Multislice Computed Tomography Basic Principles and Clinical by dsp14791


									     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

         Eberhard-Karls-University Tübingen, Department of Diagnostic Radiology, Tübingen, Germany
         Siemens AG, Medical Engineering Group, Forchheim, Germany

     Introduction                                                  improved three-dimensional rendering with diminished
         Since its clinical introduction in 1991, volumetric CT    helical artifacts [9].
     scanning using spiral or helical scanners has resulted in
     a revolution for diagnostic imaging. In addition to new          For example, the SOMATOM® Volume Zoom with a
     applications for CT, such as CT angiography and the           500 ms rotation time and the simultaneous acquisition
                                                                   of 4 slices offers an 8-fold increase of performance
     assessment of patients with renal colic, many routine
                                                                   compared to previous 1s, single-slice scanning.
     applications such as the detection of lung and liver
     lesions have substantially improved. Helical CT has              Obviously, such a quantum leap opens up a new area
     improved over the past eight years with faster gantry         in spiral CT affecting all existing applications and
     rotation, more powerful x-ray tubes, and improved             allowing the realization of new clinical applications.
     interpolation algorithms [1, 2]. However, in practice the     The key issue is correspondingly increased volume
     spiral data sets from monoslice systems suffered from         coverage per unit time at high axial resolution and a
     a considerable mismatch between the transverse (in            correspondingly improved temporal resolution [6].
     plane) and the longitudinal (axial) spatial resolution. In
     other words the isotropic 3-dimensional voxel could not          In general terms, the capabilities of spiral CT can be
     be realized apart from some very specialized cases [3].       expanded in various ways: to scan anatomical volumes
     Similarly, in routine practice a number of limitations        with standard techniques at significantly reduced scan
     still remained which prevented the scanning protocol          times; or to scan larger volumes previously not acces-
     from being fully adapted to the diagnostic needs [4].         sible in practical scan times; or to scan anatomical
                                                                   volumes with high axial resolution (narrow collimation)
         The introduction of subsecond spiral scanning with        to closely approach the isotropic voxel of high-quality
     the SOMATOM Plus 4 at the RSNA 1994 was a first               data sets for excellent 3-dimensional postprocessing
     step to facilitate routine clinical work with respect to      and diagnosis.
     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    Basic Principles
     33% longer volumes or to correspondingly reduce the              For discussion of spiral imaging the following defi-
     total scan time or to correspondingly improve axial           nition of pitch is commonly used:
                                                                           Table travel per rotation
        The latest advance has been the recent introduction          P=
     of multislice CT (MSCT) scanners. At the RSNA 1998,                  Collimation of single slice
     this new technology was introduced by several manu-
     facturers representing an obvious quantum leap in clini-         With this definition the present standard range
     cal performance [6-8]. Currently capable of acquiring         1 ≤ p ≤ 2 for single slice systems is extended to
     four channels of helical data simultaneously, MSCT            1 ≤ p ≤ 8. In Fig. 1 various sampling patterns of a 4-slice
     scanners have achieved the greatest incremental gain in       spiral are shown for representative values of the pitch.
     scan speed since the development of helical CT and            In this schematic view the projections are symbolized
     have profound implications for clinical CT scanning.          by single arrows, which for simplicity are drawn in
        Fundamental advantages of MSCT include substan-
     tially shorter acquisition times, retrospective creation of     From those examples we can draw the following
     thinner or thicker sections from the same raw data, and       conclusions:
94   electromedica 68 (2000) no. 2
                                                                                                     Figure 1
                                                                                                     Sampling Patterns of a 4-slice
                                                                                                     spiral scan at different pitch
      Rot. 1                                     Rot. 1
                                                                                                     values. At pitch 1 and 2, each
                                                                                                     z-position is sampled 4 and
      Rot. 2                                     Rot. 2                                              2 times respectively.

      Rot. 3                                     Rot. 3                                              The spacing between samples
                                                                                                     decreases from d to d/2 when
                                                                                                     going from pitch 1 to pitch 1.5,
      Rot. 4                                     Rot. 4                                              then increases again to d when
                                                                                                     increasing the pitch to 2.
                     Pitch 1                                   Pitch 2
                                                                                                     At a pitch of 4, each sample is
                                                                                                     acquired once and the sampling
      Rot. 1                                     Rot. 1                                              distance is d (d denotes the slice
      Rot. 2                                     Rot. 2

      Rot. 3                                     Rot. 3

      Rot. 4

                    Pitch 1.5                                  Pitch 4

   1. For pitch values smaller than 4 the four slices over-   center of rotation [10]. The outer detector rows cannot
lap to a certain degree after one rotation. Pitch 2 is a      be used individually. In the example of Fig. 2 the 1 mm
transparent case with double sampling. Therefore in this      slice is smeared over about 6 mm. The only way out
regime 1 ≤ p ≤ 4 the multiple sampling can be used to         is to sum the signals of the outer rows to generate thick
reduce the tube current for a desired image noise and for     slices. However, the unnecessary mechanical cuts and
a desired patient dose, respectively.
   2. However, collecting data from multiple rotations
degrades the temporal resolution of the system. There-
fore 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 non-
monotonic dependence of slice width on pitch and are                               4.7 mm                      6.6 mm
therefore not useful for practical purposes.
                                                                      sw: 4.0 mm                                         sw: 1.0 mm
   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 inde-
pendent of pitch up to a pitch of 8 and to completely
eliminate broadening of the slice width.

Design Considerations for MSCT
   It is easy to design a multi-slice scanner for a fixed                                   16 detector rows
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
                                                              Figure 2                               1.0 mm is broadened to 6.6 mm
matrix detector with elements of a fixed size or the          Fixed Array Detector. The              by smearing (right half of figure).
adaptive array principle. Both principles will be briefly     slice width is compared to the         The problem can be overcome
described and compared.                                       smearing of the slice caused           by combining several slices to a
                                                              by the cone-angle.                     wider slice (left half of figure).
   An example of a matrix detector is sketched in             It is shown that for the example       Then, however, separators
Fig. 2. In axial direction the detector is divided into 16    of a 16-row detector, the outer-       between the slices are not needed
small elements each providing a 1 mm thick slice at the       most slice of nominal thickness        for the outer slices.

                                                                                                           electromedica 68 (2000) no. 2   95
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     optical separations between the small elements, corres-                        Magnification center to detector
     pondingly reduce the geometrical efficiency and there-                         by a factor of approximately 2
     fore 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                            in mm
     corresponding waste of dose. These arguments lead to                       z
     the development of the Adaptive Array Detector [9].                                       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                                Figure 3
     of the detector rows increases with distance from the                               Design of the Adaptive Array
                                                                                         Detector. The physical width of
     center. Unnecessary dead spaces are avoided and                                     the detector is approximately
     with the corresponding prepatient collimator and the                                40 mm. Slice widths at the axis
     proper read-out schemes the following combinations of                               of rotation range from 1 mm for
     collimation can be achieved: 2 x 0.5 mm, 4 x 1 mm,                                  the inner slices to 5 mm for the
                                                                                         outer slices.
     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. Conse-
     quently with sequential imaging four 5 mm slices would
     be generated during one rotation.
                                                                   2 x 0.5 mm
        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 pro-          4 x 1.0 mm
     vides 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,       4 x 2.5 mm
     2.5, 3.0, 4.0, 5.0 up to 10 mm can be obtained by adjust-
     ing the width and the functional form of the inter-
     polator. On the other hand, a selected slice width s, like
     s = 5 mm, may be obtained from different collimator           2 x 8.0 mm
     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].
                                                                   4 x 5.0 mm
        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                           Figure 4
                                                                                         Available Collimations and
     reason is changing anatomy in axial direction, which is                             read-out schemes for the Adaptive
     particularly pronounced for large pitch applications, i. e.                         Array Detector (AAD). The
     rapid table movement. Consequently, image quality at                                dotted bar indicates the collima-
     high spatial resolution and large pitch is becoming                                 tion at the detector.
     more dependent on the flying spot technology which                                  Prepatient collimation is adjusted
     provides a quasi-instant doubling of the in plane samp-                             correspondingly.
     ling rate.
96   electromedica 68 (2000) no. 2
   In conclusion, image quality in multi-slice spiral                                              values. Obviously the AAI indeed results in slice widths
scanning must be optimized with respect to several                                                 independent of pitch; but even more important, the
factors [13]:                                                                                      functional form of the SSPs is also identical and
                                                                                                   practically independent of pitch. Slice broadening and
   1. The narrowest collimation, consistent with the
                                                                                                   long-range tails of the SSP, which prohibited the use of
coverage of a certain volume and with a certain scan                                               fast table speeds in single slice spiral scanning, can be
time, to minimize partial volume effects and to optimize                                           completely avoided. From this perspective the whole
image quality.                                                                                     range up to a pitch of 8 can be used for practical pur-
   2. Fastest rotation time to maximize z-coverage and                                             poses in multi-slice scanning [6].
to minimize motion blurring.                                                                          The second new and attractive feature of the multi-
   3. Pitch greater than 4 to preserve temporal resolu-                                            spiral AAI is illustrated in Fig. 6. From a scan with
tion and to minimize motion blurring.                                                              4 x 1 mm collimation SSPs with different width can be
                                                                                                   obtained: 1.25 mm, 2.0 mm and 4.0 mm respectively.
  4. The exploitation of flying focal spot technology to                                           Excellent agreement between theory and measurement
avoid artifacts at high spatial resolution.                                                        is observed. This feature is the basis of Combi Scans.
    To measure section profiles of multi-spiral scanning                                           This provides considerable flexibility in image recon-
a thin gold plate (thickness 50 µm) in air has been                                                struction [14] especially for imaging of the base of the
aligned orthogonal to the scanner axis and has been                                                skull and lung (Fig. 7).
scanned in spiral mode. Some of the resulting slice sen-                                              Anatomical structures that generate spiral artifacts
sitivity profiles (SSP) are shown in Fig. 5 for a 4 x 1 mm                                         are well known from single slice spirals. The most
collimation, a slice width of 2 mm and different pitch                                             difficult situations arise from bony structures (high

                                                                                                                                  Figure 5
                       Pitch 3                                  Pitch 5                                    Pitch 7                Slice Sensitivity Profiles,
                                                                                                                                  Collimation 4 x 1 mm, slice
              Measured: -FWHM = 2.05 mm                     FWHM = 2.1 mm                              FWHM = 2.1 mm
                                                                                                                                  width 2 mm, different pitch
    1                                          1                                         1                                        values.
   0.8                                        0.8                                       0.8

   0.6                                        0.6                                       0.6

   0.4                                        0.4                                       0.4

   0.2                                        0.2                                       0.2

         -4       -2      0      2        4         -4     -2      0        2       4         -4      -2      0        2      4

                                                    : calculated          : measured                                 z [SW]

                                                                                                                                  Figure 6
                                                                                                                                  Slice Sensitivity Profiles,
                                                                                                                                  Collimation 4 x 1 mm, pitch 3,
              Measured: -FWHM = 1.35 mm                    FWHM = 2.05 mm                             FWHM = 4.0 mm
                                                                                                                                  different slice widths.
    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

         -4       -2      0      2        4         -4    -2       0        2       4         -4      -2      0        2      4

                                                    : calculated                : measured

                                                                                                                                       electromedica 68 (2000) no. 2   97
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                                                        a                                                 b

                                                        c                                                 d
                                                                       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.

98   electromedica 68 (2000) no. 2
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 colli-
mation for equal z-coverage. The underlying reason
is the better elimination of partial volume artifacts.
From image quality reasons, only, the strength of multi-
slice 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                                                                        a
   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].
  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 acqui-
sition interval to the first-circulation time of the in-
jected 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      Figure 8
to be completed during the first pass of iodinated con-      MSCTA of the thoracoabdominal
trast material through the arterial system. The use of       aorta. Stanford Type B Dissection.
MSCT with CT angiography demonstrates the spatial            MPRs from images with 3 mm
                                                             slice-width and 1 mm increment.
relationship of an aneurysm with the feeding vessel, as      The spiral scan was acquired
well as the shape of the aneurysm itself, because the        with 4 x 2.5 mm collimation,
same bolus can be followed continuously throughout its       pitch 5, and 0.5 s rotation time.
course. For CT angiography of the thoracoabdominal           The total scan time for a spiral
                                                             length of 550 mm was 22 s.
aorta a collimation of 2.5 mm, a pitch of 5-6 at a rota-     Courtesy of Dr. Baum, Erlangen.
tion 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 three-
dimensional data set. A complete lower extremity study
                                                                  electromedica 68 (2000) no. 2   99
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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 thick-
                                                                 ness 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                           Left: MIP from images
                                                                 Peripheral Runoffs. Spiral scan    with 3 mm slice width,
                                                                 with 4 x 2.5 mm collimation,       2 mm increment.
                                                                 pitch 6, 0.5 s rotation time.
                                                                                                    Right: VRT from images
                                                                 The total scan time for a spiral   with 6 mm slice width,
                                                                 length of 1000 mm was 34 s.        4 mm increment.


                                                                 Figure 10                          On the VR images a stenosis in
                                                                 CTA (a, b) of visceral branch      the SMA (arrows) can be readily
                                                                 vessels (collimation 4 x 1 mm,     appreciated which was confirmed
                                                                 pitch 5, 120 cc contrast).         at conventional angiography (c).
100   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        16
                                                                 15                                                                      Slice q = 0
in 10 s allows the scanning of children without sedation.        14                                                                      Slice q = 1
This can be used to easily perform CTA of the large              13                                                                      Slice q = 2
thoracic vessels before or after surgery of complex mal-         12                                                                      Slice q = 3
formations [8].                                                  10
  Cardiac Imaging                                                 8
   Electron Beam CT scanning (EBCT) has been                      6
established as a non-invasive imaging modality for                5
imaging coronary calcification. Major clinical appli-             3                                                                  ECG-Signal
                                                                                                                                     (~ 70 bpm)
cations are the detection and quantification of coronary          2
calcium and non-invasive CT angiography (CTA) of the              0
coronary arteries [17]. Current limitations of EBCT                   0   0.5    1        1.5         2            2.5     3       3.5
imaging include the limited reproducibility of coronary                              t [sec] (0.5 sec/rotation)
calcium quantification [18], the inability to detect non-
calcified atherosclerotic plaques and the limited spatial
resolution of 3D visualizations of the coronary arteries.    Figure 11                                 Data ranges are selected with
                                                             Reconstruction with retro-                certain phase relation to the
Because of the restriction to axial, non-spiral scanning     spectively ECG-gated 4-slice              RR-intervals. 3D volume images
in ECG-synchronized cardiac investigations, acquisi-         spiral scanning.                          are generated from image stacks
tion of 3D volume images by using EBCT can only                                                        reconstructed in consecutive
provide limited z-resolution within a single breath-hold                                               heart cycles.
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      Fig. 11 shows an example of how the cardiac volume
potential to completely cover the heart volume without       is successively covered with stacks of axial images
gaps within one breath-hold [19]. Mechanical multi-          (shaded stacks) reconstructed in consecutive heart
slice CT systems with simultaneous acquisition of            cycles. All image stacks are reconstructed at identical
four slices, half-second scanner rotation and 125 ms         time-points during the cardiac cycle. At the same time,
maximum temporal resolution allows for considerably          the 4 detector slices travel along the z-axis relative to the
faster coverage of the heart volume, compared to single-     patient table. In each stack, single-slice partial scan
slice scanning. This increased scan speed allows using       data segments are generated with equidistant spacing in
thinner collimated slice widths and thus to increasing       the z-direction depending on the selected image recon-
the z-resolution of high-resolution examinations such as     struction increment [21]. Continuous volume coverage
                                                             can only be achieved, when the spiral pitch is adapted to
CTA of the coronary arteries [20].
                                                             the heart rate in order to avoid gaps between image
   ECG-synchronized multi-slice spiral scans are             stacks that are reconstructed using data from different
acquired with heart rate dependent table feed (“pitch”)      heart cycles. In order to achieve full volume coverage,
adaptation. Dedicated spiral algorithms provide 125 ms       the image stacks reconstructed in subsequent heart
(60 ms as theoretical limit) temporal resolution and are     cycles must cover all z-positions. Thus, the pitch, which
optimized with regard to volume coverage. This allows        can be used for image acquisition is limited by the
reconstruction of overlapping images (increment < slice-     patient’s RR-interval time [19].
width) at arbitrary z-positions and during any given
                                                                The cardiac multi-slice data acquisition on the
heart phase [21]. This reconstruction technique com-
                                                             SOMATOM Volume Zoom are performed using 500 ms
bines partial scan reconstruction and multi-slice spiral
                                                             full rotation time and 4 x 1 mm or 4 x 2.5 mm colli-
weighting in order to compensate for table movement
                                                             mated slice width. Non-contrast enhanced spiral scans
and to provide a well-defined slice sensitivity profile
                                                             for coronary calcium scoring are performed with 3 mm
                                                             slice-width (SW = 3 mm, SWcoll = 2.5 mm) and 1 mm
   For retrospectively ECG-gated reconstruction each         image reconstruction. For CTA of the coronary arteries
image is reconstructed using a multi-slice partial scan      and for functional heart imaging 2 different scan
data segment with an arbitrary temporal relation to the      protocols with 3 mm slice-width and with 1.25 mm
R-wave of the ECG-trace. Image reconstruction during         slice-width can be used (Fig. 12). For both protocols,
different heart phases is feasible by shifting the start     non-ionic contrast material is intravenously injected at
point of image reconstruction relative to the R-wave.        a flow rate of 3 ml/s. The delay times between start of
For a given start position, a stack of images at different   contrast injection and scan start for optimal contrast
z-positions covering a small sub-volume of the heart can     enhancement is determined individually for each patient
be reconstructed owing to mutlislice data acquisition.       by injection of a 20 ml test bolus [20].
                                                                                                                  electromedica 68 (2000) no. 2        101
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                                                                             a                                   a

                                                                             b                                   b

      Figure 12                          Both conventional angiography           Figure 13
      49-year-old male with CHD          and CT angiography                      Noninvasive CT coronary
      of the RCA. S/P posterolateral     (3D Virtuoso®, Siemens) clearly         angiography: Volume rendered
      myocardial infarction 4 months     show the patency of the RCA             image (3D Virtuoso®, Siemens)
      ago.                               at the level of the ballon angio-       depicts three high grade stenoses
                                         plasty.                                 (arrows) in the LAD and at the
      Patient underwent balloon
                                                                                 origin of the diagonal branches.
      angioplasty of subtotal stenosis   There is only minor residual
                                                                                 These findings are confirmed at
      at the beginning of the descend-   stenosis of approx. 10-20%
                                                                                 conventional angiography.
      ing part of the RCA.               (arrows).
      Patient came in for follow-up      Scan: Siemens SOMATOM®
      performed with both coronary       Volume Zoom; 140 kV, 300 mAs,
      CT angiography (a) and             collimation 4 x 1 mm;
      conventional angiography (b).      slice width 1.25 mm,
                                         increment 0.6 mm.

102   electromedica 68 (2000) no. 2
   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 repro-
ducibility 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 over-
lapping image reconstruction, however, was found to
improve the reliability of coronary calcium quantifi-
cation especially for small plaques. ECG-gated multi-                                                                          14
slice spiral CT can potentially be of high value for
coronary calcium evaluation especially for patients
undergoing lipid-lowering statin therapy and for follow-
up evaluations of patients after heart transplantation
   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 high-
resolution 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                                                                       15
holds promise to allow for the non-invasive imaging of      Figure 14                          Figure 15
rupture-prone soft coronary lesions and may have the        Virtual angioscopy (bottom         MSCTA of LAD: Non-calcified
option to lead to early onset of therapy [25].              right) of circumflex branch of     soft plaque (arrow) with density
                                                            left coronary artery. VRT (left)   of approx. 5 HU.
  High Resolution Imaging                                   and MPR (top right) images
                                                                                               This lipid-core plaque was
                                                            help navigating the virtual
                                                                                               classified as prone to rupture in
   Imaging of the temporal bone is a major challenge for    endoscope in the coronary artery
                                                                                               intracoronary ultrasound.
clinical CT and a good example of the need for high         (3D Virtuoso®, Siemens).
resolution CT. Imaging of the temporal bone is improved                                        collimation 4 x 1 mm, pitch 1.5,
using MSCT because the in-plane resolution can be                                              300 mAs, 120 cc contrast.
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/
                                                                                                    electromedica 68 (2000) no. 2   103
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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

                                                                              electromedica 68 (2000) no. 2   105

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