the phantom consisted of 5 components ; the driver , the control , the support , the balloon phantom and the ecg . the phantom 's end - diastolic phase , with the longest motion - free periods , was designed at 85% of the r - r interval despite of the changes of the hr .
different sized acrylic tubes ( 3 , 4 and 5 mm ) were used to simulate coronary arteries . inside them , artificial plaques ( + 100 hounsfield unit [ hu ] ) simulated different degrees of stenosis ( 0 , 25 , 50 and 75% ) .
these tubes were attached to the surface of a pulsating cardiac phantom ( fuyo co. , tokyo , japan ) to simulate real heart motion .
we diluted iodinated contrast medium ( angiografin , 306 mg i / ml , schering ag , berlin , germany ) with distilled water to a density of about 330 hu , and then we filled the cardiac phantom and the simulated coronary arteries with it ( fig .
the pulsating cardiac phantom equipped with the simulated coronary arteries was scanned with a 64-slice mdct ( lightspeed vct ; ge medical systems , milwaukee , wi ) .
first , we scanned the cardiac phantom with static scanning to confirm the lumen size and the degree of stenosis in the simulate coronary arteries .
the scan protocol was 120 kv , 600 ma , 0.35 second / rotation , a 0.625 mm slice - thickness , a 0.625 mm slice interval , a 0.16 fixed pitch factor and a 50 cm field of view ( fov ) for both the static and cardiac helical scanning .
the phantom and the simulated coronary arteries were scanned at 9 different hrs : 40 , 50 , 60 , 70 , 80 , 90 , 100 , 110 and 120 beats per minute ( bpm ) .
both reconstruction algorithms , the halfscan reconstruction ( single sector reconstruction , snapshot segment ) and the multisector reconstruction ( two sector reconstruction , snapshot burst ) , were applied to the 9 different hrs with the reconstructed phase at 85% of the r - r interval of the diastolic cardiac cycle of the phantom . in this study , we choose a two - sector reconstruction algorithm instead of a four - sector reconstruction algorithm because the number of sectors used for image reconstruction in the multisector reconstruction is fluctuant ( 5 ) .
the ct scanner will automatically adjust the number of sector used according to the hr .
if the four - sector reconstruction algorithm was chosen , then the number of sectors used may oscillate between one to four sectors depending on the hr .
a two - sector reconstruction algorithm was used in this study to ensure that the number of sectors used at higher hrs was constant .
the effective temporal resolution ( 175 msec ) is half of the gantry rotation time ( grt , 350 msec ) in the halfscan reconstruction , but actually the time required for one image acquisition includes the time gantry rotation for 180 plus a beam fan angle ( approximately 30 - 60 ) .
so , the temporal window is approximately around 60 - 66% of the grt for the halfscan reconstruction . in the multisector reconstruction with the m segment
, the effective temporal resolution varies between grt/2 and grt/2 m ( 6 - 9 ) .
there was a total of 324 segments of the simulated coronary arteries to be analyzed ( [ 3 segments with different degrees of stenosis + 1 non - stenotic segment ] 3 different sizes of acrylic tubes  9 different hrs 3 observers ) .
analysis was performed on an aw workstation ( aw 4.3 , ge medical systems , milwaukee , wi ) by three experienced radiologists who were kept blinded to the luminal diameters , the degree of stenosis , the hr and the reconstruction algorithm .
the radiologists measured the diameter of the patent lumen from each segmented image , instead of the stenotic rate . by doing this
, the radiologist could not know the actual diameter of each segment since the stenotic and nonstenotic segments were separately presented .
the images were reviewed with a fixed viewing window of 800/200 ( ww / wl ) and a viewing fov of 3.2 cm .
the degree of stenosis ( % ) was calculated as : ( the luminal diameter at the nonstenotic segment - the luminal diameter at the stenotic segment )  100%/the luminal diameter at the nonstenotic segment the ct scans were interpreted independently by three board - certified radiologists .
grade 1 equals a good quality image without artifacts , grade 2 equals an acceptable quality image with mild motion artifacts that do not interfere with the diagnosis and grade 3 equals a nondiagnostic , poor quality image with significant motion artifacts ( fig .
paired t tests were used to analyze the differences in image quality . p value less than 0.05 were considered to indicate a significant difference .
segments with a quality score of 3 were defined as nonassessable segments due to the nondiagnostic image quality , and the segments with a quality score of 1 or 2 were defined as assessable segments . both the numbers of nonassessable segments and the measurement error , defined as the relative error , of all the segments were recorded .
the correlations between the number of nonassessable segments , the relative error and the reconstruction algorithm , hr , luminal diameter , stenotic severity and observers were analyzed .
the phantom consisted of 5 components ; the driver , the control , the support , the balloon phantom and the ecg . the phantom 's end - diastolic phase , with the longest motion - free periods , was designed at 85% of the r - r interval despite of the changes of the hr .
different sized acrylic tubes ( 3 , 4 and 5 mm ) were used to simulate coronary arteries . inside them , artificial plaques ( + 100 hounsfield unit [ hu ] ) simulated different degrees of stenosis ( 0 , 25 , 50 and 75% ) .
these tubes were attached to the surface of a pulsating cardiac phantom ( fuyo co. , tokyo , japan ) to simulate real heart motion .
we diluted iodinated contrast medium ( angiografin , 306 mg i / ml , schering ag , berlin , germany ) with distilled water to a density of about 330 hu , and then we filled the cardiac phantom and the simulated coronary arteries with it ( fig .
the pulsating cardiac phantom equipped with the simulated coronary arteries was scanned with a 64-slice mdct ( lightspeed vct ; ge medical systems , milwaukee , wi ) .
first , we scanned the cardiac phantom with static scanning to confirm the lumen size and the degree of stenosis in the simulate coronary arteries .
the scan protocol was 120 kv , 600 ma , 0.35 second / rotation , a 0.625 mm slice - thickness , a 0.625 mm slice interval , a 0.16 fixed pitch factor and a 50 cm field of view ( fov ) for both the static and cardiac helical scanning .
the phantom and the simulated coronary arteries were scanned at 9 different hrs : 40 , 50 , 60 , 70 , 80 , 90 , 100 , 110 and 120 beats per minute ( bpm ) . both reconstruction algorithms , the halfscan reconstruction ( single sector reconstruction , snapshot segment ) and the multisector reconstruction ( two sector reconstruction , snapshot burst ) , were applied to the 9 different hrs with the reconstructed phase at 85% of the r - r interval of the diastolic cardiac cycle of the phantom . in this study , we choose a two - sector reconstruction algorithm instead of a four - sector reconstruction algorithm because the number of sectors used for image reconstruction in the multisector reconstruction is fluctuant ( 5 ) .
the ct scanner will automatically adjust the number of sector used according to the hr .
if the four - sector reconstruction algorithm was chosen , then the number of sectors used may oscillate between one to four sectors depending on the hr .
a two - sector reconstruction algorithm was used in this study to ensure that the number of sectors used at higher hrs was constant .
the effective temporal resolution ( 175 msec ) is half of the gantry rotation time ( grt , 350 msec ) in the halfscan reconstruction , but actually the time required for one image acquisition includes the time gantry rotation for 180 plus a beam fan angle ( approximately 30 - 60 ) .
so , the temporal window is approximately around 60 - 66% of the grt for the halfscan reconstruction . in the multisector reconstruction with the m segment
, the effective temporal resolution varies between grt/2 and grt/2 m ( 6 - 9 ) .
there was a total of 324 segments of the simulated coronary arteries to be analyzed ( [ 3 segments with different degrees of stenosis + 1 non - stenotic segment ] 3 different sizes of acrylic tubes  9 different hrs 3 observers ) .
analysis was performed on an aw workstation ( aw 4.3 , ge medical systems , milwaukee , wi ) by three experienced radiologists who were kept blinded to the luminal diameters , the degree of stenosis , the hr and the reconstruction algorithm .
the radiologists measured the diameter of the patent lumen from each segmented image , instead of the stenotic rate . by doing this
, the radiologist could not know the actual diameter of each segment since the stenotic and nonstenotic segments were separately presented .
the images were reviewed with a fixed viewing window of 800/200 ( ww / wl ) and a viewing fov of 3.2 cm .
the degree of stenosis ( % ) was calculated as : ( the luminal diameter at the nonstenotic segment - the luminal diameter at the stenotic segment )  100%/the luminal diameter at the nonstenotic segment the ct scans were interpreted independently by three board - certified radiologists .
grade 1 equals a good quality image without artifacts , grade 2 equals an acceptable quality image with mild motion artifacts that do not interfere with the diagnosis and grade 3 equals a nondiagnostic , poor quality image with significant motion artifacts ( fig .
segments with a quality score of 3 were defined as nonassessable segments due to the nondiagnostic image quality , and the segments with a quality score of 1 or 2 were defined as assessable segments . both the numbers of nonassessable segments and the measurement error , defined as the relative error , of all the segments were recorded .
the correlations between the number of nonassessable segments , the relative error and the reconstruction algorithm , hr , luminal diameter , stenotic severity and observers were analyzed .
at an hr of 40 and 50 bpm , the temporal window was 227 ms for both the halfscan and multisector reconstructions , so the fan angle in our study was about 53.5. with an increasing hr ( 60 - 120 bpm ) , the temporal window improved in the multisector reconstruction with the best temporal window being 127 ms , while the temporal window in the halfscan reconstruction remained unchanged at 227 ms ( fig .
in contrast , at hrs above 70 bpm ( 70 - 120 bpm ) , the multisector reconstruction achieved better image quality than the halfscan reconstruction did , except for a paradoxical reverse phenomenon at an hr of 90 bpm ( fig .
the halfscan reconstruction performed better than the multisector reconstruction despite of the tendency for a better performance of multisector reconstruction at higher hrs .
even so , at the intersection hr of 60 - 80 bpm , the two reconstruction algorithms provided equally good images . in summary ,
multisector reconstruction achieved better or equally good image quality at an hr higher than 60 bpm .
the trend for an increasing number of nonassessable segments with an increasing hr was more obvious for the halfscan than for the multisector reconstruction algorithm .
for the 324 segments , 79 ( 24.4% ) halfscan segments were nonassessable and 49 ( 15.1% ) multisector segments were nonassessable .
this advantage of multisector reconstruction was conspicuous above an hr of 80 bpm ( fig .
the number of nonassessable segments was correlated with the reconstruction algorithm ( p = 0.004 ) and hr ( p < 0.001 ) , but not with the luminal diameter , the degree of stenosis and the observer . with excluding the nonassessable segments
, we found no significant difference between the reconstruction algorithms for the accuracy of measuring the degree of stenosis ( fig .
we also used the relative error to analyze the accuracy of measuring the degree of stenosis according to the following equation : relative error ( % ) = ( the measured degree of stenosis - the true degree of stenosis)100%/true degree of stenosis so less relative error represented better accuracy . when analyzing the accuracy of stenosis detection , also referred to as the relative error , both the assessable and nonassessable segments should be included .
because the relative error of the nonassessable segments is measureless , we have to make an assumption for the relative error of the nonassessable segments .
the relative error of the nonassessable segments could be presumed to be a variable degree of error ( 100 , 150 and 200% ) because the relative error of the assessable segments varies from 0 to 196% and the relative error of the nonassessable segments should be presumed to be greater than that of the assessable ones . on the linear regression analysis , better accuracy of stenosis detection ( a smaller relative error ) was correlated with multisector reconstruction , slower hrs and a larger luminal diameter , and it was not correlated with the degree of stenosis and the observer ( table 1 ) . for example , at presumed 150% error of the nonassessable segment , better accuracy was correlated with multisector reconstruction ( p = 0.005 ) , slower hrs ( p < 0.001 ) and larger luminal diameter ( p = 0.014 ) , and it was not correlated with the degree of stenosis and the observer .
despite of the variable degree of the assumptive relative error of the nonassessable segments , the correlation remained the same . at hr of 40 - 70 bpm ; both reconstruction algorithms achieved a similar relative error .
however , multisector reconstruction achieved a better performance at an hr above 80 bpm , except at an hr of 90 bpm ( fig . 7 ) .
the volume rendering images of the two reconstruction algorithms at different hrs showed the better performance of the multisector reconstruction algorithm at higher hrs ( fig .
there was less cardiac motion related to banding artifacts for the multisector reconstruction algorithm , and especially at higher hrs .
at an hr of 40 and 50 bpm , the temporal window was 227 ms for both the halfscan and multisector reconstructions , so the fan angle in our study was about 53.5. with an increasing hr ( 60 - 120 bpm ) , the temporal window improved in the multisector reconstruction with the best temporal window being 127 ms , while the temporal window in the halfscan reconstruction remained unchanged at 227 ms ( fig .
in our study , a lower score equates with better image quality . at hrs below 60 bpm
in contrast , at hrs above 70 bpm ( 70 - 120 bpm ) , the multisector reconstruction achieved better image quality than the halfscan reconstruction did , except for a paradoxical reverse phenomenon at an hr of 90 bpm ( fig .
the halfscan reconstruction performed better than the multisector reconstruction despite of the tendency for a better performance of multisector reconstruction at higher hrs .
even so , at the intersection hr of 60 - 80 bpm , the two reconstruction algorithms provided equally good images . in summary ,
multisector reconstruction achieved better or equally good image quality at an hr higher than 60 bpm .
the number of nonassessable segments increased with an increasing hr for both algorithms . the trend for an increasing number of nonassessable segments with
an increasing hr was more obvious for the halfscan than for the multisector reconstruction algorithm .
for the 324 segments , 79 ( 24.4% ) halfscan segments were nonassessable and 49 ( 15.1% ) multisector segments were nonassessable .
this advantage of multisector reconstruction was conspicuous above an hr of 80 bpm ( fig .
the number of nonassessable segments was correlated with the reconstruction algorithm ( p = 0.004 ) and hr ( p < 0.001 ) , but not with the luminal diameter , the degree of stenosis and the observer .
with excluding the nonassessable segments , we found no significant difference between the reconstruction algorithms for the accuracy of measuring the degree of stenosis ( fig .
we also used the relative error to analyze the accuracy of measuring the degree of stenosis according to the following equation : relative error ( % ) = ( the measured degree of stenosis - the true degree of stenosis)100%/true degree of stenosis so less relative error represented better accuracy .
when analyzing the accuracy of stenosis detection , also referred to as the relative error , both the assessable and nonassessable segments should be included .
because the relative error of the nonassessable segments is measureless , we have to make an assumption for the relative error of the nonassessable segments .
the relative error of the nonassessable segments could be presumed to be a variable degree of error ( 100 , 150 and 200% ) because the relative error of the assessable segments varies from 0 to 196% and the relative error of the nonassessable segments should be presumed to be greater than that of the assessable ones . on the linear regression analysis , better accuracy of stenosis detection ( a smaller relative error ) was correlated with multisector reconstruction , slower hrs and a larger luminal diameter , and it was not correlated with the degree of stenosis and the observer ( table 1 ) . for example , at presumed 150% error of the nonassessable segment , better accuracy was correlated with multisector reconstruction ( p = 0.005 ) , slower hrs ( p < 0.001 ) and larger luminal diameter ( p = 0.014 ) , and it was not correlated with the degree of stenosis and the observer . despite of the variable degree of the assumptive relative error of the nonassessable segments , the correlation remained the same . at hr of 40 - 70 bpm ; both reconstruction algorithms achieved a similar relative error .
however , multisector reconstruction achieved a better performance at an hr above 80 bpm , except at an hr of 90 bpm ( fig . 7 ) .
the volume rendering images of the two reconstruction algorithms at different hrs showed the better performance of the multisector reconstruction algorithm at higher hrs ( fig .
there was less cardiac motion related to banding artifacts for the multisector reconstruction algorithm , and especially at higher hrs .
the temporal resolution of mdct with using the halfscan reconstruction algorithm has improved ( 6 - 9 ) and this permits reliable assessment of the main coronary branches in patients with hrs below 65 bpm ( 10 , 11 ) . however , the temporal resolution of the gated reconstruction images is of major concern when considering the heart because the motion of the coronary arteries can reach up to 69.5 mm / s during the cardiac cycle ( 10 ) .
so we confirmed that even for an hr below 65 , the halfscan algorithm provided better image quality than the multisector reconstruction algorithm did . when hrs are faster , the temporal resolution of the halfscan reconstruction algorithm will be too long for a motion - free image .
the multisector reconstruction algorithm retrospectively composes images from different cardiac cycles ( two to five ) , which theoretically could significantly reduce the effective temporal resolution ( 12 ) .
thus , marked improvement of the effective temporal resolution theoretically achieves better image quality at faster hrs .
but the maximum benefit for temporal resolution occurs when the gantry rotation and cardiac motion are fully asynchronous , which depends on the relationship between the hr and the pitch ( 13 - 15 ) , so the image quality of both reconstruction algorithms will degrade with heart rates higher than 90 bpm , as was our result .
a successful multisector reconstruction also requires no misregistration due to arrhythmia or a changing hr ( 16 ) . coronary artery visualization and analysis requires high resolution .
however , it can not be expected that the coronary artery returns to exactly the same position from one cardiac cycle to the next .
one disadvantage of multisector reconstruction is the nonconstant improvement of the temporal resolution because of synchronous gantry rotation and the cardiac cycle ; this also explains the paradoxical reverse , at an hr of 90 bpm , of both the image quality ( fig .
similar results were reported in a 32-patient study that found better image quality with an hr > 75 bpm than with an hr of 65 - 75 bpm ( 17 ) , though the temporal resolution ,
another disadvantage of multisector reconstruction is the higher radiation exposure required at a lower pitch for obtaining high spatial resolution , and this radiation exposure is estimated to be 30% higher than that of the halfscan reconstruction ( 18 ) .
there might be a certain degree of ethical concern for the unnecessary radiation dose if a lower pitch is routinely used for low hr patients for both the halfscan and multisector reconstructions .
using ecg - pulsed tube current modulation should be considered to decrease the radiation dose ( 19 ) .
two studies ( 4 , 20 ) have reported a decreased image quality with using halfscan reconstruction in mdct units when the hr exceeds 65 bpm in phantoms and patients .
one of these studies reported that an increase in image quality by multisector reconstruction when the hr exceeds 65 bpm ( variable gantry rotational speed and variable pitch values ) leads to a significant increase in imaging time with using 4-detector mdct ( 4 ) .
another study reported better sensitivity , specificity , accuracy and fewer nonassessable segments with multisegment reconstruction versus halfscan reconstruction with using a 16-slice ct scanner in 34 patients with normal hrs ( 21 ) .
the same group latter reported better diagnostic accuracy and longer vessel lengths that were free of motion artifacts with multisector reconstruction in all hr groups ( < 65 bpm , 65 - 74 bpm and > 74 bpm ) on 16-slice mdct in a study of 126 patients ( 22 ) . in a 32-patient study ( 17 ) , there was no significant improvement in image quality in any of the hr groups with employing dual - segment reconstruction versus halfscan reconstruction and using 64-detector mdct .
they proposed that the temporal resolution ( 165 ms ) using 64-detector mdct and halfscan reconstruction was similar to the temporal resolution ( approximately 160 ms ) using 16-detector mdct and multisector reconstruction ( 17 ) . in another similar 40-patient study with hrs that ranged from 61 to 87 bpm ( 23 )
, there was no significant difference of the single- and two - segment reconstruction algorithms in the number of visible segments and the quality scores .
better image quality was observed for two - segment reconstruction only at a certain hr range in that study .
however , our phantom study on a 64-detector mdct demonstrated that multisector reconstruction achieved better or equally good image quality at an hr higher than 60 bpm ( fig .
4 ) and better or equally good diagnostic accuracy for all the hr groups ( fig .
the diagnostic accuracy was significantly better for the multisector reconstruction than that for the halfscan reconstruction at hrs of 80 , 100 , 110 and 120 bpm .
this differences could have resulted from the lack of inter - heart beat variability in our study or the limited number of patients with a high hr in their studies ( 17 , 23 ) .
one advantage of our study was that the true degree of stenosis in the simulated coronary arteries was assured and this was also confirmed by static scanning .
this method should be more accurate than 2-plane coronary angiography , and more objective than expensive intracoronary ultrasound ( 24 ) .
another advantage was that the hrs in our study were well controlled by the pulsating cardiac phantom , and so we avoided the confounders of unexpected arrhythmia or changing hrs due to contrast medium injection and the vagal tone during breath - hold or that was due to anxiety .
however , this was also a limitation , since it means our results can not reflect the reality of clinical settings .
since our phantom had rigid vessel walls , it can not be a good emulation of a real heart in terms of the rate of wall motion , as well as the actual distortion of the epicardial surface .
in addition , a simulated coronary stenosis excludes artifacts from calcium plaque , making it less close to the real pathology seen in clinical settings .
finally , the diameters of our simulated coronary arteries ( 3 - 5 mm ) do not fully represent the range of diameters of the true main coronary arteries ( 1.46 - 6.09 mm ) ( 25 ) , nor do they represent the scenario of eccentric stenoses . according to our study ,
multisector reconstruction achieved equally good or better diagnostic accuracy for all the hr groups and at least equally good image quality at an hr higher than 60 bpm .
the application of multisector reconstruction for hrs between 60 and 80 bpm is acceptable , and the application of multisector reconstruction for the group with higher hrs would be beneficial .
however , the reproducibility of our results should be confirmed in the setting of inter - heart beat variability . in conclusion
, our study results showed that at higher hrs , multisector reconstruction in a 64-detector mdct achieved better temporal resolution , fewer nonassessable segments , better image quality , less cardiac motion related banding artifacts and less error in measuring the degree of stenosis .
 objectivewe wanted to evaluate the impact of two reconstruction algorithms ( halfscan and multisector ) on the image quality and the accuracy of measuring the severity of coronary stenoses by using a pulsating cardiac phantom with different heart rates ( hrs).materials and methodssimulated coronary arteries with different stenotic severities ( 25 , 50 , 75% ) and different luminal diameters ( 3 , 4 , 5 mm ) were scanned with a fixed pitch of 0.16 and a 0.35 second gantry rotation time on a 64-slice multidetector ct . both reconstruction algorithms ( halfscan and multisector ) 
 were applied to hrs of 40 - 120 beats per minute ( bpm ) at 10 bpm intervals . 
 three experienced radiologists visually assessed the image quality and they manually measured the stenotic severity.resultsfewer measurement errors occurred with multisector reconstruction ( p = 0.05 ) , a slower hr ( p < 0.001 ) and a larger luminal diameter ( p = 0.014 ) ; measurement errors were not related with the observers or the stenotic severity . 
 there was no significant difference in measurements as for the reconstruction algorithms below an hr of 70 bpm . 
 more nonassessable segments were visualized with halfscan reconstruction ( p = 0.004 ) and higher hrs ( p < 0.001 ) . 
 halfscan reconstruction had better quality scores when the hr was below 60 bpm , while multisector reconstruction had better quality scores when the hr was above 90 bpm . for the hrs between 60 and 90 bpm , 
 both reconstruction modes had similar quality scores . 
 with excluding the nonassessable segments , both reconstruction algorithms achieved a similar mean measured stenotic severity and similar standard deviations.conclusionat a higher hr ( above 90 bpm ) , multisector reconstruction had better temporal resolution , fewer nonassessable segments , better quality scores and better accuracy of measuring the stenotic severity in this phantom study . 