In this study, we investigated the mean values, variation and repeatability in an intra- and inter-observer study on MRI´s of 30 healthy volunteers using two methods based on circular and freehand regions of interests. The SNR and phantom measurements showed that the image quality was good and adequate for analysis purposes.
Regional variation of the FA and ADC absolute values was large, which has also been found in previous studies [11, 33, 36]. In our study the highest values were obtained in the corpus callosum, in concordance with Lee et al. 2009 and Brander et al. 2010. The high FA reflects the microstructure of corpus callosum in which the fibers are tightly packed and parallel to each other. The highest FA value within the pyramidal tract was found in the cerebral peduncle, which we have reported also in our previous study . The results of the frontobasal regions were very close to each other in each region.
Brain asymmetries were noticed in some regions. They were found in the corona radiata for FA, and in the posterior limb of the internal capsule for ADC, which agree with previous findings . In addition, asymmetries were found in the frontobasal area such as the uncinate fasciculus and forceps minor. Recently, Bonekamp et al. 2007 and Snook et al. 2005 have reported the existence of asymmetry in the centrum semiovale. Generally, the brain asymmetries have also been observed with other imaging modalities such as computed tomography (CT) .
The variation of the FA and ADC mean values were lower for the FM than for the CM. The results agree with the study by Bonecamp et al. 2009 and our earlier study, which included regions of the pyramidal tracts . The repeatability was better with the CM than with the FM because the freehand ROIs included the borderzones of the tracts, which have lower FA than the central tracts. However, the results were highly dependent on the region.
The interregional variation was due to the location. It depended on the density of the tracts and also the artifacts, which were represented above. In addition, relative low spatial resolution effects especially in the small regions. Variation may also be caused by several factors such as noise level, gradient stability, motion and slice position between subjects . The SNR of the b = 0 s/mm2 should be at least 20 in order to derive relatively reliable FA values . In our study, the SNR was well above 20 in all regions expect basal pons (SNR = 19.2), and measured SNR are comparable to earlier study . The variability of the intra-observer and inter-observer was relatively low at all regions. It was higher in the FA values than in the ADC values, as has been found in previous studies [1, 11, 19, 25, 39, 40]. It is known that ADC values are homogeneous throughout the healthy brain, whereas FA values change depending on the location . However, increased ADC variability was found in such regions as the cerebral peduncle and the corpus callosum, which is consistent with the study by Bonekamp . In this study, the high variability in the body of the corpus callosum resulted from the effect of cerebrospinal fluid and the small ROI diameter. On average, the intra- and inter-observer variabilities were lower with the FM than with the CM.
According to 95% limits of agreement the differences between two methods was the smallest in the genu and splenium of the corpus callosum and in the posterior limb of the pyramidal tract for FA. The result can be explained by these regions being small, compact and usually without artifacts, so that the locations and size of the ROIs were almost the same. The differences were larger in the other regions because the ROI size between circle and freehand methods varied considerably. The CM represented a small sample area, whereas the freehand ROI covers the entire area of the measured tract. In the case of the basal pons and cerebral peduncle, the sources of variation were artifacts such as air-filled cavities, which affect the FM more than the CM.
The level of repeatability was moderate in most of the regions, as has been found in previous studies [11, 22]. We found an excellent FA agreement in the posterior limb of the internal capsule and corona radiata, such as in the splenium of the corpus callosum, when using the CM . The repeatability of the FA results was lower than that of the ADC results [19, 25] because the partial volume effects and border areas had more effect on the FA values. Our results were consistent with the FM findings with the exception of the region of the body of the corpus callosum that was close to the cerebrospinal fluid. In general, the results were region-dependent. In most regions, repeatability was acceptable at the group level, but only few regions at the single-subject level.
The FA results of the DTI phantom showed more variation from the reference values when the CM was used than when the FM was used. In addition, the FA and ADC values were more variable for the FM than for the CM. Generally, the variability of both methods resembled that of the similar phantoms in previous studies . The results of SNR were a bit higher for the phantom in comparison with in vivo measures. This difference is due to the fact that it we used two loop coils with the acquisition.
The repeatability of the results was decreased by the level being chosen separately each time, but this practice is a reality in the clinical environment. In addition, the examiner displayed learning effects, for example learning to avoid the artifacts and the border areas.
More investigations are needed to characterize different methods with a larger group of volunteers. These investigations should not concentrate only on ROI-based methods, but also studies comparing them to voxel-based methods would be important. These kinds of studies could also give rise to optimal combinations of different methods producing valuable new tools for the neuroradiologists.