B31 filter and a range of kV settings (see Figure 4). Figure 4. CT number against electron density at a range of kVs 2000 80 kV 1500 100 kV 120 kV CT number (HU) Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation 1000 140 kV 500 0 -500 -1000 0 0.5 1 1.5 2 Relative electron density CT number of Catphan™ inserts A Catphan™ 500 phantom was scanned at 120 kV, 200 mA, 1 s, 3 x 9.6 mm collimation, and B31 filter. The CT number of the four sensitometry inserts in the CTP401 module was measured (see Table 9). Table 9. CT number for Catphan™ 500 sensitometry inserts Insert Measured CT # Air -969 LDPE Acrylic -90 120 Teflon 886 15 Spatial resolution Variation of spatial resolution with scan parameters Spatial resolution is quoted in terms of MTF50 and MTF10, the frequencies corresponding to the 50 % and 10 % modulation transfer function values (in cycles per cm). Larger values denote higher resolution (see Table 10). Acquisition parameters were: 120 kV, 320 mA, 1 s, 4 x 3 mm collimation, 250 mm field of view for head, and 120 kV, 300 mA, 1 s, 4 x 3 mm collimation, 380 mm field of view for body scans. Table 10. Variation of spatial resolution with scan parameters Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Parameter Reconstruction filter (head) Reconstruction filter (body) Scan type Setting MTF50 (c/cm) MTF10 (c/cm) H10s 2.8 5.0 H20s H21s 3.1 3.0 5.6 6.0 H22s 2.9 4.9 H30s 3.3 6.0 H31s 3.3 6.3 H32s H40s 3.1 3.5 5.4 6.4 H41s H42s 3.6 3.3 6.6 5.7 H45s 4.0 7.1 H50s H60s 4.2 7.2 7.1 9.4 H70h 8.7 11 H80h 8.5 11 B10s 2.7 4.5 B20s 3.2 5.2 B25s 3.7 6.1 B30s 3.5 5.8 B31s 3.2 6.1 B35s 3.4 5.9 B40s 3.9 6.5 B41s B45s B50s B60s B70s B80s Axial (H30) Helical (H30) 3.5 4.9 6.3 6.9 7.5 6.9 3.3 3.2 6.7 7.0 8.7 8.8 9.3 8.2 6.0 5.9 16 Spatial resolution Spatial resolution and image noise Figure 5 and Figure 6 show image noise and spatial resolution values for each reconstruction filter, in head and body scanning respectively. Spatial resolution is characterised by the average of the MTF50 and MTF10 values. Image noise figures were adjusted to the standard ImPACT value of 5.0 mm. Scan conditions were: 120 kV, 277 mA, 1 s, 6 x 4.8 mm collimation, resulting in a CTDIvol of 50 mGy for head scanning, and 120 kV, 173 mA, 1 s, 6 x 4.8 mm collimation, resulting in a CTDIvol of 15 mGy for body scanning. Figure 5. Image noise against spatial resolution for head scanning 50 Image Noise (HU) Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation 40 30 H10s H20s H21s H22s H30s H31s H32s H40s H41s H42s H45s H50s H60s H70h H80h 20 10 0 0.0 2.0 4.0 6.0 8.0 Mean MTF50 and MTF10 (c/cm) 17 10.0 12.0 Spatial resolution Figure 6. Image noise against spatial resolution for body scanning Image Noise (HU) 160 B10s B20s 140 B25s B30s 120 B31s B35s B40s B41s B45s B50s B60s B70s 100 80 B80s 60 40 20 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 Mean MTF50 and MTF10 (c/cm) Limiting resolution and comparison with Siemens data Scan parameters were 120 kV, 320 mA, 1 s, 4 x 3 mm collimation, H80 filter, and 120 mm reconstruction field of view (see Table 11). Table 11. Limiting spatial resolution and Siemens published specification Siemens (c/cm) ImPACT (c/cm) MTF50 - 8.6 MTF10 MTF2 14.5 12 16 18 Slice width characteristics Imaged slice thickness: axial Measured using 50µm titanium ramps inclined at 30° to the scan plane, at the centre of the field of view (see Table 12). Table 12. Axial imaged slice thickness Slice thickness(mm) Ratio Measured slice (mm) Collimation (# x mm) (measured:nominal) 2.4 (24 x 1.2) 2.5 1.02 4.8 (24 x 1.2) 4.7 0.98 7.2 (24 x 1.2) 7.1 0.98 9.6 (24 x 1.2) 9.3 0.97 14.4 (24 x 1.2) 14.2 0.99 3 (20 x 0.6) 3.0 1.00 6 (20 x 0.6) 6.0 11.5 0.99 4.8 9.1 0.97 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation 12 (1 x 12) 5 (1 x 5) 10 (1 x 10) 0.96 0.91 Table 13 shows the variation of slice thickness with detector group for 12 x 2.4 mm collimation. Table 13. Variation of axial slice thickness with detector group 1 Measured slice (mm) 2.46 Variation from mean (%) -0.3 2 3 2.54 2.46 2.8 -0.5 4 2.42 -2.0 5 2.39 -3.0 6 2.46 -0.3 7 8 2.48 2.46 0.4 -0.5 9 10 2.41 2.47 -2.3 0.1 11 2.42 -1.8 12 Mean 2.50 2.47 1.2 - Detector group 19 Slice width characteristics Imaged slice thickness: helical Measured using a 6 mm diameter, 50 µm thick gold disc, 15 mm from the centre of the field of view (to avoid ring artefacts). The full width at half maximum (FWHM) of the z-sensitivity profile is quoted to characterise the slice thickness (see Table 14). Table 14. Helical imaged slice thickness Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Collimation (mm) 20* x 0.6 Helical pitch 0.45 Table feed Nominal image Z-sensitivity Ratio actual: (mm/rotation) thickness (mm) (FWHM) (mm) nominal 2.7 0.6 0.60 1.00 20* x 0.6 20* x 0.6 0.8 1.2 4.8 7.2 0.6 0.6 0.62 0.65 1.04 1.09 20* x 0.6 1.5 9 0.6 0.65 1.08 40* x 0.6 0.45 5.4 0.6 0.63 1.05 40* x 0.6 0.8 9.6 0.6 0.64 1.07 40* x 0.6 1.2 0.64 1.06 1.5 14.4 18 0.6 40* x 0.6 0.6 0.63 1.05 24 x 1.2 24 x 1.2 0.9 0.9 25.9 25.9 1.5 3 1.67 3.17 1.12 1.06 24 x 1.2 1.5 43.2 5 5.11 1.02 20 Slice width characteristics Figure 7 shows the measured FWHM of the z-sensitivity profile of a 0.6 mm image produced from 40* x 0.6 mm collimation, at a range of helical pitches and distances from the scan isocentre. Figure 7. Helical slice thickness against distance from the isocentre 1.0 Pitch 0.45 Pitch 0.8 Pitch 1.5 0.8 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Z-sensitvity (FWHM, mm) Pitch 1.2 0.6 0.4 0.2 0.0 0 50 100 150 Distance from isocentre (mm) 21 200 Extended field of view Extended field of view In order to reconstruct a CT image, attenuation data is normally required through an object at all angles during the scanner’s rotation. The image field of view is therefore limited by the detector fan angle, and the geometry of the scanner (see Figure 8) Figure 8. Sensation Open standard and extended fields of view Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Extended field of view Standard field of view Gantry aperture . The Sensation Open features a standard scan field of view of 500 mm. It can reconstruct images up to the full 820 mm gantry bore diameter using extrapolated attenuation measurements to extend the field of view beyond the usual maximum. Any value between 500 and 820 mm can be chosen for the extended field of view size. Applications for the extended field of view include the imaging of radiotherapy and very large or obese patients. Radiotherapy patients, particularly those with breast cancer, may be treated with their arms above their head. Conventional gantries may restrict the positioning of these patients, and standard fields of view will not show the arms. The following images examine the effect of objects outside the scan field of view for radiotherapy applications using ImPACT’s body sized (340 mm) water phantom and a water filled cylinder, 94 mm in diameter, to simulate an arm within the extended field of view. The introduction of the arm into the extended field of view has minimal effect on mean CT numbers within the standard field of 22 Extended field of view view (mean value increases from -4.3 to -1.3). Some streaking is visible at narrow window widths. Figure 9 shows the simulated arm lying outside the scan field of view at 500, 650, and 820 mm image fields of view. The ‘arm’ in images b and c is slightly distorted from its circular shape, and has an average CT number of approximately -150 HU, compared to its true value of 0. Image d shows the ability of the scanner to reconstruct images all the way to the edge of the gantry bore, as the ‘arm’ is resting on the bottom of the gantry. With objects this far from the standard field of view, spatial distortion is much greater, and the CT number is further from the true value. Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Figure 9. Images with simulated arm in extended field of view, at range of window levels (WL), window widths (WW) and image fields of view (FOV) a. WL: 300 WW: 1500 FOV: 500 mm b. WL: 300 WW: 1500 FOV: 650 mm c. WL: 300 WW: 1500 FOV: 820 mm d. WL: 300 WW: 2000 FOV: 820 mm Arm resting on gantry 23 Patient couch movement Z-axis couch movement accuracy under manual control The couch was loaded with approximately 70 kg, and moved 1000 mm, as indicated on the gantry using the manual controls. The distance between the start and end positions was measured to be exactly 1000 mm. Z-axis couch movement accuracy under scanner control Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation The couch was loaded with approximately 70 kg, and a calibrated rod was scanned helically. 1.5 mm images were reconstructed at 0.2 mm intervals at each end of the rod. The z-axis positions of the images showing the first and last calibrated marks on the rod were -1066.8 and -607.8 mm, giving a measured distance of 459 mm between the marks which are 460 mm apart. Couch deflection under load The scanner was installed in a temporary location, and the couch was not securely bolted to the floor, so this test could not be performed. 24 Radiation dose CTDI100 measured free in air Table 15 shows CTDI100 measured at the scan isocentre, using 200 mA, 1 s, 4 x 3 mm collimation. Table 15. CTDI100 free in air 80 CTDI100 (head) (mGy/100mAs) 11.5 CTDI100 (body) (mGy/100mAs) 11.6 100 20.4 20.5 120 30.6 30.9 140 Not selectable 44.0 kV Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Table 16 shows relative CTDI100 values per 100 mAs, measured at 120 kV and 200 mA. Table 16. Variation of CTDI100 with scan parameters Parameter Collimation (mm) Scan time (s) † Setting Relative CTDI100 5 (1 x 5) 0.73 10 (1 x 10) 6 (20* x 0.6) 0.73 0.82 12 (20 x 0.6) 1.00 12 (40* x 0.6) 0.92 28.8 (24 x 1.2) 0.82 0.5 1 1.00 1.00 † 0.36 † 0.72 1.01 1.00 = partial scan 25 Radiation dose CTDI100 and CTDIw in acrylic phantoms Measured in 160 mm and 320 mm diameter CTDI phantoms for head and body field of view (FOV) respectively. Scan parameters were 120 kV, 200 mA, 1 s, 4 x 3 mm collimation. Results are normalised per 100 mAs (see Table 17 and Table 18). ImPACT’s measured CTDI values are compared with Siemens figures in Table 19. Table 17. CTDI100 and CTDIw in acrylic phantoms, head FOV 80 CTDICentre (mGy/100mAs) 6.9 CTDIPeriphery (mGy/100mAs) 7.6 CTDIW (mGy/100mAs) 7.4 100 13.4 14.3 14.0 120 21.2 22.4 22.0 kV Table 18. CTDI100 and CTDIw in acrylic phantoms, body FOV 80 CTDICentre (mGy/100mAs) 1.8 CTDIPeriphery (mGy/100mAs) 4.0 CTDIW (mGy/100mAs) 3.2 100 3.9 7.8 6.5 120 6.6 12.5 10.6 140 10.1 18.6 15.8 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation kV Table 19. Comparison of ImPACT's measured CTDI with Siemens values at 120 kV CTDICentre (mGy/100mAs) CTDIPeriphery (mGy/100mAs) CTDIW (mGy/100mAs) ImPACT Siemens 17.4 18.4 18.0 18.2 19.3 18.9 Ratio ImPACT:Siemens 0.96 0.95 0.95 ImPACT 5.4 10.3 8.7 Siemens Ratio ImPACT:Siemens 5.6 10.8 9.1 0.96 0.95 0.95 Head CTDI Body CTDI 26 Radiation dose X-ray beam width X-ray beam profiles measured with radiotherapy verification film at the centre of the field of view. Films were read with a scanning micro-densitometer, and the optical density measurements converted to exposure. The full width at half maximum was calculated for each exposure profile (see Table 20). Table 20. X-ray beam width Collimation (mm) (no. slices x slice width) 5 (1 x 5) Irradiated FWHM (mm) Ratio (irradiated:nominal) 5.4 1.07 10 (1 x 10) 6 (20* x 0.6) 10.5 1.05 7.2 1.20 12 (20 x 0.6) 17.5 1.46 12 (40* x 0.6) 16.0 1.33 34.8 1.21 28.8 (24 x 1.2) Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Z-axis geometric efficiency Geometric efficiency is given as the percentage of the total x-ray beam exposure falling within the z-axis detector length. The figures in Table 21 were calculated from the profiles of beam exposure in the ‘x-ray beam width section’, above. The fraction of the exposure that was within the nominal length of the selected detectors used for acquisition was calculated (see Table 21). Geometric efficiency is formally defined by an IEC standard1. Actual or physical z-axis detector length, rather than the nominal z-axis detector length would ideally be used. Any difference between the nominal and actual values will affect the accuracy of the geometric efficiency results. Table 21. Z-axis geometric efficiency Collimation (mm) (no. slices x slice width) 5 (1 x 5) Geometric efficiency (%) 94 10 (1 x 10) 6 (20* x 0.6) 71 12 (20 x 0.6) * 12 (40* x 0.6) 72 93 28.8 (24 x 1.2) 81 * value removed from report, as result is inconsistent with other values (see Appendix 2). “The geometric efficiency in z-direction is the integral of the dose profile along the z-direction , integrated over the range subtended by the detector elements used during acquisition, expressed as a percentage of the total integral of the dose profile in the z-direction. The range is defined by the geometry of the selected detector elements and by the selected post-patient collimation used during acquisition, whichever is less” (from IEC 60601-2-44 Ed 2.1) 1 27 Radiation dose Extra rotations for helical scanning In order to reconstruct the first and last images in a helical scan sequence, the z-axis scan length needs to be longer than the imaged length. The ‘extra rotations’ beyond the number required to cover the imaged length depends upon the reconstruction technique, and a number of other factors. Table 22 shows the number of extra rotations required when imaging a series 24 mm long, using a range of pitches, slice thicknesses and collimations. The rotation time was 1 s for all exposures, so the extra rotations can be calculated by subtracting the time that is required for the couch to travel 24 mm from the total exposure time. The number of extra rotations was also verified with ionisation chamber measurements; the results agreed to within 2%. Table 22. Extra rotations for helical scanning Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Collimation (mm) Image width Time to travel Helical pitch (# x width) (mm) 24 mm (s) 40* x 0.6 3 0.55 3.64 Exposure time (s) 4.95 Extra rotations 1.31 40* x 0.6 40* x 0.6 3 3 0.8 1.2 2.50 1.67 3.91 3.11 1.41 1.44 40* x 0.6 3 1.5 1.33 2.81 1.48 40* x 0.6 6 0.8 2.50 3.91 1.41 24 x 1.2 3 0.8 1.04 2.57 1.53 28 Low contrast detectability Catphan™ low contrast detectability measurements The low contrast section of a Catphan™ 500 was scanned, and the 0.3% (3 HU) contrast details scored for the smallest visible detail in 20 images by 4 observers. Figure 10 shows one of these images. The detail size quoted in Table 23 is the smallest detail that was seen in at least 50% of the images viewed by the observers. Scan conditions were 120 kV, 184 mAs, 1 s rotation, 24 x 1.2 mm collimation, 9.6 mm slice, B30 filter. This results in a CTDI at the surface of the Catphan™ of 26 mGy, this gives a comparable image quality to ImPACT’s standard settings of 25 mGy CTDI and a 10 mm slice. Table 23. Low contrast detectability, ImPACT assessment Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Smallest visible Nominal contrast detail (mm) 8 0.3% Surface dose (mGy) 25 Figure 10. Catphan™ low contrast section on Sensation Open, displayed with window level 50 HU, window width 30 HU 29 Low contrast detectability Table 24 shows Siemens low contrast results, using 120 kV, 1 x 10 mm slice and 140 mAs. Table 24. Low contrast detectability, Siemens figures Surface dose (mGy) 18 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Smallest visible Nominal contrast detail (mm) 5 0.3% 30 Appendix 1. Scanner specifications COUCH Couch top material Carbon fibre Couch top length and width (cm) 243 x 40 Horizontal movement range (cm) 200 Horizontal movement speeds (mm/sec) 1 - 150 Accuracy/reproducibility of table positioning (mm) ± 0.25 Scannable horizontal range without table top extension (cm) 157 Scannable horizontal range with table top extension(s) (cm) 157 Vertical movement range out of gantry (cm) 53 - 102 Vertical movement range in gantry (cm) 82.5 - 102 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Minimum couch top height outside gantry (cm) 53 Maximum weight allowed on couch (kg) 200 (280 option) Maximum weight on couch which still achieves stated performance specifications (kg) 200 (280 option) SCANNER GANTRY Generation 3rd Slipring Low voltage Aperture (cm) 82 Scan fields of view (cm) 50 Tilt range (degrees) ± 30 Type of positioning lights Laser Accuracy of positioning lights (mm) ±1 X-RAY GENERATOR Type High frequency Location Rotation assembly Power rating (kW) 50 kV settings available 80, 100, 120, 140 28 - 400 (1mA steps) mA range and step size 80 kV : 400mA, 100kV : 400mA 120kV : 400mA, 140kV : 360mA Max. mA at each kV 31 Appendix 1. Scanner specifications X-RAY TUBE Type and make Siemens Straton Focal spot size(s) (mm), quoted to IEC 336/93 standard 0.7 x 0.7 0.8 x 1.1 Settings at which focal spot changes. kW = kV x mA / 1000 medium 55 kW, large 60 kW Total filtration (inherent + beam shaping filter) at central axis (mm Al equivalent) 6.8 Anode heat capacity (MHU) 0.6, equiv to 30 Maximum anode cooling rate (kHU/min) 5000 Method of cooling Oil to air Guaranteed tube life 1 year unlimited guarantee DETECTION SYSTEM Detector type Solid state (Ultra Fast Ceramic) Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Number of detectors per row 672 (1344 channels) Number of elements along z-axis 40 Effective length of each element at isocentre (mm) 32 x 0.6, 8 x 1.2 Total effective length of detector array at isocentre (mm) 28.8 Future option for more slices / rotation Yes SYSTEM START-UP AND CALIBRATION Power-on to warm-up time from fully off (mins) 4 Tube warm-up time from 'cold' to operating temperature (mins) 0 Time to perform detector calibrations at warm-up (mins) 5 Recommended frequency for any additional calibration by the radiographer Not required Time to perform these additional calibrations (mins) Not required Total time from fully off to scanning in an emergency (mins) 4 32 Appendix 1. Scanner specifications SCAN PARAMETERS kV settings available 80, 100, 120, 140 28 - 400 (1mA steps) mA Range and Step size 80 kV : 400mA, 100kV : 400mA 120kV : 400mA, 140kV : 360mA Max. mA at each kV Maximum continuous scan time (s) 100 HELICAL AND AXIAL SCANNING Rotation times for axial scaning (s) * = Partial scans 0.33*(option), 0.5 (option), 0.67*, 1.0s Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Rotation times for helical scanning (s) 0.5 (option), 1.0 Axial slice widths (number x width, mm) 12 x 2.4, 4 x 3, 1 x 5, 1 x 10 Helical acquisition widths (number of channels x width, mm) 24 x 1.2, 20 x 0.6, (40* x 0.6, 20* x 0.6 option) Pitches available for routine scanning (range and increment) 0.45 - 2.0 freely selectable Recommended pitches for optimal image quality 0.45 - 2.0 freely selectable Helical interpolation algorithms available SureView, AMPR cone-beam artefact reduction Maximum number of rotations in one helical run at standard abdomen parameters 160 (324mA, 0.5s) 176 (300mA, 0.5s) 200 (273mA, 0.5s) Starting with a cold tube, the maximum helical scan distance using a 1 mm imaged slice thickness and a pitch of 1.5 (mm) 1570 Gantry tilt range for helical scanning (degrees) Not available SCANNED PROJECTION RADIOGRAPHY (SPR) Maximum SPR length (mm) 1536 SPR field dimensions (mm x mm) 500 x 1536 Angular positions of X-ray tube available for SPR AP, PA, LAT (±1° accuracy) Real time image Yes Accuracy of slice prescription from the scanogram (mm) ± 0.5 Accuracy of distance measurements from SPR's taken at isocentre (lateral and axial directions) (mm) ± 0.5 33 Appendix 1. Scanner specifications Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation IMAGE RECONSTRUCTION ON MAIN CONSOLE Reconstruction fields of view (cm) 5 - 50 Extended scan field of view (cm) 51 - 82 Reconstruction matrix 512 Minimum reconstruction interval in helical scanning (mm) 0.1 Reconstruction time from the start of data acquisition to the appearance of the 30th image of a series for a standard axial brain scan (s) 3 (up to 20 f/s) Reconstruction time from the start of data acquisition to the appearance of the 30th image of a series for an axial spine scan (s) 3 (up to 20 f/s) Reconstruction time from the start of data acquisition to the appearance of the 30th image of a series for a helical abdomen scan (s) 3 (up to 20 f/s) Simultaneous scanning and reconstruction Yes Any delay in either scanning or reconstruction when performed concurrently No Simultaneous scanning and routine analysis Yes Simultaneous scanning and archiving and/or hard copying Yes Simultaneous scanning and transfer to second console/workstation Yes 34 Appendix 1. Scanner specifications FACTORS AFFECTING IMAGE QUALITY AND DOSE Post-patient collimation for narrow slices No Automatic mA control (AEC / mA modulation) software Yes, (CAREDose 4D) - mA adjustment for patient size Yes - mA adjustment along the z-axis Yes - mA modulation during rotation Yes Number of helical gantry rotations required at each end of total imaged volume. Info. not available Adaptive filtration for noise reduction Yes (automatic) Quarter detector shift Yes Moving (dynamic/flying) focal spot, x-y plane Yes Number of imaging detectors per row 672 (1344 channels) Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Sampling frequency (Hz) Artefact reduction algorithms Cone beam correction Max. 2320 Modified beam hardening (abdomen, pelvis, shoulder), Motion correction (sequential modes), Posterior Fossa optimisation Yes, SureView and AMPR cone-beam artefact reduction 35 Appendix 1. Scanner specifications MANUFACTURER'S PERFORMANCE DATA In plane spatial resolution (lp/cm) for sharpest clinical algorithm. Acquisition parameters in brackets. MTF0: 15.4 ±10%, MTF2: 14.5 ±10% (1s, 160 mA, 120 kV, 1.2 mm) Contrast resolution: smallest rod size (mm) discernable at given parameters in 20 cm CATPHAN Spiral: 5 mm @ 0.3% @ 21 mGy (120 kV, 140 mAs 10 mm) CT number accuracy (HU) Air: ± 10, Water: ± 4 CTDI settings for std head 120 kV, 24 mm CTDI (mGy/100mAs), centre of head phantom 18.7 CTDI (mGy/100mAs), periphery of head phantom 19.8 Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation CTDI settings for std body 120 kV, 24 mm CTDI (mGy/100mAs), centre of body phantom 5.8 CTDI (mGy/100mAs), periphery of body phantom 11.1 28.8: 34.3 (s) 12 (20 x 0.6): 17 (s) 12 (40* x 0.6): 15.8 (s) 6 (20* x 0.6): 6.7 (s) 10: 10 (s) 5: 5 (s) Dose profile FWHM (mm) (focal spot size in brackets) OPERATOR'S CONSOLE Diagonal dimension of image screen (inches) Number of monitors at console (functions of each if > 1) 19 1, optional 1 x review and processing (shared database) Image area matrix dimensions 1024 x 1024 Usual range of CT Number displayed (HU) -1024 to +3071 (-10,240 to 30,710 if metal implants) Accuracy of distance measurements in x-y plane (mm) depends on pixel size Weighted CTDI (CTDIw or CTDIvol) displayed on console Yes Dose Length Product (DLP) displayed on console Yes Geometric Efficiency displayed on console when <70% Yes Control methods Mouse, keyboard 36 Appendix 1. Scanner specifications MAIN COMPUTER Make and model Siemens PC compatible with array processors Operating system Windows XP Type and speed of CPU Pentium Xeon 3.6 GHz Amount of computer RAM supplied as standard (Gbytes) 18 Maximum amount of computer RAM (Gbytes) 18 IMAGE STORAGE Total standard hard disk capacity (Gbytes) 627 Maximum hard disk capacity (Gbytes) 627 Hard disk capacity for image storage (Gbytes and no. of uncompressed 512x512 images) 146 (260,000 images) Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Hard disk capacity for storage of raw data files (Gbytes and no. of data files) 300 Archive options MOD and CD writer (standard) Capacity of a single archive disk (Gbytes and no. of images) MOD: 4.1GB (26,000 lossless images), CD-R: 0.65GB (4100 lossless images) Time to mount an archive disk or tape (s) Approx 30 for full disk Archive data transfer rate (images / s) 2-3 INDEPENDENT WORKSTATION Is a workstation provided? No, option (LEONARDO) Computer make and model Siemens Fujitsu Xeon Operating system Windows XP Type and speed (GHz) of CPU 2 x Xeon 3.2 Amount of computer RAM supplied as standard (Gbytes) 2 Maximum amount of computer RAM (Gbytes) 3 Total hard disk storage capacity supplied as standard (Gbytes) 147 Maximum total hard disk storage capacity (Gbytes) 147 Archive options CD-R standard, MOD option Capacity of a single archive disk (Gbytes) MOD: 4.1, CD-R: 0.65 Environmental requirements (max/min temperature, humidity) for workstation 10-35 ºC, 20-80% relative humidity 37 Appendix 1. Scanner specifications 3D RECONSTRUCTION ON MAIN CONSOLE (MC) AND WORKSTATION (WS) MIPs and MinIPs (maximum and minimum intensity projections) MC-standard, WS-option SSD (3D Shaded Surface Display) MC-standard, WS-option 3D Volume rendering software MC-option, WS-option 3D Virtual endoscopy MC-option, WS-option MPR (Multi-planar reconstruction) Planes available in MPR MC-standard, WS-option Axial, sagittal, coronal, oblique, curvilinear OPTIONAL FACILITIES [MC - MAIN CONSOLE, WS - WORKSTATION] Contrast injector Option Contrast media bolus tracking Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation CT fluoroscopy software and hardware Standard (CAREBolus) Option (CAREVision with HandCARE) Hard-copy imaging device Option Radiotherapy planning table top Option Carbon fibre breast board Option Means for attaching patient immobilisation devices and a stereotactic frame to the end of the couch Option Bone Mineral Densitometry CT Angiography MC-option, WS-option (Osteo CT) MC-standard basic package, WS-option Dental MC-option, WS-option (Dental CT) Coherence Dosimetrist (separate workstation) Radiotherapy CT simulation software Prospective ECG-triggered cardiac software Not available Retrospective ECG-gated cardiac software Not available CT Perfusion software MC-option, WS-option (Perfusion CT) 38 Appendix 1. Scanner specifications INSTALLATION REQUIREMENTS Environmental requirements (max/min temperature, humidity) in scanner room 15-28 ºC, 15-75% relative humidity Environmental requirements (max/min temperature, humidity) in scanner control room 15-28 ºC, 15-75% relative humidity Peak heat output from system during scanning (kW) 1.53 (add 0.07 for CT fluoro) System cooling method Water - water Air conditioning requirements for scanner room of minimum floor area None Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Minimum floor area required for the system (m²) 30 Gantry dimensions (H x W x D (mm)) and weight (kg) 1990 x 2280 x 940, 2000 kg Couch dimensions (H x W x L (mm)) and weight (kg) 1022 x 690 x 2430, 500 kg Supplementary unit dimensions (H x W x D (mm)) and weight (kg) Power unit: 1815 x 905 x 800, 550kg Cooling unit: 1815 x 905 x 860, 200kg Power supply requirements 3 phase 380-480V, 66-80kVA IMAGE TRANSFER / CONNECTIVITY Speed of scanner / workstation connections to local area networks (Mbits/s) 1000 Remote PC access to images on workstation Option DICOM services on Main Console Storage SCU, SCP Print SCU Query / retrieve SCU, SCP Modality worklist SCU Performed procedure step SCU Storage commitment SCU DICOM services on Workstation Storage SCU, SCP Print SCU Query / retrieve SCU, SCP Modality worklist management Not available Performed procedure step Not available Storage commitment SCU 39 Appendix 2. Manufacturer’s comments Medical Solutions Siemens Medical Solutions MED CTE PA P.O. box 12 66 Mrs. Maria Lewis ImPACT Group, St George's Hospital, London SW17 0QT D-91301 Forchheim Germany 8 November 2005 Dear Madam, re: ImPACT evaluation report - Siemens Somatom Sensation Open On behalf of Siemens Medical Solutions, I wish to thank you for the opportunity to review this ImPACT evaluation report. . In response to the evaluation method for Z-axis geometric efficiency, Siemens wish to present the following additional points for consideration: Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation The presented method for evaluation is different as compared to the definitions of z-axis geometric efficiency presented in the IEC standards 60601-2-44, Edition 1 or the current Edition 2.1. The report appreciates this fact at least to some degree, but for clarification we would like to add: o The measurement of a dose profile with x-ray film is very much dependant on exposure, linearity and processing of the film. IEC 61223-3-5 states that a thorough calibration of the film-method is needed to a evaluate dose profiles with film. The ImPACT report does not explain how this was is done in general or for the scanner under test. o The report claims that `the nominal z-axis detector length' was used for integration. We again have to state, as we did formerly, that we do not specify any nominal detector length or nominal length of the range of elements used during acquisition for our scanners. It is with respect that we add that we have not seen such data specified from other manufacturers either. o We assume that in this report the sum of the nominal slice width, which is typically specified for CT scanners, was used to define the range for integration. It is known in CT, that the slice width, i.e. the full-width-at-half-maximum value of the slice sensitivity profile, is not equal to the sensitive range of the corresponding detector elements. Such differences in definitions and test methods might explain discrepancies to our specified values for z-efficiency, evaluated according to IEC standard 60601-2-44, Edition 2.1, following the EU directive for medical devices. The very low value of 65% for collimation 20 x 0.6 mm presented in the ImPACT report can hardly be explained by differences in methodology. Since this value is also in conflict with the dose measurements presented in this ImPACT report, we suggest that this value is verified either by measurement or repeated evaluation of data. Division Computed Tomography Head: Dr. Bernd Montag Medical Solutions Group Executive Management: Prof. Dr. E. R. Reinhardt, Group President Dr. Hermann Requardt Götz Steinhardt Postal address: Siemens AG Office address: Henkestr. 127 D-91052 Erlangen P.O. box 12 66 Telephone (09131) 84-0 D-91301 Forchheim Erlangen Page 1 of 2 40 Appendix 2. Manufacturer’s comments We thank you for the opportunity to respond to this report, but should you have any questions, please do not hesitate to contact us accordingly. Yours sincerely, Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation Dr Christoph Suess Ph.D CT Research & Development Siemens Medical Solutions Forchheim, Germany Page 2 of 2 41 Appendix 2. Manufacturer’s comments ImPACT thanks Siemens for their response. ImPACT takes care to calibrate the x-ray film used in z-axis geometric efficiency testing. Calibration films have been exposed at a wide range of exposure intensities, using an x-ray beam with equivalent filtration to a typical CT scanner. Optical density is then converted to exposure, with a correction for background optical density. The absolute accuracy of film dose measurement is not as good as with TLDs, but for geometric efficiency measurements, the shape of the beam profile is more important than its absolute magnitude. The spatial resolution of film is much greater than TLDs, so the profile shape is better defined. Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation The nominal sizes of the detector elements used for geometric efficiency calculation are taken from the responses provided by manufacturers to our standard specification questionnaire (see Appendix 1). In the future, further questions will be asked about the number of detectors used at each collimated beam width. The z-axis geometric efficiency value measured for the 20 x 0.6 mm collimation has been removed from this report, as it does not appear to be consistent with the rest of the values. This inconsistency is suspected to relate to incorrect scan parameter selection during testing, rather than a shortcoming of the measurement technique. 42 Appendix 3. Image quality assessment and Q Image noise, scan plane spatial resolution and imaged slice width are fundamental parameters describing the amount of object information retrievable from an image, or its image quality. Radiation dose can be regarded as a 'cost' of this information. In general, it is meaningless to quote any one of these measurements without reference to the others. It is possible to incorporate dose, noise, spatial resolution and slice width into one number, using formulae derived from the relationships between image quality and dose. Figures of merit such as this can take a number of forms depending on how the various parameters are measured and quoted. ImPACT use the Q2 value, whose formula and methods of measurement are given below. Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation High Q2 values result from CT scanners that produce images with lower noise at a set spatial resolution, when dose and image width are taken into account. The parameters used in Q are standard imaging performance parameters. However it should be noted that the quantification of perceived image quality is a complicated process and as such will not be fully described by the single descriptors used for each of the parameters. Comparisons between scanners are more reliable when comparing scans reconstructed with similar convolution filters. The uncertainty in quoted values of Q2 is up to about ± 15 %, with a conservative estimate of ± 10 %. Q2 is calculated as follows: 3 fav Q2 = 2 σ z1CTDIvol σ = image noise, expressed as a percentage (for water, standard deviation in HU divided by 10), for a 5 cm2 region of interest at the centre of the field of view in the standard ImPACT water phantoms. fav = spatial resolution, given as (MTF50 + MTF10)/2, where MTF50 and MTF10 are the spatial frequencies corresponding to the 50 % and 10 % modulation transfer function values respectively (in line pairs per cm). Reconstruction filters with standard spatial resolution values are chosen to minimise the dependency of Q2 upon reconstruction filters. The reconstruction filter with MTF50 and MTF10 values as close as possible to 3.4 c/cm and 6.0 c/cm is used (c/mm used in the calculation for consistency of units with z-sensitivity). z1 = the full width at half maximum (FWHM) of the imaged slice profile (zsensitivity). This is measured using the inclined plates method (mm). CTDIvol = volume weighted CT dose index (mGy). 43 Appendix 4. About ImPACT ImPACT ImPACT (Imaging Performance Assessment of Computed Tomography) is the Department of Health’s CT evaluation facility. It is based at St George's Hospital, London, part of St George's Healthcare NHS Trust. ImPACT has developed test objects and measurement procedures suitable for inter-comparing CT scanner performance. For each CT evaluation hundreds of images are obtained from the system under test and subsequently analysed using custom written software. Dose measurements are made using ion chambers, and x-ray film is used to obtain additional x-ray dose information. Support to purchasers and users Report 05071: Siemens Somatom Sensation Open CT scanner technical evaluation The ImPACT team is available to answer any queries with regard to the details of this report, and also to offer general technical and user advice on CT purchasing, acceptance testing and quality assurance. ImPACT Bence Jones Offices St George’s Hospital London SW17 0QT T: +44 (0) 20 8725 3366 F: +44 (0) 20 8725 3969 E: [email protected] W: www.impactscan.org 44