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Diffusion-Weighted MR Imaging of Rim-Enhancing Brain Masses

Is Markedly Decreased Water Diffusion Specific For Brain Abscess?

¹ Department of Diagnostic Imaging, Brown University School of Medicine, Rhode Island Hospital, 593 Eddy St., Providence, RI 02903.
² Department of Neurosurgery, Brown University School of Medicine, Rhode Island Hospital, Providence, RI 02903.

Received October 25, 2000; accepted after revision March 21, 2001.

 
Address correspondence to G. A. Tung.

Abstract

Top Abstract Introduction Materials and Methods Results Discussion References

 
OBJECTIVE. This retrospective study investigated the specificity of restricted water diffusion for the diagnosis of brain abscess. Two of five rim-enhancing brain masses with restricted water diffusion (apparent diffusion coefficient of 0.79 [10^-3 mm²/sec] or less) were brain abscesses, but diagnoses in the other cases were metastatic squamous cell carcinoma (two cases) and radiation necrosis.

CONCLUSION. Although an important diagnostic sign, restricted water diffusion is not specific for brain abscess.

Introduction

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Several small case series have reported that markedly restricted water diffusion—an apparent diffusion coefficient (ADC) of 0.79 (10^-3 mm²/sec) or less—is a characteristic of purulent fluid in brain abscesses and that diffusion-weighted imaging might be used to distinguish brain abscess from necrotic tumor [1,2,3]. This retrospective study was performed to determine if restricted water diffusion is specific for abscess on diffusion-weighted MR imaging of the brain.

Materials and Methods

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A computer-assisted search of brain MR imaging reports, generated between January 1999 and April 2000, identified 30 rim-enhancing brain masses with pathologic or imaging follow-up in which diffusion-weighted MR imaging was performed, and the images were available for review. Of these 30 masses, five rim-enhancing masses had an ADC of 0.79 (10^-3 mm²/sec) or less and form the case material for this report. The patients included one girl, 4 years old; two men, 58 and 72 years old; and two women, each 60 years old. For these five patients, the reference standard for tissue diagnosis was brain biopsy.

These patients were imaged with routine enhanced MR imaging and diffusion-weighted echo-planar imaging on a Vision 1.5-T superconducting magnet (Siemens, Erlangen, Germany). Diffusion-weighted imaging was performed with an axial single-shot echoplanar spin-echo sequence (TR/TE, 4000/110; section thickness, 5 mm; matrix, 96 x 200; field of view, 230 x 230 mm; three b values of 0, 500 mm²/sec, and 1000 mm²/sec). Diffusion gradients were applied sequentially in three orthogonal directions to generate three sets of diffusion-weighted images. ADC map images were created from signal-intensity data on diffusion-weighted images acquired with b values of 0, 500 mm²/sec, and 1000 mm²/sec. On the ADC map image through the center of the rim-enhancing mass, the mean ADC was determined by the average of three pixel-value measurements from a 2-mm circular region of interest in the center of the mass. On the basis of case reports of eight patients with capsule-stage brain abscesses [1,2,3,4], an ADC of 0.79 (10^-3 mm²/sec) or less was used to define a mass with a "markedly decreased" ADC for the purposes of this study.

Results

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Relative to cerebrospinal fluid, the center of rim-enhancing brain masses had markedly increased signal intensity on diffusion-weighted images and decreased signal intensity on ADC map images; the mean ADC of these five masses was 0.51 (± 0.02, 10^-3 mm²/sec; range, 0.32-0.70). On MR imaging, all masses were hypointense on T1-weighted images and hyperintense on turbo spin-echo T2-weighted images relative to gray matter. On fluid-attenuated inversion-recovery images, the nonenhancing center of all masses was hyperintense compared with cerebrospinal fluid. After contrast administration, there was rim or peripheral enhancement in all five cases.

In two cases, the pathologic diagnosis was capsule-stage brain abscess. In one of these cases (Fig. 1A,1B,1C), a 4-year-old girl with a pontine glioblastoma presented with a rim-enhancing pontine mass 3 months after radiosurgery. Because of abnormal findings on diffusion-weighted MR imaging, brain biopsy and fluid aspiration were performed and revealed a Streptococcal brain abscess. Twelve days after open surgical drainage, the mean ADC of the abscess increased from 0.64 to 2.71 (10^-3 mm²/sec). In two other cases, brain biopsy revealed squamous cell carcinoma (Fig. 2A,2B,2C), both metastatic from a primary lung tumor. In another case, the pathologic diagnosis was radiation necrosis (Fig. 3A,3B).

View larger version (159K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —4-year-old girl with abscess after radiosurgery of pontine glioblastoma. Axial enhanced T1-weighted image (TR/TE, 650/15) shows large rim-enhancing pontine mass.

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —4-year-old girl with abscess after radiosurgery of pontine glioblastoma. Markedly increased signal is seen in mass (arrow) on trace-weighted diffusion-weighted image (b = 1000 mm2/sec).

View larger version (152K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —4-year-old girl with abscess after radiosurgery of pontine glioblastoma. Apparent diffusion coefficient (ADC) map image shows markedly decreased signal in mass (arrow) compared with that in cerebrospinal fluid; mean ADC was 0.64 (± 0.02, 10-3 mm2/sec). One day later, purulent fluid was aspirated at surgery.

View larger version (160K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —72-year-old man with necrotic squamous cell carcinoma metastasis. Axial T1-weighted image (TR/TE, 650/17) shows right parietal mass with irregular rim enhancement.

View larger version (134K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —72-year-old man with necrotic squamous cell carcinoma metastasis. Trace-weighted diffusion-weighted image (b = 1000 mm2/sec) shows increased signal intensity in this mass (arrow) compared with cerebrospinal fluid.

View larger version (147K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C. —72-year-old man with necrotic squamous cell carcinoma metastasis. Metastasis has marked low signal intensity (arrow) on apparent diffusion coefficient (ADC) map image; mean ADC was 0.70 (± 0.02, 10-3 mm2/sec).

View larger version (168K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —58-year-old man with surgically proven radiation necrosis after radiotherapy for residual glioblastoma. Axial fat-saturated turbo spin-echo T2-weighted image (TR/TE, 4000/90) shows mass (arrow) and postoperative changes in left occipital lobe and edema in left temporal white matter.

View larger version (133K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —58-year-old man with surgically proven radiation necrosis after radiotherapy for residual glioblastoma. Trace-weighted diffusion-weighted image (b = 1000 mm2/sec) shows marked hyperintense signal in occipital mass (arrow). The measured apparent diffusion coefficient was 0.32 (± 0.01, 10-3 mm2/sec).

Discussion

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Many factors may influence water diffusion in living tissue, including restriction (cellular compartmentalization, cell type and number, cell membrane density, and macro-molecule size and type), physicochemical properties of tissue (viscosity and temperature), and perfusion. Pus is a viscous fluid that consists of bacteria, inflammatory cells, mucoid proteins, and cellular debris. High viscosity and inflammatory cellular composition may explain restricted water diffusion in purulent fluid [1]. The binding of water to large molecules like fibrinogen in pus may also contribute to restricted water diffusion [5]. In several case reports, pyogenic brain abscess has been reported to have markedly increased signal intensity on diffusion-weighted imaging and markedly decreased signal intensity on ADC map images [1,2,3,4, 6, 7]. To our knowledge, the mean ADC of nine capsule-stage brain abscesses has been reported, and in eight cases, the measured ADC of purulent fluid has been less than that of white matter and much less than that of cerebrospinal fluid. Ebisu et al. [1] were the first to report a brain abscess with marked hyper-intensity on diffusion-weighted imaging; they reported an in vivo ADC of 0.31 (10^-3 mm²/sec) and an in vitro ADC of 0.43 (10^-3 mm²/sec) for aspirated pus. Subsequently, Desprechins et al. [2] reported two abscesses with a mean ADC of 0.28 (10^-3 mm²/sec), Noguchi et al. [3] reported four abscesses with an average ADC of 0.64 (10^-3 mm²/sec), and Ketelslegers et al. [4] reported one abscess with a mean ADC of 0.70 (10^-3 mm²/sec). In our study, two of the five masses with restricted water diffusion were brain abscesses.

Even though distinguishing brain abscess from necrotic tumor on MR imaging is difficult, our study confirms the value of diffusion-weighted imaging for the diagnosis of brain abscess but with the following caveat: in contradistinction to purulent fluid, necrotic areas in primary and metastatic tumors are usually more hemorrhagic, less viscous, and less cellular. As a result, these necrotic areas are usually hypointense on diffusion-weighted imaging and have mean diffusion coefficients four to 10 times greater than those of pus [3, 6, 8]. Krabbe et al. [6] reported the mean ADC of the necrotic component of 12 malignant gliomas and seven metastases to be 1.65 (10^-3 mm²/sec) and 2.62 (10^-3 mm²/sec), respectively, compared with a mean ADC of cerebrospinal fluid of 3.1 (10^-3 mm²/sec). In tumors, the ADC is highest in areas of cystic necrosis, followed by vasogenic peritumoral edema, nonenhancing solid tumor, and enhancing solid-tumor components [8]. However, we report two metastases, both squamous cell carcinomas, and one case of radiation necrosis with markedly increased signal intensity on diffusion-weighted imaging and a low diffusion co-efficient. We speculate that restricted diffusion in these cases was due to sterile liquefaction necrosis. There have been case reports of necrotic primary and metastatic tumors that mimic an inflammatory mass, both on gross and microscopic inspection [9]. It is also well established that therapeutic radiation causes vasculopathy and necrosis in treated tissue. Areas of sterile liquefaction necrosis may contain creamy puslike material with abundant polymorphonuclear leukocytes [9]. If fluid aspirated from areas of liquefaction necrosis can mimic pus grossly, then we postulate that some necrotic tumors or radiation necrosis may mimic a brain abscess on diffusion-weighted imaging. However, it is also important to keep in mind that pyogenic superinfection may rarely develop in a primary brain tumor, as was the case in one of our patients, or in areas of radiation necrosis [10,11,12].

We are aware of only one other report documenting a change in the signal intensity of a brain abscess on diffusion-weighted imaging after treatment. Ketelslegers et al. [4] reported an increase in the mean ADC from 0.7 (10^-3 mm²/sec) to 1.2 (10^-3 mm²/sec) after a 3-week course of IV antimicrobial treatment. In one of our cases, the signal intensity of a pontine abscess decreased significantly on diffusion-weighted imaging, and the mean ADC increased from 0.64 to 2.71 (10^-3 mm²/sec) after surgical drainage. This finding suggests that diffusion-weighted imaging may be a viable imaging technique to follow brain abscesses treated by aspiration.

In conclusion, markedly increased signal intensity of a rim-enhancing brain mass on diffusion-weighted imaging and a low ADC indicating restricted water diffusion are features that should suggest the diagnosis of brain abscess, but are not specific for this diagnosis.

References

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