high resolution x - ray computed tomography ( xct ) has gained considerably in importance and acceptance for the examination and 3d - characterization of materials and industrial specimens in recent years [ 110 ] . xct systems with cone beam geometry with a -focus or a nano - focus x - ray tube in combination with a high speed and high resolution flat panel matrix detector are widely used in materials science [ 15 ] . using cone beam xct , a specimen is placed on a rotary stage between the x - ray source and the detector . the specimen is rotated step by step , taking a projection image at each angular position . a computer cluster reconstructs the projections to a volume dataset using , for example , the filtered back projection algorithm . at each position of the resulting dataset a grey - value is calculated in the scanned sample volume , which corresponds to the effective x - ray attenuation coefficient (x , y , z ) . the major problems of cone beam xct with a polychromatic source are the limited transparency of the sample , the restricted geometrical accuracy and resolution and measurement artefacts . artefacts are structures resulting from the reconstruction algorithm of the dataset , which do not correspond to a real sample feature and which lead to problems with contrast , noise as well as measurement interpretation and dimensional accuracy . these problems are more pronounced for polychromatic cone beam xct as compared with xct with a monochromatic source . continuous improvements in x - ray tubes and xct devices have led to cone beam ct systems which can now achieve resolutions down to 1  m ( -xct ) and even below ( sub-xct ) . synchrotron radiation offers significant advantages by its adjustability , partial coherence and nearly parallel beam of high brilliance . these advantages cause fewer artefacts , improved contrast and resolution , as well as faster recording for synchrotron tomography ( sxct ) . sxct has been known since mid-1980  s and resolutions available now are below 1  m , e.g. at esrf / id19 , allowing in - situ experiments due to the short measuring period . spatial resolution of about 180  nm has recently been achieved for relatively large metallic samples on esrf / id22 by using magnifying kb - mirrors . in addition , sxct produces also phase contrast due to the high lateral coherence of the beam , so that even interfaces between phases with very low absorption contrast can be displayed . phase contrast for cone beam sub-xct was also reported but to a much lower extent . the specifications of typical xct systems ( x - ray source and voxel sizes ) are compared in table 1 . there have been several investigations of cone beam xct and sxct applied to various materials [ 7,1113 ] . xct and sxct for the 3d - characterization of inhomogeneities in steel are presented in , but no quantitative comparison was carried out . the possible applications of sub-xct are described in , but no comparison with sxct is presented . a quantitative comparison was carried out in , but the resolutions ( voxel sizes ) used are in the range of several micron ( 210  m ) and thus not in the sub-m - xct region . in   cone beam xct and sxct are compared quantitatively but no sub-xct cone beam xct measurements were performed . in summary , there are no systematic investigations on metallic samples with up - to - date spatial resolutions down to 1  m ( voxel sizes below 1  m ) published . this paper deals with the application of high resolution cone beam xct in comparison with synchrotron xct applied to fe- and al - based samples containing several phases . the xct - data are analysed with respect to measurement artefacts , detection of details , sharpness , contrast and signal - to - noise ratio . one steel and two al alloys were investigated ; all the samples included various inhomogenities such as inclusions , pores or metallic phases with a higher or lower density than the matrix resulting in distinguishable features in the corresponding ct - data - sets . a summary of these samples together with the inhomogeneities present can be found in table 2 . regions of the samples were investigated by metallographic analysis and scanning electron microscopy to identify the different inhomogeneities and phases investigated . the elemental analysis was carried out by energy dispersive x - ray analysis ( edx ) . the -focus xct - measurements were performed with two -focus cone beam xct - devices , namely a v|tome|x s constructed by ge sensing & inspection technologies phoenix|x - ray equipped with a 240  kv--focus tube and a 10241024 pixel a - si flat panel detector by perkin elmer and a rayscan 250e xct - device constructed by rayscan inc . the rayscan 250e system is equipped with a 225  kev--focus tube by viscom and a 10241024 a - si at panel matrix - detector by perkin elmer . the target for both devices is made of tungsten and the minimum size of the x - ray focal spot for both sources is about 5  m . the minimum voxel sizes for both systems are around 3  m , the actual value used was 3.5  m for all measurements . the tube energies used for scanning were between 90 and 160  kv , the current between 25 and 70  a and the number of projections 10001480 . the sub-m - ct x - ray tomograms were scanned using a nanotom 180nf ct device developed and manufactured by ge sensing & inspection technologies phoenix|x - ray with a 180  kev high power nanofocus tube and a 23002300 pixel hamamatsu detector . targets made of tungsten and molybdenum was used at a voltage of 5060  kv and a current between 325 and 400  a . the voxel size used was between 0.4 and 0.6  m , and the number of projections 15001700 . these parameters required measurement periods between 120 and 240  min . the x - ray tube of the nanotom is equipped with an external liquid cooling system to ensure stable measurement conditions and to minimize the thermal influences during the long - time scans . european synchrotron radiation facility in grenoble   provided monochromatic x - rays with 29  kv in parallel beam geometry . the sxct projections were recorded by a 2d - ccd camera with 20482048 pixels and an effective pixel size of 0.3  m . for reconstruction the samples were measured in phase contrast mode using a distance sample - to - detector of 39  mm . table 3 gives an overview of the most important measurement parameters for xct , sub-xct and sxct measurements . two xct and sub-xct measurements were performed with the steel sample ( scan a and scan b ) . the cone beam xct - data were reconstructed by means of a filtered back projection feldkamp - algorithm . for some of the cone beam xct - data a beam hardening correction was performed . the sxct data were reconstructed by using a filtered back projection algorithm for parallel beam geometry . the reconstructed xct - data were visualised and processed with the software vgstudio max 2.0 by volume graphics gmbh . various parameters were evaluated for interpreting and classifying the xct - data - sets quantitatively : sharpness , contrast , signal - to - noise ratio and a feature of the grey - value histogram ( the width of the material peak ) . in order to evaluate the sharpness of the ct - data , the grey - values for air and material were transformed to values between 0 and 65,535 in all ct - measurements . line profiles were deviated and the mean value of the rising and falling exterior edges of the metallic samples was considered as a parameter for the sharpness . the -xct , sub-xct and sxct - data were evaluated in the same way and the values determined were compared relatively to each other . line profiles were taken to quantify the contrast of the different measurements in the first step , and contrast - values were determined at certain inhomogeneities based on the following formula:(1)contrast = greyvaluematerialgreyvalueinhomogenitygreyvaluematerial by a polynomial interpolation of parts of the line profiles it was possible to determine grey - values of the material ( = grey - valuematerial ) and grey values of the inhomogeneities ( = greyvalueinhomogenity ) . the same inhomogeneities were evaluated for all three kinds of xct - measurements . the standard deviation (  ) for the material grey - values was set as the denominator for the signal - to - noise ratio ( snr ) . snr was calculated by the following formula:(2)snr=|greyvaluematerialgreyvalueinhomogenity| the data were evaluated in the following way to compare the grey - value histograms of the xct , sub-xct and the sxct - measurements . the fwhm - values ( full widths at half maximum ) were calculated for the material - peak in relation to the grey - value difference of the material- and the air - peak . these evaluations could only be done for the xct - results of the steel sample , where the absorption contrast dominates the measurements . for the al - samples this comparison was not possible , since phase contrast dominates the sxct - data . in the following the ct - results , ct cross - sectional images of slices of the steel sample measured by xct , sub-xct and sxct are presented in fig . 1 : target metallography and edx revealed non - metallic inclusions in the ct - images shown in fig . the ct - data for xct are rather diffuse due to the much larger x - ray emission spot and voxel size compared with the much sharper images obtained by sub-xct and sxct . beam hardening artefacts are visible for xct and sub-xct , whereas the sxct - data show no beam hardening manifested with more or less homogeneous grey values for steel . ring artefacts are most pronounced for the sxct - data , whereas the cone beam xct results show almost no ring artefacts . 2 shows ct - cross - sectional images of a different region of the steel sample ( scan b ) measured with a different -xct - device as compared with scan a ( see table 3 ) . in this case a beam hardening correction of the ct - data was performed so that the grey values within the material are constant similar to the sxct - data . in fig . xct gives a rather diffuse picture with a much lower resolution of details , but a rather low noise , whereas sub-xct gives much sharper data with more details , but the dataset has a higher noise level . in fig . the spatial and contrast resolutions are so good , that four different grey value contrasts appear . these contrasts can be classified as pores , low - density inclusion , steel and high - density inclusion . in both cone beam xct - methods investigated it is not possible to distinguish between pores and inclusions . these results show the much better contrast resolution of sxct as compared with cone beam xct . in fig . 4 xct - results for an alsi12ni1 sample are shown , where different metallic phases are visible . ni- and fe - aluminides appear as high - absorbing inhomogeneities within the al matrix due to the high density and atomic number of ni . in the xct - dataset areas with aluminides can be identified but an accurate segmentation and quantification is not possible as a result of blurred edges . the sub-xct and the synchrotron - xct measurements show the high - absorbing aluminides in much more detail . the xct - data for all three measurement methods show no beam - hardening and no ring artefacts . the data for sxct show not only an absorption contrast but also a phase contrast . set , where the detectable features , pores , fe - aluminides and mg2si , are shown with a higher magnification . the results for the alsi12ni1 sample show once again , that xct gives rather diffuse pictures with a much lower resolution of detail but a rather low noise , whereas sub-xct and sxct give much sharper data with more details and a better contrast . also sxct shows a phase contrast for the alsi12ni1-dataset which results in a more pronounced representation of the interfaces and edges . in addition , the sxct - data present less noise compared with those of sub-xct . different features of the ct - data were analysed qualitatively and quantitatively in order to compare the cone beam xct and synchrotron xct . measurement artefacts and detection of details are now discussed from the point of view of quality . beam - hardening artefacts can not appear in the sxct - data due to the monochromatic nature of the source used . to a certain extent , beam - hardening artefacts are present in the cone beam xct - data of steel , but these artefacts can be corrected so that the grey - values for steel are as constant as in the synchrontron xct - data . both cone beam xct and synchrotron xct - data show ring artefacts , but they do not appear very prominently . only in the sxct - data of the steel sample ring artefacts are well known and caused by faults of the detector pixels . in the detection of details -xct shows worst performance . -xct can distinguish between air and steel as well as between steel and inclusions but it is not possible to differentiate between pores and inclusions . for the al - sample -xct can tell apart between air , al - matrix and the higher absorbing ni - and fe - aluminides . it is possible to identify four different phases using sxct ( air pores , low density inclusion , steel matrix and high density inclusion ) in the steel sample . for the almg5si7 sample , sub-xct and synchrotron - ct datasets reveal four different phases : air pores , mg2si , almg - matrix and fe - phases . the spatial resolution of sub-xct and sxct seems to be very similar , but the rather high noise which is always present in the sub-xct data and the better contrast in the s - xct - data achieved by phase contrast leads to higher quality images for s - xct . the following properties were evaluated quantitatively : sharpness , contrast , signal - to - noise ratio and full width at half maximum of the material peak in the grey value histogram . the results are shown in the next figures for the three investigated materials using the various xct - methods . 7 . due to the phase contrast within sxct - data of the al - samples the sharpness values between cone beam xct and sxct can be compared qualitatively but not quantitatively . it is clear that the phase contrast gives steeper edges and therefore high sharpness values . the sharpness - data for steel - fe and for the cone beam xct results can be compared also quantitatively . as expected from the results shown in figs . 15 , the sharpness increases from xct to sub-xct and to sxct for all samples . this behaviour is more pronounced for the results of the al - based samples due to the strong phase contrast within the sxct - data . 8 , where the relative contrast for the different xct - datasets is depicted . for steel and the alsi12ni1 , the best contrast is given by sxct , whereas for the almg5si7 samples the best contrast is obtained by sub-xct . this is caused by the amount of phase contrast in the sxct - data for the almg5si7-sample . the signal- to - noise ratio ( snr ) for the different measurements is shown in fig . for -xct the measurement time was around 120  min , for sub-xct around 180  min and for sxct 15  min . the highest snr - values are for the -xct and for the sxct measurement , depending on the material and on the measurement parameters . a comparison of the snr for a definite measurement time reveals the best results for sxct . in all three cases , the snr is lowest for sub-xct since in this case the intensity of the x - ray source is lowest . a further common method for the evaluation of ct - data is an analysis of the grey - value histogram : for this evaluation the features of the air and material peaks are analysed . only an evaluation of the histogram of steel was meaningful due to the strong phase contrast obtained for the al - alloys . the results are shown in table 4 . here , where the full widths at half maximum of the material peaks relative to the distance between the air peak and the material peak for the dataset of fig . the material peak for the sub-m - ct data set is broadened due to higher noise and due to beam hardening effects . the investigations presented here show , that cone beam xct at high resolution can give comparable results to synchrotron - xct . sharpness and contrast resolution for cone beam xct can be almost as good as for sxct - systems . the biggest problem with sub-xct devices is the rather low intensity of the nano - focus tubes which results in high noise and low snr - values . however , sub-xct offers many other advantages , especially reduced measurement costs , a usually higher scanning volume , better accessibility and user - friendliness . three x - ray computed tomography methods , namely cone beam xct - systems with a -focus and a sub-m ( nano - focus ) x - ray source as well as synchrotron computed tomography , were compared . a steel sample and two al - samples ( alsi12ni1 , almg5si7 ) containing different inhomogeneities and material phases ct data features like beam - hardening , detection of details , sharpness , contrast , signal - to - noise ratio and the grey value histogram were evaluated and compared . the results arebeam - hardening artefacts were present in cone beam xct - data of the steel sample , but can be corrected fairly easily.sxct and sub-xct give data with the highest detection of details , sharpness and have the best spatial and contrast resolution . in several cases sxct and sub-xct gave very similar results.xct is always inferior with regard to detection of details , sharpness , spatial and contrast resolution.sub-xct suffers from a rather bad signal - to - noise ratio due the limited power of x - ray tubes with focal spots below 1  m . beam - hardening artefacts were present in cone beam xct - data of the steel sample , but can be corrected fairly easily . sxct and sub-xct give data with the highest detection of details , sharpness and have the best spatial and contrast resolution . xct is always inferior with regard to detection of details , sharpness , spatial and contrast resolution . sub-xct suffers from a rather bad signal - to - noise ratio due the limited power of x - ray tubes with focal spots below 1  m . all in all , sxct gives the best results , but the quality of sub-xct - data comes quite close in certain cases . this means that high resolution cone beam xct is a very useful and attractive method for materials science and industrial research , since the costs are much lower and both accessibility and user - friendliness are much better than for sxct .