four different objects with different surfaces and contours were scanned using multidetector - row helical ct . they included a mandible ( hu : 1275 ) , calvarium ( hu : 1420 ) , cup ( hu : 1997 ) , and a billiard ball ( hu : 2350 ) ( fig . 1 ) . the multidetector - row helical ct scanner ( lightspeed qx / i ; ge medical systems , milwaukee , wis . , u.s.a . ) used in this study was equipped with four row - detector arrays . along the axis of rotation ( z axis ) there were 16 rows of detector cells , with a z spacing of 1.25 mm when projected onto the axis of rotation . the following ct parameters were used : no gantry tilt , 1-cm field of view , 120 kvp , 250 ma , standard algorithm , and standard ( 180 linear ) interpolator . the 18 scanning sequences were performed with 1.25 , 2.50 , and 5.00 mm detector - row collimation , 3.0 and 6.0 pitch [ known as hq ( high quality ) and hs ( high speed ) , respectively ] , and with 0 , 25 , and 50% overlap between reconstructed slices ( table 1 ) . a total of 72 scans of four objects were obtained using 18 scanning sequences , and the images were transferred to an image processing workstation by means of lan . for each scanning sequence , the ct data were reconstructed as 3d images using shaded surface display and an aw 3.1 ( ge medical systems , milwaukee , wis . two radiologists experienced in the interpretation of reconstructed 3d images graded the 72 images on a scale from 1 ( worst ) to 5 ( best ) for each of four preset criteria , thus producing a mean score for each rating criterion and an overall mean score . the four rating criteria for each object are shown in table 2 . the mean score for each rating criterion in each scanning sequence was calculated by adding the score ( averaged for the two items , as in the definition ) of the two raters for each object and dividing it by eight ( four objects and two raters ) . an anova ( analysis of variance ) model provided by sas v6.12 ( sas institute inc . , cary , n.c . was used to compare the mean score for each rating criterion and the overall mean score according to the levels of the scanning parameters . the anova model was selected using a backward elimination method which started at the initial model containing the main effects and all interaction effects of less than three - order , and progressed to the final model , eliminating insignificant higher - order interaction . the mean score and 95% confidence interval for the image quality in each scanning sequence were estimated using the selected model . additionally , significant differences between the levels in each scanning parameter were assessed by means of multiple comparison using the tukey method . four different objects with different surfaces and contours were scanned using multidetector - row helical ct . they included a mandible ( hu : 1275 ) , calvarium ( hu : 1420 ) , cup ( hu : 1997 ) , and a billiard ball ( hu : 2350 ) ( fig . 1 ) . the multidetector - row helical ct scanner ( lightspeed qx / i ; ge medical systems , milwaukee , wis . , u.s.a . ) used in this study was equipped with four row - detector arrays . along the axis of rotation ( z axis ) there were 16 rows of detector cells , with a z spacing of 1.25 mm when projected onto the axis of rotation . the following ct parameters were used : no gantry tilt , 1-cm field of view , 120 kvp , 250 ma , standard algorithm , and standard ( 180 linear ) interpolator . the 18 scanning sequences were performed with 1.25 , 2.50 , and 5.00 mm detector - row collimation , 3.0 and 6.0 pitch [ known as hq ( high quality ) and hs ( high speed ) , respectively ] , and with 0 , 25 , and 50% overlap between reconstructed slices ( table 1 ) . a total of 72 scans of four objects were obtained using 18 scanning sequences , and the images were transferred to an image processing workstation by means of lan . for each scanning sequence , the ct data were reconstructed as 3d images using shaded surface display and an aw 3.1 ( ge medical systems , milwaukee , wis . two radiologists experienced in the interpretation of reconstructed 3d images graded the 72 images on a scale from 1 ( worst ) to 5 ( best ) for each of four preset criteria , thus producing a mean score for each rating criterion and an overall mean score . the mean score for each rating criterion in each scanning sequence was calculated by adding the score ( averaged for the two items , as in the definition ) of the two raters for each object and dividing it by eight ( four objects and two raters ) . an anova ( analysis of variance ) model provided by sas v6.12 ( sas institute inc . , cary , n.c . was used to compare the mean score for each rating criterion and the overall mean score according to the levels of the scanning parameters . the anova model was selected using a backward elimination method which started at the initial model containing the main effects and all interaction effects of less than three - order , and progressed to the final model , eliminating insignificant higher - order interaction . the mean score and 95% confidence interval for the image quality in each scanning sequence were estimated using the selected model . additionally , significant differences between the levels in each scanning parameter were assessed by means of multiple comparison using the tukey method . the results of anova ( table 3 ) showed that mean scores for each rating criterion , and the overall mean score according to the level of each scanning parameter were significantly different ( p < 0.05 ) . in terms of smoothness and overall accuracy , the interaction effect between pitch and the extent of overlap between reconstructed slices was also significant ( p < 0.05 ) . that is , the effect of pitch on smoothness varied significantly according to the extent of overlap , and vice versa . the multiple comparison method ( table 4 ) showed that for each rating criterion and with regard to overall mean score , differences in image quality occurred at all levels of detector - row collimation and pitch ( p < 0.05 ) . however , for definition and smoothness , significantly different image quality was observed only when overlap of reconstructed slices was between 0% and 50% ( p < 0.05 ) . in terms of overall accuracy and overall mean score , there were , on the other hand , significant differences in image quality between an overlap of 0% and 50% , and also between overlap of 25% and 50% ( p < 0.05 ) . using the fitted anova model , we estimated the mean score and 95% confidence intervals of each scanning sequence for each rating criterion and the overall mean score ( figs . 2 - 5 ) . among the 18 sequences , that in which detector - row collimation , pitch , and overlap of reconstructed slices were , respectively , 1.25 mm , 3.0 , and 50% , showed the highest score for each rating criterion , and highest overall mean score ( 4.94 ) . 7 ) ; and as the degree of overlap between reconstructed slices increased ( fig . 8) . the results of anova ( table 3 ) showed that mean scores for each rating criterion , and the overall mean score according to the level of each scanning parameter were significantly different ( p < 0.05 ) . in terms of smoothness and overall accuracy , the interaction effect between pitch and the extent of overlap between reconstructed slices was also significant ( p < 0.05 ) . that is , the effect of pitch on smoothness varied significantly according to the extent of overlap , and vice versa . the multiple comparison method ( table 4 ) showed that for each rating criterion and with regard to overall mean score , differences in image quality occurred at all levels of detector - row collimation and pitch ( p < 0.05 ) . however , for definition and smoothness , significantly different image quality was observed only when overlap of reconstructed slices was between 0% and 50% ( p < 0.05 ) . in terms of overall accuracy and overall mean score , there were , on the other hand , significant differences in image quality between an overlap of 0% and 50% , and also between overlap of 25% and 50% ( p < 0.05 ) . using the fitted anova model , we estimated the mean score and 95% confidence intervals of each scanning sequence for each rating criterion and the overall mean score ( figs . 2 - 5 ) . among the 18 sequences , that in which detector - row collimation , pitch , and overlap of reconstructed slices were , respectively , 1.25 mm , 3.0 , and 50% , showed the highest score for each rating criterion , and highest overall mean score ( 4.94 ) . 7 ) ; and as the degree of overlap between reconstructed slices increased ( fig . 8) . multidetector - row helical ct offers the advantage of simultaneous scanning of multiple slices at different z locations with more than two row - detector arrays ( 5 - 8 ) . in single - slice ct , x - ray beam collimation ( or the thickness of the x - ray beam ) affects both z volume coverage speed and high z - axis resolution ( or slice thickness ) . thick x - ray collimation is preferred for high volume coverage speed , while thin collimation is desirable for high z - axis resolution . here , the term ' volume coverage speed ' ( more exactly , volume coverage speed versus performance ) refers to the ability to rapidly scan a large longitudinal ( z ) volume with high longitudinal ( z - axis ) resolution and few image artifacts ( 7 ) . however , one of the great advantages of multidetector - row helical ct is the multiple row - detector arrays ( number of acquisition channels ) which permit further division of the total x - ray beam , known as x - ray beam collimation , into multiple subdivided beams , known as detector row collimation . in multidetector - row helical ct , while total x - ray collimation still determines volume coverage speed , detector - row collimation thus determines longitudinal ( z - axis ) resolution , i.e. slice thickness . detector rowcollimation , d , and x - ray beam collimation , d , have the following relationship : where n is the number of detector rows . at present , there are two commonly used patterns of detector - row array ( detector technology ) . in ge medical systems scanners , the 16 detector rows are evenly spaced at 1.25-mm intervals along the z - axis and have a maximum scan coverage of 20 mm per rotation , known as the ' matrix detector ' , while in a siemens scanner ( siemens medical systems , forchheim , germany ) , eight detector rows are unevenly but symmetrically spaced at 1 , 1.5 , 2.5 and 5 mm along the z - axis and have a maximum scan coverage of 20 mm per rotation , known as the ' adaptive array detector ' . in this study , mean scores for each rating criterion , and overall mean scores , varied significantly according to the levels of the scanning parameters employed ( detector - row collimation , pitch , and overlap between the reconstructed slices ) . the rating criteria we selected , namely definition , smoothness , and overall image accuracy , were the same as those chosen by kasales et al . furthermore , when the multiple comparison method was used to determine significant differences between the levels for each scanning parameter , detector - row collimation of 1.25 , 2.50 , and 5.00 mm gave rise to significant differences in image quality , which improved as detector - row collimation decreased ( table 4 , fig . this result is consistent with that of conventional or single - slice helical ct ( 8 , 10 ) . however , because both the number of images acquired , and scanning time increase as detector - row collimation decreases , this latter can not be reduced sufficiently to increase image quality . the pitch for single - slice helical ct is defined as the table - translating distance per gantry rotation , s , divided by x - ray beam collimation , d ( eq . 2a ) ; it differs from that for multidetector - row helical ct , which is defined as the table - translating distance per gantry rotation , s , divided by detector - row collimation , d ( eq . 2b ) ( 7 ) . the concepts of hq ( high quality ) and hs ( high speed ) pitch , introduced in multidetector - row helical ct , refer to preferred pitch and are based on the fact that 3.0 ( hq ) pitch provides high image quality and 6.0 ( hs ) pitch offers high - speed scanning while preserving tolerably high image quality . in terms of image quality , hq and hs pitches correspond respectively to the 1.0 and 2.0 pitches of single - slice helical ct ( 7 ) . hq pitch is available for applications which demand good contrast resolution and few image artifacts , while hs pitch is used for applications in which high volume - coverage speed and thin - slice scan scanning are required ( 7 ) . in single - slice helical ct , helical ct , however , the image quality contour is ' valley - shape ' : quality improves , in other words , at a pitch of 3.0 , deteriorating and forming a plateau until a pitch of 6.0 is reached . in our study , there was a significant difference in image quality between the two levels of pitch ( fig . 7 ) , image quality at 3.0 being significantly superior to that at 6.0 ( table 4 ) . by means of this study , we demonstrated that the quality of reconstructed 3d images was higher at a pitch of 3.0 than at 6.0 . since pitches of 3.0 and 6.0 were the only ones used in this study , the ' valley - shape ' image quality apparent in axial images could not , however , be demonstrated in reconstructed 3d images . hs pitch of 6.0 has , though , recently become popular in reconstructed 3d imaging and ct angiography , as well as in chest ct , because of its tolerable image quality , fast scan time , and the lower - level of radiation emitted ( 17 , 18 ) . increases in the degree of overlap between reconstructed slices require reductions in the actual reconstruction interval , and theoretically this should improve longitudinal ( z - axis ) resolution by decreasing the effects of partial volume averaging in the z - axis direction ( 12 ) . this phenomenon has been confirmed several times in conventional or single - slice helical ct ( 10 , 11 , 13 , 14 ) . in addition , ney et al . reported that in determining the quality of the reconstructed 3d images , the degree of overlap between reconstructed slices is more important than detector - row collimation ( 10 ) . the choice of optimal overlap between reconstructed slices is governed , however , by several factors . there is a trade - off between practical issues such as imaging processing time and data storage requirements secondary to the number of images to review versus maximal longitudinal ( z - axis ) resolution ( 3 ) . in our study , with regard to overlap between reconstructed slices , there was a significant difference in quality between images obtained with overlap of 0% and of 50% , and between some of images obtained with overlap was of 25% and of 50% ( in terms only of overall accuracy and overall mean score ) ( table 4 , fig . there was , though , no significant difference in image quality between 0% and 25% overlap , and in order to obtain significantly different quality , substantial overlapping , of about 50% , is thus required with regard to smoothness and overall accuracy , the interaction effect between the pitch ( overlap between reconstructed slices ) and overlap between reconstructed slices ( pitch ) was significant , and was marginally significant in determining the overall mean score ( table 3 ) . the significant interaction effect between the two means that the effect of pitch is significantly different depending upon the degree of overlap between reconstructed slices , and vice versa . for example , with regard to smoothness and overall image accuracy , at a pitch of 6.0 there is little change in image quality as overlap between reconstructed slices increases , but at a pitch of 3.0 , there is perceivable change in image quality as this occurs ( fig . the quality of reconstruction 3d images obtained using multidetector - row helical ct and various scanning techniques is best when 1.25-mm detector - row collimation , 3.0 pitch , and 50% overlap between reconstructed slices are the parameters used . image quality improved as detector - row collimation decreased ; with a pitch of 3.0 as compared with one of 6.0 , and as overlap between reconstructed slices increased .
objectiveto determine which multidetector - row helical ct scanning technique provides the best - quality reconstructed 3d images , and to assess differences in image quality according to the levels of the scanning parameters used.materials and methodsfour objects with different surfaces and contours were scanned using multidetector - row helical ct at three detector - row collimations ( 1.25 , 2.50 , 5.00 mm ) , two pitches ( 3.0 , 6.0 ) , and three different degrees of overlap between the reconstructed slices ( 0% , 25% , 50% ) . 
 reconstructed 3d images of the resulting 72 sets of data were produced using volumetric rendering . 
 the 72 images were graded on a scale from 1 ( worst ) to 5 ( best ) for each of four rating criteria , giving a mean score for each criterion and an overall mean score . 
 statistical analysis was used to assess differences in image quality according to scanning parameter levels.resultsthe mean score for each rating criterion , and the overall mean score , varied significantly according to the scanning parameter levels used . with regard to detector - row collimation and pitch , all levels of scanning parameters gave rise to significant differences , while in the degree of overlap of reconstructed slices , there were significant differences between overlap of 0% and of 50% in all levels of scanning parameters , and between overlap of 25% and of 50% in overall accuracy and overall mean score . among the 18 scanning sequences , the highest score ( 4.94 ) 
 was achieved with 1.25 mm detector - row collimation , 3.0 pitch , and 50% overlap between reconstructed slices.conclusioncomparison of the quality of reconstructed 3d images obtained using multidetector - row helical ct and various scanning techniques indicated that the 1.25 mm , 3.0 , 50% scanning sequence was best . quality improved as detector - row collimation decreased ; as pitch was reduced from 6.0 to 3.0 ; and as overlap between reconstructed slices increased .