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12 SALMINEN ET AL.

SNR). For example, the left putamen demonstrated a the lateral ventricle (Figure 3) shows deviation of the
subtle negative percent change between nobase and b~0 data point, while the b ≥ 680 s/mm data points
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high-b (-1.8%). Further study is needed to determine fall on a straight line. This effect is indicative of a CSF
whether this very small difference is due to a subtle PVE in this white matter region and monoexponential
effect of CSF-like signals (e.g., Virchow-Robin spaces). decay in the underlying white matter parenchyma
Similar phenomena can occur in white matter. A over the range of b values from 680 – 1412 s/mm .
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semilog graph of the isotropic DWI signal versus b This monoexponentiality is important because it
value in white matter anterior to the temporal horns of enables accurate tensor measurement over a subset

Figure 2. Scatterplots demonstrating the difference between mean diffusivity (MD) measurements in superior temporal gray matter of the
right hemisphere (rh.superiortemporal) using the three different schemes: standard (all four b values), nobase (no b~0 data), and highb
(no b~0 data and no b ≥ 680 s/mm data). The diagonal black dashed lines are lines of identity. Figures 2a and 2b show that the standard
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MD is overestimated with respect to the other measures. The agreement between nobase and high-b in Figure 2c indicates that nobase
effectively corrects for CSF PVEs. In Figure 2c, the slope is 0.965 +/- 0.041, and the y-intercept is 0.0293 x 10-3 +/- 0.0505 x 10-3 (+/- 1.0
standard error in each case), indicating agreement in the nobase and hi-b measures. Note that MD values are in units of mm /s. Each
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data point represents a ROI measurement for one participant in the study. Linear regression lines are shown (solid red), with regression
equations and R statistics given as text.
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