Regular articleHigh b value diffusion-weighted imaging is more sensitive to white matter degeneration in Alzheimer's disease
Introduction
In diffusion-weighted imaging (DWI), a monoexponential relationship has been assumed between the degree of diffusion-weighting as expressed by the b value and the signal intensity. However, previous studies have shown that in biological tissues such as those of the brain signal decay due to diffusion does not necessarily follow the simple monoexponential model. When the b value is extremely low (<300 s/mm2, approximately), a component with higher diffusivity, which likely reflects capillary perfusion, is observed (Le Bihan et al., 1988). More recently, it has been shown that when the b value exceeds approximately 2000 s/mm2, the rate of the diffusion-induced signal decay of brain tissue becomes lower with increasing b value Niendorf, et al., 1996, Assaf and Cohen, 1998, Mulkern et al., 1999. This non-monoexponential signal decay at high diffusion-weighting suggests that within each imaging voxel, there are two or more components with different apparent diffusion coefficients (ADCs) in the brain parechyma. In this multicomponent diffusion model, the diffusion-weighted image is weighted toward the fast ADC component at low b values, and toward the slow ADC component at high b values. In addition, at such high b values, dissociation of gray matter and white matter emerges: the ADC of white matter becomes significantly lower than that of gray matter DeLano et al., 2000, Yoshiura et al., 2001. It appears that the slow component of the white matter has a lower ADC than that of the gray matter. The origin of the slow component in the white matter is still unknown, though some pathological process in the white matter may be closely related to changes in the slow component rather than in the fast component. In such cases, changes in the slow component may be a sensitive marker for white matter abnormalities.
Several reports have shown that there is abnormal elevation of white matter ADC in the Alzheimer's disease (AD) brain Hanyu et al., 1997, Sandson et al., 1999, Kantarci et al., 2001, which may reflect Wallerian-type axonal degeneration secondary to cortical degeneration. We hypothesized that DWI at a high b value is more sensitive to white matter degeneration in AD than is DWI with a conventional b value, and therefore, we studied the effect of high b value on the DWI of AD brains.
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Materials and methods
We studied seven patients with AD (two males and five females, mean age 60.7 ± 4.9 years, range 53–68 years) and seven age-matched healthy volunteers (five males and two females, mean age 58.9 ± 6.1 years, range 52–68 years). The experimental procedures were approved by the ethical committee of our university hospital, and written informed consent was obtained from each subject. Scores on the Mini-Mental State Examination (MMSE) of the AD patients ranged from 13 to 28 (mean 18.1 ± 4.9). AD
Results
The results of the Dmean measurements are summarized in Table 1. No significant effect of side (i.e., right or left) on the measured Dmean values was observed in the parietal or in the occipital white matter (P > 0.05, respectively). Therefore, Dmean values from the right and left ROIs were combined in the statistical analyses. A highly significant effect of b value on the Dmean values in both the parietal and the occipital white matter was found (P < 0.0001, respectively): the Dmean decreased
Discussion
Significant decrease of Dmean at higher b values (2000 and 4000 s/mm2) in comparison with the Dmean value at the conventional b value (1000 s/mm2) (Table 1) confirms previously published results DeLano et al., 2000, Yoshiura et al., 2001. In the multicomponent diffusion model, this phenomenon represents the emergence of the slow diffusion component in the white matter, which occurs at heavier diffusion weighting. Elevation of Dmean in the parietal white matter of AD patients in comparison with
Conclusion
We have shown that high b value DWI significantly improves the lesion-to-normal contrast and the CNR of AD-related white matter degeneration in comparison with that by conventional DWI. High b value DWI is completely noninvasive, requires no drug injection or radioactive agents, and can be easily incorporated in routine clinical imaging studies. The significance of high b value DWI in clinical management of neurodegenerative diseases remains to be proven in future clinical studies.
Acknowledgements
This work was supported in part by a Grant-in-Aid for Young Scientists from the Japan Society for the Promotion of Science (No. 14770461) and by the Magnetic Health Science Foundation.
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