Report 05070
GE LightSpeed RT CT scanner
technical evaluation
November 2005
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GE LightSpeed RT CT
scanner technical evaluation
Assessed at Weston Park Hospital, Sheffield, UK, November 2004
David Platten
Nicholas Keat, Maria Lewis, Sue Edyvean
ImPACT
Bence Jones Offices
St George’s Hospital
London SW17 0QT
Tel: 020 8725 3366
Fax: 020 8725 3969
Report 05070: GE LightSpeed RT CT scanner technical evaluation
e-mail: impact@impactscan.org
For more information on ImPACT visit www.impactscan.org
© Crown Copyright 2005
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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: hmsolicensing@cabinet-office.x.gsi.gov.uk.
Contents
Contents .....................................................................................................4
Summary.....................................................................................................5
GE LightSpeed RT.........................................................................................5
Clinical scans .............................................................................................6
Dose and image quality.............................................................................7
Scanner Q2 values.........................................................................................8
Image noise ................................................................................................9
Variation of image noise with scan parameters.............................................9
Variation of image noise with reconstruction filter .......................................10
Inter-slice noise variation.............................................................................11
CT number accuracy and uniformity .....................................................12
CT number accuracy and uniformity............................................................12
CT number and electron density linearity....................................................14
Spatial resolution.....................................................................................15
Variation of spatial resolution with scan parameters ...................................15
Spatial resolution and image noise..............................................................16
Limiting resolution and comparison with GE data .......................................17
Slice width characteristics......................................................................18
Imaged slice thickness: axial .......................................................................18
Imaged slice thickness: helical ....................................................................19
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Extended field of view .............................................................................20
Extended field of view..................................................................................20
Patient couch movement ........................................................................22
Z-axis couch movement accuracy under scanner control ...........................22
Couch deflection under load........................................................................22
Radiation dose .........................................................................................23
CTDI100 in air................................................................................................23
CTDI100 and CTDIw in acrylic phantoms .....................................................24
X-ray beam width.........................................................................................25
Z-axis geometric efficiency ..........................................................................25
Low contrast detectability.......................................................................26
Appendix 1. Scanner specifications ......................................................27
Appendix 2. Manufacturer’s comments.................................................39
Appendix 3. Image quality assessment and Q......................................40
Appendix 4. ImPACT................................................................................41
ImPACT .......................................................................................................41
Support to purchasers and users ................................................................41
Summary
GE LightSpeed RT
The GE LightSpeed RT is a third generation 4-slice helical CT scanner with a
gantry opening that is wider than those found on standard diagnostic CT
systems.
A generator rating of 53.2 kW enables a maximum current of 440 mA at 120 kV.
Rotation times of 1, 2, 3 and 4 seconds are available.
Routine beam collimations available from the system’s 20 mm, 16 x 1.25 mm
element detector bank are 4 x 1.25, 4 x 2.5, 4 x 3.75 and 4 x 5 mm. Narrow
collimations of 2 x 0.625 and 1 x 1.25 mm are also available (see Figure 1).
Figure 1. LightSpeed RT z-axis detector layout
Report 05070: GE LightSpeed RT CT scanner technical evaluation
16 x 1.25 mm
z-axis
The wide gantry aperture of 80 cm allows flexibility with patient scanning
position, in particular for patients being scanned as part of the radiotherapy
treatment planning process. 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.
In addition to having a larger gantry bore, the RT also allows images to be
reconstructed with a larger field of view than a standard CT system. Traditional
reconstruction methods on the RT allow images up to 50 cm to be
reconstructed. In addition, extended field of view reconstruction enables images
with a field of view of 65 cm to be generated. These images can be used to
visualise anatomy that lies beyond the standard 50 cm field of view, again
helpful for radiotherapy treatment planning. However, it should be noted that the
portion of the image in the extended field of view will have a reduced image
quality with respect to spatial resolution, noise and CT number accuracy,
compared to that in the primary field of view.
5
Clinical scans
The scan settings chosen for the six clinical scans, defined in ImPACT report
MDA/98/25, were given by GE for the LightSpeed RT, and said by GE to be
representative of protocols in clinical use. In addition, helical protocols for head
and abdomen were used.
Results in italics are mean results for the following four-slice CT scanners: GE
LightSpeed RT, GE LightSpeed Plus, Philips Mx8000, Siemens Sensation 4
and Toshiba Aquilion. Note that the mean z-sensitivity figures may be a result of
collimations that differ from those used for the LightSpeed RT (see Table 1).
Table 1. Clinical scan tables
250
120
Helical
brain
120
200
1
5
(4 x 2.5)
Inner ear
140
120
1
0.63
(2 x 0.63)
270
1
Helical
inner ear
140
70
1
0.63
(2 x 0.63)
Axial
abdomen
120
190
1
5
(4 x 5)
Helical
abdomen
120
Low noise
spine
120
High res
spine
120
250
290
200
1
1
1
5
(4 x 5)
0.75
1
1.5
SOFT
250
SOFT
120
EDGE
120
EDGE
380
STANDARD
380
2.5
(4 x 2.5)
120
1.25
(4 x 1.25)
120
6
STANDARD
STANDARD
BONE+
MTF10 (c/cm)
5
(4 x 5)
Standard
brain
MTF50 (c/cm)
SOFT
Pitch
250
Noise (HU)
5
(4 x 5)
Z-sens (mm)
Slice thickness (mm)
(Collimation (# x mm))
1
Recon filter
Time (s)
320
Recon FOV (mm)
mA
kV
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Post fossa 120
Results
CTDIvol (mGy)
Scan parameters
Scan
56.8
4.7
2.6
3.1
5.7
54.9
4.6
3.2
3.2
5.9
47.9
4.7
2.8
3.1
5.7
46.9
6.3
2.9
3.1
5.8
52.7
4.9
2.6
3.2
5.8
43.5
0.9
97
7.8
13.4
51.2
0.8
141
9.1
14.4
25.4
1.0
111
6.3
11.9
16.2
4.7
22
3.9
6.3
15.4
6.6
21
3.8
6.3
14.8
6.4
20
3.8
6.5
15.8
6.5
19
3.7
6.4
29.9
2.3
26
3.5
6.1
32.1
2.4
25
3.7
6.3
24.4
1.1
257
7.8
10.8
30.5
1.7
181
7.6
11.0
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 3 for more details).
The Q2 values presented in this section (see Table 2 and Table 3) are for head
and body imaging. Standard acquisition and reconstruction parameters are
used where possible to minimise slight variations that occur for different kV,
slice thickness, scan time and reconstruction filter. These standard settings are
listed below:
•
•
•
•
•
Report 05070: GE LightSpeed RT CT scanner technical evaluation
•
•
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)
Small focal spot size.
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 four-slice CT scanners: GE LightSpeed
RT, GE LightSpeed Plus, Philips Mx8000, Siemens Sensation 4 and Toshiba
Aquilion.
7
Dose and image quality
Scanner Q2 values
Table 2. Q2 value for head scanning
Scanner
Filter
GE LightSpeed RT
SOFT
Mean
mAs for z-sens Noise
50 mGy (mm)
(HU)
MTF50
MTF10
(c/cm)
(c/cm)
Q2
253
4.7
3.4
3.2
5.8
5.9
269
4.7
3.7
3.4
6.2
5.8
MTF50
MTF10
(c/cm)
(c/cm)
Table 3. Q2 value for body scanning
Scanner
Filter
GE LightSpeed RT
SOFT
Q2
151
4.7
20
3.5
6.0
2.0
165
6.5
18
3.5
6.0
2.0
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Mean
mAs for z-sens Noise
15 mGy (mm)
(HU)
8
Image noise
Variation of image noise 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, 270 mA, 1 s scan time, 20 mm
collimation resulting in 4 x 5 mm images. The SOFT reconstruction filter was
used.
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Table 4. Variation of image noise with scan parameters
Parameter
Setting
Relative noise
Adjusted relative
noise
mA
10
20
40
80
160
270
440
6.06
4.05
2.76
1.90
1.31
1.00
0.77
1.17
1.10
1.06
1.03
1.01
1.00
0.98
1
2
3
4
1.25 (1 x 1.25)
1.25 (2 x 0.625)
5 (4 x 1.25)
10 (4 x 2.5)
15 (4 x 3.75)
1.00
0.70
0.57
0.50
2.03
2.65
2.04
1.41
1.15
1.00
0.99
0.99
1.01
1.01
0.94
1.02
1.00
1.00
20 (4 x 5)
5 (2 x 2.5)
10 (2 x 5)
15 (2 x 7.5)
20 (2 x 10)
5 (1 x 5)
10 (1 x 10)
1.00
1.44
0.99
0.81
0.70
1.00
0.69
1.00
1.02
0.99
1.00
0.99
1.00
0.98
Scan time (s)
Collimation (mm)
(# x mm)
9
Image noise
Variation of image noise with reconstruction filter
Results are acquired using head and body sized uniform water phantoms (see
Table 5). Scans are axial, parameters are:
120 kV, 320 mA, 1 s scan time, 20 mm collimation resulting in 4 x 5 mm
images.
Table 5. Variation of image noise with reconstruction filter
Parameter
Recon. filter
(head)
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Recon. filter
(body)
Setting
Relative noise
SOFT
STANDARD
LUNG
DETAIL
BONE
EDGE
BONEPLUS
SOFT
STANDARD
LUNG
DETAIL
BONE
EDGE
BONEPLUS
1.00
1.26
5.85
1.80
5.39
10.61
7.81
1.00
1.27
5.86
1.80
5.40
10.58
7.82
10
Image noise
Inter-slice noise variation
Axial scans were made of ImPACT’s head phantom, and the noise from each
slice in a 4 x 5 mm and 4 x 1.25 mm slice acquisition was compared (see Table
6 and Table 7).
Results are for 120 kV, 270 mA, 1 s scan time and SOFT reconstruction filter.
Table 6. Variation of noise in each slice in a 4 x 5 mm axial acquisition
Detector group
Noise (HU)
Variation from
mean (%)
1
2
3
4
Mean
2.96
2.85
2.52
2.74
2.77
6.8
3.1
-8.9
-1.0
-
Results are for 120 kV, 200 mA, 1 s scan time and SOFT reconstruction filter.
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Table 7. Variation of noise in each slice in a 4 x 1.25 mm axial acquisition
Detector group
Noise (HU)
Variation from
mean (%)
1
2
3
4
Mean
7.70
7.89
7.71
8.05
7.84
-1.7
0.6
-1.6
2.7
-
11
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) 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 also at four positions 1 cm in from the
inside of the periphery of the phantoms corresponding to north, east, south and
west compass points.
Acquisition parameters for head scanning were 120 kV, 200 mA, 1 s scan time,
1 x 5 mm slice, SOFT filter, 250 mm scan and reconstruction field of view (see
Table 8).
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Table 8. CT number accuracy and uniformity for head scanning
Position
CT number
Difference from
centre (HU)
Centre
N
E
S
W
-1.24
0.96
1.10
1.09
1.10
2.20
2.34
2.33
2.34
Acquisition parameters for body scanning were 120 kV, 190 mA, 1 s scan time,
1 x 5 mm slice, SOFT filter, 500 mm scan field of view and 380 mm
reconstruction field of view (see Table 9).
Table 9. CT number accuracy and uniformity for body scanning
Position
CT number
Difference from
centre (HU)
Centre
N
E
S
W
1.96
0.56
1.44
1.74
1.48
-1.40
-0.52
-0.22
-0.48
12
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. Acquisition
parameters for the 480 mm phantom were 120 kV, 200 mA, 1 s, 1 x 5 mm slice,
SOFT filter (axial scan). The acquisition parameters for head and body as given
for Table 8 and Table 9.
Figure 2 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 %, 1020 % …. 80-90 % of the phantom’s internal diameter.
Acquisition parameters for the 480 mm phantom were 120 kV, 200 mA, 1 s, 1 x
5 mm slice, SOFT filter (axial scan). The acquisition parameters for head and
body as given for Table 8 and Table 9.
Figure 2. CT number in annular ROI at increasing diameter from the
isocentre for head, body and 480 mm phantoms
15
CT number (HU)
Report 05070: GE LightSpeed RT CT scanner technical evaluation
10
5
0
-5
Head
Body
Large FOV
-10
-15
0
40
80
120
160
200 240 280 320
ROI Diameter (mm)
13
360
400
440
480
CT number accuracy and uniformity
CT number and electron density linearity
An RMI 467 electron density phantom was scanned with 200 mA, 1 s scan time,
1 x 5 mm slice, SOFT filter and a range of kV settings (see Figure 3).
Figure 3. CT number against electron density at a range of kVs
2000
80 kV
100 kV
120 kV
140 kV
CT number (HU)
1500
1000
500
0
Report 05070: GE LightSpeed RT CT scanner technical evaluation
-500
-1000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Relative electron density
14
1.4
1.6
1.8
Spatial resolution
Variation of spatial resolution with scan parameters
Spatial resolution is characterised by the average of the MTF50 and MTF10
values, the frequencies corresponding to the 50 % and 10 % modulation
transfer function values respectively (in cycles per cm).
Acquisition parameters were 120 kV, 1 s scan time, head mode, 10 mm
collimation resulting in 4 x 2.5 mm images. 200 mA was used for small focal
spot, and 300 mA for large focus (see Table 10).
Table 10. Variation of spatial resolution with scan parameters
Parameter
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Small focus
Large focus
Scan type
Setting
MTF50 (c/cm)
MTF10 (c/cm)
SOFT
STANDARD
LUNG
DETAIL
BONE
EDGE
BONEPLUS
SOFT
STANDARD
LUNG
DETAIL
BONE
EDGE
BONEPLUS
Axial (SOFT)
Helical (SOFT)
3.2
3.6
7.3
4.0
7.0
7.8
9.3
3.1
3.4
7.1
3.8
6.3
8.1
8.8
3.2
3.2
5.8
6.5
8.3
8.0
11.0
13.4
12.0
5.7
6.4
8.3
7.8
11.3
12.8
11.6
5.8
5.8
15
Spatial resolution
Spatial resolution and image noise
Figure 4 and Figure 5 show image noise and spatial resolution values for each
reconstruction filter, in head and body scanning respectively. Head scan
conditions are 120 kV, 253 mA, 1 s scan time, 4 x 5 mm collimation resulting in
a CTDIvol of 50 mGy. Body scan conditions are 120 kV, 151 mA, 1 s scan time,
4 x 5 mm collimation resulting in a CTDIvol of 15 mGy. Spatial resolution is
characterised by the average of the MTF50 and MTF10 values.
Figure 4. Image noise against spatial resolution for head scanning
35
30
SOFT
LUNG
BONE
BONEPLUS
Image Noise (HU)
25
STANDARD
DETAIL
EDGE
20
15
Report 05070: GE LightSpeed RT CT scanner technical evaluation
10
5
0
0
2
4
6
8
Mean MTF50 and MTF10 (c/cm)
16
10
12
Spatial resolution
Figure 5. Image noise against spatial resolution for body scanning
140
120
STANDARD
LUNG
DETAIL
BONE
EDGE
BONEPLUS
100
Image Noise (HU)
SOFT
80
60
40
20
0
0
2
4
6
8
10
12
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Mean MTF50 and MTF10 (c/cm)
Limiting resolution and comparison with GE data
ImPACT used settings of 120 kV, 200 mA, 3 s scan time, 10 mm collimation,
resulting in 4 x 2.5 mm images. A scan and reconstruction field of view of
120 mm was used, together with the EDGE reconstruction filter (see Table 11).
Table 11. Limiting spatial resolution
MTF50
MTF10
MTF0 (GE), MTF2 (ImPACT)
GE (c/cm)
ImPACT (c/cm)
10.5
13.9
15.4
8.6
13.2
14.4
17
Slice width characteristics
Imaged slice thickness: axial
Measured using 50 µm titanium ramps inclined at 30° to the scan plane, at the
centre of the field of view (see Table 12 to Table 15).
Table 12. Axial slice thickness, small focal spot
Nominal slice (mm)
(# x mm)
Report 05070: GE LightSpeed RT CT scanner technical evaluation
0.625 (2 x 0.625)
1.25 (1 x 1.25)
1.25 (4 x 1.25)
2.5 (4 x 2.5)
3.75 (4 x 3.75)
5 (4 x 5)
2.5 (2 x 2.5)
5 (2 x 5)
7.5 (2 x 7.5)
10 (2 x 10)
5 (1 x 5)
10 (1 x 10)
Measured slice (mm)
Ratio
(measured:nominal)
0.93
1.1
1.1
2.4
3.6
4.7
2.3
4.9
7.3
9.6
4.8
9.8
0.74
0.89
0.88
0.96
0.95
0.93
0.93
0.97
0.97
0.96
0.97
0.98
Table 13. Axial slice thickness, large focal spot
Nominal slice (mm)
(# x mm)
1.25
1.25
2.5
3.75
5
2.5
5
7.5
10
5
10
(1 x 1.25)
(4 x 1.25)
(4 x 2.5)
(4 x 3.75)
(4 x 5)
(2 x 2.5)
(2 x 5)
(2 x 7.5)
(2 x 10)
(1 x 5)
(1 x 10)
Measured slice (mm)
Ratio
(measured:nominal)
1.1
1.1
2.3
3.5
4.7
2.4
4.8
7.2
9.6
4.8
9.7
0.86
0.90
0.94
0.94
0.94
0.95
0.96
0.96
0.96
0.97
0.97
18
Slice width characteristics
Table 14. Variation of axial slice thickness with detector group, 4 x 5 mm,
large focal spot
Detector group
Measured slice
(mm)
Variation from
mean (%)
1
2
3
4
Mean
4.60
4.87
4.78
4.48
4.68
-1.9
4.0
2.2
-4.3
-
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Table 15. Variation of axial slice thickness with detector group,
4 x 1.25 mm, large focal spot
Detector group
Measured slice
(mm)
Variation from
mean (%)
1
2
3
4
Mean
1.14
1.13
1.13
1.10
1.12
1.5
0.2
0.3
-1.9
-
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. The full width at half maximum (FWHM) of the z-sensitivity
profile is quoted to characterise the slice thickness (see Table 16).
Table 16. Helical slice thickness (z-sensitivity)
Collimation (mm)
(# slices x slice width)
Mode
(pitchx)
Table feed
(mm/rotation)
Nominal image
thickness (mm)
Z-sensitivity
(FWHM) (mm)
Ratio
actual:nominal
20 (4 x 5)
20 (4 x 5)
20 (4 x 2.5)
20 (4 x 2.5)
0.75
1.5
0.75
1.5
15
30
15
30
5
5
2.5
2.5
5.1
6.4
2.5
3.2
1.0
1.3
1.0
1.3
19
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 6).
Figure 6. LightSpeed RT standard and extended fields of view
Extended field of view
Standard
field of view
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Gantry bore
The LightSpeed RT has a standard field of view of 500 mm. It can also
reconstruct images with a 650 mm diameter using extrapolated attenuation
measurements (Figure 6).Applications for this extended field of view include the
scanning of radiotherapy and very large patients.
A water filled cylinder of 94 mm diameter containing a 10 mm diameter Teflon
rod was used to represent an arm. This was positioned in the scanner next to
ImPACT’s body sized water phantom and scanned at various distances from
the body phantom to simulate the presence of an arm in the extended field of
view.
Figure 7(a) shows the body phantom without the simulated arm present. Figure
7(b) has the arm alongside the body phantom. The outline of the arm is well
defined, and the CT number of the water in the arm is close to 0 HU. Artefact is
visible around the Teflon insert in Figure 7(c). Also, the CT number of the water
in the arm has dropped by 200 HU. In the small portion of the arm present with
in the reconstructed field of view in Figure 7(d) there is a further drop in CT
number of the water, and some geometric distortion of the arm outline.
20
The CT number of the water in the body phantom is constant in each image,
and unaffected by the presence of the arm.
Figure 7. Images with simulated arm in extended field of view, all at image
fields of view of 650 mm
(b) arm on border of 500 mm
field of view
(c) arm on border of 650 mm
field of view
(d) arm almost outside 650 mm
field of view
Report 05070: GE LightSpeed RT CT scanner technical evaluation
(a) no ‘arm’
21
Patient couch movement
Z-axis couch movement accuracy under scanner
control
The couch was loaded with approximately 70 kg, and a rod of known
dimensions containing calibrated incremental markers was scanned helically.
1.25 mm images were reconstructed at 0.63 mm intervals at each end of the
rod. The z-axis positions of the images showing the first and last calibrated
markers on the rod were 133.98 and 33.98, giving a measured distance of
100 mm between markers which were known to be 100 mm apart.
Couch deflection under load
Report 05070: GE LightSpeed RT CT scanner technical evaluation
The couch top was scanned with the loaded couch at zero z-position. The
couch was then moved 112.9 cm into the gantry, as indicated by the scanner.
The scan was repeated in this extended position. Using the images, the vertical
position of the couch top was measured for the two couch positions. The loaded
couch was shown to deflect vertically by 1.9 mm at the scan plane in the course
of moving 112.9 cm.
22
Radiation dose
CTDI100 in air
Table 17 shows CTDI100 measured at the scan isocentre using 200 mA, 1 s
scan time and 1 x 10 mm collimation.
Table 17. CTDI100 in air
kV
CTDI100 (head)
(mGy/100mAs)
CTDI100 (body)
(mGy/100mAs)
80
100
120
140
13.2
21.1
30.2
42.1
9.2
15.9
23.9
34.6
Table 18 shows relative CTDI100 values per 100 mAs, measured at 120 kV and
200 mA.
Table 18. Variation of CTDI100 with scan parameters
Parameter
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Focal spot
Collimation (mm)
(# slices x slice
width)
Scan time (s)
Setting
Relative CTDI100
Small
Large
1.25 (1 x 1.25)
1.25 (2 x 0.63)
5 (1 x 5)
10 (1 x 10)
15 (2 x 7.5)
20 (4 x 5)
1
2
3
4
1.00
1.04
2.44
1.28
1.24
1.00
0.93
0.86
1.00
1.00
1.00
1.00
23
Radiation dose
CTDI100 and CTDIw in acrylic phantoms
Measured in 160 mm and 320 mm diameter CTDI phantoms for head and body
fields of view (FOV) respectively. Scan parameters are 120 kV, 200 mA, 1 s
scan time and 1 x 10 mm collimation (see Table 19 to Table 21).
Table 19. CTDI100 and CTDIw in acrylic phantoms, head FOV
kV
CTDICentre
(mGy/100mAs)
CTDIPeriphery
(mGy/100mAs)
CTDIW
(mGy/100mAs)
80
100
120
140
6.9
12.6
19.3
28.0
7.9
13.5
20.1
28.7
7.6
13.2
19.8
28.5
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Table 20. CTDI100 and CTDIw in acrylic phantoms, body FOV
kV
CTDICentre
(mGy/100mAs)
CTDIPeriphery
(mGy/100mAs)
CTDIW
(mGy/100mAs)
80
100
120
140
1.6
3.3
5.6
8.7
4.3
7.8
12.1
17.7
3.4
6.3
9.9
14.7
Table 21. ImPACT and manufacturer dose comparison at 120 kV
ImPACT figures are for 4x5 mm collimation, large focus
Phantom, position
ImPACT CTDI100
(mGy/100mAs)
GE CTDI100
(mGy/100mAs)
Head, centre
17.3
Head, periphery
18.0
Head, weighted
17.8
Body, centre
5.0
Body, periphery
10.8
Body, weighted
8.9
* GE's CTDI specification is currently under review
24
*
*
*
*
*
*
Ratio ImPACT:GE
*
*
*
*
*
*
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 densitometer (see Table 22).
Table 22. Irradiated length, large focal spot
Collimation (mm)
Irradiated FWHM (mm)
(# slices x slice width)
1.25
1.25
5
10
15
20
(2x0.63)
(1x1.25)
(4x1.25)
(4x2.5)
(4x3.75)
(4x5)
Ratio
(irradiated:nominal)
2.1
3.6
7.1
11.5
16.3
20.0
1.65
2.89
1.42
1.15
1.09
1.00
Z-axis geometric efficiency
Geometric efficiency is expressed as the percentage of the x-ray beam
exposure falling within the nominal collimated slice width (see Table 23).
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Table 23. Geometric efficiency, large focal spot
Collimation (mm)
(# slices x slice width)
1.25
1.25
5
10
15
20
(2x0.63)
(1x1.25)
(4x1.25)
(4x2.5)
(4x3.75)
(4x5)
Geometric
efficiency (%)
55
34
70
85
91
97
25
Low contrast detectability
The low contrast section of a Catphan™ 500 was scanned, as shown in Figure
8. The 0.3% (3 HU) contrast details scored for the smallest visible detail in 20
images by 4 observers. The detail size quoted in Table 24 is the smallest detail
that was seen in at least 50% of the images viewed by the observers. Scanning
conditions were 120 kV, 200 mA, 1 s scan time, 2 x 10 mm collimation, 10 mm
image, SOFT reconstruction filter. This results in a CTDI at the surface of the
Catphan™ of 25 mGy.
The GE figures use 120 kV, 135 mAs, 10 mm image and STANDARD
reconstruction filter. They are obtained from a statistical not visual analysis.
Table 24. Low contrast detectability
Smallest visible Nominal contrast
detail (mm)
(HU)
ImPACT
GE (statistical)
5
5
3
3
Surface dose
(mGy)
25
13.3
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Figure 8. Catphan™ low contrast section on LightSpeed RT, displayed
with window level 50 HU, window width 30 HU
26
Appendix 1. Scanner specifications
Table 25. Couch
GE LightSpeed RT
Couch top material
Carbon fibre
Couch top length and width (cm)
239 x 42
Horizontal movement range (cm)
170
Horizontal movement speeds (mm/sec)
up to 100
Accuracy/reproducibility of table positioning
(mm)
± 0.25
Scannable horizontal range without table top
extension (cm)
170 (Axial),
160 (Helical & Scout)
Scannable horizontal range with table top
extension(s) (cm)
170 (Axial),
160 (Helical & Scout)
Vertical movement range out of gantry (cm)
51 - 99
Vertical movement range in gantry (cm)
72.8 - 99.1
Minimum couch top height outside gantry (cm)
51
Maximum weight allowed on couch (kg)
205
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Maximum weight on couch which still achieves
stated performance specifications (kg)
180 (±0.25mm)
205 (±1mm)
Table 26. Scanner gantry
GE LightSpeed RT
Generation
3rd
Slipring
Low voltage
Aperture (cm)
80
Scan fields of view (cm)
25 and 50
Tilt range (degrees)
± 30
Type of positioning lights
Laser
Accuracy of positioning lights (mm)
± 1 at any laser to patient distance
27
Appendix 1. Scanner specifications
Table 27. X-ray generator
GE LightSpeed RT
Type
High frequency
Location
Rotation assembly
Power rating (kW)
53.2
kV settings available
80, 100, 120, 140
10 - 440
(5mA steps)
mA range and step size
80kV: 400 mA
100kV: 420 mA
120kV: 440 mA
140kV: 380 mA
Max. mA allowed for each kV
Table 28. X-ray tube
GE LightSpeed RT
Type and make
GE Performix Pro
Focal spot size(s) (mm), quoted to IEC 336/93
standard
0.6 x 0.7
0.9 x 0.9
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Settings at which focal spot changes.
kW = kV x mA / 1000
24 kW
Total filtration (inherent + beam shaping filter) at
central axis
(mm Al equivalent)
6.8 (70kV, head)
9.5 (70kV, body)
Anode heat capacity (MHU)
6.3
Maximum anode cooling rate (kHU/min)
840
Method of cooling
Glycol to air
Guaranteed tube life
1 year unlimited guarantee
Table 29. Detection system
GE LightSpeed RT
Solid state
(HiLight / Lumex)
Detector type
Number of detectors per row
888 (plus 18 reference detectors)
Number of elements along z-axis
16
Effective length of each element at isocentre
(mm)
16 x 1.25
Total effective length of detector array at
isocentre (mm)
20
Option for more slices / rotation
8-slice option available in future
28
Appendix 1. Scanner specifications
Table 30. System start-up and calibration
GE LightSpeed RT
Power-on to warm-up time from fully off (mins)
2
Tube warm-up time from 'cold' to operating
temperature (mins)
0.5
Time to perform detector calibrations at warmup (mins)
Included in tube warm-up
Recommended frequency for any additional
calibration by the radiographer
Once every 24 hours
Time to perform these additional calibrations
(mins)
13 (inc warm-up)
Total time from fully off to scanning in an
emergency (mins)
<3
Table 31. Scan parameters
GE LightSpeed RT
kV settings available
80, 100, 120, 140
10 - 440
(5mA steps)
Report 05070: GE LightSpeed RT CT scanner technical evaluation
mA Range and Step size
80kV: 400 mA
100kV: 420 mA
120kV: 440 mA
140kV: 380 mA
Max. mA allowed for each kV
Maximum continuous
scan time (s)
120
29
Appendix 1. Scanner specifications
Table 32. Helical and axial scanning
GE LightSpeed RT
Rotation times for axial scaning (s)
* = Partial scans
1, 2, 3, 4
Rotation times for helical scanning (s)
1
2 x 0.625, 1 x 1.25,
4 x 1.25, 1 x 2.5,
4 x 3.75, 4 x 5
Axial slice widths (number x width, mm)
Helical acquisition widths (number of channels x
width, mm)
Info. not available
Pitches available for routine scanning (range
and increment)
2 slice : 1
4-slice : 0.75 & 1.5
Recommended pitches for optimal image quality
2 slice : 1
4-slice : 0.75 & 1.5
Helical interpolation algorithms available
SmartHelical & MDMP, including CrossBeamTM &
HyperplaneTM
70sec (350-440 mA)
90sec (280-415 mA)
110sec (240-345 mA)
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Maximum number of rotations in one helical run
at standard abdomen parameters
Starting with a cold tube, the maximum helical
scan distance using a 1 mm imaged slice
thickness and a pitch of 1.5 (mm)
1600
Gantry tilt range for helical scanning (degrees)
± 30
Table 33. Scanned projection radiography (SPR)
GE LightSpeed RT
Maximum SPR length (mm)
1600
SPR field dimensions (mm x mm)
500 x 1600
Angular positions of X-ray tube available for
SPR
0 - 359°
(1° steps)
Real time image
No
Accuracy of slice prescription from the
scanogram (mm)
± 0.25
Accuracy of distance measurements from SPR's
taken at isocentre (lateral and axial directions)
(mm)
< 2 x image pixel size
30
Appendix 1. Scanner specifications
Table 34. Image reconstruction on main console
Report 05070: GE LightSpeed RT CT scanner technical evaluation
GE LightSpeed RT
Reconstruction fields of view (cm)
9.6 - 50
Extended scan field of view (cm)
65
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)
30 (with IBO)
Reconstruction time from the start of data
acquisition to the appearance of the 30th image
of a series for an axial spine scan (s)
<7 Sec
Reconstruction time from the start of data
acquisition to the appearance of the 30th image
of a series for a helical abdomen scan (s)
<7 Sec
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
31
Appendix 1. Scanner specifications
Table 35. Factors affecting image quality and dose
GE LightSpeed RT
Post-patient collimation for narrow slices
No
Automatic mA control
(AEC / mA modulation) software
3D Dose Modulation
- 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
Low signal correction
Quarter detector shift
Yes
Moving (dynamic/flying) focal spot, xy plane
No
Number of imaging detectors per row
888
Sampling frequency (Hz)
Artefact reduction algorithms
Iterative Bone Option (IBO), Recon of thick slices from
thinner ones
GE Proprietary algorithms (SmartHelical & MDMP,
including CrossBeamTM & HyperplaneTM)
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Cone beam correction
984 @ 1s scan (hardware capable to 1760)
32
Appendix 1. Scanner specifications
Table 36. Manufacturer's performance data
GE LightSpeed RT
In plane spatial resolution (lp/cm) for sharpest
clinical algorithm. Acquisition parameters in
brackets.
Contrast resolution: smallest rod size (mm)
discernable at given parameters in 20 cm
CATPHAN
MTF0: 15.4
MTF10: 13.9
(10cm DFOV, Edge alg, Small Focus)
5mm @ 0.3% @ 13.3 mGy with 95%CL: 120kVp,
135mAs, 10mm, Std alg
CT number accuracy (HU)
Water : ± 3
CTDI100 settings for std head
120 kVp, 20 mm
CTDI100 (mGy/100mAs), centre of head
phantom
Under review
CTDI100 (mGy/100mAs), periphery of head
phantom
Under review
Report 05070: GE LightSpeed RT CT scanner technical evaluation
CTDI100 settings for std body
120 kVp, 20 mm
CTDI100 (mGy/100mAs), centre of body
phantom
Under review
CTDI100 (mGy/100mAs), periphery of body
phantom
Under review
Dose profile FWHM (mm) (focal spot size in
brackets)
20: 20.6(l)
15: 16.5 (s)
10: 11.5 (s)
5: 6.7 (s)
1.25: 3.5 (s)
2 x 0.63:1.9(s)
Table 37. Operator's console
GE LightSpeed RT
Diagonal dimension of image screen (inches)
19
Number of monitors at console (functions of
each if > 1)
2 (acquisition / review and processing)
Image area matrix dimensions
512 x 512, 768 x 768, 1024 x 1024
Usual range of CT Number displayed (HU)
-31743 to +31743
Accuracy of distance measurements in x-y
plane (mm)
< 2 times
image 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, trackball, keyboard
33
Appendix 1. Scanner specifications
Table 38. Main computer
GE LightSpeed RT
Make and model
HP XW8200
Operating system
Linux SMP Kernel 2.6
Type and speed of CPU
2 x 3.2 GHz
Amount of computer RAM supplied as standard
(Gbytes)
2
Maximum amount of computer RAM (Gbytes)
12
Table 39. Image storage
GE LightSpeed RT
Total standard hard disk capacity (Gbytes)
291
Maximum hard disk capacity (Gbytes)
291
Hard disk capacity for image storage (Gbytes
and no. of uncompressed 512x512 images)
146
(250,000 images)
Hard disk capacity for storage of raw data files
(Gbytes and no. of data files)
36(2881) 4-slice axial raw data files)
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Archive options
Capacity of a single archive disk (Gbytes and
no. of images)
MOD [images] & DVD [scan data, protocols] (standard)
4.6 (9400 losslessly compressed 512x512 images or
700 raw data files)
Time to mount an archive disk or tape (s)
5-6 in background operation
Archive data transfer rate
(images / s)
1 (read)
0.7 (write)
34
Appendix 1. Scanner specifications
Table 40. Independent workstation
GE LightSpeed RT
Is a workstation provided?
Yes
Computer make and model
HP XW8200
Operating system
Fedora Linux (Kernel 2.6)
Type and speed (GHz) of CPU
2 x 3.4
Amount of computer RAM supplied as standard
(Gbytes)
2
Maximum amount of computer RAM (Gbytes)
4
Total hard disk storage capacity supplied as
standard (Gbytes)
182
Maximum total hard disk storage capacity
(Gbytes)
182
CD-R standard
MOD optional
Archive options
Capacity of a single archive disk (Gbytes)
4.6
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Environmental requirements (max/min
temperature, humidity) for workstation
10-40 ºC,
20-80 % relative humidity
Table 41. 3D reconstruction display on main console (MC) and
workstation (WS)
GE LightSpeed RT
MIPs and MinIPs (maximum and minimum
intensity projections)
MC-standard
WS-standard
SSD (3D Shaded Surface Display)
MC-option
WS-standard
3D Volume rendering software
MC-option
WS-standard
3D Virtual endoscopy
MC-option
WS-standard
MPR (Multi-planar reconstruction)
MC-standard
WS-standard
Planes available in MPR
Axial, para-axial, sagittal, coronal, oblique, curvilinear
35
Appendix 1. Scanner specifications
Table 42. Optional facilities (MC = main console, WS = workstation)
GE LightSpeed RT
Contrast injector
Option
Contrast media bolus tracking
Standard (SmartPrep)
CT fluoroscopy software and hardware
Option (SmartStep)
Hard-copy imaging device
Option
Option
(RT flat pad and Exact couch top)
Radiotherapy planning table top
Carbon fibre breast board
Option
Means for attaching patient immobilisation
devices and a stereotactic frame to the end of
the couch
Option
(Exact couch)
MC-Not available
WS-option
(BMD)
Bone Mineral Densitometry
MC-standard
WS-standard
AVA
(Vessel Assessment) option on WS
CT Angiography
MC-option
WS-option
(Dentascan)
Dental
MC-Not available
WS-option
(CT sim)
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Radiotherapy CT simulation software
Prospective ECG-triggered cardiac software
MC-option
WS-option
(SmartScore)
Retrospective ECG-gated cardiac software
MC-Not available
MC & WS-option (Cardiac Imaging)
MC-option
WS-option
(CT Perfusion)
CT Perfusion software
36
Appendix 1. Scanner specifications
Table 43. Installation requirements
GE LightSpeed RT
Environmental requirements (max/min
temperature, humidity) in scanner room
15-26 ºC, 30-60% relative humidity
Environmental requirements (max/min
temperature, humidity) in scanner control room
15-26 ºC, 30-60% relative humidity
Peak heat output from system during scanning
(kW)
15.3
System cooling method
Output to air
Air conditioning requirements for scanner room
of minimum floor area
Recommended
Minimum floor area required for the system (m²)
28
Gantry dimensions
(H x W x D (mm)) and weight (kg)
Couch dimensions
(H x W x L (mm))
and weight (kg)
1992x2439x1007
1701kg
1121x610x2387
330kg
Power unit:
1067x711x559
350kg
Supplementary unit dimensions
(H x W x D (mm)) and weight (kg)
3 phase 380-480V, 150kVA
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Power supply requirements
37
Appendix 1. Scanner specifications
Table 44. Image transfer and connectivity
GE LightSpeed RT
Speed of scanner / workstation connections to
local area networks (Mbits/s)
100
Remote PC access to images on workstation
Option
DICOM services on Main Console
Storage
SCU, SCP
Print
SCU
Query / retrieve
SCU, SCP
Modality worklist
Option
Performed procedure step
Option
Storage commitment
Not available
DICOM services on Workstation
Storage
SCU, SCP
Print
SCU
Query / retrieve
SCU, SCP
Not available
Performed procedure step
Not available
Storage commitment
Not available
Report 05070: GE LightSpeed RT CT scanner technical evaluation
Modality worklist management
38
Appendix 2. Manufacturer’s comments
GE Healthcare are generally in agreement with the data in this report. They
have made the following comments:
Image Noise (page9)
Our current LS RT product includes a lower noise Data Acquisition Subsystem
(DAS) that was not available on the unit tested in November 2004. This DAS
should improve the noise data under low signal conditions.
Extended Field of View (pages 20 – 21)
The primary requirement of the Wide View feature is to permit visualization of
patient anatomy outside of the standard 50 cm DFOV to assist with radiation
treatment planning. There is no expectation or requirement by GE to reproduce
image quality to the same level and degree as within the 50 cm FOV.
Report 05070: GE LightSpeed RT CT scanner technical evaluation
GE recommends that the 100 mm water cylinder should remain adjacent to the
larger phantom as they both move from isocentre to wider fields of view. We
believe this phantom positioning is more representative of true clinical scenarios
(arm or breast) used in radiation treatment planning.
39
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.
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.
Report 05070: GE LightSpeed RT CT scanner technical evaluation
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
algorithms with standard spatial resolution values are chosen to minimise the
dependency of Q2 upon reconstruction algorithms. The reconstruction algorithm
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).
40
Appendix 4. 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
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.
Report 05070: GE LightSpeed RT CT scanner technical evaluation
ImPACT
Bence Jones Offices
St George’s Hospital
London SW17 0QT
T: +44 (0) 20 8725 3366
F: +44 (0) 20 8725 3969
E: impact@impactscan.org
W: www.impactscan.org
41