AJR 2003; 181:1711-1714
© American Roentgen Ray Society
Diffusion-Weighted MRI of Cerebral Toxoplasma Abscess
Crispina H. Chong-Han1,
Selina C. Cortez2 and
Glenn A. Tung1
1 Department of Diagnostic Imaging, Brown University School of Medicine, Rhode
Island Hospital, 593 Eddy St., Providence, RI 02903.
2 Department of Pathology, Our Lady of Fatima Hospital, 200 High Service Ave.,
North Providence, RI 02904.
Received February 25, 2003;
accepted after revision June 3, 2003.
Address correspondence to G. A. Tung.
Abstract
OBJECTIVE. This retrospective study reports the diffusion-weighted
MRI appearance of Toxoplasma abscesses, rim-enhancing cerebral masses
resulting from toxoplasmosis infection. In all patients, the signal intensity
of the abscess core on diffusion-weighted MRI was equal to or less than that
of normal, unaffected cerebral white matter and the measured apparent
diffusion coefficient was greater than that of unaffected white matter.
Histopathology revealed necrotic tissue in the center of these abscesses but
no purulent fluid.
CONCLUSION. Unlike pyogenic abscesses, the core tissue of
rim-enhancing Toxoplasma abscesses shows no restriction of water
diffusion.
Introduction
For the characterization of a rimenhancing cerebral mass,
diffusion-weighted MRI provides useful diagnostic information. With rare
exception, water diffusion in the center of a peripherally enhancing primary
brain tumor or metastasis is either increased or normal relative to that in
the unaffected white matter
[1–3].
In contrast, restricted water diffusion in the center of a capsule-stage
pyogenic abscess has been attributed to the high viscosity and cellularity of
purulent fluid
[1–5].
Few investigations have been conducted on the features of nonpyogenic
infectious masses on diffusion-weighted MRI
[6,
7]. We sought to study the
appearance of rim-enhancing cerebral Toxoplasma abscesses on
diffusion-weighted MRI.
Materials and Methods
A computer-assisted search of the radiologic information system for reports
of brain MRI examinations performed from January 1999 to March 2002 containing
the keyword "toxoplasmosis" generated 27 cases. The medical
records for these patients were requested, but the records of five patients
were not available for review. In another five patients, the final diagnosis
was a disease other than toxoplasmosis. Of the 17 remaining patients with a
clinical diagnosis of toxoplasmosis, 11 either had biopsy-proven toxoplasmosis
or had documented marked improvement of symptoms after specific treatment for
toxoplasmosis; six patients were lost to follow-up and were excluded. The
imaging history of six of the remaining 11 confirmed cases of rim-enhancing
toxoplasmosis included diffusion-weighted MRI, and these images formed the
basis of our report.
All six patients were imaged on a 1.5-T superconducting magnet (Vision,
Siemens, Erlangen, Germany) using echoplanar diffusion-weighted MRI.
Diffusion-weighted imaging was performed with an axial single-shot echoplanar
spin-echo sequence (TR/TE, 4,000/110; section thickness, 5 mm; matrix, 96
x 200; field of view, 230 x 230 mm; three b values of 0, 500, and
1,000 mm2/sec). Diffusion gradients were applied sequentially in
three orthogonal directions to generate three sets of diffusion-weighted MRIs.
Apparent diffusion coefficient (ADC) map images were created from signal
intensity data on diffusion-weighted images.
We evaluated the single largest rim-enhancing Toxoplasma abscess
in each patient, performing a qualitative assessment of the signal intensity
of the center or core of the abscess, its wall, and the surrounding edema and
comparing them with the signal intensity of unaffected white matter in the
contralateral cerebral hemisphere. Using a 3-mm circular region of interest,
we quantified the ADCs for the center and wall of the abscess and the
surrounding edema from the ADC map in four patients. The ADC of
normal-appearing white matter contralateral to the abscess was also measured
in a similar manner.
Results
All six patients were HIV-positive, with ages ranging from 34 to 61 years.
Five were men. Multiple rim-enhancing lesions were present in five of the six
patients (range, 1–4 lesions) and ranged in size from 0.8 to 2.5 cm in
diameter.
All six cases of Toxoplasma abscess had a similar appearance on
diffusion-weighted MRI (Figs.
1A,
1B,
1C,
2A,
2B,
3A,
3B,
4A,
4B,
4C). The center of the abscess
was only slightly hyperintense on diffusion-weighted imaging compared with
normal contralateral white matter, and the wall was relatively hypointense
compared with the center of the abscess. Surrounding edema was hyperintense on
diffusion-weighted imaging. On the ADC map images, the center of the abscess
had a signal intensity that was similar or higher than that of the unaffected
white matter, whereas the abscess wall was slightly hypointense. Edema
surrounding the abscess was hyperintense on the ADC map but not as high in
signal intensity as ventricular cerebrospinal fluid.
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Fig. 1A. —54-year-old man with cerebral toxoplasmosis that responded to
treatment with sulfamethoxazole and trimethoprim. Axial enhanced T1-weighted
image (TR/TE, 690/17) shows rim-enhancing mass in left inferior parietal lobe
with eccentric central contrast enhancement.
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Fig. 1B. —54-year-old man with cerebral toxoplasmosis that responded to
treatment with sulfamethoxazole and trimethoprim. Axial diffusion-weighted
image shows that center of abscess is isointense (asterisk) and
surrounding edema (arrow) is hyperintense.
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Fig. 1C. —54-year-old man with cerebral toxoplasmosis that responded to
treatment with sulfamethoxazole and trimethoprim. Quantitative analysis of
apparent diffusion coefficient (ADC) map image shows center of abscess
(asterisk) has slightly greater ADC value (mean, 1.10 x
10–3 mm2/sec) than does contralateral white matter
(mean, 0.85 x 10–3 mm2/sec). Mean ADC value
of surrounding edema (arrow) is 1.64 x 10–3
mm2/sec, nearly twice that of normal white matter.
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Fig. 2A. —34-year-old woman with biopsy-proven Toxoplasma
abscess. Axial contrast-enhanced T1-weighted image (TR/TE, 650/17) shows right
frontal rim-enhancing mass (arrow) and smaller enhancing lesion in
parietal lobe.
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Fig. 2B. —34-year-old woman with biopsy-proven Toxoplasma
abscess. Apparent diffusion coefficient (ADC) map image shows diffusion in
abscess core (asterisk) is increased (mean ADC, 2.13 x
10–3 mm2/sec) relative to unaffected white matter,
whereas diffusion in abscess wall (straight arrow) is relatively
decreased (mean ADC, 0.82 x 10–3 mm2/sec).
Diffusion in edema (curved arrow) surrounding abscess is greater than
that of unaffected white matter but lower than that of abscess core (mean ADC,
1.74 x 10–3 mm2/sec).
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Fig. 3B. —61-year-old man with biopsy-proven Toxoplasma
abscess. Apparent diffusion coefficient (ADC) map image shows that signal
intensity in abscess core tissue (asterisk; mean ADC, 1.15 x
10–3 mm2/sec) is qualitatively similar to that of
right basal ganglia. ADC ratio was 1:4.
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Fig. 4C. —50-year-old man with biopsy-proven Toxoplasma
abscess. Apparent diffusion coefficient (ADC) map image shows signal intensity
of core of Toxoplasma abscess (asterisk; mean ADC, 0.69
x 10–3 mm2/sec) is equal to that of
uninvolved white matter. ADC ratio was 1.1.
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ADC data for the four patients in whom it was measured are summarized in
Table 1. The mean ADC (±
SD) of the center of the Toxoplasma abscess was 1.22 (± 0.50)
x 10–3 mm2/sec. The mean ADCs of the wall of
the abscess and the edema surrounding the lesion were 0.74 (± 0.14)
x 10–3 mm2/sec and 1.64 (± 0.13)
x 10–3 mm2/sec, respectively. The
normal-appearing contralateral white matter had an ADC of 0.82 (± 0.12)
x 10–3 mm2/sec. The ADC value of the center
of the abscess was equal to (n = 1) or greater than (n = 3)
the normal contralateral white matter; in no patient was water diffusion
restricted. The ADC ratio of the abscess core to unaffected white matter was
greater than 1.0 in all patients, ranging from 1.1 to 3.7. In three of four
patients, the wall of the Toxoplasma abscess had a slightly lower ADC
than did the normal white matter.
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TABLE 1 Apparent Diffusion Coefficient (ADC) Values for Toxoplasma
Abscess, Edema, and Unaffected White Matter in Four Patients Who Were
HIV-Positive
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In three patients, histopathologic specimens of the Toxoplasma
abscess were available for review, and a representative example is shown in
Figure 5A,
5B. In these patients, the
abscess had a necrotic center containing cellular debris, macrophages, and
other inflammatory cells, but pus is conspicuously absent. Unlike the pyogenic
abscess, the wall of the Toxoplasma abscess was not encapsulated by a
collagenous wall but contained intracellular and extracellular tachyzoites,
the actively dividing and infectious form of the parasite. Abundant vasogenic
edema normally surrounds the abscess wall.
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Fig. 5A. —Photomicrographs of histopathologic specimens of cerebral
Toxoplasma abscess in 50-year-old man. (H and E, magnification
unknown) Center of abscess consists of necrotic tissue and inflammatory cells
with karyorrhexis (curved arrow) and nuclear debris (straight
arrow). Neither pus nor viable organisms are present.
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Fig. 5B. —Photomicrographs of histopathologic specimens of cerebral
Toxoplasma abscess in 50-year-old man. (H and E, magnification
unknown) Wall of abscess consists of viable acute inflammatory cells including
macrophages and Toxoplasma tachyzoites (short arrow) and
encysted bradyzoites (long arrow).
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Discussion
A rim-enhancing brain mass may be caused by a primary or metastatic
neoplasm, abscess, resolving hematoma, tumefactive demyelination, or radiation
necrosis. In many cases, the clinical history of a patient can narrow the
differential diagnosis, but it still may be difficult to distinguish a
neoplasm from an abscess on conventional MRI. In recent years,
diffusion-weighted MRI has been shown to be helpful in the differentiation of
these peripherally enhancing masses. Markedly restricted water diffusion is
characteristic of the purulent abscess core, a finding in contrast to the
relatively increased water diffusion observed in the necrotic center of a
peripherally enhancing brain neoplasm
[1–5].
A rim-enhancing mass may also be the presentation of cerebral
toxoplasmosis, the prevalence of which has increased during the AIDS epidemic
of the past two decades. However, the HIV status of a patient with a
rim-enhancing cerebral mass may not be known at the time of clinical
presentation. Although a cerebral Toxoplasma abscess may have the
same appearance as a pyogenic abscess on contrast-enhanced MRI, our study
shows that, unlike water diffusion in the center of a pyogenic abscess, water
diffusion is not restricted in the center of the Toxoplasma abscess.
The absence of restricted water diffusion can be established both
qualitatively and quantitatively by comparing the appearance of the abscess
center with that of the normal white matter. In all of our patients, water
diffusion in the center of the Toxoplasma abscess was either equal to
or greater than that of the unaffected white matter. Chang and Ernst
[6] reported a single case of
toxoplasmosis in which the measured ADC was slightly greater than that of the
normal white matter. In a review of cases of AIDS-associated infections,
Zimmerman [7] stated that
toxoplasmosis is heterogeneous in signal intensity on diffusion-weighted
MRI.
The neuropathology of cerebral toxoplasmosis may explain our observation.
The gross pathology of a cerebral mass lesion caused by toxoplasmic
encephalitis may resemble a pyogenic abscess and has been referred to as a
Toxoplasma abscess in the pathology literature
[7,
8]. However, there are distinct
histopathologic differences between the pyogenic and Toxoplasma
abscesses. When toxoplasmosis invades the brain of an immunocompromised
patient, acute encephalitis may ensue. If the infection progresses, central
necrosis with variable petechial hemorrhage may develop and become encircled
by a ring of free tachyzoites, encysted bradyzoites, and inflammatory cells.
The core of a Toxoplasma abscess consists primarily of necrotic
tissue and does not have the viscous, proteinaceous, and inflammatory debris
of purulent fluid. This feature is important because it has been postulated
that the high viscosity of mucoid proteins or other large molecules in pus
binds water and restricts its free diffusion
[4].
In conclusion, the core of a rim-enhancing Toxoplasma abscess does
not exhibit restricted water diffusion and, in contrast to pyogenic abscess,
cerebral Toxoplasma abscess may resemble a metastasis or primary
brain tumor on diffusion-weighted MRI.
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Abstract
Introduction
