Impact of the z-flying focal spot on resolution and artifact behavior for a 64-slice spiral CT scanner

Transcription

1 Eur Radiol (2006) 16: DOI /s COMPUTER TOMOGRAPHY Yiannis Kyriakou Marc Kachelrieβ Michael Knaup Jens U. Krause Willi A. Kalender Impact of the z-flying focal spot on resolution and artifact behavior for a 64-slice spiral CT scanner 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, Erlangen, Germany Willi.Kalender@imp.unierlangen.de Tel.: Fax: 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 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 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 phenomenon. 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

2 1207 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 mm and 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 R F by R F = z/tanφ, where φ denotes the anode angle. Note that the variation in R F due to α is negligible. As derived in [6], α is given ¼ 1 4 Δβ R FD R D ; (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 B C 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 α

3 1208 where β phys is the angle within the fan, R D the distance of the isocenter to the detector, and R FD the distance of the undeflected focal spot to the detector (R FD = R D + R F ). 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 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. Considering the single focus case, the sampling distance in the z-direction scaled to the isocenter is the physical slice thickness S ¼ ΔbR F = R FD. Consequently, the rays should be longitudinally separated by 1 2 S when intersecting the isocenter, as shown in Fig. 3b. Since the table increment per reading is much smaller than z, we ¼ 1 4 S R FD R D (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 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 mm collimation mode. Reconstruction Fig. 3 Geometry of the flying focal spot (FFS) deflection (sketches are not drawn to scale) 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

4 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 S nom = 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 mm collimation was used to evaluate the impact of the zffs on slice sensitivity. A nominal reconstruction width of S nom = 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. Holder 2 cm a SSP delta phantom Gold Disc (inside) 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 ¼jFfSSPgj: (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 S nom = 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 mm scan with zffs, and the second was a scan without zffs but with a physical collimation of 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 mm was not available for measurements, the experiments considered cases with and without the zffs at a collimation of mm and 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 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

5 1210 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 cm 2 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 mm collimation and to compare it with the 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 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 S nom =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 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 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 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 S nom =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

6 1211 FWHM [mm] pitch 0.6 mm 1.0 mm 2.0 mm 3.0 mm a FWHM Impact of the zffs on longitudinal spatial resolution 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] mm 1.0 mm 2.0 mm 3.0 mm b pitch FWTM Fig. 6 Full width at half maximum (FWHM) and full width at tenth maximum (FWTM) as a function of the pitch value for S nom =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 S nom =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 S nom. 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 S nom =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). Noise issues 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 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].

7 without zffs with zffs MTF u / (lp/mm) a MTF without and with zffs. b Bar-pattern without 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 c Bar-pattern with zffs 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 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 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 mm collimation for the same FWHM of 0.7 mm depicts that this configuration provides slightly better results than the zffs. Fig. 8 Reconstructed slices of simulated scans of the FORBILD head phantom without and with z-flying focal spot (zffs) for collimations of mm and 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 mm mode. This confirms our theoretical considerations [9] 50/100 50/100 50/100 50/200 σ= 29 HU 50/200 σ= 22 HU 50/200

8 1213 Fig. 9 Reconstructed slices of a measured 20 cm water phantom without and with z-flying focal spot (zffs) for the mm and 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 50/200 σ= 24 50/200 σ= 24 a without zffs b with 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 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 forschung/bildgebung/frm_bildmed.htm. 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 mm collimation. The full width at half maximum (FWHM) was in all cases 0.7 mm. Respective noise levels are equivalent No noise added 32 x 0.6 mm (w/o zffs) x 0.6 mm (with zffs) 64 x0.3 mm (w/o zffs) 50/100 50/100 50/100 With noise added 50/200 σ= 20 HU 50/200 σ= 20 HU 50/200 σ= 20 HU

9 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 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 mm

10 1215 collimation showed better windmill artifact suppression as compared with the zffs. Of course, the single-sampling scheme of 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 mm collimation, an increased noise level was the cost for the maximal spatial resolution as compared with the collimation of 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. References 1. Silver MD, Taguchi K, Hein IA, Han HS, Kazama M, Mori I (2003) Windmill artifact in multislice CT. Proc SPIE 5032: Taguchi K, Aradate H, Saito Y (2004) The cause of the artifact in 4-slice helical computed tomography. Med Phys 31(7): Kalender WA (2005) Computed Tomography, 2nd edn. Wiley-VCH, New York 4. Hsieh J (2003) Analytical models for multi-slice helical CT performance parameters. Med Phys 30(2): Schardt P, Deuringer J, Freudenberger J, Hell E, Knuepfer W, Mattern D, Schild M (2004) New x-ray tube performance in computed tomography by introducing the rotating envelope tube. Med Phys 32(9): Kachelrieß M, Knaup M, Penßel C, Kalender WA (2005) Flying focal spot (FFS) in cone-beam CT. In: Records of the 2004 IEEE Medical Imaging Conference pp Flohr T, Stierstorfer K, Ulzheimer S, Bruder H, Primak AN, McCollough CH (2005) Image reconstruction and image quality evaluation of a 64-slice CT scanner with focal spot. Med Phys 32(8): Kachelrieß M, Schaller S, Kalender WA (2000) Advanced single-slice rebinning in cone-beam spiral CT. Med Phys 27: Kachelrieß M, Kalender WA (2005) Presampling, algorithm factors, and noise in CT. Med Phys 32(5): Taguchi T, Aradate H (1998) Algorithm for image reconstruction in multislice helical CT. Med Phys 25(4):