AJR 2003; 181:1705-1709
© American Roentgen Ray Society
Hypointensity on Diffusion-Weighted MRI of the Brain Related to T2 Shortening and Susceptibility Effects
Akio Hiwatashi1,
Toshibumi Kinoshita,
Toshio Moritani,
Henry Z. Wang,
David A. Shrier,
Yuji Numaguchi,
Sven E. Ekholm and
Per-Lennart Westesson
1 All authors: Department of Diagnostic Radiology, University of Rochester
Medical Center, Box 648, 601 Elmwood Ave., Rochester, NY 14642-8648.
Received January 6, 2003;
accepted after revision April 28, 2003.
Address correspondence to A. Hiwatashi
(Akio_Hiwatashi{at}urmc.rochester.edu).
Introduction
Because diffusion-weighted images are inherently T2-weighted, changes in
tissue T2 can influence the appearance of diffusion-weighted images
independent of tissue diffusibility. The effect of prolonged T2—that is,
of T2 "shine-through"—is well known. Less well described are
the effects of T2 shortening on the appearance of diffusion-weighted images.
The goal of this article is to show examples of how T2 shortening influences
the appearance of diffusion-weighted images.
Basic Concepts
The signal intensity (SI) on a diffusionweighted image is
influenced by T2, the apparent diffusion coefficient (ADC), b value
(b), spin density (SD), and TE and is calculated as follows:
where SIb = 0 is the signal intensity on the
spin-echo echoplanar image (T2-weighted image where b = 0). However,
where k is a constant.
Moreover, for TR >> T1,
From this theory, we may consider the three conditions that may
occasionally be confused in the interpretation of diffusionweighted
images.
T2 Shine-Through
This well-known phenomenon causes hyperintensity on diffusion-weighted
images as a result of T2 prolongation, such as in subacute to chronic
infarction [1]. A decreased
apparent diffusion coefficient can be observed, which results in the
accentuation of hyperintensity on diffusion-weighted images.
T2 Washout
The term "T2 washout" implies isointensity on
diffusion-weighted images due to a balance between hyperintensity on
T2-weighted images and an increased apparent diffusion coefficient
[2]. T2 washout is often seen
in vasogenic edema. To the best of our knowledge, there have been no
systematic reports about pathologic conditions that are characterized by
isointensity on diffusion-weighted images caused by a balance of hypointensity
on T2-weighted images and a decreased apparent diffusion coefficient.
T2 Blackout
The term "T2 blackout" indicates hypointensity on
diffusion-weighted images caused by hypointensity on T2-weighted images. T2
blackout is commonly seen in some hematomas
[3]. The cause of T2 blackout
is predominantly susceptibility effects. However, other conditions show
hypointensity on diffusion-weighted images independent of tissue
diffusibility, and this pictorial essay illustrates several of these
conditions.
Normal Iron Deposition
In the normal adult brain, the globus pallidus, substantia nigra, red
nucleus, and dentate nucleus may show hypointensity on T2-weighted images
[4] (Fig.
1A,
1B,
1C). This hypointensity may be
caused by susceptibility effects that result from physiologic senile iron
deposition. On diffusion-weighted images, these areas can also show
hypointensity. Apparent diffusion coefficient maps may appear normal, and it
might be inaccurate to calculate the apparent diffusion coefficient because of
the susceptibility effects produced by local field inhomogeneity.
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Fig. 1C. —90-year-old man without neurologic deficit. Apparent
diffusion coefficient map shows normal appearance of deep nuclei, which proves
that hypointensity seen in B is effect of hypointensity on T2-weighted
image where b is zero rather than increased diffusibility. This example
underscores how hypointensity on T2-weighted image can influence appearance of
diffusion-weighted image independent of tissue diffusibility.
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Hemorrhage
In hemorrhage, oxyhemoglobin evolves sequentially: first to
deoxyhemoglobin, then to methemoglobin, and finally to hemosiderin or
ferritin. Oxyhemoglobin is diamagnetic and has been reported to show
hyperintensity on T2- and diffusion-weighted images with a decreased apparent
diffusion coefficient. Being paramagnetic, deoxyhemoglobin, intracellular
methemoglobin, hemosiderin, and ferritin may cause hypointensity on T2- and
diffusion-weighted images (Fig.
2A,
2B,
2C). A decreased apparent
diffusion coefficient in hemorrhage with intact RBC membranes and an increased
apparent diffusion coefficient after lysis of RBC membranes have been reported
[5]. The latter can be related
to dilution of methemoglobin in the extracellular fluid. However, measurements
of diffusion in areas with susceptibility effects can be problematic because
of local field distortions.
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Fig. 2A. —49-year-old man with acute hemorrhage. T2-weighted fast
spin-echo image shows hypointense hemorrhage (arrow) in left temporal
lobe with areas of surrounding hyperintensity (arrowheads), which is
consistent with edema.
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Fig. 2B. —49-year-old man with acute hemorrhage. Diffusion-weighted (b
= 1,000) image shows hypointensity in central part of hemorrhage (large
arrow) and hyperintensity in region of edema (arrowhead).
Additionally, there are curvilinear areas of marked hyperintensity (small
arrows) that most likely represent regions of focal susceptibility
artifact caused by paramagnetic effects of blood products.
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Fig. 2C. —49-year-old man with acute hemorrhage. Apparent diffusion
coefficient (ADC) map shows heterogeneous values of ADC within hemorrhage
(arrow) and thin rim of increased ADC along margin
(arrowhead), which is consistent with edema. This example shows how
hypointensity on T2-weighted image and susceptibility effects can combine to
produce complex appearance in and around cerebral hemorrhage.
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Infections
Bacterial Abscess
It is well known that the cavity of bacterial abscess will show
hyperintensity on diffusion-weighted images with a decreased apparent
diffusion coefficient, probably because of pus formation. Bacterial abscess
often has the rim that has been reported to show hypointensity on T2-weighted
images, probably ascribed to paramagnetic free radicals released from
macrophages and microhemorrhage. The rim also shows hypointensity on
diffusion-weighted images and T2-weighted images where b is zero, and this
finding might be more prominent on these images than on conventional
T2-weighted images. Because this hypointensity is one of the characteristic
findings of bacterial abscesses, evaluating diffusion-weighted and T2-weighted
images where b is zero in addition to the conventional T2-weighted images is
useful (Fig. 3A,
3B,
3C,
3D).
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Fig. 3C. —38-year-old man with bacterial abscesses. Apparent diffusion
coefficient map shows hypointensity in cavities, hyperintensity in rims
(arrows), and hyper-intensity in surrounding areas.
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Fig. 3D. —38-year-old man with bacterial abscesses. Diffusion-weighted
(b = 1,000) image shows hyperintensity in cavities, hypointensity in rims
(arrows), and isointensity in surrounding areas. Hyperintensity in
cavities is ascribed to restricted diffusion. Isointensity in surrounding
edema is ascribed to balance between hyperintensity on T2-weighted image and
increased diffusibility. Hypointensity in rims is ascribed to both
hypointensity on T2-weighted image and increased diffusibility. This example
also shows how tissue T2 and diffusibility can influence appearance of
diffusion-weighted image.
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Toxoplasmosis
Abscess due to toxoplasmosis has been reported to show homogeneous
hypointensity on diffusion-weighted images with an increased apparent
diffusion coefficient [6].
However, the signal intensity on diffusion-weighted images may be variable in
the acute phase of toxoplasmosis. In the chronic phase, the lesion shows
hypointensity on T2-weighted images, probably because of granulomatous changes
that also cause hypointensity on diffusion-weighted images and T2-weighted
images where b is zero (Fig.
4A,
4B,
4C).
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Fig. 4C. —28-year-old woman with chronic toxoplasmosis.
Diffusion-weighted (b = 1,000) image shows hypointensity (arrow).
This example also shows how hypointensity on T2-weighted image can influence
appearance of diffusion-weighted image independent of tissue
diffusibility.
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Aspergillosis
In disseminated aspergillosis, infiltration to the arteries is seen causing
hemorrhagic infarction or cerebritis. The latter may induce abscess formation.
On the other hand, granulomatous aspergillosis is more fibrotic than
disseminated aspergillosis. This lesion shows hypointensity on T2-weighted
images that is probably caused by hemorrhage or iron accumulation
[7]. Because hemorrhage and
iron deposition are characteristics of aspergillosis, diffusion-weighted
images and T2-weighted images where b is zero are helpful for detecting these
lesions (Fig. 5A,
5B,
5C,
5D).
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Fig. 5A. —48-year-old woman with aspergillosis. Unenhanced CT scan
shows small hyperdense lesion (arrow), consistent with hemorrhage,
and surrounding hypodense areas, consistent with subacute infarction in left
frontal lobe.
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Fig. 5C. —48-year-old woman with aspergillosis. Apparent diffusion
coefficient map shows hypointensity in portion of hemorrhage (arrow)
due to susceptibility effects, whereas surrounding area shows heterogeneous
intensity.
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Fig. 5D. —48-year-old woman with aspergillosis. Diffusion-weighted (b =
1,000) image shows small hypointense lesion (arrow) with areas of
surrounding hyperintensity, which are consistent with subacute infarction.
This example shows how hypointensity on T2-weighted image (susceptibility
effects) and diffusibility can combine to produce complex appearance in
hemorrhagic infarction.
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Metastasis
Most metastatic tumors are hyper- or isointense on T2-weighted images, but
some, such as metastatic adenocarcinoma, show hypointensity
[8] (Fig.
6A,
6B,
6C). The mechanism is
uncertain, but it has been ascribed to the presence of blood, mucin, iron,
necrosis, or calcium. Hemorrhagic metastases such as melanoma, lung carcinoma,
choriocarcinoma, thyroid carcinoma, and renal cell carcinoma may also show
hypointensity on diffusion-weighted images and T2-weighted images where b is
zero because of susceptibility effects. These images are useful for detecting
the hemorrhage, necrosis, or inflammation in a tumor.
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Fig. 6A. —61-year-old woman with metastasis from adenocarcinoma of
lung. T2-weighted fast spin-echo image shows heterogeneous mass with
surrounding hyperintensity, which is consistent with edema in left cerebellar
hemisphere. Peripheral portion of this tumor shows hypointensity
(arrow) that is consistent with noncystic–necrotic component,
whereas central portion shows hyperintensity (arrowhead) that is
consistent with cystic–necrotic component.
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Fig. 6B. —61-year-old woman with metastasis from adenocarcinoma of
lung. Apparent diffusion coefficient map shows hypointensity (arrow)
in peripheral noncystic–necrotic portion and hyperintensity
(arrowhead) in cystic–necrotic portion.
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Fig. 6C. —61-year-old woman with metastasis from adenocarcinoma of
lung. Diffusion-weighted (b = 1,000) image shows hypointensity throughout
tumor. Cause of this hypointensity of peripheral noncystic–necrotic
portion is related to hypointensity on T2-weighted images, and cause of
hyperintensity of central cystic–necrotic portion is related to
increased diffusibility. This example also shows how hypointensity on
T2-weighted image and increased diffusibility can combine to produce complex
appearance in metastatic adenocarcinoma.
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Conclusion
The interpretation of hypointensity on diffusion-weighted images requires
correlation with apparent diffusion coefficient maps and T2-weighted images
where b is zero to gain an understanding of the underlying pathophysiologic
conditions. Hypointensity on diffusion-weighted images due to T2 shortening
and susceptibility effects occurs in a variety of conditions such as those in
which iron deposition, hemorrhage, and granulomatous tissue are
characteristic. Knowledge of these pitfalls will improve our ability to
correctly interpret diffusion-weighted images.
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Introduction
Basic Concepts
