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ImPACT
Technology Update No.1, 2nd Edition
MDA 02024
£30
Multi-Slice CT Scanners
January 2002
Introduction
x-ray
tube
The first edition of this leaflet was issued in May 1999,
when multi-slice CT was in its infancy. This second
edition has been produced to include new scanner
models and updated information on multi-slice scanner
operation.
The first 3rd generation multi-slice CT scanner, the
Elscint CT Twin, was launched in 1992. The scanner had
helical capabilities and the ability to acquire two
transaxial slices simultaneously, using two parallel
banks of detectors.
Single
detector
bank
In the second half of 1998, four manufacturers (GE,
Siemens, Toshiba and Picker, now Philips) extended this
concept by launching multi-slice CT scanners. All were
3rd generation helical scanners, with low voltage slip
rings, capable of acquiring four CT slices in one x-ray
tube rotation (see Fig. 1). Additional four-slice models
have been introduced by Toshiba and GE. Some basic
specifications of the systems currently available are
shown in Table 1.
4 parallel
detector
banks
Fig. 1. Single and multi-slice scanner concepts
compared
Note that Fig. 1 is simplified, the configuration shown could
only offer a minimal choice of slice widths. In practice,
between 8 and 34 detector banks are used in different
groupings, but a maximum of 4 slices can be acquired
simultaneously.
GE recently introduced the eight-slice LightSpeed Ultra,
and other manufacturers have announced 16 slice
scanners that should be available by the end of 2002.
These scanners are not discussed in this leaflet.
Feature
GE
Lightspeed S
[LS Plus]
Philips
Mx8000
Z-axis detector array length (mm)
20
20
20
32
32
Min. slice width (mm)
0.63 (x2)
0.5 (x2)
0.5 (x2)
0.5 (x4)
0.5 (x4)
Max. slice width (mm)
10 (x2)
10 (x2)
10 (x2)
8 (x4)
8 (x4)
Min. tube rotation time (sec/rev)
0.8 [0.5]
0.5
0.5
0.5
0.75
512 x 512
1024 x 1024
512 x 512
512 x 512
512 x 512
Generator output (kW)
53.2
60
60
60
36 / 48
Anode heat capacity (MHU)
6.3
6.5
5.3
7.5
4
Anode cooling rate (MHU/min)
0.84
0.73
0.73
1.4
1.4
Reconstruction matrix (max)
Siemens
Toshiba
Toshiba
Volume Zoom Aquilion Multi Asteion Multi
Table 1. Comparison of Multi-slice CT Systems
ImPACT Technology Update, 2nd Edition: Multi-Slice CT Scanners
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GE
Clinical Applications of Multi-Slice CT
16 x 1.25
The clinical advantages of multi-slice technology can be
broadly divided into two categories.
20mm
Their speed can be utilised for fast imaging of large
volumes of tissue with wide slices. This is particularly
useful in studies where patient motion is a limiting
factor. With a four-slice system and a 0.5 second
rotation, it is possible to acquire volume data up to 8
times faster than with a single slice, 1 second scanner.
Applications in this area include trauma, thoracic,
geriatric and paediatric examinations. Fast rotation times
on multi-slice systems also expand the capabilities for
ECG gated cardiac studies and cardiac calcification
scoring.
The other main advantage of multi-slice systems is their
ability to acquire a large number of thin slices quickly,
making routine abdomen acquisitions with 2-3 mm slice
widths possible. The increased z-axis resolution provides
high quality 3D visualisations, for applications such as
CT angiography and virtual endoscopy.
Multi-slice systems also have applications in CT
fluoroscopy. By simultaneous imaging of a number of
slices, real time image display for up to 4 slices is
possible, resulting in improved localisation for
interventional procedures.
X-ray tube thermal loading for a given patient volume is
lower for multi-slice than for single-slice systems
because of the greater length covered per rotation.
Volume coverage for a single helical run can therefore
be increased.
Another clinical advantage of multi-slice systems is the
ability to reconstruct broad slices from a narrow
acquisition width. This permits partial volume artefacts
to be reduced without the increase in noise that would
result from reconstructing narrow slices.
Detector Arrays and Slice Width
Although all the systems discussed in this brochure are
capable of producing four slices in one x-ray tube
rotation, the arrangement of detectors along the z-axis
and the available slice widths vary between systems.
Fig. 2 shows the three different detector array designs
currently available.
There is a large variation in the number and width of
detector banks between the different designs. This
affects:
•
•
•
•
Minimum slice width available
Number of slices at minimum width
Range of slice widths available
Maximum length imaged in one rotation
Table 2 shows the possible combinations of slice width
for each scanner, when scanning in sequential (standard
axial) mode.
2
Philips and Siemens
5
2.5 1.5 1 1 1.5 2.5
5
20mm
Toshiba
4 x 0.5
15 x 1
15 x 1
32mm
z-axis
Fig. 2. CT multi-slice system detector array designs
(Distances given as effective size at isocentre)
Another aspect that must be considered is the efficiency
of the various detector array designs. Due to gaps
between the detector banks, it could be predicted that a
design employing a larger number of banks will be less
efficient in terms of both dose and imaging. In practice,
however, the gaps are relatively small and so other
factors, such as irradiation beyond the imaged length,
have a greater impact on performance.
Scanners with detector banks that have greater z-axis
coverage may potentially have problems with artefacts
due to the geometry of the greater cone angle employed.
GE
Philips and Siemens
2 x 0.7
4 x 1.25
4 x 2.5
4 x 3.75
4x5
2 x 7.5
2 x 10
2 x 0.5
4x1
4 x 2.5
4x5
2x8
2 x 10
Toshiba
4 x 0.5
4x1
4x2
4x3
4x4
4x5
4x8
2 x 10
Table 2. Available slice widths in sequential mode
Helical Pitch Definitions
Currently, manufacturers of multi-slice systems are
employing two different definitions of pitch. ImPACT
use the terminology Pitchx (x-ray beam pitch) and
Pitchd (detector pitch) to differentiate between the two:
Pitch x =
table travel per rotation
x - ray beam collimation
Pitch d =
table travel per rotation
detector acquisition width
Detector acquisition width is defined at the isocentre.
ImPACT Technology Update, 2nd Edition: Multi-Slice CT Scanners
Therefore,
Pitchd = Pitchx x No. of slices acquired simultaneously,
and, on a single slice scanner, Pitchd = Pitchx
Pitchx is determined solely by the x-ray collimation and
table speed, whereas Pitchd will also depend on the
number of slices acquired per rotation, as shown in the
following examples:
Example 1:
Irradiated width: 20 mm,
Table speed: 20 mm/rot,
Detector acquisition width: 4 x 5 mm
Results in: Pitchx = 1
Example 2:
Pitchd = 4
Irradiated width: 20 mm,
Table speed: 20 mm/rot,
Detector acquisition width: 2 x 10 mm
Results in: Pitchx = 1
Pitchd = 2
On a single slice system, example 1 would be similar in
terms of dose and image quality to a 5 mm nominal slice
with a table speed of 5 mm/rotation. This would result in
a pitch value of 1, by either definition. Using Pitchd, the
number of slices acquired per rotation must be known
before any inference can be made about image quality or
dose at the quoted pitch value.
Philips use the first definition, Pitchx, whilst the other
manufacturers are currently using the second definition,
Pitchd.
Philips have also introduced the nomenclature of Pitch
Quad, Pitch Dual and Pitch Single, to indicate the
number of slices acquired per rotation. In the examples
given above, pitch would be quoted as: Pitch Quad = 1
for Example 1 and Pitch Dual = 1 for Example 2. It
should be noted that the value of pitch does not change,
because of their adoption of Pitchx.
Helical Interpolation Algorithms
To reconstruct an axial image from a helical data set,
single-slice scanners have commonly used 180° linear
interpolation algorithms. With this type of algorithm, the
z-sensitivity profile (imaged slice width) for a helical
scan with Pitchx = 1 is similar to that of an image
acquired in sequential mode.
Currently, on multi-slice scanners, the various
manufacturers employ different approaches to helical
interpolation. Some favour the approach commonly used
on single slice scanners of interpolating over a fixed
number of projection data points regardless of pitch.
With this approach, as pitch is increased, the noise
remains constant but the z-sensitivity increases. Others
ImPACT Technology Update, 2nd Edition: Multi-Slice CT Scanners
employ a z-filter interpolation method, where the
interpolation is performed over a fixed z-axis distance.
The latter approach results in a constant z-sensitivity
over a range of pitches, but increased noise with pitch if
the tube current is not altered. In both cases, non-linear
weighting functions may be applied to the interpolated
data.
Image Quality
In sequential mode, multi-slice systems should have
largely the same image quality as equivalent single slice
scanners. The efficiency of individual detector banks
may vary however, resulting in different noise levels
between the slices. The cone beam geometry may lead to
unequal z-axis sensitivities for different slices.
As explained in the previous section, the relationship of
noise and slice width with pitch is not always the same
on multi-slice as on single-slice scanners. For systems
that employ z-filter interpolation to keep slice width
constant with pitch, the image noise will increase with
increasing pitch if the tube current is held constant.
These systems will, however, automatically adjust the
mA as pitch is changed, resulting in constant dose, noise
and slice width.
The level of interpolation artefact in helical mode shows
an overall increasing trend with increasing pitch.
However, the relationship is not as straightforward as on
single-slice systems, with certain pitch values
theoretically resulting in a reduced level of artefact.
There are differences of opinion between the
manufacturers on the subject of pitch optimisation.
Some manufacturers recommend specific pitch values
for “optimum image quality”, which in this context is
thought to refer mainly to the level of helical
interpolation artefacts. Toshiba recommend Pitchd values
of 2.5, 3.0, 3.5 or 4.5, with four slices per rotation, for
optimal image quality. Philips also recommends specific
pitches, including Pitchx values of 0.875 and 1.25. On
GE multi-slice systems, only two pitches are available.
HQ (High Quality) and HS (High Speed) modes
correspond to Pitchd = 3 and 6 respectively on their four
slice models. Siemens claim there are no preferred pitch
values on their multi-slice systems, with Pitchd settings
of between 1 and 8 freely selectable.
It is important to note that, in order to keep helical
artefacts on a multi-slice scanner down to similar levels
as those produced on a single-slice CT system, it may be
necessary to use a Pitchx of less than 1. Thus, a four-slice
scanner with the same rotation time as a single-slice
scanner would not be able to obtain images of the same
quality at four times the speed, but more typically at
about three times the speed. It is possible, however, to
use a lower tube current, so that, in spite of the partially
overlapping helices, there would be no increase in
average dose compared to a pitch of 1 for the same
amount of image noise.
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Whereas helical interpolation artefacts are an important
issue for multi-slice scanners, especially in structures
that change rapidly in the z-axis, patient movement
artefacts are likely to be reduced because of the
increased examination speeds.
Dose
The dose considerations for a multi-slice scanner are
broadly similar to those of a single slice scanner. There
are, however, some important differences.
Dose utilisation in the z-axis tends to be somewhat
poorer on multi-slice than on single-slice scanners. This
is mainly because the x-ray beam width is generally
slightly broader than the total imaged width, in order to
achieve uniform irradiation over all the detector banks.
Whereas on a good single-slice scanner the geometric
efficiency may be close to 100% for all slice widths, on
a multi-slice scanner it is more likely to be 80 – 90% for
most slices, falling to around 70% for 1 mm and 50% for
0.5 mm slice widths.
All helical scanning necessitates extra irradiation at each
end of the helical run to obtain sufficient interpolation
data to reconstruct the required volume. On multi-slice
scanners, four helices are acquired simultaneously, so
the extra irradiated length is likely to be longer, and the
resultant dose length product slightly greater, than on a
single-slice scanner.
On a single-slice scanner, tube current is normally held
constant for changes in pitch, since there is no variation
in noise with pitch. Thus, patient dose falls as pitch
increases. On a multi-slice scanner using z-filter
interpolation, noise increases with pitch. Manufacturers
overcome this effect by incorporating an automatic rise
in tube current as the pitch increases, so that patient dose
will remain constant for all pitches.
When comparing doses quoted by different scanner
manufacturers, it is important to consider whether the
mA quoted is the actual tube current or an “effective
mA” value, which is the tube current divided by Pitchx,
also sometimes called “mA per slice”.
CTDI can be measured using the same methodology as
for single-slice systems. The dose measured should be
divided by the total irradiated width, e.g. 20 mm for a
scanner acquiring 4 x 5 mm slices. Even though the
irradiated slice width (up to 32 mm) is quite large
relative to the length of the standard 10 cm CT chamber,
the resultant underestimate in dose has been found to be
only 2% at most.
Speed and Volume Coverage
All the 4-slice systems have sub-second rotation times
available. On the GE LightSpeed Plus, Philips, Siemens
and Toshiba Aquilion systems, the minimum 360° scan
time is 0.5 second, while on the GE LightSpeed S and
Toshiba Asteion systems it is 0.7 - 0.8 second.
The maximum volume coverage possible in one helical
run will be determined by a combination of the tube
characteristics and the maximum length covered per
rotation. The Toshiba tube has the highest anode thermal
capacity and the highest anode cooling rate (Table 1).
The Toshiba scanner is also capable of imaging 32 mm
of the patient in a single rotation compared with 20 mm
for the other systems.
The product of the increased z-axis coverage and the
faster rotation can offer scanning times that are in the
region of 4 to 8 times faster than for a single slice
scanner. Manufacturers have quoted higher potential
speed gains than this, but it is considered unlikely that
these will occur in routine clinical practice. Indeed, to
obtain image quality comparable to single-slice
scanning, it is more likely that scanning times would be
3 to 6 times faster.
Software, Upgrades and Ease of Use
Multi-slice scanner software continues to undergo major
developments. User interfaces are being refined as the
manufacturers get more clinical feedback, and different
clinical applications are coming to light.
Some manufacturers offer multi-slice capabilities as
optional upgrades to existing single-slice scanners. For
centres with one of these systems, the most cost efficient
route to multi-slice scanning will probably be achieved
through this path.
Multi-slice scanners offer increased options in scan
protocol selection, such as collimated slice width,
imaged slice width, pitch and interpolation algorithm.
Drawing the correct compromise between flexibility of
settings and pre-defined combinations of these
parameters will be important in both staff training and
clinical ease of use
ImPACT, St. George's Hospital, Tooting, London SW17 0QT
Tel. 020 8725 3366 Fax. 020 8725 3969
e-mail: [email protected]
website: www.impactscan.org
ImPACT is the UK's national CT evaluation centre, providing publications, information and advice on all aspects of CT scanning. Funded by the
Medical Devices Agency, it is part of a comprehensive medical imaging device evaluation programme.
Crown Copyright, 2002
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ImPACT Technology Update, 2nd Edition: Multi-Slice CT Scanners