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British Journal of Radiology (2004) 77, 387-394
© 2004 British Institute of Radiology
doi: 10.1259/bjr/23641059
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Full Paper

Infectious meningitis: prospective evaluation with magnetization transfer MRI

P Kamra, MD1, R Azad, MD1, K N Prasad, MD2, S Jha, MD3, S Pradhan, MD3 and R K Gupta, MD1

Departments of 1 Radiology, 2 Microbiology and 3 Neurology, Sanjay Gandhi Post-Graduate Institute of Medical Sciences, Lucknow, Uttar Pradesh, India–226014


   Abstract
Top
 Abstract
Introduction
 Methods and materials
 Results
 Discussion
 Conclusion
 References
 
The study was performed with the aim of prospectively characterizing infectious meningitis of different aetiology using magnetization transfer (MT) MRI. Spin-echo (SE) T1, T2 and pre- and post-contrast T1 weighted MT images in 100 patients with aetiologically proven meningitis were evaluated for the visibility and enhancement of the meninges on pre- and post-contrast T1 weighted MT images, respectively. The MT ratio (MTR) was calculated from the thickened meninges in tuberculous meningitis. In addition, the percentage difference in the mean signal intensity (SI) of the meninges and adjacent brain parenchyma was calculated and compared between different groups using 2-tailed student's t-test. T1 weighted MT images were highly sensitive (96%) in the detection of abnormal meningeal enhancement. Meninges were visible on pre-contrast T1 weighted MT images only in patients with tuberculous meningitis. The MTR from meninges in tuberculous infection was 19.10±1.02, and the percentage difference in the mean SI of the meninges and the adjacent T2 normal brain parenchyma was significantly higher (p<0.05) in the tuberculous group compared with that in the non-tuberculous group. MT MRI is an important technique for the detection and characterization of infectious meningitis of different aetiology. Visibility of the meninges on pre-contrast T1 weighted MT images may be considered highly suggestive of tuberculous meningitis.


   Introduction
Top
 Abstract
 Introduction
Methods and materials
 Results
 Discussion
 Conclusion
 References
 
Cranial meninges (dura, arachnoid, and pia) may be involved in various pathological processes leading to infectious, carcinomatous, chemical/reactive, and inflammatory meningitis [1]. Infection is a common cause of meningeal disease and may be caused by any of the common infectious agents including bacteria, fungi, viruses and granulomatous agents [2].

The imaging features of meningitis are non-specific, demonstrating abnormal meningeal enhancement. MRI is superior to CT in the evaluation of patients with suspected meningitis [3]. Gadolinium diethylene triamine pentaacetic acid (Gd-DTPA) enhanced MRI is found to be more effective than non-enhanced MR and intravenous contrast-enhanced CT for demonstrating meningitis and its complications [4]. Magnetization transfer (MT) has received attention as an imaging sequence which may improve image contrast and tissue specificity on MR studies [5, 6]. Post-contrast T1 weighted MT imaging has improved the sensitivity of detection of meningitis, and it has been demonstrated that the detection of abnormal meningeal enhancement on conventional T1 weighted images significantly increases with the use of a higher dose of contrast agent and/or concurrent use of MT imaging [7–9]. A recent retrospective study on the role of MT imaging in meningitis showed significantly different magnetization transfer ratio (MTR) in different aetiologic types [10]. The purpose of this study was to characterize meningitis of different aetiology using MT prospective imaging.


   Methods and materials
Top
 Abstract
 Introduction
 Methods and materials
Results
 Discussion
 Conclusion
 References
 
The study population consisted of 100 patients with meningitis (65 tuberculous, 17 pyogenic, 9 fungal, and 9 viral). 57 patients were men and 43 were women; ages ranged from 2 years to 65 years. The final diagnosis of tuberculosis was based on typical cerebrospinal fluid (CSF) features (cellularity and biochemistry) in 40; positive enzyme linked immunosorbent assay (ELISA, ANDATB, ANDA Biologicals, Italy) for anti-tubercular antibodies in CSF in 15; positive polymerase chain reaction (PCR, Perkin-ELMER 9600, Roche, USA) for Mycobacterium antigen in CSF in 8; positive CSF culture in 6; fine needle aspirate from axillary lymph nodes positive for acid fast bacilli (AFB) in 3; and response to anti-tuberculous drugs in the remaining 33 patients [11, 12]. All patients with fungal meningitis had cryptococcal infection of the central nervous system (CNS). Cryptococcal meningitis was confirmed by demonstration of the yeast on India ink staining of CSF smear, raised cryptococcal antigen titers in CSF by latex agglutination test and a positive culture of the organism [13, 14]. Nine patients had viral meningoencephalitis (Japanese meningoencephalitis in six, herpes meningoencephalitis in three). Viral meningitis was confirmed by a rise in the serum antibody titer for the specific virus [15, 16]. Of the patients with pyogenic meningitis, the diagnosis was based on the CSF profile, demonstration of bacteria on Gram staining of CSF smear and positive culture of organisms.

All MR examinations were performed on a 1.5 Tesla MR system (Magnetom, Siemens, Erlangen, Germany) using a circularly polarized head coil. The head of the patient was restrained to prevent any movement during and inbetween the acquisition of images. The children were given sedation in appropriate doses. Conventional spin echo (SE) T1 weighted (repetition time (TR)/echo time (TE)/number of excitations (n)=1000 ms/14 ms/3), proton density (PD) and T2 weighted (2200 ms/20 ms, 80 ms/1) images were obtained in the axial plane (5 mm slice thickness, 0.5 mm interslice gap, and a 192 x 256 matrix). The pulse sequence used for MT contrast consisted of an off resonance saturation pulse immediately before the 90° pulse (excitation pulse to saturate the magnetization of protons with restricted motion). The bandwidth of the saturation pulse was 250 Hz with a frequency offset of 1.5 KHz. For T1 weighted MT MRI only a saturation pulse was added, the other parameters were identical to the conventional SE T1 weighted image. The TR and TE parameters were chosen to minimize the T1 and T2 weighting effect [17]. Post-contrast T1 weighted MT images were also obtained in all cases, after injecting Gd-DTPA (Magnevist, Schering, Germany) intravenously in a dose of 0.1 mmol kg–1 body weight. Informed consent was obtained from the patients and guardians of the children who were unable to give consent for the examination.

MR images were evaluated by two radiologists (PK, RA) independently for the presence of abnormal meningeal enhancement on post-contrast T1 weighted MT images. They were blinded to the presence of meningitis. Meningeal enhancement, when present on more than three contiguous MR images, asymmetric enhancement extending deep into the base of the sulci and thicker, long segmental (>3 cm) or nodular meningeal enhancement was considered abnormal [18–20]. The visibility of the abnormally enhancing meninges on pre-contrast T1 weighted MT image was observed and compared between different aetiological groups.

Choosing thick, nodular enhancing meninges, the MT ratio from thickened meninges was calculated in all patients with tuberculous meningitis. Signal intensity from the same region of interest (ROI) was measured from the conventional T1 weighted image without (Mo) and with (Ms) an off-resonance pulse. The MTR was calculated using the formula [21]: Go


It was calculated by placing multiple ROIs with single pixels and the mean and standard deviations from all ROIs was calculated. Consistency and reliability of the measurement were confirmed by obtaining the values repeatedly.

The signal intensity of the adjacent brain parenchyma on T1 weighted MT images (normal appearing on T2 and T1 weighted MT images) was calculated from multiple ROIs and mean obtained. The percentage difference in the mean signal intensity of the meninges and adjacent brain parenchyma was calculated and compared between different groups using a 2-tailed Student's t-test for non-paired data. Data were analysed by SPSS (Version 10, SPSS Inc., Chicago, IL) statistical software and a value of p<=0.05 was considered significant.


   Results
Top
 Abstract
 Introduction
 Methods and materials
 Results
Discussion
 Conclusion
 References
 
Abnormal meningeal enhancement was noted in all patients with tuberculous (n=65), pyogenic (n=17) and fungal meningitis (n=9). It was noted in five out of nine patients with viral meningitis. Thus, the overall sensitivity of detection of abnormal meningeal enhancement was 96%, sensitivity for tuberculous, pyogenic and fungal meningitis was 100% and 55% for viral meningitis.

Routine SE T1 and T2 weighted imaging did not show any obvious meningeal thickening in any of the 100 patients. In all patients with tuberculous meningitis, abnormal meninges were visible on pre-contrast T1 weighted MT images as mild to moderate hyperintense signal around the brain stem (Figure 1Go) in the basal cisterns and supratentorial subarachnoid space, consistent with thickened pia-arachnoid, which showed enhancement after administration of contrast material. However, pre-contrast T1 weighted MT images did not reveal any obvious meningeal abnormality in patients in the non-tuberculous group (viral, Figure 2Go; pyogenic, Figure 3Go; and fungal, Figure 4Go). The MT ratio of the pre-contrast MT hyperintense meninges in tuberculous infection was 19.10±1.02.



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  		Figure 1. Tuberculous meningitis. (a) T2 and (b) pre-contrast T1 weighted axial images show presence of hydrocephalus. (c) Pre-contrast T1 weighted magnetization transfer (MT) image shows thick basal meninges as hyperintense signal around the brain stem (small arrows), not seen on T1 weighted image (b). (d) Post-contrast T1 weighted MT axial demonstrate thick enhancement of the basal meninges and exudates.

 


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  		Figure 2. Herpes simplex encephalitis. (a) T2 weighted axial image shows abnormal increased signal intensities in both temporal lobes involving gray and white matter. These are hypointense on (b) T1 weighted image and mildly hyperintense on (c) T1 weighted magnetization transfer (MT) image. No abnormal meninges are seen on pre-contrast images. (d) Post-contrast T1 weighted MT image shows abnormal meningeal enhancement in both hemispheres along with left ependymal enhancement.

 


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  		Figure 3. Pyogenic meningitis. (a) T2, (b) pre-contrast T1, and (c) T1 weighted magnetization transfer (MT) axial images do not reveal any abnormality except for the compressed ventricular system. Abnormal meningeal enhancement is seen on (d) post-contrast T1 weighted MT image.

 


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  		Figure 4. Cryptococcal meningitis (a) T2 weighted axial image shows presence of dilated Virchow Robin spaces (VRS) in bilateral basal ganglia region along with presence of few larger lesions (gelatinous pseudocysts) in bilateral cerebral parenchyma, one of which in the right frontal region shows enhancement on (d) post-contrast T1 weighted magnetization transfer (MT) image suggestive of cryptococcoma. Pre-contrast T1 (b) and T1 weighted MT (c) images do not reveal any meningeal abnormality. Abnormal meningeal enhancement is however noted on post-contrast T1 weighted MT image (d).

 
The percentage difference in the mean signal intensity of the meninges and the adjacent T2 normal brain parenchyma was significantly higher (p<0.05) in the tuberculous group (21.21%±1.98). However in the non-tuberculous group (4.06%±1.01 viral, 4.39±2.39 fungal and 6.15%±2.18 pyogenic) this difference was not statistically significant (p>0.05).


   Discussion
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
Conclusion
 References
 
Meningitis is perhaps the most familiar and most feared among CNS infections. The detection of abnormal meningeal enhancement on MRI is indicative of meningitis in a specific clinical setting. In normal meninges, enhancement is visualized as a thin, markedly discontinuous rim covering the surface of the brain that is typically most prominent parasagittally. The enhancement is primarily in the dura and venous structures. The arachnoid is thin and avascular. However, vascular enhancement of the normal, delicate pia is too subtle to visualize [2]. Thin, linear, well-demarcated and symmetrical enhancement may sometimes be seen in the sulci, due to enhancing veins. Abnormal meningeal enhancement is usually not symmetrical and is usually not as sharply demarcated and more importantly it extends deep into the base of the sulci [19]. Cohen et al [20] reported that when meningeal enhancement was present on more than three contiguous 1.5 T spin-echo MR images, it was highly correlated with substantial intracranial abnormality. Thicker, longer, or more intensely enhancing segments, as well as nodular meningeal enhancement, are abnormal.

Experimental studies by Mathews et al [4] have shown that early infection of the meninges can be demonstrated on contrast-enhanced MRI and that the degree of abnormal meningeal enhancement seen with imaging correlates with the degree of inflammatory cellular infiltration of the meninges. Important factors determining the detection of abnormal meningeal enhancement are the imaging technique and the dose of contrast agent [20].

Post-contrast T1 weighted MT imaging as a technique improves the visualization of normally enhancing structures as well as facilitates the early diagnosis of meningitis [7, 8]. It also shows greater CNS involvement than is apparent on conventional SE images, specifically, at delineating generalized meningeal enhancement as well as focal areas of brain involvement not seen on non-contrast T2 weighted images or conventional post-contrast T1 weighted images [10, 22, 23]. We found that post-contrast T1 weighted MT images are highly sensitive (96%) in the detection of meningitis. Abnormal meningeal enhancement was seen on post-contrast T1 weighted MT images in all patients with tuberculous, pyogenic and cryptococcal meningitis (100% sensitivity). However, it was seen in 5 out of 9 (55% sensitivity) patients with viral encephalitis.

Leptomeninges are usually not visible on pre-contrast conventional SE images. Runge et al [9] found that the inflamed meninges were not visible even on pre-contrast T1 weighted MT images in experimental animals in whom pyogenic meningitis had been introduced and early meningitis could be seen only on post-contrast studies. However, Meyer et al [24] have reported the importance of obtaining pre-contrast T1 weighted MT images. They found that a significant number of patients had increased signal intensity on pre-contrast T1 weighted MT images that was not seen on pre-contrast T1 weighted images without MT. A retrospective study by Gupta et al [10] on the role of MT MRI in CNS tuberculosis found that inflamed meninges in tuberculous meningitis were visible in all patients on pre-contrast T1 weighted MT images. They also found significantly different MTR from thickened meninges in tuberculous meningitis than from the meninges in pyogenic, fungal and viral meningitis.

On prospectively evaluating 100 patients with meningitis, we found that inflamed meninges were visible on pre-contrast T1 weighted MT images as distinct basal periparenchymal hyperintensity in all patients with tuberculous meningitis. These were not or barely visible in the non-tuberculous group. Thus, visibility of meninges on pre-contrast T1 weighted MT images may be considered highly suggestive of tuberculous meningitis. Visibility of the lesion on T2 and T1 weighted MT images depends on the signal intensity difference between the lesion and the surrounding normal parenchyma [25]. Review of the literature does not reveal any definite data regarding the percentage difference in signal intensity between the lesions and surrounding brain parenchyma and its relationship with its visibility on T1 weighted MT images. We found that on T1 weighted MT image, the percentage difference in the mean signal intensity between the meninges and the adjacent (T2 and MT normal) brain parenchyma was found to be significantly higher (>20%) in the tuberculous group compared with that in the non-tuberculous group (<10%) and may explain the difference in visibility.

Kumar et al [26] have also calculated the MTR difference between the surrounding normal brain parenchyma and MT visible lesion to account for the lesion visibility on MT T1 weighted images. We calculated the MTR from the thickened meninges in tuberculous meningitis and found it to be 19.10±1.02, which is similar to that reported in the previous study (19.49±1.22). However, we could not reliably and reproducibly calculate the MTR from the meninges in the non-tuberculous group, which could be due to the fact that the meninges were not much thickened in pyogenic, viral and fungal meningitis and moreover, were more often noted in the cerebral convexity. In tuberculous meningitis, tuberculomas along with exudates are found in the meninges, which are composed of cellular infiltrate, degenerated and partly caseated fibrin, tubercles, and rarely, bacilli. These are probably responsible for the different MTR between brain parenchyma and inflamed meninges, and hence, its visibility on pre-contrast T1 weighted MT images.

Untreated tuberculous meningitis progresses through three stages and is ultimately fatal. The insidious onset and the great variation of presenting signs and symptoms may cause a serious delay in the diagnosis of tuberculous meningitis. It has been suggested that when tuberculous meningitis is suspected on the basis of clinical signs, CSF examination and imaging findings, anti-tuberculous therapy should be initiated without delay [27], as it takes a long time to obtain positive culture results and also, that the microbiological diagnosis may not be possible in many patients [28–32]. The detection of meninges on pre-contrast T1 weighted MT images appears to pinpoint the diagnosis of tuberculous meningitis and may lead to early initiation of treatment and thus, reduction in the morbidity and mortality.


   Conclusion
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusion
References
 
In conclusion, MT MRI is an important technique for the detection and characterization of infectious meningitis of different aetiology. Visibility of the meninges on pre-contrast T1 weighted MT images may be considered to be highly suggestive of tuberculous meningitis.

Received for publication November 13, 2002. Revision received August 20, 2003. Accepted for publication October 20, 2003.


   References
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusion
 References
 

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