Fractional anisotropy--threshold dependence in tract-based diffusion tensor analysis: evaluation of the uncinate fasciculus in Alzheimer disease

Abstract

Background and purpose: Tract-based analysis can be used to investigate required tracts extracted from other fiber tracts. However, the fractional anisotropy (FA) threshold influences tractography analysis. The current study evaluated the influence of the FA threshold in measuring diffusion tensor parameters for tract-based analysis of the uncinate fasciculus in subjects with Alzheimer disease (AD).

Materials and methods: Subjects included 30 patients with AD and 10 healthy controls. We acquired tractographies of the uncinate fasciculus by using different FA thresholds. We measured mean FA and the apparent diffusion coefficient (ADC) along the uncinate fasciculus for different FA thresholds and evaluated the correlation between diffusion tensor parameters (FA, ADC) and the Mini-Mental State Examination (MMSE) scores.

Results: The uncinate fasciculus showed lower mean FA and higher mean ADC values in cases with more severe AD. A higher FA threshold led to a lower mean ADC value and a higher mean FA value along the uncinate fasciculus, whereas the relative order of measured values according to the severity of AD was not influenced by the FA threshold. An FA threshold of 0.2 showed higher correlation between mean ADC values and MMSE scores. FA thresholds of 0.15 and 0.20 showed higher correlation between mean FA values and MMSE scores.

Conclusions: Appropriate selection of the FA threshold leads to higher correlation between diffusion tensor parameters and the severity of AD. For tract-based analysis of degenerative diseases such as AD, appropriate selection of the FA threshold for tractography is important.

Fig 1.

Tractographies of the uncinate fasciculus were drawn by using diffusion-weighted images (EPI sequence: TR = 2300 ms, TE = 122 ms, b = 1000 s/mm², 6-axis encoding). A and B, Tractographies, coronal (A) and near sagittal (B) views, of the uncinate fasciculus are obtained with the seed area (blue) on the coronal plane to include the white matter of the substantia innominata and the external capsule just cranial to the middle cerebral artery (arrow, note the flow void) and with the target area (pink) on coronal planes in the white matter of the temporal lobe just caudal to the middle cerebral artery (arrow).

Fig 2.

Tractographies of uncinate fasciculus with 4 different FA thresholds. FA thresholds were set at 0.1, 0.15, 0.2, and 0.25. There is a tendency for lower thresholds to reveal a larger number of tract lines and higher thresholds to reveal fewer tract lines and concentrated tracts.

Fig 3.

The correlations between the MMSE score and the mean FA value (A) and mean ADC value (B) of the uncinate fasciculus with their linear regression lines are shown. Tractographies of the uncinate fasciculus are drawn under 4 different FA thresholds (0.10, 0.15, 0.20, and 0.25). A higher FA threshold results in a lower mean ADC value and a higher mean FA value along the uncinate fasciculus. For mean FA values, the Rs are 0.59 for an FA threshold of 0.25, 0.73 for a threshold of 0.20, 0.68 for a threshold of 0.15, and 0.56 for a threshold of 0.10. For mean ADC values, Rs are −0.58 for an FA threshold of 0.25, −0.70 for a threshold of 0.20, −0.71 for a threshold of 0.15, and −0.66 for a threshold of 0.10. An FA a threshold of 0.20 shows higher correlation between mean FA values and the MMSE score, whereas FA thresholds of 0.15 and 0.2 show a higher correlation between mean ADC and MMSE scores.

Fig 4.

On the tract construction of the uncinate fasciculus, a lower threshold results in a larger number of tract lines and a higher threshold results in a smaller number and concentrated tracts. Ellipsoid displays of diffusion tensor parameters on the axial plane near the temporal stem with different FA thresholds (0.25 and 0.10) are shown. The size of the ellipsoid represents the ADC value, and the shape of the ellipsoids represents the anisotropy. The color represents the direction of the eigenvector. The ellipsoids near the center of the uncinate fasciculus tend to have a smaller diffusivity and larger anisotropy.