Fractional anisotropy of the corticospinal tract in normal adults Preliminary study using a diffusion tensor imaging technique☆
Xuejin Sun, Jihong Hu, Yin Mo, Qing Gu, Yi Liu, Yi Lu, Conghui Ai

Department of MRI, First Affiliated Hospital of Kunming Medical College, Kunming  650032, Yunnan Province, China
Xuejin Sun☆, Doctor, Associate professor, Department of MRI, First Affiliated Hospital of Kunming Medical College, Kunming  650032, Yunnan Province, China

Corresponding author: Xuejin Sun, Department of MRI, First Affiliated Hospital of Kunming Medical College, Kunming  650032, Yunnan Province, China
sunxuejinkm@126.com

Received: 2009-09-26   Accepted: 2009-12-01   (20090114005/WLM)

Sun XJ, Hu JH, Mo Y, Gu Q, Liu Y, Lu Y, Ai CH. Fractional anisotropy of the corticospinal tract in normal adults: preliminary study using a diffusion tensor imaging technique. Neural

Abstract
BACKGROUND: The corticospinal tract is an important tract for conducting motor function. The majority of studies focus on lesions of the corticospinal tract on appearance and function, whereas observation of normal corticospinal pathways can also improve understanding of lesion outcomes. 
OBJECTIVE: To observe the normal adult corticospinal tract using a diffusion tensor imaging technique to analyze fractional anisotropy (FA) in different levels of the brain.
DESIGN, TIME AND SETTING: Neuroimaging observation was performed in the MRI Department, First Affiliated Hospital of Kunming Medical College in China, from October 2005 to October 2008.
PARTICIPANTS: A total of 30 healthy adults were selected from the Department of MRI, First Af-filiated Hospital of Kunming Medical College in China, from October 2005 to October 2008, and people with nervous system symptoms and signs were excluded.
METHODS: Participants with normal conventional MRI results underwent diffusion tensor imaging examination in a 1.5 T GE MRI (slice thickness 4-5 mm, slice gap 0) for gradient data acquisition from 15 directions. The scanning involved the entire brain from the inferior medulla oblongata to the inferior cranial plate. Imaging post-processing was performed to obtain FA values; a paired t-test was applied for statistical analysis. 
MAIN OUTCOME MEASURES: FA values of the bilateral corticospinal tract in the medulla ob-longata, pons, cerebral peduncle, basal ganglia, corona radiata, and centrum semiovale.
RESULTS: FA values in the medulla oblongata and centrum semiovale were similar (P > 0.01). FA values of left corticospinal tract were significantly greater than the right side in the pons, cerebral peduncle, basal ganglia and corona radiata (P < 0.01). 
CONCLUSION: FA values vary by brain levels, including pons, cerebral peduncle, basal ganglia, and corona radiata. Moreover, FA values of the left corticospinal tract pathway were greater than the right side, which may relate to right handedness.
Key Words: corticospinal tract; fractional anisotropy; diffusion tensor imaging; neuroimaging; neural regeneration

INTRODUCTION
  
The corticospinal tract (CST) arises in the motor cortex (primary motor cortex or B4 region and premotor area or B6 region), passes down through the centrum semio-vale and corona radiata to the anterior middle part of the limb of internal capsule, through the cerebral peduncle, ventrally passing the pons and entering the medulla oblongata ventral pyramid to form the py-ramidal tract. Most of the CST crosses to the opposite side to form the lateral CST, and small bundles of them travel downward to form the anterior CST[1-2]. CST lesions can affect appearance and function[3-13], but the normal CST pathway remains poorly understood.
Diffusion tensor imaging (DTI) is an ad-vanced technique that produces in vivo im-ages of white matter fibers in the brain to measure their location and structure. Frac-tional anisotropy (FA) is a scalar value be-tween zero (isotropic diffusion) and one (diffusion is fully restricted along all other directions) that describes the degree of anisotropy of a diffusion process. DTI FA values can provide microstructural informa-tion of white matter, reflect structural char-acteristics, and supply functional anatomic evidence for white matter in the brain[14-25]. FA values may vary along the CST tract. We used DTI techniques to analyze CST travel and FA values in different levels of the brain (medulla oblongata, pons, cerebral peduncle, basal ganglia, corona radiata and centrum semiovale) in 30 healthy adults.
 

SUBJECTS AND METHODS

Design
Neuroimaging observation.
Time and setting
This study was performed at the MRI Department, First Affiliated Hospital of Kunming Medical College in China, from October 2005 to October 2008.
Subjects
A total of 30 healthy adults who underwent physical examination in the same department were selected, with 16 males (average age, 58 years) and 14 females (average age, 50 years). People with a history of nervous system or cardio-cerebrovascular diseases, nervous system dysfunction confirmed by physical or neurological examination, structural and signal abnor-malities in the brain, or lesions in the brain confirmed by MRI were excluded. All subjects voluntarily participated in the study. Written informed consent was obtained, and the experiment was in accordance with the Admin-istrative Regulations on Medical Institutions formulated by the State Council of the People's Republic of China[26].
Methods
Conventional MRI examination
Conventional MRI examination was performed using a GE1.5T MR scanner to identify abnormal structures and signals in the brain. The parameters were as follows: T1WI: echo time (TE) = mini full, repetition time (TR) =  2 100 ms, inversion time (TI) = 750 ms, echo train length = 8, Bandwidth = 31.25, field of view (FOV) = 22 cm, Slice thickness/spacing = 6.0 mm/2.0 mm, matrix = 224 × 384, NEX = 2; T2WI: TE = 90 ms, TR = 3 000 ms, echo train length = 21, matrix = 224 × 384. Frequency direction (Freq dir) = S/I, NEX = 2, FOV = 24 cm.
DTI examination
Subjects with normal MRI were subjected to DTI for gra-dient data acquisition from 15 directions (Slice = 31, slice thickness = 4-5 mm, slice gap = 0). The scanning in-volved the entire brain from the inferior medulla oblon-gata to the inferior cranial plate. The parameters are as follows: TE = mini full, TR = 10 000 ms, Matrix = 128 × 128, NEX = 2, Freq dir = S/I, Phase FOV = 1.0, FOV = 24 cm, B value = 1 000 s/mm2, Shots = 1.
DTI post-processing
DTI data of the entire brain were processed using post-processing software from SUN. A total of 31 slices of FA gray maps of the entire brain were obtained. Along the CST route, the medulla oblongata, pons, cerebral pe-duncle, basal ganglia, corona radiata and centrum semiovale were selected as regions of interest (ROI) by at least two attending physicians. ROI was defined in the white matter fiber bundles at the transverse plane of the CST on FA gray maps. Moreover, ROI size was confined in the transverse plane of the CST (Figure 1). That is, the transverse plane of the CST in the medulla oblongata and pons (Figures 1A, B); central part of cerebral pe-duncle (Figure 1C); anterior 1/2 of the posterior limb of the internal capsule (Figure 1D), as well as the center of the corona radiata and centrum semiovale (fiber bundles upward along the anterior 1/2 of the posterior limb of internal capsule, Figures 1E, F). FA values were meas-ured by attending physicians.
 

 
Main outcome measures
FA values of CST in different levels of the brain.
Statistical analysis
Data were analyzed by SPSS 11.0 (SPSS, Chicago, IL, USA) and expressed as Mean ± SD. FA values of bilat-eral CST pathway at different levels were compared us-ing a t-test. P < 0.01 was considered statistically signifi-cant.

RESULTS

Quantitative analysis of participants
All 30 participants were included in the final analysis.
FA values of CST pathway
Individual differences were observed in FA values of the CST pathway. For example, in the medulla oblongata, the left CST FA values in some subjects (18/30) were slightly greater than the right side, and the right CST FA values in several subjects (10/30) were slightly greater than the left side, while 2 subjects exhibited similar FA values on both sides. In pons, cerebral peduncle, basal ganglia, and corona radiata, a majority (24/30) of sub-jects exhibited greater FA values on the left, and 6 sub-jects had greater FA values on the right. No significant differences were observed in centrum semiovale FA values between two sides; however, the majority of sub-jects displayed slightly greater left FA values than the right. Moreover, the FA values in the centrum semiovale were lower than pons, cerebral peduncle, and corona radiata.
The mean total FA values were greater on the left than the right (0.590 vs. 0.574, P < 0.01). With the exception of the anterior part of anterior 1/2 of the posterior limb of the internal capsule, medulla oblongata, centrum semiovale, FA values of the left side in different levels were significantly greater than the right (P < 0.01;  Table 1).

DISCUSSION

The present study utilized DTI technique to investigate bilateral CST travel and FA value changes in different levels of the brain, especially in the medulla oblongata, pons, cerebral peduncle, basal ganglia, corona radiata, and centrum semiovale. FA values of the left CST were greater than the right on different levels. FA value corre-lates to the anisotropy of each organization, i.e., the greater the anisotropy, the greater FA values are. Therefore, the anisotropy of the left tract is greater than the right, possibly due to structural directionality and fiber bundle density[27].
Moreover, FA values showed individual differences on the left and right side, again potentially related to hand-edness. In addition, FA values were lower in the centrum semiovale than the pons, cerebral peduncle, and corona radiata, indicating a gradual reduction from the basal ganglia to the head and the spinal cord, potentially indi-cating CST degeneration[28-30].
In conclusion, FA values of the bilateral CST pathway in healthy adults are different in the brainstem and midbrain. Moreover, the left FA values are greater than the right, possibly due to right handedness. However, as the large age span here may result in bias, further study is required to determine age and gender differences in FA values.

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 (Edited by Maio YW, Zhang GX/Su LL/Song LP)