Download Multislice Computed Tomography: Basic Principles and Clinical

New Applications with the Assistance of Innovative Technologies
Multislice Computed Tomography:
Basic Principles and Clinical Applications
A. F. Kopp 1, K. Klingenbeck-Regn 2, M. Heuschmid 1, A. Küttner 1, B. Ohnesorge 2,
T. Flohr 2, S. Schaller 2, C. D. Claussen 1
1
Eberhard-Karls-University Tübingen, Department of Diagnostic Radiology, Tübingen, Germany
2
Siemens AG, Medical Engineering Group, Forchheim, Germany
Introduction
Since its clinical introduction in 1991, volumetric CT
scanning using spiral or helical scanners has resulted in
a revolution for diagnostic imaging. In addition to new
applications for CT, such as CT angiography and the
assessment of patients with renal colic, many routine
applications such as the detection of lung and liver
lesions have substantially improved. Helical CT has
improved over the past eight years with faster gantry
rotation, more powerful x-ray tubes, and improved
interpolation algorithms [1, 2]. However, in practice the
spiral data sets from monoslice systems suffered from
a considerable mismatch between the transverse (in
plane) and the longitudinal (axial) spatial resolution. In
other words the isotropic 3-dimensional voxel could not
be realized apart from some very specialized cases [3].
Similarly, in routine practice a number of limitations
still remained which prevented the scanning protocol
from being fully adapted to the diagnostic needs [4].
The introduction of subsecond spiral scanning with
the SOMATOM Plus 4 at the RSNA 1994 was a first
step to facilitate routine clinical work with respect to
scannable volumes, total scan time and axial resolution
[5]. Compared the 1 sec scanners which were standard
at that time, the 750 ms rotation time allowed one to scan
33% longer volumes or to correspondingly reduce the
total scan time or to correspondingly improve axial
resolution.
The latest advance has been the recent introduction
of multislice CT (MSCT) scanners. At the RSNA 1998,
this new technology was introduced by several manufacturers representing an obvious quantum leap in clinical performance [6-8]. Currently capable of acquiring
four channels of helical data simultaneously, MSCT
scanners have achieved the greatest incremental gain in
scan speed since the development of helical CT and
have profound implications for clinical CT scanning.
Fundamental advantages of MSCT include substantially shorter acquisition times, retrospective creation of
thinner or thicker sections from the same raw data, and
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electromedica 68 (2000) no. 2
improved three-dimensional rendering with diminished
helical artifacts [9].
For example, the SOMATOM® Volume Zoom with a
500 ms rotation time and the simultaneous acquisition
of 4 slices offers an 8-fold increase of performance
compared to previous 1s, single-slice scanning.
Obviously, such a quantum leap opens up a new area
in spiral CT affecting all existing applications and
allowing the realization of new clinical applications.
The key issue is correspondingly increased volume
coverage per unit time at high axial resolution and a
correspondingly improved temporal resolution [6].
In general terms, the capabilities of spiral CT can be
expanded in various ways: to scan anatomical volumes
with standard techniques at significantly reduced scan
times; or to scan larger volumes previously not accessible in practical scan times; or to scan anatomical
volumes with high axial resolution (narrow collimation)
to closely approach the isotropic voxel of high-quality
data sets for excellent 3-dimensional postprocessing
and diagnosis.
Basic Principles
For discussion of spiral imaging the following definition of pitch is commonly used:
Table travel per rotation
P=
Collimation of single slice
With this definition the present standard range
1 ≤ p ≤ 2 for single slice systems is extended to
1 ≤ p ≤ 8. In Fig. 1 various sampling patterns of a 4-slice
spiral are shown for representative values of the pitch.
In this schematic view the projections are symbolized
by single arrows, which for simplicity are drawn in
parallel.
From those examples we can draw the following
conclusions:
Rot. 1
Rot. 1
Rot. 2
Rot. 2
Rot. 3
Rot. 3
Rot. 4
Rot. 4
Pitch 1
Figure 1
Sampling Patterns of a 4-slice
spiral scan at different pitch
values. At pitch 1 and 2, each
z-position is sampled 4 and
2 times respectively.
The spacing between samples
decreases from d to d/2 when
going from pitch 1 to pitch 1.5,
then increases again to d when
increasing the pitch to 2.
Pitch 2
Rot. 1
Rot. 1
Rot. 2
Rot. 2
Rot. 3
Rot. 3
At a pitch of 4, each sample is
acquired once and the sampling
distance is d (d denotes the slice
collimation).
Rot. 4
Pitch 1.5
1. For pitch values smaller than 4 the four slices overlap to a certain degree after one rotation. Pitch 2 is a
transparent case with double sampling. Therefore in this
regime 1 ≤ p ≤ 4 the multiple sampling can be used to
reduce the tube current for a desired image noise and for
a desired patient dose, respectively.
Pitch 4
center of rotation [10]. The outer detector rows cannot
be used individually. In the example of Fig. 2 the 1 mm
slice is smeared over about 6 mm. The only way out
is to sum the signals of the outer rows to generate thick
slices. However, the unnecessary mechanical cuts and
2. However, collecting data from multiple rotations
degrades the temporal resolution of the system. Therefore for imaging of moving organs with high image
quality, a pitch smaller than 4 should be avoided.
3. The distance between neighboring samples varies
with pitch periodically. Consequently, 180° LI and 360°
LI spiral interpolators would yield a corresponding nonmonotonic dependence of slice width on pitch and are
therefore not useful for practical purposes.
Design Considerations for MSCT
It is easy to design a multi-slice scanner for a fixed
slice collimation, the challenge is to design the detector
in such a way as to meet the clinical requirement of
different slice collimations adjustable to the diagnostic
needs. There are basically two different approaches, the
matrix detector with elements of a fixed size or the
adaptive array principle. Both principles will be briefly
described and compared.
An example of a matrix detector is sketched in
Fig. 2. In axial direction the detector is divided into 16
small elements each providing a 1 mm thick slice at the
6.6 mm
sw: 4.0 mm
sw: 1.0 mm
FOV
4. The distance between neighboring samples never
exceeds the slice collimation up to a pitch of 8. This
opens up the possibility to realize a slice width independent of pitch up to a pitch of 8 and to completely
eliminate broadening of the slice width.
4.7 mm
z
16 detector rows
Figure 2
Fixed Array Detector. The
slice width is compared to the
smearing of the slice caused
by the cone-angle.
1.0 mm is broadened to 6.6 mm
by smearing (right half of figure).
The problem can be overcome
by combining several slices to a
wider slice (left half of figure).
It is shown that for the example
of a 16-row detector, the outermost slice of nominal thickness
Then, however, separators
between the slices are not needed
for the outer slices.
electromedica 68 (2000) no. 2
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optical separations between the small elements, correspondingly reduce the geometrical efficiency and therefore the dose efficiency of the system. In summary,
the matrix detector is well suited to scan at 4 x 1 mm
collimation but not more than 4. Wider collimations
4 x 2 mm, etc. can be realized by signal combination
during read out but at the expense of dead zones and a
corresponding waste of dose. These arguments lead to
the development of the Adaptive Array Detector [9].
1
1.5
2.5
5
Magnification center to detector
by a factor of approximately 2
in mm
z
appr. 40 mm
The design of the Adaptive Array Detector (AAD)
takes into account the cone beam constraints for optimal
image quality, optimizes the dose efficiency and in
conjunction with an Adapted Axial Interpolator (AAI)
provides a flexible selection of slice widths [6, 11, 12].
The design principle is depicted in Fig. 3. Narrow
detector elements are close to the center; the width
of the detector rows increases with distance from the
center. Unnecessary dead spaces are avoided and
with the corresponding prepatient collimator and the
proper read-out schemes the following combinations of
collimation can be achieved: 2 x 0.5 mm, 4 x 1 mm,
4 x 2.5 mm, 4 x 5 mm, 2 x 8 mm and 2 x 10 mm (Fig. 4).
These combinations represent the collimation of the
X-ray beam at center: e. g. a 4 x 5 mm collimation
means a X-ray beam width at center of 20 mm. Consequently with sequential imaging four 5 mm slices would
be generated during one rotation.
In spiral imaging the variety of axial sampling
patterns as a function of pitch allows both: obtaining
slice widths independent of pitch and reconstructing a
multiplicity of slice widths from a scan with collimation
narrower than s. To achieve this the AAI-scheme provides a set of linear and nonlinear interpolators which
are adapted to the desired pitch, collimation and slice
width. To give some examples: from a spiral scan with
4 x 1 mm collimation slice widths of 1, 1.25, 1.5, 2.0,
2.5, 3.0, 4.0, 5.0 up to 10 mm can be obtained by adjusting the width and the functional form of the interpolator. On the other hand, a selected slice width s, like
s = 5 mm, may be obtained from different collimator
settings, like 4 x 1 mm or 4 x 2.5 mm. This is important
to remember for clinical applications, as the narrower
collimation is preferable for image quality reasons, i. e.
the reduction of partial volume effects [9].
Yet another design criterion should be emphasized:
spiral scanning at large pitch, e. g. large table velocity,
implies a more severe inconsistency between direct
projections and the complementary projections, taken
half a rotation later with a quarter detector offset. The
reason is changing anatomy in axial direction, which is
particularly pronounced for large pitch applications, i. e.
rapid table movement. Consequently, image quality at
high spatial resolution and large pitch is becoming
more dependent on the flying spot technology which
provides a quasi-instant doubling of the in plane sampling rate.
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electromedica 68 (2000) no. 2
Figure 3
Design of the Adaptive Array
Detector. The physical width of
the detector is approximately
40 mm. Slice widths at the axis
of rotation range from 1 mm for
the inner slices to 5 mm for the
outer slices.
2 x 0.5 mm
4 x 1.0 mm
4 x 2.5 mm
2 x 8.0 mm
4 x 5.0 mm
Figure 4
Available Collimations and
read-out schemes for the Adaptive
Array Detector (AAD). The
dotted bar indicates the collimation at the detector.
Prepatient collimation is adjusted
correspondingly.
values. Obviously the AAI indeed results in slice widths
independent of pitch; but even more important, the
functional form of the SSPs is also identical and
practically independent of pitch. Slice broadening and
long-range tails of the SSP, which prohibited the use of
fast table speeds in single slice spiral scanning, can be
completely avoided. From this perspective the whole
range up to a pitch of 8 can be used for practical purposes in multi-slice scanning [6].
In conclusion, image quality in multi-slice spiral
scanning must be optimized with respect to several
factors [13]:
1. The narrowest collimation, consistent with the
coverage of a certain volume and with a certain scan
time, to minimize partial volume effects and to optimize
image quality.
2. Fastest rotation time to maximize z-coverage and
to minimize motion blurring.
The second new and attractive feature of the multispiral AAI is illustrated in Fig. 6. From a scan with
4 x 1 mm collimation SSPs with different width can be
obtained: 1.25 mm, 2.0 mm and 4.0 mm respectively.
Excellent agreement between theory and measurement
is observed. This feature is the basis of Combi Scans.
This provides considerable flexibility in image reconstruction [14] especially for imaging of the base of the
skull and lung (Fig. 7).
3. Pitch greater than 4 to preserve temporal resolution and to minimize motion blurring.
4. The exploitation of flying focal spot technology to
avoid artifacts at high spatial resolution.
To measure section profiles of multi-spiral scanning
a thin gold plate (thickness 50 µm) in air has been
aligned orthogonal to the scanner axis and has been
scanned in spiral mode. Some of the resulting slice sensitivity profiles (SSP) are shown in Fig. 5 for a 4 x 1 mm
collimation, a slice width of 2 mm and different pitch
Pitch 3
Anatomical structures that generate spiral artifacts
are well known from single slice spirals. The most
difficult situations arise from bony structures (high
Pitch 5
Measured: -FWHM = 2.05 mm
FWHM = 2.1 mm
FWHM = 2.1 mm
1
1
1
0.8
0.8
0.8
0.6
0.6
0.6
0.4
0.4
0.4
0.2
0.2
0.2
0
-4
-2
0
2
4
-4
-2
0
: calculated
Measured: -FWHM = 1.35 mm
Figure 5
Slice Sensitivity Profiles,
Collimation 4 x 1 mm, slice
width 2 mm, different pitch
values.
Pitch 7
2
4
-4
-2
0
2
4
z [SW]
: measured
FWHM = 2.05 mm
Figure 6
Slice Sensitivity Profiles,
Collimation 4 x 1 mm, pitch 3,
different slice widths.
FWHM = 4.0 mm
1
1
1
0.8
0.8
0.8
0.6
0.6
0.6
0.4
0.4
0.4
0.2
0.2
0.2
0
-4
-2
0
2
4
-4
-2
: calculated
0
2
4
-4
-2
0
2
4
: measured
electromedica 68 (2000) no. 2
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New Applications with the Assistance of Innovative Technologies
a
c
b
Figure 7
Combi Scan: lung study with
4 x 1 mm collimation, pitch 6.
Reconstruction of 5 mm images
for soft tissue (a) and standard
lung window (b);
1.25 mm high resolution
images for detection of
interstitial disease (c).
HR-MPR (d) clearly depicts
segmental anatomy.
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electromedica 68 (2000) no. 2
d
contrast) which are strongly inhomogeneous in axial
direction. A demanding example is the base of the skull
with bony structures abruptly ending in an image plane.
Phantom studies have shown that a larger pitch and
narrower collimation is much more favorable for
suppressing artifacts than a lower pitch and wider collimation for equal z-coverage. The underlying reason
is the better elimination of partial volume artifacts.
From image quality reasons, only, the strength of multislice spiral scanning is the ability to cover anatomical
volumes with narrowest collimation and thereby to
minimize partial volume effects. This is only a matter of
scanning technique (collimation) and is independent
of the selected slice width. For optimization of image
quality we derive the rule: the narrowest collimation
should be selected which is consistent with volume
coverage and scan time. This generally results in large
pitch values (e. g. from 4 to 6) which are also helpful to
avoid motion blurring [13].
Clinical Applications
The advantages of MSCT are important to many
applications of CT scanning, including survey exams in
oncologic or trauma patients and the characterization of
focal lung and liver lesions through the creation of thin
sections retrospectively. However, the greatest impact
has been on CT angiography, cardiac imaging, virtual
endoscopy, and high resolution imaging [15].
a
CT Angiography
A fundamental advantage of a multislice scanner
over monoslice systems is its ability to obtain a first
circulation study of a rapidly injected contrast bolus
with thinner images. Determining circulation time
either by a preliminary minibolus or by online bolus
tracking software is important in matching the acquisition interval to the first-circulation time of the injected bolus. As a result of the shorter acquisition time,
the contrast dose can be significantly reduced.
CT angiography of the intracranial vessels benefits
from the quick and detailed scanning technology of
MSCT. At 1 mm-collimation, the circle of Willis can be
scanned within 10 seconds. This permits the entire scan
to be completed during the first pass of iodinated contrast material through the arterial system. The use of
MSCT with CT angiography demonstrates the spatial
relationship of an aneurysm with the feeding vessel, as
well as the shape of the aneurysm itself, because the
same bolus can be followed continuously throughout its
course. For CT angiography of the thoracoabdominal
aorta a collimation of 2.5 mm, a pitch of 5-6 at a rotation speed of 0.5 s are used. Volume coverage is from
the thoracic inlet to the inguinal region, a 50 to 55 cm
area that can be covered in an acquisition interval of
approx. 20 s (Fig. 8). Reconstructions with 50% overlap
are used, generating 400-450 images for the threedimensional data set. A complete lower extremity study
Figure 8
MSCTA of the thoracoabdominal
aorta. Stanford Type B Dissection.
MPRs from images with 3 mm
slice-width and 1 mm increment.
The spiral scan was acquired
with 4 x 2.5 mm collimation,
pitch 5, and 0.5 s rotation time.
The total scan time for a spiral
length of 550 mm was 22 s.
Courtesy of Dr. Baum, Erlangen.
electromedica 68 (2000) no. 2
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▲
can be performed from the level of the renal vascular
pedicles to the ankles. A 2.5 mm collimation, a pitch of
6 and 50% overlap reconstructions are used (Fig. 9).
A first-pass circulation study can be obtained without
venous overlay. The total number of images generated
is 800 to 1000. With multislice CT operating at pitch 6
and collimation of 4 x 1 mm, a CTA of the pulmonary
arteries can be acquired in less than 25 s. Image thickness of 1.25 mm significantly increases detectability of
subsegmental emboli in comparison to monoslice spiral
CT using 2 to 3 mm image thickness [16]. CTA of the
visceral branch vessels can be performed significantly
faster and/or with increased spatial resolution. Using a
Figure 9
Peripheral Runoffs. Spiral scan
with 4 x 2.5 mm collimation,
pitch 6, 0.5 s rotation time.
Left: MIP from images
with 3 mm slice width,
2 mm increment.
The total scan time for a spiral
length of 1000 mm was 34 s.
Right: VRT from images
with 6 mm slice width,
4 mm increment.
Figure 10
CTA (a, b) of visceral branch
vessels (collimation 4 x 1 mm,
pitch 5, 120 cc contrast).
c
On the VR images a stenosis in
the SMA (arrows) can be readily
appreciated which was confirmed
at conventional angiography (c).
a
100
b
electromedica 68 (2000) no. 2
collimation of 1 mm allows for detection of even minor
branches (Fig. 10). The ability to cover the entire chest
in 10 s allows the scanning of children without sedation.
This can be used to easily perform CTA of the large
thoracic vessels before or after surgery of complex malformations [8].
Cardiac Imaging
Electron Beam CT scanning (EBCT) has been
established as a non-invasive imaging modality for
imaging coronary calcification. Major clinical applications are the detection and quantification of coronary
calcium and non-invasive CT angiography (CTA) of the
coronary arteries [17]. Current limitations of EBCT
imaging include the limited reproducibility of coronary
calcium quantification [18], the inability to detect noncalcified atherosclerotic plaques and the limited spatial
resolution of 3D visualizations of the coronary arteries.
Because of the restriction to axial, non-spiral scanning
in ECG-synchronized cardiac investigations, acquisition of 3D volume images by using EBCT can only
provide limited z-resolution within a single breath-hold
scan. Retrospectively ECG-gated single-slice spiral
scanning does not allow for sufficient continuous volume
coverage within reasonable scan times. Retrospectively
ECG-gated multi-slice spiral scanning, however, has the
potential to completely cover the heart volume without
gaps within one breath-hold [19]. Mechanical multislice CT systems with simultaneous acquisition of
four slices, half-second scanner rotation and 125 ms
maximum temporal resolution allows for considerably
faster coverage of the heart volume, compared to singleslice scanning. This increased scan speed allows using
thinner collimated slice widths and thus to increasing
the z-resolution of high-resolution examinations such as
CTA of the coronary arteries [20].
ECG-synchronized multi-slice spiral scans are
acquired with heart rate dependent table feed (“pitch”)
adaptation. Dedicated spiral algorithms provide 125 ms
(60 ms as theoretical limit) temporal resolution and are
optimized with regard to volume coverage. This allows
reconstruction of overlapping images (increment < slicewidth) at arbitrary z-positions and during any given
heart phase [21]. This reconstruction technique combines partial scan reconstruction and multi-slice spiral
weighting in order to compensate for table movement
and to provide a well-defined slice sensitivity profile
[19].
For retrospectively ECG-gated reconstruction each
image is reconstructed using a multi-slice partial scan
data segment with an arbitrary temporal relation to the
R-wave of the ECG-trace. Image reconstruction during
different heart phases is feasible by shifting the start
point of image reconstruction relative to the R-wave.
For a given start position, a stack of images at different
z-positions covering a small sub-volume of the heart can
be reconstructed owing to mutlislice data acquisition.
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Slice q = 0
Slice q = 1
Slice q = 2
Slice q = 3
ECG-Signal
(~ 70 bpm)
0
0.5
1
1.5
2
2.5
3
3.5
t [sec] (0.5 sec/rotation)
Figure 11
Reconstruction with retrospectively ECG-gated 4-slice
spiral scanning.
Data ranges are selected with
certain phase relation to the
RR-intervals. 3D volume images
are generated from image stacks
reconstructed in consecutive
heart cycles.
Fig. 11 shows an example of how the cardiac volume
is successively covered with stacks of axial images
(shaded stacks) reconstructed in consecutive heart
cycles. All image stacks are reconstructed at identical
time-points during the cardiac cycle. At the same time,
the 4 detector slices travel along the z-axis relative to the
patient table. In each stack, single-slice partial scan
data segments are generated with equidistant spacing in
the z-direction depending on the selected image reconstruction increment [21]. Continuous volume coverage
can only be achieved, when the spiral pitch is adapted to
the heart rate in order to avoid gaps between image
stacks that are reconstructed using data from different
heart cycles. In order to achieve full volume coverage,
the image stacks reconstructed in subsequent heart
cycles must cover all z-positions. Thus, the pitch, which
can be used for image acquisition is limited by the
patient’s RR-interval time [19].
The cardiac multi-slice data acquisition on the
SOMATOM Volume Zoom are performed using 500 ms
full rotation time and 4 x 1 mm or 4 x 2.5 mm collimated slice width. Non-contrast enhanced spiral scans
for coronary calcium scoring are performed with 3 mm
slice-width (SW = 3 mm, SWcoll = 2.5 mm) and 1 mm
image reconstruction. For CTA of the coronary arteries
and for functional heart imaging 2 different scan
protocols with 3 mm slice-width and with 1.25 mm
slice-width can be used (Fig. 12). For both protocols,
non-ionic contrast material is intravenously injected at
a flow rate of 3 ml/s. The delay times between start of
contrast injection and scan start for optimal contrast
enhancement is determined individually for each patient
by injection of a 20 ml test bolus [20].
electromedica 68 (2000) no. 2
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Figure 12
49-year-old male with CHD
of the RCA. S/P posterolateral
myocardial infarction 4 months
ago.
Patient underwent balloon
angioplasty of subtotal stenosis
at the beginning of the descending part of the RCA.
Patient came in for follow-up
performed with both coronary
CT angiography (a) and
conventional angiography (b).
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electromedica 68 (2000) no. 2
a
a
b
b
Both conventional angiography
and CT angiography
(3D Virtuoso®, Siemens) clearly
show the patency of the RCA
at the level of the ballon angioplasty.
There is only minor residual
stenosis of approx. 10-20%
(arrows).
Scan: Siemens SOMATOM®
Volume Zoom; 140 kV, 300 mAs,
collimation 4 x 1 mm;
slice width 1.25 mm,
increment 0.6 mm.
Figure 13
Noninvasive CT coronary
angiography: Volume rendered
image (3D Virtuoso®, Siemens)
depicts three high grade stenoses
(arrows) in the LAD and at the
origin of the diagonal branches.
These findings are confirmed at
conventional angiography.
The high spatial resolution, the absence of motion
artifacts and the good overall image quality of the
clinical MSCT images of the entire heart volume let
appear multislice spiral CT as a promising modality for
the non-invasive evaluation of coronary calcification.
The scan time needed to acquire continuous ECG-gated
multislice spiral CT image data is significantly reduced
compared to EBCT (≈ factor 2.5) and single slice CT
(≈ factor 5). Data with 3 mm slice width can be used for
volumetric coronary calcium scoring. This alternative
scoring method has the potential to improve the reproducibility of repeat calcium scoring compared to the
conventional Agatston-score. Phantom studies have
shown that non-overlapping sequential scanning is an
important contributor to the inter-scan variability of
Agatston- and volumetric Ca-scores due to partial
volume errors in plaque registration [22]. ECG-gated
volume coverage with multi-slice spiral CT and overlapping image reconstruction, however, was found to
improve the reliability of coronary calcium quantification especially for small plaques. ECG-gated multislice spiral CT can potentially be of high value for
coronary calcium evaluation especially for patients
undergoing lipid-lowering statin therapy and for followup evaluations of patients after heart transplantation
[23].
In contrast to sequential CT scanning z-resolution
of ECG-gated spiral images with 3 mm slice-width can
be improved by using overlapping reconstruction with
1 mm slice increment. Moreover, the fast scan speed
allows covering the entire heart with 1.25 mm slices
within a single breath-hold (10 cm in 25-35 s).
3D reconstruction with 1.25 mm slice-width and
sub-millimeter image increment allows generating highresolution visualizations of the coronary arteries, which
may be suitable for a highly accurate diagnosis of
coronary artery disease (Fig. 13, 14) [24]. Even more
important, the first results indicate that MSCTA not
only allows non-invasive imaging of coronary plaques
but also assessment of plaque composition (i. e., soft,
fibrous, calcified) (Fig. 15). Thus, this new technology
holds promise to allow for the non-invasive imaging of
rupture-prone soft coronary lesions and may have the
option to lead to early onset of therapy [25].
High Resolution Imaging
Imaging of the temporal bone is a major challenge for
clinical CT and a good example of the need for high
resolution CT. Imaging of the temporal bone is improved
using MSCT because the in-plane resolution can be
greatly increased. The temporal bone is a structure of
high intrinsic contrast and is routinely scanned with
thin collimation. Both the 2 x 0.5 mm and the 4 x 1 mm
mode on the SOMATOM Volume Zoom are applicable
(pitch 2 or 3.5) and the tube current can be reduced
to below 200 mAs (Fig. 16). The reconstruction kernels
enable maximum spatial resolution up to 24 line pairs/
14
15
Figure 14
Virtual angioscopy (bottom
right) of circumflex branch of
left coronary artery. VRT (left)
and MPR (top right) images
help navigating the virtual
endoscope in the coronary artery
(3D Virtuoso®, Siemens).
Figure 15
MSCTA of LAD: Non-calcified
soft plaque (arrow) with density
of approx. 5 HU.
This lipid-core plaque was
classified as prone to rupture in
intracoronary ultrasound.
Scan:
collimation 4 x 1 mm, pitch 1.5,
300 mAs, 120 cc contrast.
electromedica 68 (2000) no. 2
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cm. At this resolution, the delicate structures of the
middle and inner ear are sharply delineated and even
subtle changes can be assessed. Another advantage of
using MSCT to image the temporal bone is that the
slice thickness is reduced to 1 mm or below, thus minimizing the partial volume effects and further increasing
image quality of subtle structures [26].
Impact on Radiological Practice
The large number of images generated by MSCT is a
major issue for workstation performance, film display,
and PACS archiving. The images should be reviewed
in a cine paging format at a workstation for diagnostic
purposes [15]. Film hard copy will only be used for
reference display. Planning for PACS must take into
account this exponential growth in image data generated
by this new technology. Rapid generation of 3D data sets
is important for inclusion in the diagnostic study interpretation and for demonstration to referring clinicians.
The hundreds or thousands of thin sections acquired
using MSCT are incompatible with traditional viewing
practice; thus MSCT will force a rapid transition of
radiology from two-dimensional to volumetric imaging.
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[14] Schöpf, U. J., Becker, C. R., Brüning, R., Huber, A. M., Hong, C.:
Multidetector-array spiral CT imaging of focal and diffuse lung
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thorax. Radiology 1999; 213 (P):73.
Figure 16
High resolution image of the
temporal bone.
Note the excellent definition
of the ossicles at a collimation
thickness of 0.5 mm
(200 mAs, 120 kV).
[17] Achenbach, S., Moshage, W., Ropers, D., Nossen, J., Daniel, W. G.:
Value of electron-beam computed tomography for the noninvasive
detection of high-grade coronary-artery stenoses and occlusions.
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Prime Time? Radiology 1998; 208:571-2.
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schneller, retrospektiv EKG-synchronisierter Mehrschichtspiral-CT.
Radiologe 2000; 40:111-7.
[20] Kopp, A. F., Ohnesorge, B., Flohr, T. et al: Multidetektor CT
des Herzens: Erste klinische Anwendung einer retrospektiv EKGgesteuerten Spirale mit optimierter zeitlicher und örtlicher Auflösung
zur Darstellung der Herzkranzgefäße. Fortschr. Röntgenstr. 2000;
172:1-7.
104
electromedica 68 (2000) no. 2
[21] Ohnesorge, B., Flohr, T., Becker, C., Kopp, A. F., Schoepf, U. J.,
Baum, U., Knez, A., Klingenbeck-Regn, K., and Reiser, M. F.: Cardiac
Imaging with ECG-Gated Multi-Slice Spiral CT – Initial Experience.
Radiology 2000, in Press.
[22] Ohnesorge, B., Flohr, T., Becker, C. R., Kopp, A. F., Knez, A.:
Comparison of EBCT and ECG-gated multislice spiral CT: a study
of 3D Ca-scoring with phantom and patient data. Radiology 1999;
213 (P):402.
[23] Becker, C. R., Knez, A., Ohnesorge, B., Flohr, T., Schoepf, U. J.,
Reiser, M.: Detection and quantification of coronary artery calcifications with prospectively ECG triggered multirow conventional CT and
electron beam computed tomography: comparison of different methods
for quantification of coronary artery calcifications. Radiology 1999;
213 (P):351.
[24] Kopp, A. F., Georg, C., Schröder, S., Claussen, C. D.: CT-Angiographie der Herzkranzgefäße bei koronarer 3-Gefäß-Erkrankung.
Fortschr.Röntgenstr. 2000; 172:M3-M4.
[25] Kopp, A. F., Ohnesorge, B., Flohr, T., Schroeder, S., Claussen, C. D.:
Multidetector-row CT for the noninvasive detection of high-grade
coronary artery stenoses and occlusions: first results. Radiology 1999;
213 (P):435.
[26] Klingenbeck-Regn, K., Schaller, S., Flohr, T., Ohnesorge, B.,
Kopp, A. F., Baum, U.: Subsecond multislice computed tomography:
basics and applications. Eur.J.Radiol. 1999; 31:110-24.
Author’s address:
Andreas F. Kopp, M.D.
Eberhard-Karls-University Tübingen
Department of Diagnostic Radiology
D-72076 Tübingen/Germany
Tel: +49-(0)-70 71-29-8 20 87
Fax: +49-(0)-70 71-29-58 45
e-mail: [email protected]
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