Download Report 05071 Siemens Somatom Sensation Open CT scanner

Report 05071
Siemens Somatom Sensation Open
CT scanner technical evaluation
November 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
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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
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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
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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.
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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).
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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