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AJNR Am J Neuroradiol 22:1738–1742, October 2001

Restricted Diffusion within Ring Enhancement Is NotPathognomonic for Brain Abscess

Marius Hartmann, Olav Jansen, Sabine Heiland, Clemens Sommer, Kristin Munkel, and Klaus Sartor

BACKGROUND AND PURPOSE: Recent experience suggests that diffusion-weighted MRimaging may be decisive in the differential diagnosis of ring-enhancing cerebral lesions. Wheth-er restricted diffusion within a ring-enhancing cerebral mass lesion is pathognomonic for ab-scess was studied.

METHODS: Seventeen patients with ring-enhancing cerebral lesions (three abscesses, sixglioblastomas, eight metastases) on conventional contrast-enhanced T1-weighted images wereexamined with echo-planar diffusion-weighted MR imaging. Apparent diffusion coefficient(ADC) maps and the ADCs were calculated for all lesions. Lesions with signs of intralesionalhemorrhage on unenhanced T1-weighted images were excluded.

RESULTS: The central portion of all six glioblastomas and seven of eight metastases showedunrestricted diffusion, whereas two of three abscesses showed restricted diffusion (low ADCvalues) in their cavity. However, restricted diffusion also was found in one metastasis, and oneabscess within a postoperative cavity showed unrestricted diffusion within a larger nondepen-dent portion.

CONCLUSION: In patients with ring-enhancing cerebral mass lesions, restricted diffusionmight be characteristic but is not pathognomonic for abscess, as low ADC values also may befound in brain metastases.

With conventional MR imaging, the differential di-agnosis of ring-enhancing cerebral lesions is diffi-cult and often impossible. However, recent reportsclaim that brain abscesses can be differentiatedfrom cystic or necrotic brain tumors with diffusion-weighted (DW) MR imaging (1–5), a method thatprovides unique information about water diffusion.While the centers of most primary and secondarynecrotic or cystic brain tumors are hyperintense onapparent diffusion coefficient (ADC) maps corre-sponding to unrestricted diffusion, restricted dif-fusion with low ADC values within abscess cavi-ties was reported by several authors. Yet, the reasonfor restricted diffusion in brain abscesses is poorlyunderstood. Restricted diffusion might reflect highviscosity of proteinaceous fluid with high concen-tration of inflammatory cells, but these conditions

Received December 1, 1999; accepted after revision June 5,2001.

From the Departments of Neuroradiology (M.H., O.J., S.H.,K.S.), Neuropathology (C.S.), and Neurosurgery (C.M.), Uni-versity of Heidelberg Medical School, Heidelberg, Germany.

Presented at the annual meeting of the American Society ofNeuroradiology, Atlanta, April 2000.

Address reprint requests to Marius Hartmann, MD, Depart-ment of Neuroradiology, University of Heidelberg MedicalSchool, Im Neuenheimer Feld 400, D-69120 Heidelberg,Germany.

q American Society of Neuroradiology

are not confined to abscesses and might be presentin various other brain diseases. Therefore, we usedDW imaging in patients with ring-enhancing brainlesions to determine whether restricted diffusionwithin a ring-enhancing cerebral mass lesion waspathognomonic for abscess.

MethodsFrom October 1998 to September 1999, 17 consecutive pa-

tients with a cystic or necrotic cerebral mass lesion that showedring enhancement on contrast-enhanced T1-weighted imageswere examined using DW imaging. Lesions with signs of in-tralesional hemorrhage on unenhanced T1-weighted imageswere excluded. Stereotactic aspiration biopsy or surgery wasperformed for all patients. The final diagnosis was pyogenicabscess for three patients, glioblastoma multiforme for six pa-tients, and metastasis for eight patients.

All MR examinations were performed with a 1.5-T imagercapable of echo-planar imaging (Marconi, Cleveland, Ohio).Conventional and DW MR imaging were performed during thesame study for each patient. Conventional MR images wereobtained with transverse proton density– and T2-weighted se-quences (3000/20/2 [TR/TE/excitations]; matrix, 192 3 256;field of view, 23 3 23 cm2; section thickness, 6 mm; sectiongap, 1 mm), transverse T1-weighted sequences (650/25/ 2; ma-trix, 192 3 256; field of view, 23 3 23 cm2; section thickness,6 mm; section gap, 1 mm), and transverse contrast-enhancedT1-weighted sequences (0.1 mmol/kg of gadopentetate dime-glumine [Magnevist; Schering, Berlin, Germany]). The con-trast-enhanced images were obtained following DW imaging.Isotropic DW images were obtained in the transverse plane by

AJNR: 22, October 2001 BRAIN ABSCESS 1739

FIG 1. Metastatic adenocarcinoma. The T2-weighted image (A) shows a hypointense mass lesion surrounded by massive edema. Onthe contrast-enhanced T1-weighted image (B), there is ring enhancement presumably due to central necrosis. With diffusion weighting(C), the central part of the tumor becomes markedly hyperintense, while the ADC map (D) reveals low values, indicating restricteddiffusion. Histopathologic section (E): Note that besides tumor cells, additional geographic necrosis (N), lymphocytes (arrowheads), andconnective tissue (arrow) are present (hematoxylin and eosin stain; original magnification, 3 200).

using a single-shot echo-planar spin-echo pulse sequence(3000/120/1; matrix, 128 3 64; field of view, 31.5 3 17 cm;section thickness, 6 mm; section gap, 0.5 mm) with four bvalues (0, 333, 666, 1000 s/mm2). The total duration of thediffusion-sensitizing gradients was 80 m. To obtain isotropicdiffusion weighting, a gradient scheme was used according toHeid and Weber (6). The maximum diffusion gradient was 26mT/m. The ADC maps were calculated and the ADC value ofthe cystic or necrotic portion was measured in a region ofinterest of at least 1 cm2. On visual inspection, the signal in-tensities of the lesions on the DW images and on the ADC

maps were interpreted relative to the contralateral brainparenchyma.

ResultsDifferentiation between abscess, metastasis, and

glioblastoma was not possible by conventional MRimaging alone. The necrotic or cystic portion of sixmetastases and of all glioblastomas (mean tumorgroup) was markedly hypointense on DW images,

AJNR: 22, October 20011740 HARTMANN

FIG 2. Pyogenic brain abscess. On the T2-weighted image (A), the lesion has a thickhypointense wall with a hyperintense center, consistent with necrosis. Dense ring en-hancement is noted on the contrast- enhanced T1-weighted image (B). The DW image(C) and the ADC map (D) reveal restricted diffusion within the center.

FIG 3. Pyogenic brain abscess with para-falcial subdural empyema. On T2-weightedimage (A) the lesion has a thick hypoin-tense wall. The contrast-enhanced T1-weighted image (B) shows ring enhance-ment. Evidence for restricted diffusionwithin the abscess cavity and the parafal-cial empyema is provided by the DW im-age (C) and the ADC map (D).

AJNR: 22, October 2001 BRAIN ABSCESS 1741

FIG 4. Mean ADC values 6 SD (31023 mm2/s) of the meantumor group (n 5 13), adenocarcinoma outlier (n 5 1), meanabscess group (n 5 2), and nondependent and dependent por-tions of the abscess outlier (n 5 1).

with corresponding marked hyperintensity on theADC maps. The mean ADC value of the centralportion was 2.65 6 0.34 3 1023 mm2/s. However,in one metastasis (adenocarcinoma), the centralportion was markedly hyperintense on DW imagesrelative to normal brain parenchyma. On the ADCmaps, there was a corresponding low signal inten-sity (Fig 1) and the ADC value was 0.85 6 0.143 1023 mm2/s. Two abscesses showed markedlyhyperintense signal on DW images and were mark-edly hypointense on the ADC maps (Figs 2 and 3).One abscess showed extraparenchymal extensioncorresponding to a parafalcial subdural empyemathat had low ADC values as well (Fig 3). On quan-titative analysis, the mean ADC value of the centralportion of both abscesses was 0.58 6 0.16 3 1023

mm2/s. The central portion of the third abscess ina patient with resection of a right temporal metas-tasis 2 months earlier showed a fluid-fluid levelwithin the cavity. The DW imaging intensity of thegravity-dependent portion was markedly hyperin-tense and showed markedly hypointense signal onthe ADC map, indicating restricted diffusion. TheDW imaging intensity of the nondependent portionwas hypointense with corresponding marked hy-perintensity on the ADC map. The ADC value ofthe gravity-dependent area was 1.15 6 0.12 3 1023

mm2/s; for the nondependent portion, 2.54 6 0.193 1023 mm2/s (Fig 4).

DiscussionIn most patients with brain tumors, symptoms

and signs (often even neuroradiologic findings) are

not pathognomonic. However, because of the var-iable clinical course and prognosis of the diseasesthat must be included in the differential diagno-sis—untreated brain abscesses, for example, can berapidly fatal—an early and correct diagnosis is nec-essary for choosing the appropriate treatment. Theintroduction of CT has led to a dramatic decreasein the mortality rate from brain abscesses (7) be-cause of early and improved diagnosis with exactlocalization of the mass. Although there are variousCT features that help differentiate brain abscessesfrom primary or secondary solid brain tumors, in-farction, and noninfectious inflammatory mass le-sions, determining the true nature of ring-enhanc-ing cerebral lesions remains difficult. Even with theadvent of MR imaging, this diagnostic dilemma didnot disappear. The differential diagnosis of ring-enhancing cerebral lesions continues to be a chal-lenge, as such lesions include glioblastoma, metas-tasis, pyogenic abscess, subacute ischemic infarction,resolving hematoma, and demyelinating disease.

Recent experience suggests that DW MR imag-ing might be decisive in the differential diagnosisof ring-enhancing cerebral lesions. In 1996, Ebisuet al (2) reported finding restricted diffusion withinthe cavity of a brain abscess. This phenomenonmight be related to the high viscosity and cellular-ity of pus, which causes restriction of water protonmobility. However, in 1997, Krabbe et al (8) re-ported the opposite: increased diffusion within anabscess. Further studies confirmed restricted diffu-sion in abscesses with high signal intensity in thecentral cavity and correspondingly low ADC values(1, 3–5). Yet, the ADC values between and withinthese studies varied widely. It is possible that var-iable concentrations of inflammatory cells and bac-teria, different etiologic organisms, and the age ofthe abscess with variable viscosity of the abscessfluid were responsible for these conflicting data.Nevertheless, these studies strongly suggest that thediagnosis of a brain abscess can be made if thereis restricted diffusion within a cystic ring-enhanc-ing cerebral lesion with low ADC values. Our ex-ample of a postoperative abscess that developedwithin the resection cavity and showed unrestricteddiffusion within the larger nondependent portion,with a small area of restricted diffusion within thesmaller dependent portion (Fig 5), underlines theusefulness of DW imaging in the differential di-agnosis of ring-enhancing lesions.

On the other hand, our example of a metastaticadenocarcinoma with restricted diffusion within thenecrosis shows that low ADC values within a ring-enhancing lesion are not pathognomonic for brainabscess. The reason for restricted diffusion withinthe metastasis in our study is unclear. Increasedprotein concentration in the form of highly viscousmucin might cause restricted diffusion. This, how-ever, contradicts the results of Krabbe et al (8), whofound increased ADC values (mean, 2.62 6 0.313 1023 cm2/s) in four metastases from adenocar-cinoma. The adenocarcinoma ‘‘outlier’’ in our

AJNR: 22, October 20011742 HARTMANN

FIG 5. Postoperative brain abscess. TheT2-weighted image (A) shows a hyperin-tense cavity surrounded by massive ede-ma. The contrast-enhanced T1-weightedimage (B) shows ring enhancement. Withdiffusion weighting (C), the gravity-depen-dent portion was markedly hyperintense(arrow) and showed markedly hypointensesignal on the ADC map, (arrow), indicatingrestricted diffusion (D). The DW imagingintensity of the nondependent portion washypointense with corresponding markedhyperintensity on the ADC map.

study contained, in addition to tumor cells, addi-tional geographic necrosis, lymphocytes, and con-nective tissue, but no mucin was found during his-topathologic analysis (Fig 1E). Thus, we concludethat low ADC values are not pathognomonic formetastatic adenocarcinoma. Small hemorrhages orhemosiderin deposits are unlikely, as one could ex-pect marked hypointensity because of susceptibilityeffects on the DW images, and we excluded lesionswith possible subacute blood products that werehyperintense on noncontrast T1-weighted images.On the other hand, the exclusion of these lesionsfrom our study is a potential limitation. From dif-fusion measurements in intracranial hematomas, weknow that intracellular hemoglobin states (intracel-lular oxy-, intracellular deoxy-, and intracellularmethemoglobin) cause restricted diffusion in hy-peracute, acute, and early subacute hematomas de-spite their marked differences on conventional MRimages (9). Therefore, bloody lesions would be dis-played as identical to pyogenic abscess, based onsignal intensity, and these findings may beconfounding.

ConclusionIn patients with ring-enhancing cerebral mass le-

sions, low ADC values are not 100% specific forbrain abscess, as restricted water diffusion also

might be found in brain metastases. The dilemmaconcerning the differential diagnosis of ring-en-hancing cerebral mass lesions continues.

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