Diffusion-Weighted Imaging of the Normal Brain

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2.1 Introduction

Diffusion-weighted (DW) images are usually ob- tained in three orthogonal orientations using spin- echo type single-shot DW echo-planar imaging with b-values around 0 and 1,000 s/mm

²

. These three planes are combined into isotropic DW images, and apparent diffusion coefﬁcient (ADC) maps are calcu- lated on a pixel-by-pixel basis (Fig. 2.1). To avoid mis- interpretations, it is important to recognize the nor- mal ﬁndings on DW images and ADC maps.

2.2 Adult Brain

2.2.1 Low Signal in Basal Ganglia

Isotropic DW imaging in adult brain often shows low signal intensity in the basal ganglia (Fig. 2.1). This low signal is caused by normal iron deposition. The hypointensity on DW images of these areas is essen- tially related to T2 contrast, which is also shown on b

₀

images. ADC maps usually show the areas as iso-in- tense, but it can be hyper- or hypointense depending on the paramagnetic susceptibility artifact of iron deposition.

2.2.2 Diffusion-Weighted Imaging of Gray and White Matter

Gray matter on DW images is generally hyperintense when compared with white matter. ADC of gray (0.76±0.13 ×10

^-3

mm

²

/s) and white matter (0.77±

0.18 ×10

^-3

mm

²

/s) are, however, identical in the adult brain [1]. There are several reports about ADC in- creasing with age [2–7], but this increase is minimal and has been observed in all parts of the brain. It is usually more apparent in the white matter and lentiform nucleus than in the rest of the brain. Focal areas of DW hyperintensities are often seen in poste- rior limbs of internal capsule, corticospinal tracts, medial lemniscus and the decussation of the superi- or cerebellar peduncles (Fig. 2.1). These DW hyperin- tensities are caused by T2 contrast and represent nor- mal ﬁndings without clinical signiﬁcance. ADC maps are usually isointense in these areas.

2.2.3 Choroid Plexus

The choroid plexus occasionally shows prominent hyperintensity on DW imaging associated with mild elevation of ADC. In these situations the ADC is often higher than in white matter, but lower than in cere- brospinal ﬂuid. The high DW signal is believed to represent gelatinous cystic changes of the choroid plexus, which can occur with age (Fig. 2.2).

Chapter 2 7

Diffusion-Weighted Imaging of the Normal Brain

In collaboration with A. Hiwatashi and J. Zhong

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Chapter 2 8 Diffusion-Weighted Imaging of the Normal Brain

Figure 2.1 a–f

Normal adult brain of a 40-year-old male without neurological deﬁcits. a Isotropic DW image is obtained by combining

b

₀

image and three orthogonal unidirectional images (x, y, z axis).The bilateral globi pallidi have low signal on DW image

as a result of physiological iron deposition (arrows). Corticospinal tracts have mildly high signal on DW image (arrow-

heads). Gray matter shows mildly high signal compared to white matter. These signal changes on isotropic DW imaging

are normal and are caused by T2 contrast. b ADC map shows homogeneous ADC values in globi pallidi, corticospinal

tracts, gray and white matter. c b

₀

image shows low signal in globi pallidi (arrows), high signal in corticospinal tracts

(arrowheads), and the gray–white matter contrast. d–f Diffusion weighting is applied in x axis (d), y axis (e), and z axis (f)

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Chapter 2 9 Diffusion-Weighted Imaging of the Normal Brain

2.3 Pediatric Brain

2.3.1 Diffusion-Weighted Imaging and ADC of the Pediatric Brain

The normal brain of neonates and infants has signif- icantly higher ADC values than the adult brain [8–13]

(Fig. 2.3). ADC in neonates and infants varies markedly within different areas of the brain and is higher in white matter (1.13 ×10

^-3

mm

²

/s) than in gray matter (1.02 ×10

^-3

mm

²

/s) [13]. ADC at birth is higher in subcortical white matter (1.88 ×10

^-3

mm

²

/s) than in both the anterior (1.30 ×10

^-3

mm

²

/s) and posterior limbs of the internal capsule (1.09×

10

^-3

mm

²

/s). It is also higher in cortex and the caudate nucleus (1.34 × 10

^-3

mm

²

/s) than in the thalamus and the lentiform nucleus (1.20 ×10

^-3

mm

²

/s) [13]. With the exception of the cerebrospinal ﬂuid (CSF), there

is a trend of decreasing ADC with increasing matura- tion in most areas of the pediatric brain. These ADC changes seem to reﬂect a combination of different factors, including a reduction of overall water con- tent, cellular maturation and white matter myelina- tion. In neonates and infants, ischemia is usually global and can therefore resemble the normal image with elevated DW signal and decreased ADC. White matter diseases can also be mimicked by the normal, age-related appearance of DW imaging and ADC. Out of necessity, the ADC values will therefore have to be age related for a correct interpretation of the DW im- ages of the pediatric brains.

Figure 2.2 a–d

Cystic changes in the choroid

plexus. a DW image shows hyper-

intensity in cystic changes of the

left choroid plexus (arrow). b ADC

values of the cystic changes are

lower than those of the CSF, which

may represent viscous gelatinous

materials, but higher than those

of brain parenchyma (arrow). cT2-

weighted image shows the cystic

changes as hyperintensity (ar-

row). d Gadolinium-enhanced T1-

weighted image with magnetiza-

tion transfer contrast reveals no

enhancement in it (arrow)

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Chapter 2 10 Diffusion-Weighted Imaging of the Normal Brain

2.4 Conclusion

Good knowledge of the DW appearance of the nor- mal adult and pediatric brain and variations is neces- sary to avoid misinterpretation. In children it is also important to match the ﬁndings with those of normal children of the same age.

References

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diffusion during the 1st year of life. Radiology 222:405–409 Figure 2.3 a, b

Normal neonatal brain. a The ap- pearance of the pediatric brain on DW images varies with age. In neonates it is normal to have low DW signal intensities in the frontal deep white matter (arrows).b ADC values of the corresponding areas are high in neonatal brain, espe- cially in the white matter (arrows).