CHAPTER 14 MRI Evaluation of Meningiomas

INTRODUCTION

Magnetic resonance imaging (MRI) is the state of art in the preoperative diagnosis of meningiomas. The diagnostic accuracy of standard MRI is approximately 95%. Further, contrast-enhanced MRI has the highest ability to detect and characterize meningiomas radiologically.¹ This high rate of diagnostic accuracy can be attributed to special properties of the MRI technique presented in this chapter that provide vast information on meningiomas.

ROUTINE MRI SEQUENCES

Conventional MR imaging sequences are usually standard in many centers. Multiplanar T1- and T2-weighted images (WI), and postcontrast T1-WI with 5 mm section thickness usually provide sufficient information to make a definitive radiologic diagnosis and a preoperative morphologic evaluation. However, meningiomas in special localizations, such as in the parasellar region, or in the pontocerebellar angle cistern, need to be studied with higher spatial resolution. In these specific localizations, 2 to 3 mm section thickness with more in-plane resolution than in routine brain imaging not only provides precise information on localization of the meningioma, but also expedites the definition of anatomic neighborhood including the relationship to vascular structures and cranial nerves.

Spin-echo (SE) is still the preferable sequence to obtain both precontrast and postcontrast T1-WI, despite relatively poor performance in the post-fossa on contrast-enhanced studies. However, turbo spin echo (TSE) or fast spin echo (FSE) has been the preferred technique for T2-WI. FLAIR, which is part of routine brain MRI, is a heavily T2-WI sequence in which free water is suppressed to highlight meningiomas near the brain surface or in the ventricles. Postcontrast high spatial resolution 3D GRE T1-WI could be implemented in the routine imaging protocol to take advantage of multiplanar and real-time postprocessing and to acquire artefact-free images, especially in the posterior fossa. In this sequence, the relationship of the meningioma to vascular structures is also better demonstrated.

Fat-saturation techniques after contrast enhancement are used to delineate meningiomas from surrounding fatty tissues such as the orbit and the medullary bone. It is also very useful to evaluate the fat content in the tumor if the meningioma displays hyperintensity on precontrast routine T1-WI.

Gradient-echo T2* (GRE T2*), due to its inherent ability to demonstrate susceptibility, could be added to the routine protocol to evaluate calcification and hemorrhage because the distinction between these two is not possible on routine sequences. Moreover, the recently described susceptibility weighted imaging (SWI) sequence, containing both phase and magnitude information of a 3D GRE sequence, is very sensitive to hemorrhage which could be a useful marker to differentiate it from calcification.

MRI OF THE DURA MATER RELATED TO MENINGIOMAS

Meningiomas arise from the meningothelial cells of the arachnoid membranes, which are attached to the inner layer of the dura mater. This close relationship between the meningioma and the dura mater can be demonstrated on MR imaging. Normal dura mater is a non-neuronal connective tissue and appears as a hypointense thin layer adjacent to the hypointense inner table of bone on T1- and T2-WI (Fig. 14-1). After contrast injection, dura and the dural reflections of falx cerebri, tentorium, and cavernous sinus enhance as a thin discontinuous layer (Fig. 14-2).² The demonstration of the dura mater and the adjacent meningioma on MRI supplies information about the extra-axial location and the origin of meningioma. In this respect, MRI is advantageous over computed tomography (CT) imaging, in which bone and enhanced dura both appear as hyperdense structures and cannot be distinguished from each other. The sensitivity of MRI is two to three times higher than that of CT for imaging dural and meningeal pathologies.³ Dural attachment of a meningioma is usually broad and the angle between the tumor and the dura is an obtuse one, indicating the extra-axial location of the meningioma (Figs. 14-2 and 14-3). Rarely the dural attachment may be narrow and a pedunculated meningioma can be demonstrated, more so in cases with large subarachnoid spaces where the peduncle becomes more conspicuous.

FIGURE 14-1 On contrast-enhanced (CE) T1-WI, the dura mater appears as a hypointense line compared to the brightly enhancing meningioma as demonstrated in a cavernous sinus meningioma (A) and a petroclival meningioma (B).

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

FIGURE 14-2 Meningiomas show a broad based attachment to the underlying dura, which is seen as a thin enhancing line adjacent to the tumor, along the brain surface and dural duplications like the tentorium. (Images in A and B are coronal; C is sagittal CE T1-WI.)

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

FIGURE 14-3 Meningiomas present an obtuse angle between the tumor and the underling dura mater which reflects their broad dural base and extra-axial localization. (Image in A is an axial and B and C are sagittal CE T1-WI.)

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

Pathologic enhancement of the dura mater and the adjacent inner table of bone, known as pachymeningeal enhancement, may be due to abnormally increased capillary permeability, increased blood volume, edema, tumor invasion of the dura mater, or as a result of surgery.² The local pachymeningeal enhancement adjacent to a meningioma which tapers smoothly away from the tumor is designated as a dural “tail” (Fig. 14-4). The dural “tail,” which is depicted radiologically only on MRI, is reported in 52% to 72% of meningiomas. Nevertheless, it is nonspecific, and has also been associated with lymphomas, sarcoidosis, schwannomas, metastatic tumors, syphilitic gumma, and hemangiopericytomas.⁴^–¹⁰ A dural “tail” may also be demonstrated, without contrast injection, on MR FLAIR sequences. Both meningiomas and the dural “tail” are demonstrated on FLAIR sequences, yet the dural “tail” usually has higher signal intensity than the tumor.¹¹

FIGURE 14-4 Meningiomas classically present a dural “tail” and this is observed in various localizations in the brain. CE T1-W images of sphenoid wing (A), foramen magnum (B), convexity (C, D, E), sphenopetroclival (F), and petroclival (G) meningiomas in this figure all have an enhancing prominent dural tail.

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

MRI FINDINGS SUPPORTING EXTRA-AXIAL LOCALIZATION OF MENINGIOMAS

The extra-axial localization of a mass is an important diagnostic finding, albeit it is not always simple to appreciate this finding, more so when a dural relation cannot be detected (Fig. 14-5). A cerebrospinal fluid (CSF) interface between a mass and the brain surface is another finding supporting the extra-axial localization (Fig. 14-6). Occasionally the subarachnoid space around a meningioma may enlarge and acquire the form of a cyst. Another finding reflecting the extra-axial localization of a meningioma is the “buckling” or compression and displacement of adjacent brain cortex and the white matter (Fig. 14-7).

FIGURE 14-5 Demonstration of the dural relation can aid in the anatomic classification of the meningioma. Although the exact origin of the meningioma cannot be distinguished on axial contrast enhanced T1-WI (A), the demonstration of a dural tail near its dural attachment on the hyperostotic anterior clinoid process on coronal images (arrow in B) clearly indicate that this is a clinoidal meningioma.

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

FIGURE 14-6 The cerebrospinal fluid ring surrounding meningiomas indicates their extra-axial localization. The clinoidal (A) and convexity (B, C, D) meningiomas demonstrated in this figure all present a CSF rim that appears hyperintense on T2-WI (A, B), hypointense on T1-WI (C), and FLAIR images (D).

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

FIGURE 14-7 Buckling of the surrounding cortex is another indication of extra-axial localization. This is well demonstrated on three convexity meningiomas. Please note the peritumoral edema as demonstrated by the hyperintensity in the FLAİR image (A), the peritumoral CSF cleft as demonstrated on T1-WI of a convexity meningioma (arrow in B), and another conveity meningioma (arrows in C and D).

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

BORDER AND CONTOUR OF MENINGIOMAS

The borders and contours of meningiomas are usually regular and the margins are sharp as a result of the extra-axial location of the tumor which is surrounded by a CSF ring (Fig. 14-8). Despite the extra-axial location, meningiomas may indent into the adjacent brain cortex and so exhibit irregular margins. A meningioma that has multiple lobulated outgrowths on its contour is designated as “mushrooming” meningioma (Fig. 14-9).

FIGURE 14-8 Meningiomas are extra-axial tumors, and therefore are demarcated from the surrounding brain tissue usually with clear, sharp, and regular margins, as demonstrated in this T2-WI of a parasagittal meningioma.

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

FIGURE 14-9 Irregular margins with nodular outgrowths, which is known as “mushrooming,” may be observed in meningiomas as demonstrated in these CE T1-WI of a giant sphenoid wing (A, B, C) and a tuberculum sella meningioma (D).

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

CONTRAST ENHANCEMENT OF MENINGIOMAS ON MRI

MRI contrast agents are gadolinium chelates that shorten the T1 relaxation times in certain tissues to increase signal intensity on T1-WI and produce contrast enhancement that improves the sensitivity of MRI for residual, recurrent, and multiple meningiomas.¹² On early MR studies, some small meningiomas with signal intensities close to that of the brain cortex on T1- and T2-WI were not recognized if contrast medium was not used.¹ Routine use of contrast media is necessary to increase the sensitivity for meningioma detection. Meningiomas that are homogeneous on T1-WI and T2-WI enhance in general intensely and in a uniform manner (Fig. 14-10). Calcified, cystic, necrotic, and hemorrhagic meningiomas enhance less intensely, and more heterogeneously (Fig. 14-11). No correlation was established between contrast enhancement and the histopathologic features of meningiomas.¹³ Contrast media are among strong radiodiagnostic tools; however, caution is recommended in patients with poor kidney functions, as cases of nephrogenic systemic fibrosis have been related to some ingredients of MR contrast agents.

FIGURE 14-10 In general, meningiomas enhance homogeneously and intensely after contrast medium injection on T1-WI, as seen in the case of a petroclival meningioma. Note that the dural tail also enhances brightly (arrow).

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

FIGURE 14-11 Intratumoral calcification and vasculature may cause heterogeneous enhancement in meningiomas. CE T1-WI of the olfactory groove meningioma demonstrated in A and B revealed a heterogenous signal inside the tumor due to both calcification and prominent intratumoral vasculature. The MRI signal was also heterogeneous in this giant sphenoid wing meningioma due to intratumoral vessels (C, D). Also microcystic-syncytial meningiomas may enhance heterogeneously as demonstrated in E and F.

(Courtesy of Canan Erzan, MD, Marmara University School of Medicine, Istanbul, Turkey.)

EDEMA RELATED TO MENINGIOMAS ON MRI

Despite their extra-axial location, meningiomas can cause mild to extensive vasogenic white-mater edema in up to 75% of cases.¹⁴ Calcified meningiomas exhibit less vasogenic edema.¹⁵ Many factors were associated with the development of edema. These include shape of meningioma, mechanical injury to the adjacent brain tissue, compression of nearby vasculature, angiogenic factors, large surface area of the tumor, invasive pattern of brain–tumor interface and hyperintensity of meningioma on T2-WI.¹⁴^–¹⁶ Nevertheless, the exact cause of edema is still unknown. Newer techniques such as diffusion tensor imaging may provide additional information. Vasogenic edema in the white matter presents in the form of “finger edema” and is hypointense on T1-WI, and of hyperintense and of homogeneous signal intensity on FLAIR and T2-WI (Fig. 14-12). Small and isointense meningiomas amid edema can be depicted only after contrast medium administration.