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Normal adult brain of a 40-year-old male without neurological deficits.a Isotropic DW image is obtained by combining b0 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 (arrowheads). 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 b0 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 xaxis (d),y axis (e), and zaxis (f)
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). c T2-weighted image shows the cystic changes as hyperintensity (arrow). d Gadolinium-enhanced T1-weighted image with magnetization transfer contrast reveals no enhancement in it (arrow)
2.3 Pediatric Brain
2.3.1 Diffusion-Weighted Imaging and ADC of the Pediatric Brain
The normal brain of neonates and infants has significantly 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 x 10-3 mm2/s) than in gray matter (1.02x 10-3 mm2/s) . ADC at birth is higher in subcortical white matter (1.88 x 10-3 mm2/s) than in both the anterior (1.30 x 10-3 mm2/s) and posterior limbs of the internal capsule (1.09 x 10-3 mm2/s). It is also higher in cortex and the caudate nucleus (1.34 x 10-3 mm2/s) than in the thalamus and the lentiform nucleus (1.20x 10-3 mm2/s) . With the exception of the cerebrospinal fluid (CSF), there is a trend of decreasing ADC with increasing maturation in most areas of the pediatric brain. These ADC changes seem to reflect a combination of different factors, including a reduction of overall water content, 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 images of the pediatric brains.
Normal neonatal brain. a The appearance 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, especially in the white matter (arrows). These ADC changes seem to reflect a combination of factors, including a reduction of overall water content, cellular maturation, and white matter myelination
Good knowledge of the DW appearance of the normal adult and pediatric brain and variations is necessary to avoid misinterpretation. In children it is also important to match the findings with those of normal children of the same age.
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Pitfalls and Artifacts of DW Imaging
In collaboration with A. Hiwatashi and J.Zhong
There are many inherent artifacts and pitfalls in diffusion-weighted (DW) imaging of the brain that are important to recognize to avoid misinterpretations.
3.2 Influence of ADC
and T2 on the DW Appearance
Diffusion-weighted images are inherently T2 weighted and changes in T2 signal characteristics will thus influence the appearance of DW images independent of tissue diffusibility [1-16]. The effect of T2 prolongation, so-called "T2 shine-through", is well known. Less well known is the balance between apparent diffusion coefficient (ADC) and T2, sometimes called T2 washout. Also the effect of T2 shortening, or T2 blackout, and magnetic susceptibility effects will influence the DW appearance in many situations. This chapter will illustrate and discuss the effects of T2 and ADC on DW images.
The signal intensity (SI) on DW images is influenced by T2,ADC,the b factor,the spin density (SD) and the echo time (TE), and is calculated as follows:
where k is a constant, TR is repetition time, and SIb=0 is the signal intensity on the spin-echo echo-planar image (b0 image) [1,2,5,7,8,10,12,16].
To evaluate the tissue T2 and ADC, we should pay attention to the images discussed below as well as isotropic DW images and b0 images [3-5,7,8,10,11, 13-16].
3.2.2 Apparent Diffusion Coefficient Maps
To evaluate the diffusibility, ADC is calculated as: ADC=-n (SI/SIb=0)/b
Subsequently, increased ADC causes decreased SI on DW images, and decreased ADC causes increased SI on DW images [3-5,7,10,15,16].
3.2.3 Exponential Images
To remove the T2-weighted contrast, the DW image can be divided by the b0 image to create an "exponential image" [4,7,10,15].
The signal intensity (SIeDWI) on the exponential image is calculated as:
Therefore, this image can eliminate the effect of T2. Contrary to ADC maps, hyperintensity on exponential DW images means decreased ADC, and hy-pointensity means increased ADC.
3.3 Clinical Conditions 3.3.1 T2 Shine-through
This is a well-known phenomenon that causes hy-perintensity on DW images by means of T2 prolon gation [3-5,7,8,10,11,15,16]. If ADC is decreased at the same time, this can result in an accentuation of the hyperintensity on DW images (Figs. 3.1, 3.2 and 3.3).
Figure 3.1 a-e
T2 shine-through in a 35-year-old female with multiple sclerosis and weakness of the lower extremities. a T2-weighted image shows several hyperintense lesions,with the largest one in the right frontal lobe (arrow). b On T1-weighted image the lesion was hypointense (arrow) and did not enhance with contrast (not shown).c On DW image the lesion is hyperintense (arrow). d ADC map also shows hyperintensity in the lesion (1.2X10-3 mm2/s; arrow). e Exponential image eliminates the T2 effect and shows the lesion to be hypointense (arrow).This confirms that the hyperintensity on DW image is due to a T2 shine-through
T2 shine-through in a 45-year-old female with seizures caused by anaplastic astrocytoma. a T2-weighted image shows a hyperintense lesion in the left frontal lobe (arrow). b On T1-weighted image the lesion is hypointense with peripheral hy-perintense area (arrow).The lesion did not enhance with contrast (not shown).c DW image shows hyperintensity (arrow). d ADC map also shows hyperintensity in the lesion (0.98-1.35X10-3 mm2/s;arrow).e Exponential image eliminates the T2 effect and shows the lesion to be hypointense (arrow).This confirms that the hyperintensity on DW image is due to a T2 shine-through
T2 shine-through and restricted diffusion in a 56-year-old male with right-sided weakness due to acute infarction.MR imaging obtained 24 hours after the onset of symptoms.a FLAIR image shows a hyperintense lesion in the left middle cerebral artery territory. b On T1-weighted image the lesion is hypointense. c On T2-weighted image (b0) the lesion is hyperintense. d DW image also shows hyperintensity in the lesion. e ADC map shows hypointensity in the lesion (0.27-0.45X10-3 mm2/s).f On the exponential image,which eliminates the T2 effect,the lesion remains hyperintense.This confirms that the DW hyperintensity is due to both restricted diffusion and T2 prolongation
3.3.2 T2 Washout
This implies that isointensity on DW images is the result of a balance between hyperintensity on T2-weighted images and increased ADC [13,14,16]. This is often seen in vasogenic edema, where the combination of increased ADC and hyperintensity on T2-
weighted images will result in isointensity on DW images (Fig. 3.4).
To the best of our knowledge there have been no systematic reports on pathological conditions with isointensity on DW images, caused by a balance of hypointensity on T2-weighted images and decreased ADC.
Figure 3.4 a-d
T2 washout in a 45-year-old female with hypertension, seizures and posterior reversible encephalopathy syndrome. a FLAIR image shows hyperintense lesions in the bilateral occipital lobe (arrows). b T2-weighted image (b0) also shows hyperintensity of the lesions (arrows). c DW image shows mild hyperintensity in the lesions. d ADC map shows hyper-intensity of the lesions (1.18-1.38X10-3 mm2/s; arrows). With the strong T2 prolongation one would expect more hyperintensi-ty on the DW image, but the T2 shine-through effect is reduced by the hyperintensity on the ADC, resulting in a balance between increased diffusibility and hyperintensity on the T2-weighted image (T2 wash-out)
3.3.3 T2 Blackout cally seen in some hematomas [9,16]. Paramagnetic susceptibility artifacts may occur in this situation This indicates hypointensity on DW images caused (Figs. 3.5 and 3.6). by hypointensity on T2-weighted images and is typi-
T2 blackout in lung cancer metastasis in a 62-year-old male with adenocarcinoma of the lung. a T2-weighted image shows a hypointense mass (arrow) with surrounding edema in the left cerebellar hemisphere. b Gadolinium-enhanced T1-weighted image shows heterogeneous enhancement of the mass (arrow). c T2-weighted image (b0) also shows hypointensity in the lesion with surrounding hyperintense edema (arrow). d ADC map shows central hyperintensity (1.63-2.35X10-3 mm2/s;arrowhead) and peripheral hypointensity (1.13-1.38X10-3 mm2/s;arrow) of the mass.There is also hyperintensity of the surrounding tissue, consistent with vasogenic edema. e DW image shows heterogeneous hypointensity of the mass (arrow) and isointensity of the surrounding edema.The DW hypointensity of the mass (arrow) is due to the increased diffusibility and hypointensity on T2-weighted image.The isointensity in the surrounding edema is due to the balance between the increased diffusibility and hyperintensity on T2-weighted image (T2 washout)
Figure 3.6 a-d
T2 blackout from susceptibility artifacts in acute hemorrhage (deoxyhemoglobin and intracellular met-hemoglobin) in a 74-year-old male with left-sided weakness.MR imaging was obtained 24 hours after the onset of symptoms. a T2-weight-ed image shows hypointense lesions in the right frontoparietal lobes (arrows deoxyhemoglobin and intracellular met-hemoglobin) with areas of surrounding hyperintensity consistent with edema (arrowheads). b T1-weighted image shows the heterogeneous lesion with hypointensity (arrow deoxyhemoglobin) and hyperintensity (arrowheads intracellular met-hemoglobin).c DW image shows hypointensity (arrows deoxy-hemoglobin and intracellular met-hemoglobin) and hyperintensity in region of edema (arrowhead).The surrounding hyperintense rims (smallarrowheads) are due to magnetic susceptibility artifacts.d ADC could not be calculated accurately in the T2 "dark"hematoma due to magnetic susceptibility artifacts (arrows).The surrounding areas of hypointensity (arrowhead) probably correspond to cytotoxic edema surrounding the hematoma. This example shows how T2 hypointensity from susceptibility effects can produce a complex appearance in and around cerebral hemorrhage
Numerous artifacts can be generated during acquisition of DW images. There are five main artifacts of single-shot DW echo-planar imaging:
1. Eddy current artifacts due to echo-planar imaging phase-encode and readout gradients, and motion-probing gradient pulses for diffusion weighting
2. Susceptibility artifacts
3. N/2 ghosting artifacts
4. Chemical shift artifacts
5. Motion artifacts
We will discuss each artifact separately.
3.4.1 Eddy Current Artifacts
Eddy currents are electrical currents induced in a conductor by a changing magnetic field. Eddy currents can occur in patients and in the MR scanner itself, including cables or wires, gradient coils, cryoshields and radiofrequency shields . Eddy currents are particularly severe when gradients are turned on and off quickly, as in echo-planar imaging pulse sequences. Gradient waveforms are distorted due to eddy currents,which results in image artifacts, including spatial blurring and misregistration. In single-shot DW echo-planar imaging, eddy currents are due to both echo-planar imaging gradients and motion-probing gradients, which lead to image distortions (Fig. 3.7). Correction of image distortion is essential to calculate ADC values and especially to quantify anisotropy with diffusion tensor imaging. Correction methods: (1) correction of distortion by using post-processing [18-21], (2) pre-emphasis or pre-compensation, purposely distorting the gradient-driving currents [22,23], (3) shielded gradients, redesigning the magnet to incorporate shielding coils between the gradient coils and main windings .
3.4.2 Susceptibility Artifacts
Single-shot echo-planar imaging is sensitive to susceptibility artifacts, especially frequency and phase errors due to paramagnetic susceptibility effects. These artifacts are seen near the skull base, especially near the air in the sinus and mastoid (Fig. 3.8). Susceptibility artifacts are more severe along the phase-encoding direction and phase encoding should thus be along the anterior-posterior direction for axial DW images. Coronal and sagittal DW images are helpful in detecting lesions in certain locations, such as the hippocampus and brain stem, and to identify susceptibility artifacts (Fig. 3.9). Increased matrix size leads to elongation of readout time, which causes even larger image distortions. Correction methods: (1) multi-shot echo-planar imaging (to reduce the readout time, to enable high-resolution scan) [25,26],(2) line scan [27,28],(3) singleshot fast spin echo (SSFSE) [29,30], (4) periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) [31,32],(5) sensitive encoding (SENSE)/array spatial and sensitivity-encoding technique (ASSET), undersampling of k-space enables effective band width and shortens readout time, providing thin section and high-resolution matrix .
Misregistration due to eddy current artifact. a, b Misregistration artifact is noted in the occipital regions (arrows) on DW image (a) and the ADC map (b). Gradient waveforms are distorted due to eddy currents, which results in this misregistration
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