remarkable advances in stereotactic radiosurgery and functional neurosurgery have been made along with improvements in high - tech imaging modalities . however , distortion of the image has become another issue that needs to be overcome when performing surgical interventions , especially in the aspect of precision . this problem was not seen with x - ray films of ventriculography and computerized tomography ( ct ) in the earlier era . until now , the errors in the 1.5-tesla ( t ) mri were acceptable in radiosurgery and functional neurosurgery , whereas the errors in the 3.0 t mri have not been accepted in general . besides , interpersonal differences in the practice of radiosurgery have been ignored . therefore , we measured the errors in the 1.5 t and 3.0 t mris and evaluated the feasibility of the 3.0 t mri . in addition we compared interpersonal differences in defining coordinates using the gamma plan workstation and also compared the errors according to the x , y , z coordinates and according to the mri sequences . a cylindrical mri phantom ( elekta , stockholm , sweden ) was used for this study and is shown in fig . 1 . it was made of acrylic plastic and was 180.5 mm in diameter and 131 mm in height . in this volume , there were 190 marker points which were 2 mm in diameter and 2.5 mm in height respectively . we filled the volume with cooking oil to minimize dielectric resonance and chemical shift effects4,7 ) . ct ( ge discovery ct 750 hd , wi , usa ) , 1.5 t mri ( ge signa hdxt 1.5 t , wi , usa ) , and 3.0 t mri ( philips achieva 3.0 t tx , eindhoven , the netherlands ) were used for this study . three sequences ( t1 weighted images , t2 , and flair ) were scanned by each mri . the images obtained from the ct and mris were transferred to the gamma plan workstation . and then two expert neurosurgeons defined the 3 dimensional coordinates ( x , y , z ) . in the workstation , the coordinates were defined by manual mouse click with the maximal magnification view ( fig . we assumed the ct images had no distortion error because they used x - rays . we also assumed that the error in transferring the images to the picture archiving and communication system ( pacs ) was trivial . therefore , we compared the coordinates of the mri with those of the ct to figure out the errors of the mri . we defined the error of each coordinate as such : x = x coordinate in ct - x in mri , y = y coordinate in ct - y in mri , z = z coordinate in ct - z in mri . and the 3-dimensional error ( r ) was defined as a localization error : we calculated all the errors in each sequence of the 1.5 t and 3.0 t mris . first , we compared each coordinates in the ct images defined by a different expert neurosurgeon . second , we compared the errors ( x , y , z , and r ) in the 1.5 t and 3.0 t mris according to the respective sequences . third , we compared r in the 1.5 t and 3.0 t mris according to centrality since the central area in an mri is considered as having lesser distortion than peripheral areas . we defined the central area as being within 20 mm from the center of the phantom images because almost all of the functional neurosurgery targets were in this area . fourth , we compared the errors in each axis in the 1.5 t and 3.0 t mris according to the respective sequences . last , we compared the errors in each sequence in the 1.5 t and 3.0 t mris according to the respective axes . we thought that we could figure out the accuracies of each of the sequences and each axis with the last two analyses . we used medcalc ( version 12 , medcalc software bvba , belgium ) to compare the errors of the coordinates . pearson correlation coefficient , with p - value was obtained to compare interpersonal differences . the 95% confidence interval for r a paired t - test was used to compare the errors in all of the sequences of the 1.5 t and 3.0 t mris . bonferroni 's correction was done due to repeated performances of t - test . for these t - tests , corrected p - value < 0.017 threshold level of significance was applied . to compare errors in each axis and each mri sequence , we used the repeated measurement anova test . and tukey - kramer post - hoc test was performed for statistically valuable test in anova . for anova , a cylindrical mri phantom ( elekta , stockholm , sweden ) was used for this study and is shown in fig . 1 . it was made of acrylic plastic and was 180.5 mm in diameter and 131 mm in height . in this volume , there were 190 marker points which were 2 mm in diameter and 2.5 mm in height respectively . we filled the volume with cooking oil to minimize dielectric resonance and chemical shift effects4,7 ) . ct ( ge discovery ct 750 hd , wi , usa ) , 1.5 t mri ( ge signa hdxt 1.5 t , wi , usa ) , and 3.0 t mri ( philips achieva 3.0 t tx , eindhoven , the netherlands ) were used for this study . three sequences ( t1 weighted images , t2 , and flair ) were scanned by each mri . the images obtained from the ct and mris were transferred to the gamma plan workstation . and then two expert neurosurgeons defined the 3 dimensional coordinates ( x , y , z ) . in the workstation , the coordinates were defined by manual mouse click with the maximal magnification view ( fig . we assumed the ct images had no distortion error because they used x - rays . we also assumed that the error in transferring the images to the picture archiving and communication system ( pacs ) was trivial . therefore , we compared the coordinates of the mri with those of the ct to figure out the errors of the mri . we defined the error of each coordinate as such : x = x coordinate in ct - x in mri , y = y coordinate in ct - y in mri , z = z coordinate in ct - z in mri . and the 3-dimensional error ( r ) was defined as a localization error : we calculated all the errors in each sequence of the 1.5 t and 3.0 t mris . first , we compared each coordinates in the ct images defined by a different expert neurosurgeon . second , we compared the errors ( x , y , z , and r ) in the 1.5 t and 3.0 t mris according to the respective sequences . third , we compared r in the 1.5 t and 3.0 t mris according to centrality since the central area in an mri is considered as having lesser distortion than peripheral areas . we defined the central area as being within 20 mm from the center of the phantom images because almost all of the functional neurosurgery targets were in this area . fourth , we compared the errors in each axis in the 1.5 t and 3.0 t mris according to the respective sequences . last , we compared the errors in each sequence in the 1.5 t and 3.0 t mris according to the respective axes . we thought that we could figure out the accuracies of each of the sequences and each axis with the last two analyses . we used medcalc ( version 12 , medcalc software bvba , belgium ) to compare the errors of the coordinates . pearson correlation coefficient , with p - value was obtained to compare interpersonal differences . the 95% confidence interval for r a paired t - test was used to compare the errors in all of the sequences of the 1.5 t and 3.0 t mris . bonferroni 's correction was done due to repeated performances of t - test . for these t - tests , corrected p - value < 0.017 threshold level of significance was applied . to compare errors in each axis and each mri sequence , we used the repeated measurement anova test . and tukey - kramer post - hoc test was performed for statistically valuable test in anova . for anova , pearson correlation coefficients and p - value were r=1.0 and < 0.001 , respectively in all axes . the mean error in the ct images between the two neurosurgeons ( r ) was 0.480.22 mm . the maximal error was 1.2 mm , which was in the y axis , and smallest error out of all the axes was 0 . the errors in the 1.5 t mri were 1.550.48 mm ( t1 sequence ) , 0.750.38 mm ( t2 ) , and 1.070.57 mm ( flair ) , and those in the 3.0 t mri were 2.350.53 mm ( t1 ) , 2.180.76 mm ( t2 ) , and 2.160.77 mm ( flair ) . almost all of the differences between the 1.5 t and 3.0 t mris were statistically significant but z in t1 , x in t2 , and z in flair were not . the errors in the central area ( within 20 mm from the center in the 3-dimensional space ) showed statistically significant differences in all the sequences . these errors and p - values the range of mean errors of all coordinates in each mri sequences were 0.3 - 1.20 mm ( 1.5 t ) and 0.43 - 1.78 mm ( 3.0 t ) in t1 sequence , 0.29 - 0.58 mm ( 1.5 t ) and 0.31 - 1.85 mm ( 3.0 t ) in t2 , and 0.28 - 0.79 mm ( 1.5 t ) and 0.31 - 1.66 mm ( 3.0 t ) in flair . the error in the t2 was the smallest out of all of the three sequences . the errors and p - values are shown in fig . results of the differences between 1.5 t and 3.0 t mri in all sequences were shown in table 3 . the range of mean errors of all coordinates in each axis were 0.28 - 0.34 mm ( 1.5 t mri ) and 0.31 - 0.52 mm ( 3.0 t ) in the x axis , 0.36 - 0.98 mm ( 1.5 t ) and 0.66 - 1.78 mm ( 3.0 t ) in the y axis , and 0.40 - 1.20 mm ( 1.5 t ) , and 1.30 - 1.85 mm ( 3.0 t ) in the z axis . 6 . for y and z axes , tukey - kramer post - hoc tests were performed . and the results of the differences between 1.5 t and 3.0 t mri in y and z axes were shown in table 3 . pearson correlation coefficients and p - value were r=1.0 and < 0.001 , respectively in all axes . the mean error in the ct images between the two neurosurgeons ( r ) was 0.480.22 mm . the maximal error was 1.2 mm , which was in the y axis , and smallest error out of all the axes was 0 . the errors in the 1.5 t mri were 1.550.48 mm ( t1 sequence ) , 0.750.38 mm ( t2 ) , and 1.070.57 mm ( flair ) , and those in the 3.0 t mri were 2.350.53 mm ( t1 ) , 2.180.76 mm ( t2 ) , and 2.160.77 mm ( flair ) . almost all of the differences between the 1.5 t and 3.0 t mris were statistically significant but z in t1 , x in t2 , and z in flair were not . the errors in the central area ( within 20 mm from the center in the 3-dimensional space ) showed statistically significant differences in all the sequences . these errors and p - values the range of mean errors of all coordinates in each mri sequences were 0.3 - 1.20 mm ( 1.5 t ) and 0.43 - 1.78 mm ( 3.0 t ) in t1 sequence , 0.29 - 0.58 mm ( 1.5 t ) and 0.31 - 1.85 mm ( 3.0 t ) in t2 , and 0.28 - 0.79 mm ( 1.5 t ) and 0.31 - 1.66 mm ( 3.0 t ) in flair . the error in the t2 was the smallest out of all of the three sequences . the errors and p - values are shown in fig . results of the differences between 1.5 t and 3.0 t mri in all sequences were shown in table 3 . the range of mean errors of all coordinates in each axis were 0.28 - 0.34 mm ( 1.5 t mri ) and 0.31 - 0.52 mm ( 3.0 t ) in the x axis , 0.36 - 0.98 mm ( 1.5 t ) and 0.66 - 1.78 mm ( 3.0 t ) in the y axis , and 0.40 - 1.20 mm ( 1.5 t ) , and 1.30 - 1.85 mm ( 3.0 t ) in the z axis . 6 . for y and z axes , tukey - kramer post - hoc tests were performed . and the results of the differences between 1.5 t and 3.0 t mri in y and z axes were shown in table 3 . mri has been the most accurate neuroimaging modality for showing details of the brain anatomy until now , and the technology including the various sequences have rapidly developed . . however , distortion and accuracy of the mri can be a concern in functional neurosurgery and radiosurgery . the generally accepted accuracy of the 1.5 t mri is about 1 mm3,18 ) . in our study , we knew that the difference between 1.5 t and 3.0 t mri was various according to sequences and stereotactic axes . as shown in table 3 , the difference between 1.5 t and 3.0 t mri was largest in z axis of t2 sequence . among the all axes , the error was smallest in x axis and largest in z axis in both 1.5 t and 3.0 t mri . however , among the all mri sequences , the error in t2 sequence was not reliable in 3.0 t mri even though the error was smallest in 1.5 t . we thought that the distortion of 3.0 t mri rapidly increased in z axis and t2 sequence . the primary factor that introduces distortion in mris is the degree of homogeneity in the magnetic field and nonlinear magnetic field gradients13,17 ) . additional factors such as chemical shifts , strength of the magnetic field and the tissue susceptibility effect should be considered when it comes to discussing the accuracy of mris9,11,12 ) . on a practical level , however , patient factors and chemical shift artifacts , and tissue susceptibility effects did not need to be considered in our study . the lack of water or tissue of the brain in our study was able to minimize susceptibility artifacts that depend on the sequences . we focused on the acceptability of distortion of the 3.0 t mri in functional neurosurgery and stereotactic surgery . in the early era of the mri , some authors reported on the discrepancies between ct and mri . lunsford et al.10 ) reported that the difference in the x or y coordinate was 0 - 2 mm in a small number of patients . bradford et al.2 ) reported that the mean differences in the x and y were 1.0 mm and 3.75 mm , respectively . only a mean difference of 0.03 mm in the x coordinate and 1.7 mm in the y coordinate were reported by andoh et al.1 ) . villemure et al.16 ) compared mri and ventriculography in the x and y coordinates in patients who underwent functional neurosurgery and reported a difference of 0 - 3 mm16 ) . the homogeneity in the mri magnetic field may be interfered with by imperfect construction of the magnet by the manufacturer , a temporally fluctuating power supply , thermal instability , and internal or external ferromagnetic objects14 ) . this inhomogeneity in the magnetic field could evoke distortions of the mri and cause the discrepancy between mris and x - ray based imaging modalities . however , these mechanical problems , nowadays , have been solved to the point that there are no objections to the 1.5 t mri being accepted for use in frame - based stereotactic surgery . even the 7.0 t mri recently showed relatively small hardware - related distortions , which may be acceptable for image coregistration or direct tissue - targeting procedures , although this very strong magnetic modality has still yet to used for scientific purposes4 ) . the distortions of the mri are dependent on imaging parameters including the specific pulse sequence , acquisition orientation , field of view ( fov ) , bandwidth ( bw ) and available distortion correction algorithms5,14,15 ) . our study had the limitation that only the corrected parameters in house were used for the mri scan . the parameters were routine since quality assurance had been performed by radiologists and physicians and the maximal cost - effective ones were selected . we thought that using our routine parameters was meaningful because we have used these in real practice . continuous quality assurance process in our institute should be performed to obtain more accurate and consistent images . in these settings , the t2 sequence of 1.5 t mri was thought to be the most reliable imaging because the errors in each mri sequence of the t2 images were the smallest errors regardless of the coordinates . the centrality of the coordinates did not influence the degree of errors8 ) . in our study , the differences between errors in the central area ( < 20 mm away from the center ) and those in the peripheral areas were not statistically significant . it has been thought that inhomogeneity of the magnetic field according to the isocenter of the magnet in mris had been resolved . in fact , manufacturers of mri provide distortion correction procedures as part of their standard acquisition . however , we thought that the overall errors in the 3.0 t mri had not been corrected enough . . geometric accuracy of the stereotactic references is absolutely paramount in any imaging technique19 ) . in the literatures , the reference deviations in both the t1 and t2 sequences were similar in the 1.5 t and 3.0 t mris6,19 ) . in our study , the deviation of the stereotactic references was not considered because it was generally within the acceptable error range . besides , additional errors in each procedure step could be expected . in fact , the sum of errors in all the steps of the procedure during performance of radiosurgery or functional neurosurgery may be the real error . mri has been the most accurate neuroimaging modality for showing details of the brain anatomy until now , and the technology including the various sequences have rapidly developed . . however , distortion and accuracy of the mri can be a concern in functional neurosurgery and radiosurgery . the generally accepted accuracy of the 1.5 t mri is about 1 mm3,18 ) . in our study , we knew that the difference between 1.5 t and 3.0 t mri was various according to sequences and stereotactic axes . as shown in table 3 , the difference between 1.5 t and 3.0 t mri was largest in z axis of t2 sequence . among the all axes , the error was smallest in x axis and largest in z axis in both 1.5 t and 3.0 t mri . however , among the all mri sequences , the error in t2 sequence was not reliable in 3.0 t mri even though the error was smallest in 1.5 t . we thought that the distortion of 3.0 t mri rapidly increased in z axis and t2 sequence . the primary factor that introduces distortion in mris is the degree of homogeneity in the magnetic field and nonlinear magnetic field gradients13,17 ) . additional factors such as chemical shifts , strength of the magnetic field and the tissue susceptibility effect should be considered when it comes to discussing the accuracy of mris9,11,12 ) . on a practical level , however , patient factors and chemical shift artifacts , and tissue susceptibility effects did not need to be considered in our study . the lack of water or tissue of the brain in our study was able to minimize susceptibility artifacts that depend on the sequences . we focused on the acceptability of distortion of the 3.0 t mri in functional neurosurgery and stereotactic surgery . in the early era of the mri , some authors reported on the discrepancies between ct and mri . lunsford et al.10 ) reported that the difference in the x or y coordinate was 0 - 2 mm in a small number of patients . bradford et al.2 ) reported that the mean differences in the x and y were 1.0 mm and 3.75 mm , respectively . only a mean difference of 0.03 mm in the x coordinate and 1.7 mm in the y coordinate were reported by andoh et al.1 ) . villemure et al.16 ) compared mri and ventriculography in the x and y coordinates in patients who underwent functional neurosurgery and reported a difference of 0 - 3 mm16 ) . the homogeneity in the mri magnetic field may be interfered with by imperfect construction of the magnet by the manufacturer , a temporally fluctuating power supply , thermal instability , and internal or external ferromagnetic objects14 ) . this inhomogeneity in the magnetic field could evoke distortions of the mri and cause the discrepancy between mris and x - ray based imaging modalities . however , these mechanical problems , nowadays , have been solved to the point that there are no objections to the 1.5 t mri being accepted for use in frame - based stereotactic surgery . even the 7.0 t mri recently showed relatively small hardware - related distortions , which may be acceptable for image coregistration or direct tissue - targeting procedures , although this very strong magnetic modality has still yet to used for scientific purposes4 ) . the distortions of the mri are dependent on imaging parameters including the specific pulse sequence , acquisition orientation , field of view ( fov ) , bandwidth ( bw ) and available distortion correction algorithms5,14,15 ) . our study had the limitation that only the corrected parameters in house were used for the mri scan . the parameters were routine since quality assurance had been performed by radiologists and physicians and the maximal cost - effective ones were selected . we thought that using our routine parameters was meaningful because we have used these in real practice . continuous quality assurance process in our institute should be performed to obtain more accurate and consistent images . in these settings , the t2 sequence of 1.5 t mri was thought to be the most reliable imaging because the errors in each mri sequence of the t2 images were the smallest errors regardless of the coordinates . the centrality of the coordinates did not influence the degree of errors8 ) . in our study , the differences between errors in the central area ( < 20 mm away from the center ) and those in the peripheral areas were not statistically significant . it has been thought that inhomogeneity of the magnetic field according to the isocenter of the magnet in mris had been resolved . in fact , manufacturers of mri provide distortion correction procedures as part of their standard acquisition . however , we thought that the overall errors in the 3.0 t mri had not been corrected enough . hence , any deviations in the references would have direct implications on target accuracy . geometric accuracy of the stereotactic references is absolutely paramount in any imaging technique19 ) . in the literatures , the reference deviations in both the t1 and t2 sequences were similar in the 1.5 t and 3.0 t mris6,19 ) . in our study , the deviation of the stereotactic references was not considered because it was generally within the acceptable error range . besides , additional errors in each procedure step could be expected . in fact , the sum of errors in all the steps of the procedure during performance of radiosurgery or functional neurosurgery may be the real error . there is no interpersonal difference in running a workstation to determine the stereotactic coordinates using a phantom . the distortion in the 3.0 t mri is bigger than in the 1.5 t , and that is not acceptable regardless of the centrality in the coordinates . among the stereotactic coordinates , the x coordinate has the least error regardless of the mri sequences . and the t2 mri sequence in 1.5 t mri has the smallest error regardless of the coordinates . considering the biological effect of vessels , variable consistency of tissue , cerebrospinal fluid , etc on mri distortion , more practical data obtained from future studies on real patients could support and clarify the experimental results of this study .
objectivethe aims of this study are to identify interpersonal differences in defining coordinates and to figure out the degree of distortion of the mri and compare the accuracy between ct , 1.5-tesla ( t ) and 3.0 t mri.methodswe compared coordinates in the ct images defined by 2 neurosurgeons . 
 we also calculated the errors of 1.5 t mri and those of 3.0 t . 
 we compared the errors of the 1.5 t with those of the 3.0 t . 
 in addition , we compared the errors in each sequence and in each axis.resultsthe mean difference in the ct images between the two neurosurgeons was 0.480.22 mm . 
 the mean errors of the 1.5 t were 1.550.48 mm ( t1 ) , 0.750.38 ( t2 ) , and 1.070.57 ( flair ) and those of the 3.0 t were 2.350.53 ( t1 ) , 2.180.76 ( t2 ) , and 2.160.77 ( flair ) . 
 the smallest mean errors out of all the axes were in the x axis : 0.28 - 0.34 ( 1.5 t ) and 0.31 - 0.52 ( 3.0 t ) . the smallest errors out of all the mri sequences were in the t2 : 0.29 - 0.58 ( 1.5 t ) and 0.31 - 1.85 ( 3.0t).conclusionthere was no interpersonal difference in running the gamma plan to define coordinates . 
 the errors of the 3.0 t were greater than those of the 1.5 t , and these errors were not of an acceptable level . 
 the x coordinate error was the smallest and the z coordinate error was the greatest regardless of the mri sequence . 
 the t2 sequence was the most accurate sequence .