INTRODUCTION

Meningiomas are typically diagnosed by their characteristic appearance on conventional imaging. However, 10% to 15% of meningiomas demonstrate an atypical appearance, with rimlike enhancement, cystic components, hemorrhage, marked peritumoral edema, or parenchymal invasion.¹ These meningiomas may resemble malignant intra-axial neoplasms with cystic or necrotic components. Further, other extra-axial pathology, such as metastatic disease or lymphoma, may mimic the appearance of meningiomas. After complete surgical excision, 5-year recurrence rates for benign, atypical, and malignant meningiomas have been reported as 3%, 38%, and 78%, respectively.² For these reasons, advanced noninvasive imaging techniques may provide useful diagnostic information to aid in surgical and treatment planning and in predicting prognosis.³

DIFFUSION-WEIGHTED MAGNETIC RESONANCE IMAGING

Diffusion describes the thermally induced behavior of molecules moving in a random pattern. Diffusion-weighted magnetic resonance imaging (DWI) measures the diffusivity of hydrogen atoms in biologic tissue. The diffusivity is quantified by means of an apparent diffusion coefficient (ADC). Water diffusion is highly dependent on the ratio of extracellular to intracellular space in biologic tissue. DWI has proven utility in acute stroke management, and has been used to evaluate primary brain neoplasms. Studies have shown a correlation between ADC values, tumor cellularity, and tumor grade. Primary brain tumors with higher cellularity or higher grades typically have lower ADC values when compared with lower grade neoplasms.^4–6 Low ADC values in malignant tumors likely reflect an underlying histologic pattern of densely packed tumor cells, thus decreasing the relative fraction of extracellular space and inhibiting the motion, or diffusivity, of water molecules.^7,8

Delineation of meningioma from surrounding tissue on DWI and ADC maps is inferior to that by contrast-enhanced T1-weighted MR imaging.⁹ Meningiomas typically appear isointense to normal white matter on DWI, with ADC values only slightly higher than those of white matter.^4,8 The ADC values of solid gliomas, metastases, and meningiomas are all within a similar range.⁹

Studies analyzing the potential role of DWI in distinguishing typical from atypical/malignant meningiomas have yielded varying results^10–13 (Figs. 15-1 and 15-2). These studies have in common relatively small sample sizes, which may underlie their differing results. A study examining the potential role of DWI in the evaluation of peritumoral edema adjacent to meningiomas found that mean ADC values of peritumoral edema did not differ significantly between typical and atypical meningiomas.¹¹ In addition, ADC values of peritumoral edema do not differ significantly in meningiomas, astrocytomas, or metastatic disease.¹² Thus the role of DWI in the imaging workup of meningiomas is still evolving and should be studied in larger series.

FIGURE 15-1 Meningothelial meningioma (grade 1) arising from the planum sphenoidale. Axial T1-weighted (A) and coronal T2-weighted (B) MR images demonstrate a homogeneous mass isointense to gray matter (arrows) with a significant mass effect on the frontal horns of the lateral ventricles. Vasogenic edema is seen in the left frontal lobe (arrowhead). Avid enhancement is seen on postcontrast T1-weighted imaging (C). On diffusion-weighted imaging (DWI), (D) the lesion is hyperintense despite its low grade. Dynamic-susceptibility perfusion color-scaled map (E) demonstrates profound hypervascularity in the tumor. Signal intensity curves (F) can be plotted from the perfusion maps over time. Area under the curve represents rCBV. The calculated ratio of rCBV within the tumor (green curve) to that of normal appearing white matter (red curve) was approximately 9.0.

FIGURE 15-2 Atypical meningioma (grade II) in a 43-year-old woman. Axial T1-weighted (A) and sagittal T2-weighted (B) MR images demonstrate a parasagittal heterogeneous mass (arrows), arising from the anterior falx cerebri. Significant vasogenic edema is seen in the underlying white matter extending into the corpus callosum (arrowhead). On DWI (C), the lesion demonstrates restricted diffusion at its periphery (arrows). Avid enhancement is seen on post contrast T1-weighted MRI (D). Dynamic-susceptibility perfusion color-scaled map (E) demonstrates hypervascularity in the tumor. The corresponding signal intensity curve plotted over time (F) demonstrates a steep drop in signal when the contrast bolus reaches the tumor (rapid wash-in) with a relatively slow return of the curve to baseline (wash-out). This pattern reflects the typical appearance of meningiomas on angiography, namely a hypervascular tumor blush throughout the arterial phase, persisting well into the late venous phase, with slow washout.

PERFUSION-WEIGHTED MRI

The overall principle of MR perfusion in oncologic imaging is that tumor growth occurs in the setting of increased metabolic demands required by rapid cell growth and increased cell turnover. This, in turn, leads to new blood vessel formation, or angiogenesis, which results in higher blood volume and blood flow in the tumor bed.¹⁴ Given the correlation between microvascular density and cerebral blood volume (rCBV), and the further correlation of microvascular density and tumor grade, higher grade tumors would be expected to have higher rCBV.

The best established method of evaluating tissue perfusion using MR imaging is dynamic-susceptibility imaging, which exploits the T2* susceptibility effects of a paramagnetic contrast agent during its first pass through tissue. Using this method, the loss of T2* signal is proportional to the concentration of gadolinium within a particular tissue. Measurement of rCBV using dynamic contrast-enhanced MR has shown good correlation with tumor grade according to the World Health Organization (WHO) tumor grading scheme.¹⁵

Meningiomas are highly vascular tumors that demonstrate intense contrast enhancement on conventional imaging. Accordingly, meningiomas have elevated rCBV values, which exceed those of high-grade gliomas.^16–18 Attempts to prospectively assess the grade of meningiomas using rCBV values have been unsuccessful¹⁹ (see Figs. 15-1 and 15-2).

Another approach to MR perfusion imaging, arterial spin labeling (ASL), is an endogenous contrast approach in which the patient’s own blood is magnetically labeled with an MR prepulse before the blood’s entry into the imaging volume.²⁰ Because of the use of a purely diffusible tracer, ASL techniques may allow absolute quantification of blood flow unaffected by disruption of the blood–brain barrier (BBB), a common problem using contrast reagent-based techniques for assessing tumor perfusion (Fig. 15-3).

FIGURE 15-3 Meningioma in a 39-year-old woman. Axial T1-weighted (A), T2-weighted (B), and postcontrast T1-weighted (C) MR images depict an avidly enhancing mass (arrows) arising from the planum sphenoidale with extensive “bat-wing” appearing vasogenic edema (arrowheads). Perfusion color-scaled maps using dynamic-susceptibility (D) and arterial spin labeling. (E) techniques demonstrate hypervascularity in the tumor. The corresponding signal intensity curve plotted over time. (F) demonstrates a steep drop in signal when the contrast bolus reaches the tumor (rapid wash-in) with a relatively slow return of the curve to baseline (wash-out). MR spectroscopy (G) demonstrates an inverted doublet peak centered at 1.47 ppm (arrow), corresponding to high alanine content in the tumor.

(E, G courtesy of Michael Lipton, MD, New York, NY.)

Assessment of endothelial permeability using perfusion-weighted magnetic resonance imaging has proven useful in distinguishing meningioma tumor grade. Specifically, atypical meningiomas were found to have a significantly higher degree of degree of permeability than typical meningiomas. It has been hypothesized that the elevated permeability seen in atypical meningiomas may be related to capillary leakiness and the larger size of endothelial gap junctions when compared with typical meningiomas.¹⁹

Occasionally, meningiomas cannot be distinguished from an isolated dural metastasis by conventional imaging. Because meningiomas have high rCBV, a relatively low rCBV in a dural lesion should suggest metastasis rather than meningioma.²¹ On the other hand, hypervascular metastases, such as those from renal cell carcinoma, melanoma, or Merkel carcinoma, may present with elevated rCBV, which may be indistinguishable from that of meningioma.^21,22

PROTON MAGNETIC RESONANCE SPECTROSCOPY

Proton MR spectroscopy (MRS) is a noninvasive imaging technique that gives additional information about the biochemical content of living tissue, which is often a useful adjunct to anatomic imaging findings.^23–25 Spectroscopy data are analyzed as a spectrum. Each spectral peak is characterized by its resonance frequency, height, width, and area. Height or area under the peak may be calculated to give a relative measure of the concentrations of protons. The major brain metabolites detected by proton MRS are choline, creatine, N-acetyl aspartate (NAA), lactate, myoinositol, glutamine and glutamate, lipid, and the amino acids leucine and alanine.²⁶

Using MRS, the majority of meningiomas can be confidently distinguished from other intracranial tumors on the basis of an increased amount of alanine (see Fig. 15-3). In meningiomas, pyruvate kinase is inhibited by L-alanine, resulting in an increased pool of pyruvate, which may then be converted to alanine.²⁷ The alanine peak, defined as an inverted doublet centered at 1.47 ppm, has been found to be relatively specific for meningioma.^28–30 However, in some series, alanine was not found; necrotic areas may show less alanine.^27,31,32 An absence or low quantity of lipid at 0.9 and 1.30 ppm in meningiomas is useful in distinguishing them from gliomas, which characteristically have higher lipid content.^33,34

MRS may also be useful in distinguishing typical from atypical/malignant meningiomas. In a study comparing MRS characteristics of 25 benign meningiomas, WHO grade I, with five meningiomas of WHO grade II and III,³⁵ the mean choline-to-creatine ratio was significantly higher in the nonbenign meningiomas (7.85 ± 3.23) compared with the benign group (2.56 ± 1.26). The same study³⁵ reported a methylene peak at 1.3 ppm, correlating highly with intratumoral necrosis and high tumor grade. An increase in choline concentration, reflected by a large choline peak on MRS, likely reflects increased membrane synthesis or increased cellularity. In gliomas, a correlation between high concentrations of choline and malignant grade has been well documented.^36,37 The presence of necrosis in meningiomas, characterized by the methylene peak, is highly suggestive of malignancy in the absence of prior embolization.³⁸

Increased levels of lactate and lipid, indicating areas of infarction, characterize the MRS of meningiomas treated with embolization.³⁹ In patients undergoing embolization without surgery, long-term follow-up with MRS indicated subsequent fatty degeneration within the tumor.⁴⁰