Eur Radiol (2006) 16: 1206–1215
DOI 10.1007/s00330-005-0118-9
Yiannis Kyriakou
Marc Kachelrieβ
Michael Knaup
Jens U. Krause
Willi A. Kalender
Received: 13 July 2005
Revised: 25 November 2005
Accepted: 2 December 2005
Published online: 6 April 2006
# Springer-Verlag 2006
Y. Kyriakou . M. Kachelrieβ .
M. Knaup . J. U. Krause .
W. A. Kalender (*)
Institute of Medical Physics,
University Erlangen-Nürnberg,
Henkestr. 91,
91052 Erlangen, Germany
e-mail: [email protected]
Tel.: +49-9131-8522310
Fax: +49-9131-8522824
COMPUTER TOMOG RAPHY
Impact of the z-flying focal spot on resolution
and artifact behavior for a 64-slice
spiral CT scanner
Abstract The effect of the z-flying
focal spot (zFFS) technology was
evaluated by simulations and measurements with respect to resolution
and artifact behavior for a 64-slice
spiral cone-beam computed tomography (CT) scanner. The zFFS alternates
between two z-positions of the X-ray
focal spot, acquiring two slices per
detector row, which results in double
sampling in the z-direction. We implemented a modified reconstruction
that is able to obtain images as they
would be without zFFS. A delta
phantom equipped with a thin gold
disc was used to measure slice sensitivity profiles (SSP), and a high-contrast bar phantom was used to quantify
Introduction
The use of multislice spiral computed tomography (MSCT)
scanners provides several advantages for most clinical applications. Modern MSCT scanners offer isotropic submillimeter resolution at considerably shorter scan time.
Demanding applications, such as pure arterial CT angiography (CTA) of the carotid arteries and circle of Willis
benefit from the extended scan range. Of great importance
for cardiac scans is the increased temporal resolution,
which goes hand in hand with the higher longitudinal
resolution.
Even so, MSCT systems still suffer from windmill artifacts, which are disturbing for applications where highest
possible image quality is required. Windmill artifacts in
spiral CT were discussed in previous studies [1, 2]. Several
causes were given: (1) changeovers in pairs of data used in
z-interpolation, (2) cone angle, and (3) sparse longitudinal
sampling. First, let us consider the windmill artifact phe-
the resolution in the x/z-plane with and
without zFFS. The zFFS decreases the
full width at half maximum (FWHM)
of the SSPs by a factor of about 1.4.
The double z-sampling allows the
separation of 0.4 mm bars in the
z-direction compared with 0.6 mm in
the case without zFFS. The zFFS
effectively reduces windmill artifacts
in the reconstructed images while
maintaining the transverse resolution,
even at the largest available pitch
value of 1.5.
Keywords Spiral CT . Image
quality . Windmill artifacts .
z-flying focal spot
nomenon. These artifacts appear as streak-like patterns that
rotate around structures of high longitudinal gradients when
scrolling through the volume. Artifact strength depends on
the magnitude of high contrast transitions but is generally
independent of location. Silver et al. [1] showed that artifact
magnitude increases with thin nominal slice widths and
large pitch values.
We concentrated on the underlying cause for these artefacts, which is aliasing. Aliasing artifacts generally occur
when the Nyquist condition is not fulfilled. According to
the Shannon sampling theorem, at least two sample points
should be taken per spatial resolution element. In case this
is not satisfied, aliasing artifacts affect the sampled data.
Double sampling is not easy to achieve, especially for CT
imaging where the spacing of the detector samples is
slightly larger than the active width of the detector elements. For the x/y-plane, a workaround is the quarter detector offset. Shifting the detector array (channel direction)
by one quarter of the detector sampling distance results in
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opposing rays that interlace and, by combining opposing
views, one effectively doubles the in-plane sampling [3].
An alternative is deflecting the focal spot between adjacent
detector readouts. A flying focal spot (FFS) can be used to
double the sampling density in the rotation direction
(αFFS).
The improvement of sampling in the longitudinal direction can be achieved by the choice of optimized small
pitch values, such that complementary data acquired in
different rotations interleave in the z-direction [4]. Another
approach that is not restricted to certain pitch values is the
use of a z-flying focal spot (zFFS) in the axial direction. A
periodic motion of the focal spot in the z-direction allows
the doubling of the sampling density, thus satisfying the
sampling theorem.
We distinguish here between two sampling directions:
the transversal, which is defined by the view angle α, and
the axial or z-direction. Since the FFS in azimuthal direction
(αFFS) has been used by CT scanners for more than one
decade, this work concentrates on the zFFS. In this paper,
we investigate the effect of the zFFS by simulations and
measurements with respect to resolution and artifact
behavior.
Materials and methods
Materials
CT scanner
The Sensation 64 CT scanner (Siemens Medical Solutions,
Forchheim, Germany) allows measuring 64 slices of 0.6 mm
thickness. The scanner is equipped with an array detector
consisting of 40 rows of different sizes in the longitudinal
direction. The central 32 rows correspond to a slice width of
0.6 mm each whereas the outer eight rows, four on each
side, provide a slice width of 1.2 mm. The given detector
configuration allows for the realization of the collimations
32×0.6 mm and 24×1.2 mm (in the isocenter).
Fig. 1 Rotating vacuum vessel X-ray tube with focal spot detection
capability. The focal spot wobbles between two different positions
on the anode plate controlled by an electromagnetic deflection
system
axial or z-direction. The deflection parameters for both
directions are ∂α and ∂z; the focal spot deflection angle and
deflection length as shown in Fig. 3a,b, respectively. The
deflection angle ∂α is used to improve in-plane sampling
whereas ∂z determines the axial sampling properties of the
scanner. As shown in Fig. 1, a deflection of ∂z in z-direction
changes the distance to the isocenter RF by ∂RF = ∂z/tanφ,
where φ denotes the anode angle. Note that the variation in
RF due to ∂α is negligible. As derived in [6], ∂α is given by
1
RFD
@α ¼ Δβ
;
4
RD
(1)
Flying focal spot technology
The scanner is equipped with a Straton X-ray tube [5],
which has an electromagnetic beam detection system for
the focal spot position (Fig. 1). The periodic motion of
the focal spot on the anode plate, which has an anode
angle of typically 7–9°, results in a motion both in the
radial and z-direction, as shown in Fig. 2. We here define
a projection to be the collection of adjacent readings that
comprise a full FFS cycle, as illustrated in Fig. 2.
Movement of the αFFS and the zFFS results in four
readings or view angles.
We distinguish between two sampling directions: the
transversal, which is defined by the view angle α, and the
D
C
α
B
A
Fig. 2 Geometry of the focal spot deflection (sketch is not drawn to
scale). The drawing demonstrates the combined in-plane (αFFS)
and z-flying focal spot (zFFS) movement. One FFS cycle therefore
results in four readings (corresponding to each focal spot position) in
the case that both FFSs are used. Consequently, in the case that only
the zFFS is on, the FFS cycle will consist of two readings
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sidering the single focus case, the sampling distance in the
z-direction scaled to the isocenter is the physical slice
thickness S ¼ ΔbRF =RFD . Consequently, the rays should
be longitudinally separated by 12 S when intersecting the
isocenter, as shown in Fig. 3b. Since the table increment per
reading is much smaller than ∂z, we obtain
1 RFD
@z ¼ S
4 RD
(2)
by simple geometrical considerations.
The values of ∂α and ∂z remain the same, even if the
deflection is switched on for both directions. However, the
focal spot itself will wobble in a rectangular fashion relative
to the anode disc, as shown in Fig. 2. As already mentioned,
this gives four physical readings per FFS cycle. A more
detailed description of the FFS detection influence on the
reconstruction geometry is given in Kachelrieß et al. [6] and
Flohr et al. [7].
Acquisition mode
The scanner allows for two acquisition modes using the
32×0.6 mm collimation: one with zFFS only, and a combined mode using in-plane and zFFS. The first is used when
the gantry rotation time is ≤0.5 s and the second for 1.0 s
rotation time. In both cases, 64 overlapping 0.6 mm slices
per rotation are acquired. This will be referred to in the
following as the 2·32×0.6 mm collimation mode.
Reconstruction
Fig. 3 Geometry of the flying focal spot (FFS) deflection (sketches
are not drawn to scale)
where βphys is the angle within the fan, RD the distance of
the isocenter to the detector, and RFD the distance of the
undeflected focal spot to the detector (RFD = RD + RF).
Regarding the z-direction, the zFFS switches between two
z-positions acquiring two slices per detector row, which in
effect provides the double sampling required by the Nyquist
theorem. In practice, two subsequent readings with a collimation of 32×0.6 mm are combined into one 64-slice projection with a sampling distance of 0.3 mm at the isocenter.
Concerning the z-direction, the correct selection of the
magnitude of ∂z should double the sampling density. Con-
A modified advanced single-slice rebinning (ASSR) [8] algorithm was used for off-line reconstruction (SyngoExplorer,
VAMP GmbH, Erlangen, Germany). All measurements
were conducted with the zFFS enabled since the Sensation
64 does not allow the double z-sampling to be switched off.
The reconstruction was modified such that we were able to
obtain images as they would be without zFFS. In order to
achieve this and mimic a standard scan (without zFFS),
every other reading was skipped during reconstruction.
Image quality evaluation
Slice sensitivity profiles
To quantify resolution for the z-direction, we performed
slice sensitivity profile (SSP) measurements and simulations. For SSP measurements, we used a thin (50 μm) gold
disc of 2.0 mm imbedded in a cylinder of water-equivalent
1209
material (QRM GmbH, Möhrendorf, Germany). The phantom represents an approximation for a delta impulse in the
z-direction, as is shown in Fig. 4a. The SSP phantom was
positioned centrally in the field of measurement, and the
scan was performed at 120 kVand 200 mAs. Tube current is
automatically adapted to provide constant dose at any spiral
pitch. Regarding the corresponding simulations, these were
performed with the software package ImpactSim (VAMP
GmbH, Erlangen, Germany).
The collected images of the SSP phantom were
evaluated using the software package ImpactIQ (VAMP
GmbH). Since the water-equivalent support of the gold disc
yields a constant background of about 0 HU, contrast was
measured as the mean CT value within a region of interest
(ROI) centered about the disc. SSPs were extracted from
the given ROI stacks by automated tracking of the mean
ROI value through the volume. The resulting curves represent the contrast in absolute CT numbers as a function of
the z-position of the reconstructed image relative to the
actual position of the gold disc.
To quantify the measured resolution in z-direction, the
resulting FWHM was determined for all measurement
setups. In practice, the FWHM of the SSP defines the
measured slice width. This was calculated for each case,
with and without zFFS, to quantify the influence of the
zFFS. Further, dependence of the FWHM and full width at
tenth maximum (FWTM) on the pitch value was examined.
For this case, we reconstructed the images at four nominal
slice thicknesses Snom= 0.6 mm, 1.0 mm, 2.0 mm, and
3.0 mm. Pitch values were varied from 0.5 to 1.5 in steps of
0.1. The reconstruction increment was 0.1 mm.
A scan of the SSP phantom with the 2·32×0.6 mm
collimation was used to evaluate the impact of the zFFS on
slice sensitivity. A nominal reconstruction width of Snom=
0.6 mm and reconstruction increment of 0.1 mm were
chosen for the reconstruction without and with the zFFS.
The resulting SSPs were compared with respect to their
FWHM and shape.
2 cm
Holder
Gold Disc (inside)
a
SSP delta phantom
b
High-contrast resolution
bar-pattern phantom
Fig. 4 Phantoms for measuring the slice sensitivity profile and
spatial resolution
Longitudinal resolution
The modulation transfer function (MTF) allows quantification of the spatial resolution capabilities of the system and
the corresponding scan mode. Similar to the SSP evaluation, a comparison of the MTF with and without the zFFS
was carried out. The MTF in the z-direction is given by the
Fourier transform of the SSP
MTF ¼jF fSSPgj:
(3)
For the MTF calculation, we took the SSP generated with a
reconstruction slice width of 0.6 mm.
Additionally, we used a high-contrast bar phantom for
visual evaluation of the achievable z-resolution, as shown
in Fig. 4b. The test pattern consists of several groups of
poly-methyl-methacrylate (PMMA) bars, which are fixed
on a supporting disc. The width of the bars is equal to the
spacing in between and is increased in steps of 0.1 mm
from 0.1 mm to 1.5 mm. For a qualitative evaluation of the
spatial resolution, we performed measurements of the testpattern phantom shown in Fig. 4b. It was placed such that
the 0.4 mm, 0.5 mm, and 0.6 mm bars were oriented in the
z-direction near the isocenter. The nominal slice width was
Snom = 0.6 mm and the reconstruction increment 0.1 mm.
Noise
Spiral scan simulations of the head phantom were performed for two different setups with respect to noise behavior. The first considered a 2·32×0.6 mm scan with zFFS,
and the second was a scan without zFFS but with a physical
collimation of 64×0.3 mm, both providing a z-coverage of
19.2 mm. We adapted the interpolation between detector
rows in order to ensure that the reconstruction provided the
same SSP with an FWHM of 0.6 mm for both cases. This
FWHM was selected because it is the thinnest effective
slice width available for the zFFS and allows for an investigation of noise behavior at the maximum resolution level.
The noise was measured in the respective difference images.
Since a scanner that offers a collimation of 64×0.3 mm
was not available for measurements, the experiments considered cases with and without the zFFS at a collimation of
32×0.6 mm and 2·32×0.6 mm, respectively. Noise evaluations were performed with a standard 20 cm water phantom
at a pitch value of 1.3. Two spiral scans were carried out
with 2·32×0.6 mm collimation and 120 kV tube voltage at
150 mAs and 300 mAs, respectively. The 300 mAs scan
was reconstructed using the modified ASSR reconstruction
in order to mimic a standard scan without zFFS. Two scans
were necessary since the modified ASSR reconstruction
leaves out every other reading. For the case of the 150 mAs
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scan, reconstruction was performed with the zFFS using all
readings.
A standard convolution kernel B40s was used in both
cases. We calculated the standard deviation σ of the CT
values in a 10 cm2 circular ROI in the center of the phantom
for 40 slices. The average σ over all single images of the
reconstructed volume was evaluated for both cases. An
important precondition for comparison of the noise level is
that spatial resolution in the z-direction is equivalent for both
cases, as mentioned above. The comparison was conducted
at an identical FWHM of 1.0 mm. For this comparison, a
reconstruction at maximum resolution was explicitly
avoided. This will be discussed in the Discussion section.
Artifact evaluation
Investigation of the windmill artifact content was also conducted by simulation and measurement. Simulation offers
the additional possibility to examine the 64×0.3 mm collimation and to compare it with the 32×0.6 mm mode with
and without the zFFS. Again, the interpolation between
detector rows was adapted such that evaluation of the
reconstructed images was carried out at an FWHM of
0.7 mm for all cases.
For further investigation of the windmill artifact phenomenology, we analyzed a number of randomly chosen
clinical head CT exams and focused on artifact-prone
regions, such as the orbit and the base of the skull, where
high-contrast transitions are present. Data selected herein
were scanned with a pitch value of 1.2. In contrast to simulations, measured patient data were reconstructed without
adapting the interpolation between detector rows. This
means that the respective reconstructions did not provide
the same FWHM.
Results
a Simulated SSP.
b Measured SSP.
Fig. 5 Simulated and measured slice sensitivity profiles (SSPs)
without and with z-flying focal spot (zFFS) activated for the physical
collimation of 32×0.6 mm. The full width at half maximum
(FWHM) of the measured SSP reconstructed without zFFS was
approximately 1.38 times wider than the FWHM with zFFS
Impact of the zFFS on slice sensitivity
Slice sensitivity profiles
SSPs generated using the zFFS are thinner compared to the
simulated case without zFFS. Simulations (see Fig. 5a)
showed that double sampling in the z-direction is able to
naturally improve axial spatial resolution.
Measurements confirmed the results of the simulations.
Figure 5b shows that SSP with the zFFS depicts an FWHM
decreased by a factor of about 1.38 as compared with the case
without zFFS. For this comparison, the nominal slice width
chosen for reconstruction was Snom=0.6 mm. Regarding the
case with zFFS, the measured SSP is bell-shaped, without far
reaching tails, with an FWHM of 0.66 mm. Differences to the
simulation may be explained by the non-ideal focus and
collimators used by the measurement setup. It should be
noted here that these improvements were achieved without
modifying the kind of interpolation between detector rows
(standard linear interpolation).
Dependence of SSP parameters on pitch
The results for FWHM and FWTM of the SSPs did not vary
significantly with the pitch for all reconstructed slice
thicknesses. As shown in Fig. 6a, for Snom=0.6 mm,
1.0 mm, 2.0 mm, and 3.0 mm, values of the measured
FWHM did not noticeably change, even at higher pitch
values of up to 1.5. The range of FWHM variation was
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3.5
Impact of the zFFS on longitudinal spatial resolution
3
FWHM [mm]
2.5
2
1.5
1
0.5
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
pitch
0.6 mm
1.0 mm
a
2.0 mm
3.0 mm
FWHM
5
4.5
4
The quantitative effect on spatial resolution in the z-direction
is given by the corresponding measured MTFs (see Fig. 7a).
The MTF for the case of double z-sampling proved that a
higher resolution is obtained compared with the reconstruction without zFFS. As shown in Fig. 7a, the 50%, 10%,
and 5% values of the MTF amount to 0.52 lp/mm, 1.12 lp/
mm, and 1.3 lp/mm, respectively. These results correspond
to an achievable resolution of typically about 0.44 mm
(10% MTF value) in the case where the zFFS is employed.
We compared these quantitative results given by the
MTF to the qualitative impression provided by the resolution patterns of the bar-pattern phantom. The bar-pattern
phantom was placed such that the resolution in z-direction
could be evaluated in both cases, with and without the
zFFS. Visual evaluation of the reconstructed slices of the
bar phantom confirmed our numerical results, as demonstrated in Fig. 7b,c. Double z-sampling allowed the separation of the 0.4 mm bars in the z-direction in contrast to
0.6 mm in the case without zFFS.
FWTM [mm]
3.5
3
Noise issues
2.5
2
1.5
1
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
pitc h
0.6 mm
1.0 mm
b
2.0 mm
3.0 mm
FWTM
Fig. 6 Full width at half maximum (FWHM) and full width at tenth
maximum (FWTM) as a function of the pitch value for Snom=0.6 mm,
1.0 mm, 2.0 mm, and 3.0 mm. The pitch varied from 0.5 to 1.5 in
steps of 0.1. For the computed tomography (CT) scanner evaluated,
the measured FWHM is independent of the pitch. All scans herein
used the z-flying focal spot (zFFS)
1.19%, 1.13%, 0.49%, and 0.8% for Snom=0.6 mm, 1.0 mm,
2.0 mm, and 3.0 mm, respectively, corresponding to a mean
variation of 0.9% within the measured interval of pitch
values. The average of the FWHM values for the given
pitch interval deviated about 5% from Snom.
Figure 6b shows the respective results for FWTM
values. The average of FWTM values for the same pitch
interval as above was 1.33 mm, 1.98 mm, 3.22 mm, and
4.34 mm for Snom=0.6 mm, 1.0 mm, 2.0 mm, and 3.0 mm,
respectively. Mean variation of FWTM over all measurement series was <2.3%.
FWTM and FWHM of SSPs confirmed their independence of pitch. All SSP results shown were measured in the
center of rotation (COR).
Simulated images of the head phantom were evaluated with
respect to noise level, as shown in Fig. 8. Respective noise
values were obtained from an ROI in difference images.
The precondition for this comparison is the same FWHM of
0.6 mm. The lowest noise level was provided by the
hypothetical CT system providing the collimation of
64×0.3 mm and amounted to 22 HU. On the other hand,
the system with a physical collimation of 0.6 mm using the
zFFS resulted in a higher noise level. In the zFFS case, a
standard deviation of 29.0 HU was assessed, which corresponds to a factor 1.3 increase in noise for the same
resolution level. This discrepancy between both scanners is
the expected behavior since it is of disadvantage in terms of
dose usage and noise when the spatial resolution is pushed
to the maximum [9].
Based on the equivalence of slice thickness (FWHM=1.0 mm
for both cases), a comparison of the noise level was performed based on measurements of the water phantom. Evaluation of the water phantom images generated with the use
of the zFFS (see Fig. 9b) resulted in a mean standard deviation of σ ¼ 24:0 HU; the corresponding reconstruction
without zFFS shown in Fig. 9a amounted to the same value.
Standard deviation of the noise was 0.49 HU and 0.51 HU
with and without the zFFS, respectively. Consequently,
there is no appreciable difference with respect to noise
between the standard sampling and the double sampling
schemes. This meets the theoretical expectation that noise
is independent of sampling but depends on the algorithm’s
z-interpolation function only [9].
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1
without zFFS
with zFFS
0.9
0.8
0.7
MTF
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
1.2
u / (lp/mm)
1.4
1.6
1.8
2
a MTF without and with zFFS.
b Bar-pattern without
zFFS
c
Bar-pattern with
zFFS
Fig. 7 Spatial resolution evaluation: a Modulation transfer function
(MTF) without and with z-flying focal spot (zFFS). In the case of
double z-sampling, the corresponding MTF shows a higher resolution than the MTF generated without the zFFS. b Reconstructed
images of the bar pattern phantom in the z-direction without and
with zFFS. Double z-sampling allowed the separation of 0.4 mm
bars as compared with 0.6 mm in the case of no zFFS (C:400,
W:100)
Effect of the zFFS on windmill artifacts
other hand, the case without zFFS and single sampling was
strongly affected by streak-like windmill artifacts arising
from the high-contrast transitions in the phantom. However,
visual impression of the images resulting in the case of the
64×0.3 mm collimation for the same FWHM of 0.7 mm
depicts that this configuration provides slightly better
results than the zFFS.
Simulations for the head phantom with and without the use
of the zFFS showed that double z-sampling generally
reduces aliasing artifacts (see Fig. 10). The occurrence of
windmill artifacts in simulated images remained low, even
for the largest pitch value, when the zFFS was used. On the
Fig. 8 Reconstructed slices
of simulated scans of the
FORBILD head phantom without and with z-flying focal spot
(zFFS) for collimations of
64×0.3 mm and 2·32×0.6 mm,
respectively. Both reconstructions provided identical slice
sensitivity profiles (SSP) and
thus the same full width at half
maximum (FWHM) of 0.6 mm.
At this resolution level, the
corresponding noise level is
higher in the case of the zFFS as
compared with the 64×0.3 mm
mode. This confirms our theoretical considerations [9]
50/100
50/200
50/100
σ= 29 HU
50/200
50/100
σ= 22 HU
50/200
1213
Fig. 9 Reconstructed slices of a
measured 20 cm water phantom
without and with z-flying focal
spot (zFFS) for the 32×0.6 mm
and 2·32×0.6 mm collimations
at a full width at half maximum
(FWHM) of 1.0 mm. Visual
impression confirms that the
noise level is equivalent in both
cases
σ= 24
50/200
a
wit hout zFFS
To validate the simulation results, measurements were
performed with respect to windmill artifact susceptibility.
Clinical examples for abrupt attenuation transitions along
the scan direction are spiral scans of the skull, especially the
skull base and orbita region. As demonstrated in Fig. 11, for
a head scan, the images reconstructed without using the zFFS
clearly show windmill artifacts near the skull bones. The
effect becomes more obvious when scrolling through the
image volume, where windmill artifacts can be identified
as rotating structures. The right column in Fig. 11 shows
b
wit h zFFS
the corresponding reconstructions with double z-sampling.
In this case also, windmill artifacts were significantly
reduced in agreement with simulation results. Visual image
impression confirmed the significant benefits offered by
double z-sampling due to the zFFS as compared with
standard reconstructions with single sampling per slice
width. To obtain a more vivid impression of windmill
artifacts and their suppression, a movie of the case shown
in Fig. 11 is provided at http://www.imp.uni-erlangen.de/
forschung/bildgebung/frm_bildmed.htm.
2.32 x 0.6 mm (with zFFS)
64 x 0.3 mm (w/o zFFS)
No noise added
32 x 0.6 mm (w/o zFFS)
50/100
50/100
50/100
With noise added
Fig. 10 Simulated images of
the FORBILD head phantom
without and with z-flying focal
spot (zFFS). Windmill artifacts
are largely suppressed in the
case of zFFS and the 64×0.3 mm
collimation. The full width at
half maximum (FWHM) was in
all cases 0.7 mm. Respective
noise levels are equivalent
σ= 24
50/200
50/200
σ= 20HU
50/200
σ= 20HU
50/200
σ= 20HU
1214
Fig. 11 Reconstructed head images without (left) and with (right)
z-flying focal spot (zFFS). As demonstrated in the right column
images, windmill artifacts are successfully suppressed
Discussion and conclusions
The zFFS technology allows for controlled motion of the
focal spot in the z-direction. Its primary goal is to switch
between two discrete focus positions and thereby double
the number of samples and simultaneously acquired slices.
On the scanner investigated here, which offers 32 central
0.6 mm slices, the zFFS technique provides 64 overlapping
0.6 mm slices per rotation. This finer sampling scheme
corresponds with the sampling density, which a hypothetical 64×0.3 mm collimation would offer.
Our results show that this approach to double sampling
improves longitudinal resolution and artifact behavior as
compared with the case without zFFS for arbitrary pitch
values. Simulations and measured results agreed in the
finding that SSPs with zFFS were approximately 1.4 times
narrower than the corresponding SSPs without use of the
zFFS. Additionally, both the resulting MTF and visual
evaluation of a test-pattern phantom verified separation of
the 0.4 mm bars in the z-direction for the CT scanner
evaluated here. This was achieved without modification of
the interpolation between detector rows.
Windmill artifacts that emerge from high-density objects
and appear to rotate around their origin when going through
the stack of images are caused by inadequate axial sampling. Our results showed that these artifacts are an aliasing
problem and not a cone-beam issue, as it is often assumed
Windmill artifacts persist, even if an exact three-dimensional (3D) cone-beam reconstruction algorithm is used.
Exact algorithms correct cone-beam problems such as
shadows but have no effect on the windmill problem [2].
Conventional approaches try to suppress windmill artifacts by a decreased pitch or by increasing reconstruction
slice width relative to collimation. “Optimized” small pitch
values are chosen in such a manner that data acquired in
different rotations interleave in the z-direction [4, 10].
Besides the restriction to small pitch values and thus to low
volume coverage speed, an major disadvantage of these
approaches is that the optimized sampling is provided only
for the isocenter. This is an important difference to the
zFFS, where improved sampling is offered also for offcenter regions at any pitch value [7]. The benefits of the
zFFS are evident in a wide range of the scan field of view,
which can be defined by a limiting radius of about 180 mm,
as reported in Flohr et al. [7]. This is the radial distance of the
intersection points of the rays measured by adjacent detector
elements at the two different z-positions (see Fig. 3b).
Another approach suggested by Silver et al. [1] was to
blur the data before interpolation. This is a common method
in digital sampling; a prefiltering of sparsely sampled data
with a smoothing function serves to decrease aliasing. This
method provides a cosmetic effect instead of a real windmill
artifact suppression method since the original sampled data
do not conform with the sampling theorem. Additionally,
every kind of data preblurring is performed at the expense
of axial resolution. Since modern CT imaging aims for
high isotropic resolution at sub-millimeter slice thicknesses,
such methods are limited in their application.
The double z-sampling achieved by the zFFS is in
conformance with the Nyquist theorem. Therefore, aliasing
artifacts are clearly suppressed even for the high pitch
values, which were used in this paper for simulations and
measurements. Nevertheless, the hypothetical 64×0.3 mm
1215
collimation showed better windmill artifact suppression as
compared with the zFFS. Of course, the single-sampling
scheme of 64×0.3 mm does not satisfy the Nyquist
theorem. However, reconstruction of a 0.6 mm slice for a
physical collimation of 0.3 mm represents a smoothing in
the z-direction, which counteracts aliasing.
Spatial resolution is commonly pushed to the limit dictated by the detector element’s size. This means that the
effective slice width or the FWHM of the SSP should equal
the physical slice width, in this case, 0.6 mm. Our simulations illustrated that in the case of an FWHM of 0.6 mm
for the 2·32×0.6 mm collimation, an increased noise level
was the cost for the maximal spatial resolution as compared
with the collimation of 64×0.3 mm. It appears that for a
given scanner, reconstruction at maximum spatial resolution is not the optimum case with respect to the relation
between noise, dose, and spatial resolution.
In summary, it can be stated that once the measured signal
is inadequately sampled, it is relatively difficult to perform
correction of windmill artifacts without loss of spatial
resolution. Double z-sampling, on the contrary, satisfies the
sampling theorem and significantly reduces these artifacts at
measurement. Spatial resolution and image quality can be
greatly improved with the focal spot deflection, even at high
pitch values and the thinnest effective slice thicknesses [7].
This ensures sub-millimeter resolution and simultaneously a
reduction of the disturbing windmill artifacts. The zFFS
technology helps to improve image quality for demanding
clinical applications such as CT angiography, 3D image
post-processing, and virtual endoscopy.
Acknowledgement We gratefully acknowledge support from
Siemens Medical Solutions who provided a Sensation 64 CT scanner
to our institute for experimental and clinical work.
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