MAGNETIC RESONANCE IMAGING / IMAGERIE PAR RÉSONANCE MAGNÉTIQUE
Nonischemic causes of hyperintense signals on
diffusion-weighted magnetic resonance images:
a pictorial essay
Jeffrey M. Hinman, MD; James M. Provenzale, MD
D
iffusion-weighted magnetic resonance imaging (MRI) allows for tissue
characterization on the basis of microscopic intravoxel incoherent
(diffusional) motion of water. Diffusion-weighted MRI is performed by
applying a pair of pulsed magnetic field gradients in association with a
90°–180° spin-echo pulse; this results in varying degrees of signal loss
according to the rate of diffusion (i.e., random motion) of water molecules
within tissue. The rate of signal loss depends on a number of factors,
including some that are operator dependent (e.g., strength and duration of
gradient pulses) and some that are tissue specific (e.g., microscopic structure of the tissue).
The hyperintense appearance of acute cerebral infarction on diffusionweighted magnetic resonance images has been well outlined in numerous
reports.1 In this pictorial essay, increased signal intensity on diffusionweighted images due to causes other than acute cerebral ischemia are reviewed.
Hinman, Provenzale — Division
of Neuroradiology, Department
of Radiology, Duke University Medical
Center, Durham, NC
Address for correspondence: Dr. James
M. Provenzale, Box 3808, Department
of Radiology, Duke University Medical
Center, Durham, NC 27700; fax 919
684-7138; prove001@mc.duke.edu
Submitted Apr. 22, 2000
Revision requested Sept. 26, 2000
Resubmitted Oct. 8, 2000
Accepted Oct. 17, 2000
Can Assoc Radiol J 2000;51(6):351-6.
HYPERINTENSE SIGNAL DUE TO NORMAL ANISOTROPIC DIFFUSION
© 2000 Canadian Association of Radiologists
In the normal brain, water does not diffuse uniformly in all directions, but
is restricted by the presence of highly ordered white-matter pathways.
Diffusion is relatively unimpeded along such pathways, but is relatively
restricted perpendicular to the path of a white-matter tract; this phenomenon is termed anisotropy. In clinical practice, anisotropy becomes an important consideration when diffusion-weighted MRI is performed using a
diffusion gradient in a single direction and the rate of tissue water diffusion (measured as the apparent diffusion coefficient [ADC]) is sampled in
only 1 direction.1 Even normal diffusional motion along white-matter pathways that are perpendicular to the applied diffusion gradient will appear
to be restricted (i.e., have a decreased ADC) and can simulate restricted diffusion due to other causes (e.g., infarction) (see Fig. 1). This problem is alleviated by sampling the ADC in many directions, usually 3,1 and a summed
image that gives the average diffusional motion for all sampled directions
(termed a trace-weighted image) can be produced that is relatively free of
anisotropy effects (Fig. 1). It is also possible to compute the ADC in each
voxel and display these values as a map; ADC maps display signal intensity solely on the basis of relative differences in tissue diffusion and allow
quantitative measurements of the ADC. Such maps are useful for measuring the degree of diffusion abnormality in lesions, which can be helpful in
characterizing lesions and obtaining a rough estimate of infarction age.
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Table of Contents
INCREASED SIGNAL INTENSITY DUE TO T2-PROLONGATION EFFECTS
Unlike the signal intensities displayed on ADC maps, the appearance of
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tissues on diffusion-weighted images is dependent not
only on signal properties associated with the random
motion of water molecules but also on T2-relaxation
properties. Approximately 7–10 days after infarction
occurs, ADC values return to normal levels again, even
though the infarct is hyperintense on T2-weighted
images. At this point and for some weeks thereafter,
infarcts can appear bright on diffusion-weighted
images, despite normal ADC values, because of T2-prolongation effects.1 This phenomenon has been termed
A
B
C
D
E
F
FIG. 1: Diffusion-weighted magnetic resonance images in a 63-year-old woman with a 1-day history of right hemiparesis showing 2
causes of hyperintense signal (acute cerebral infarction and anisotropy effects).
A: T2-weighted image shows 2 foci of hyperintense signal (arrowheads) within the posterior limb of the left internal capsule, consistent
with infarction.
B: Axial trace-weighted diffusion-weighted image, which represents a composite of diffusion gradients applied in 3 directions (and is
therefore relatively free of anisotropy effects), shows a region of hyperintense signal consistent with acute infarction.
C: Axial diffusion-weighted image obtained with diffusion gradient applied solely in the superior–inferior direction shows regions of
hyperintense signal (arrows) that are not seen on trace-weighted image in B and represent anisotropy effects due to the diffusion gradient being applied perpendicular to the course of white matter tracts.
D: Axial diffusion-weighted image obtained with diffusion gradient applied solely in the right–left direction shows regions of hyperintense signal (arrows) that are not seen on the trace-weighted image in B or on the single-direction diffusion gradient image in C. The
regions of hyperintense signal represent anisotropy effects.
E: Axial diffusion-weighted image obtained with diffusion gradient applied solely in the anterior–posterior direction shows regions of
hyperintense signal (arrows) that are not seen in B, C or D and are due to anisotropy effects.
F: Axial apparent diffusion coefficient (ADC) map reveals areas of decreased signal (arrows) corresponding to the regions of hyperintense signal seen in B, confirming acute infarction.
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the T2 shine-through effect (Fig. 2). If this effect is not
considered when evaluating an infarct that is bright on
diffusion-weighted images, a subacute or chronic infarct can be mistakenly determined to be acute. ADC
maps have no contribution from T2 effects and are useful to differentiate areas of truly restricted diffusion
from T2 shine-through (Fig. 2).
HYPERINTENSE APPEARANCE DUE TO NONISCHEMIC LESIONS
CNS infections
Cerebral abscesses in diffusion-weighted imaging studies have been reported to have a hyperintense appearance relative to normal tissue, reflecting restricted dif-
fusion and confirmed by lower ADC values on ADC
maps.2 Restricted water diffusion, which has been
postulated to be caused by the viscous nature of pus,2
has been seen within large abscesses (Fig. 3). However, in our experience hyperintense signal also seen
on diffusion-weighted images of small abscesses is
likely due to T2 prolongation, or shine-through, rather than restricted diffusion because ADC values are
not decreased.
Diffusion-weighted MRI of herpesvirus type 1 encephalitis has also shown hyperintense lesions (Fig. 4)
in association with decreased ADC values.3 Restricted
microscopic water diffusion noted in patients with
encephalitis has been attributed to cytotoxic edema
secondary to cell death.
B
A
FIG. 2: Magnetic resonance images in a 72-year-old man with a
10-day history of ataxia showing hyperintense signal due to T2prolongation effects (so-called T2 shine-through) associated with
subacute infarct rather than restricted diffusion associated with
acute infarction.
A: Axial T2-weighted image shows region of hyperintense signal
(arrow) in the left cerebellar hemisphere, consistent with infarction.
B: Axial trace-weighted diffusion-weighted image shows region
of hyperintense signal in left cerebellar hemisphere (arrow).
Relative contributions of restricted diffusion (seen in acute infarction) and T2-prolongation effects are not evident.
C: Axial ADC map shows region of hyperintense signal seen
in B (arrow) appears normal. Normal ADC values within this
region indicate that the lesion seen in B is subacute (rather than
acute) and that the hyperintense signal represents T2 shinethrough effect.
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Seizure foci
Although infrequently seen on diffusion-weighted
imaging, epileptogenic foci in patients with complex
partial status epilepticus are a non-ischemic cause of
A
hyperintense signal. The hyperintense appearance
of epileptic regions correlates with low ADC values
and indicates the presence of cytotoxic edema
(Fig. 5).4 The hyperintensity is thought to be due to
changes in the proportion of water in extracellular
B
C
FIG. 3: Magnetic resonance images of a cerebral abscess in a 9-year-old boy with headache show increased signal intensity due to
restricted diffusion.
A: Axial T2-weighted image shows left hemisphere mass with hypointense rim (arrowheads), commonly seen with abscesses.
B: Axial diffusion-weighted image shows hyperintense signal (arrowhead) within central portion of lesion. Vasogenic edema (arrows)
appears hypointense on this image because of increased water diffusion.
C: Axial ADC map shows decreased signal intensity within lesion (arrows) indicating restricted diffusion. ADC values measured
approximately 77% of normal white matter. Vasogenic edema increased signal intensity due to increased water diffusion.
A
B
FIG. 4: Magnetic resonance images in a 1-year-old boy with herpesvirus encephalitis (reprinted from Tsuchiya K, et al.,3 with permission from the American Journal of Roentgenology).
A: Axial fluid-attenuated inversion-recovery image shows mildly increased signal intensity in left frontal and temporal lobes (arrow).
B: Axial diffusion-weighted image shows increased signal intensity consistent with cytotoxic edema (arrows) due to encephalitis.
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and intracellular compartments, possibly reflecting
failure of the sodium–potassium adenosine triphosphatase pump caused by energy depletion.5 As such,
diffusion-weighted imaging abnormalities due to
continuous seizures are potentially reversible, but if
prolonged, continuous seizures can cause irreversible changes, as seen on follow-up T2-weighted images.
A
B
Trauma
Cranial trauma produces lesions that are hyperintense
on diffusion-weighted imaging, a result of cytotoxic
edema (Fig. 6). Such lesions exhibit decreased ADC
values on ADC maps6 and could potentially be mistaken for cerebral infarction. In the absence of a clinical
history, the distribution of lesions is a primary means
C
FIG. 5: MRI scans in an 8-year-old girl with status epilepticus.
A: Axial T2-weighted image shows normal signal intensity in right temporal lobe.
B: Axial diffusion-weighted image shows increased signal intensity in right temporal lobe (arrows) and right thalamus (arrowhead)
due to cytotoxic edema.
C: Axial ADC map shows region of decreased signal intensity in the right temporal lobe (arrows) and right thalamus (arrowhead) consistent with restricted diffusion.
A
B
C
FIG. 6: MRI in a 56-year-old woman with diffuse axonal injury caused by trauma following an automobile crash.
A: Axial T2-weighted image shows subtle region of increased signal intensity within splenium of corpus callosum (arrow) consistent
with diffuse axonal injury.
B: Axial diffusion-weighted image obtained at the same time as A shows increased signal intensity within splenium of corpus callosum (arrow). Because the lesion is hyperintense in A, the increased signal intensity seen here could be due to restricted diffusion or to
T2-prolongation effects.
C: Axial ADC map shows restricted diffusion within splenium of corpus callosum, indicating that increased signal intensity in B is due
to cytotoxic edema. ADC values within splenium of corpus callosum were approximately 25% decreased when compared with normal
white matter.
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A
B
C
FIG. 7: Magnetic resonance images of a neoplasm in a 52-year-old man with glioblastoma multiforme.
A: Axial T2-weighted image shows masses in the left frontal white matter (arrows) and genu of corpus callosum (arrowhead), with surrounding vasogenic edema.
B: Axial diffusion-weighted image obtained at the same time as A shows the masses (arrows) to have increased signal intensity.
C: Axial ADC map shows that the lesions (arrows) have slightly increased signal intensity (rather than decreased signal intensity
expected in restricted diffusion), indicating that increased signal intensity in B is due to T2-prolongation effects. ADC values actually
measured slightly higher than normal white matter.
of distinguishing these entities. Traumatic injury typically occurs at a few specific locations: at the grey–
white matter junction and corpus callosum (diffuse
axonal injury), cerebral cortex (contusion) and basal
ganglia and thalamus (deep grey matter injury).
Neoplasms
Most reports of diffusion-weighted imaging of cerebral
neoplasms have indicated better water diffusion
through neoplasms than through normal brain tissue.7
However, neoplasms can occasionally have a hyperintense appearance on diffusion-weighted MRI (Fig. 7).
Although, in some cases, this may be due to dense
small-cell packing and restricted extracellular diffusion,8 the hyperintense appearance of tumours on
diffusion-weighted images is likely most often due to
T2 shine-through effects (Fig. 7).
SUMMARY
A number of entities other than acute cerebral infarction can produce bright signal intensity on diffusionweighted magnetic resonance images, and an understanding of the range of possible diagnoses for these
hyperintense lesions is important for radiologists who
must interpret these images.
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