In vivo Parametric T 1/R 1 Imaging Correlation with Myelin Density and Microstructure Properties of Rat Corpus Callosum
Xiao Wang1,2,3, Xiao-hong Zhu1, Yi Zhang1, and Wei Chen1

1Center for Magnetic Resonance Research, University of Minnesota Medical School, Minneapolis, MN, United States, 2Diagnostic Radiology, University of Minnesota Medical School, Minneapolis, MN, United States, 3Transitional Year Residency Program, Hennepin County Medical Center, Minneapolis, MN, United States

Synopsis

Corpus callosum (CC) is a prominent white matter commissure of the brain bridging two cerebral hemispheres and communicating between the cortical and subcortical neurons. It is known that the fiber composition and microstructure of CC varies anteriorly to posteriorly (1, 2). Due to different spatial scale, co-register of macro-morphologic MR image with micro-morphologic histology transmission electron microscopy (TEM) of CC is extremely strenuous and challenging yet necessary and important. In the present study, we performed an extensive and near point to point comparison between MR T1/R1 imaging in vivo and histological TEM of the entire CC in normal rat. It shows that there is a significantly positive correlation between R1 and myelin density and negative correlation between R1 and the axon diameter in normal rat corpus callosum. The overall results indicate that T1/R1 images are tightly correlated to myelin density and provide robust assessment of myelin density and axon size in vivo, thus, should provide valuable information of the microstructure properties of the tissue. Moreover, all measures are highly inhomogeneous in CC.

Purpose

To perform an extensive and near point to point comparison investigation between MR T1/R1 imaging in vivo and histological transmission electron microscopy (TEM) of the entire corpus callosum (CC) in normal rat, and to explore the relationship between T1/R1 and both myelin density and axon size.

Materials and Methods

There were twelve male Sprague-Dawley rats weighing 338 ± 36 g included in the present study. MR T1/R1 imaging were performed using a 9.4T/31cm magnet interfaced with VNMRJ consoles (Varian) and a 1H surface coil. The rat brain anatomic images were acquired with a fast spin echo (FSE) sequence (TE=10ms; TR=4sec; FOV=3.2×3.2cm; matrix=256×256; thickness=1 mm; 8 echo train length). A modified FSE sequence combined with the global saturation-recovery preparation was used to image the parametric longitudinal relaxation time (T1) or rate (R1=1/ T1) of central sagittal slice of corpus callosum with 10 saturation recovery times ranging from 0.1 to 11.9s, resulting in a voxel size of 0.125×0.125×1 mm³ and a total acquisition time of 15 minutes. This R1 measurement was repeated 4 to 6 times for each rat. One of the rats was sacrificed and the central medial sagittal of entire CC was cut off, processed and examined with the TEM (Joel 1200 EX II, Japan, histology slice thickness = 65nm). Due to the size limitation, the entire CC was cut into 4 segments for better chemical process and TEM study purpose. Each TEM image (MegAview 3 camera, ResAlta Research Technologies Corp) is equivalent to an actual sample area of 13.1 × 9.8 um2. Three representative TEM images (magnification of x10000) were randomly taken within each TEM mesh grid (90×90 um2), resulting in about 800 TEM images in total. Myelin density, axon diameter and myelin thickness of CC were analyzed for each TEM images using NIH ImageJ. These parameters were averaged in each TEM mesh grid, co-registered and compared to R1 image with similar spatial resolution.

Results

Figure 1 shows anatomic sagittal T2-weighted image of the representative rat that underwent histology analysis, 1-D perpendicular projection of R1 values to the longitudinal main body of the CC from the rat which underwent the TEM analysis (red) and that of the mean and SD of R1 values of 12 rats (black). Figure 2 shows significant positive Pearson correlation between the CC myelin density calculated from the TEM images and R1 measured using the MRI from the same rat in each individual segment, T1/R1 images of 12 rats created using the MRI technique and myelin density image calculated from the TEM images of CC in the rat underwent histology analysis overlaying on its T2-weighted sagittal anatomic image. Figure 3 shows negative correlation between the axon diameter in CC measured from the TEM images and R1 measured using the MRI from the same rat in each segment. Figure 4 displays representative TEM images, the corresponding processed binary images, and axon size distribution histograms of TEM images in CC. Figure 5 shows linear correlation between the myelin thickness of axons and myelin density, between the axon size and numbers of the axons per TEM image throughout the entire CC.

Discussion

The proton parametric longitudinal relaxation rate (R1) or time (T1) is a basic yet crucial parameter reflecting the biophysical property of tissue and is determined by the chemical composition and its exchange with the surrounding environment. It not only plays an important role in modulating the magnetic resonance imaging (MRI) contrast by judiciously optimizing acquisition parameters such as TR and TE; but also serves as a biomarker for varieties of diseases including strokes (3, 4), tumors (5-7) and multiple sclerosis (8-10) etc. There is a significantly positive correlation between R1 and myelin density of CC in normal rat brain, demonstrating in Figure 2. It is consistent with previous comparison studies of MRI T1 mapping of postmortem human brain or spinal cord with multiple sclerosis and microscopic histology based myelin content measurement, which have demonstrated that T1 is highly correlated with myelin content (11-14). The present study maybe more challenging in a sense of narrower dynamic range of T1 ([(T1-maximum-T1- minimum) /T1-maximum] <14%) of normal CC than that in the multiple sclerosis patient (>0.66 (11), >0.34 (12) and >0.23 (13)). Nevertheless, the unequivocal statistically significant correlation between R1 and myelin density indicates that T1/R1 images are indeed very sensitive to myelin density and is a good indicator of the density of myelin. A weak and less significant negative correlation between the axon diameter and R1 which is also coincident with previous reports (15-17). The underlying mechanism is complicated but is likely still a myelin-dependent phenomenon (17). This could be partially explained by Figure 5, numbers of axons decreased per TEM image results in reduced myelin density with increased axon diameter although myelin thickness slightly contributes positively to myelin density.

Conclusion

In summary, we have performed an extremely strenuous and challenging TEM and MRI comparison study of entire CC of normal rat brain at high spatial resolution (100×100 um2 in-plane resolution). T1/R1 images are highly correlated to myelin density and provide a good indicator of the density of myelin in vivo. The parametric T1/R1 images should provide useful information of the microstructure property of the tissue.

Acknowledgements

NIH grants NS057560, NS041262, NS070839, P41 RR08079 & EB015894, P30 NS057091 & NS076408 and WM Keck Foundation. The authors thank Ms. Fang Zhou and Mr. Grant M Barthel for their technical assistance and discussion.

References

1. Aboitiz F, Scheibel AB, Fisher RS, Zaidel E. Fiber composition of the human corpus callosum. Brain research. 1992 Dec 11;598(1-2):143-53. PubMed PMID: 1486477.

2. Barazany D, Basser PJ, Assaf Y. In vivo measurement of axon diameter distribution in the corpus callosum of rat brain. Brain : a journal of neurology. 2009 May;132(Pt 5):1210-20. PubMed PMID: 19403788. Pubmed Central PMCID: 2677796.

3. DeWitt LD, Kistler JP, Miller DC, Richardson EP, Jr., Buonanno FS. NMR-neuropathologic correlation in stroke. Stroke; a journal of cerebral circulation. 1987 Mar-Apr;18(2):342-51. PubMed PMID: 3564090. 4. Wang X, Zhu XH, Zhang Y, Chen W. Simultaneous Imaging of CBF Change and BOLD with Saturation-Recovery-T1 Method. PloS one. 2015;10(4):e0122563. PubMed PMID: 25905715.

5. Englund E, Brun A, Larsson EM, Gyorffy-Wagner Z, Persson B. Tumours of the central nervous system. Proton magnetic resonance relaxation times T1 and T2 and histopathologic correlates. Acta radiologica: diagnosis. 1986 Nov-Dec;27(6):653-9. PubMed PMID: 3028046.

6. Just M, Thelen M. Tissue characterization with T1, T2, and proton density values: results in 160 patients with brain tumors. Radiology. 1988 Dec;169(3):779-85. PubMed PMID: 3187000.

7. Kurki T, Komu M. Spin-lattice relaxation and magnetization transfer in intracranial tumors in vivo: effects of Gd-DTPA on relaxation parameters. Magnetic resonance imaging. 1995;13(3):379-85. PubMed PMID: 7791547.

8. Lacomis D, Osbakken M, Gross G. Spin-lattice relaxation (T1) times of cerebral white matter in multiple sclerosis. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 1986 Apr;3(2):194-202. PubMed PMID: 3713485.

9. Ormerod IE, Miller DH, McDonald WI, du Boulay EP, Rudge P, Kendall BE, et al. The role of NMR imaging in the assessment of multiple sclerosis and isolated neurological lesions. A quantitative study. Brain : a journal of neurology. 1987 Dec;110 ( Pt 6):1579-616. PubMed PMID: 3427402.

10. Larsson HB, Frederiksen J, Kjaer L, Henriksen O, Olesen J. In vivo determination of T1 and T2 in the brain of patients with severe but stable multiple sclerosis. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 1988 May;7(1):43-55. PubMed PMID: 3386521.

11. Mottershead JP, Schmierer K, Clemence M, Thornton JS, Scaravilli F, Barker GJ, et al. High field MRI correlates of myelin content and axonal density in multiple sclerosis--a post-mortem study of the spinal cord. Journal of neurology. 2003 Nov;250(11):1293-301. PubMed PMID: 14648144.

12. Schmierer K, Scaravilli F, Altmann DR, Barker GJ, Miller DH. Magnetization transfer ratio and myelin in postmortem multiple sclerosis brain. Annals of neurology. 2004 Sep;56(3):407-15. PubMed PMID: 15349868.

13. Bot JC, Blezer EL, Kamphorst W, Lycklama ANGJ, Ader HJ, Castelijns JA, et al. The spinal cord in multiple sclerosis: relationship of high-spatial-resolution quantitative MR imaging findings to histopathologic results. Radiology. 2004 Nov;233(2):531-40. PubMed PMID: 15385682.

14. Schmierer K, Wheeler-Kingshott CA, Tozer DJ, Boulby PA, Parkes HG, Yousry TA, et al. Quantitative magnetic resonance of postmortem multiple sclerosis brain before and after fixation. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 2008 Feb;59(2):268-77. PubMed PMID: 18228601. Pubmed Central PMCID: 2241759.

15. Yarnykh VL, Yuan C. Cross-relaxation imaging reveals detailed anatomy of white matter fiber tracts in the human brain. NeuroImage. 2004 Sep;23(1):409-24. PubMed PMID: 15325389.

16. Thiessen JD, Zhang Y, Zhang H, Wang L, Buist R, Del Bigio MR, et al. Quantitative MRI and ultrastructural examination of the cuprizone mouse model of demyelination. NMR in biomedicine. 2013 Nov;26(11):1562-81. PubMed PMID: 23943390.

17. Harkins KD, Xu J, Dula AN, Li K, Valentine WM, Gochberg DF, et al. The microstructural correlates of t in white matter. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 2015 Apr 28. PubMed PMID: 25920491.

Figures

Figure 1 (a) Anatomic sagittal T2-weighted image of the representative rat that underwent histology analysis. (b) Schematic perpendicular projection (yellow solid lines) to the longitudinal axis of the CC (outlined with yellow dash line), 1-D projection of R1 values (red) from the rat which underwent the TEM analysis and the mean and SD of R1 values (black) of 12 rats. The entire CC was cut into four segments for the histology specimen process and TEM images data acquisition.

Figure 2 (a) Significant positive Pearson correlation between the CC myelin density calculated from the TEM images and R1 measured using the MRI from the same rat in each individual segment. (b) The T1/R1 image created using the MRI technique and myelin density image calculated from the TEM images of CC in the same rat overlaying on its T2-weighted sagittal anatomic image. (c) The T1/R1 images of CC of the other 11 rats overlaying on their corresponding T2-weighted anatomic sagittal images.

Figure 3 Negative correlation between the axon diameter in CC measured from the TEM images and R1 measured using the MRI from the same rat in each segment.

Figure 4 Three representative TEM images, the corresponding processed binary images, and axon size distribution histograms of TEM images from segment 1, 2, 3 and 4. The axon size appears to be smaller in segments 1 and 4, and tends to be larger in segments 2 and 3.

Figure 5 (a) Strong and positive linear correlation between the myelin thickness of axons and myelin density throughout the entire CC. (b) Negative correlation between the axon diameter and numbers of the axons per TEM image. p value and R2 are indicated in the figures.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
1465