Although typically not necessary for the diagnosis of intracranial meningiomas, advanced imaging techniques, including perfusion and diffusion imaging, spectroscopy, and nuclear medicine imaging, can help confirm the diagnosis of intracranial meningiomas, especially for meningiomas that do not exhibit the typical anatomic imaging findings. Advanced imaging techniques also have the potential to be able to differentiate between the subtypes of meningiomas, predict clinical aggressiveness of the tumor, and better characterize response to treatment. Although no advanced imaging technique has been able to definitively subclassify meningiomas, current results are encouraging and may be helpful in surgical planning.
Key points
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Conventional anatomic imaging is typically adequate to distinguish intracranial meningiomas from other tumors, but advanced imaging techniques can help confirm the diagnosis in equivocal cases.
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Meningiomas demonstrate elevated cerebral blood flow/cerebral blood volume on perfusion imaging, with the perfusion characteristics helpful to distinguish it from other intracranial neoplasms, as well as potentially characterizing the subtype of meningioma.
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Different types of meningiomas have been shown to have differences in diffusivity, but more recent larger sample studies have not shown diffusion imaging as a reliable way of differentiating meningioma histopathologic types.
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Spectroscopy provides molecular information with regard to meningiomas and can potentially reflect genetic heterogeneity within a tumor and aid biopsy planning.
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Radionuclides that bind somatostatin receptors can be a sensitive and specific method of characterizing meningiomas.
Background
Meningiomas arise from the arachnoid meningothelial cells. Intracranially, they are extra-axial masses that are typically T1 isointense to slightly hypointense and T2 hyperintense to cortex, demonstrating avid and often homogeneous postcontrast enhancement. Differentiating meningiomas from other intracranial masses typically involve identifying the extra-axial nature of the mass, with evidence of hyperostosis of the adjacent osseous structures, as well as presence of calcification within the mass helpful for confirming the diagnosis of meningioma. The imaging appearance of meningiomas on MRI is typically characteristic enough that routine imaging sequences of T1-weighted and T2-weighted sequences, as well as postcontrast T1-weighted sequences are sufficient in making the diagnosis.
Differentiating different subtypes of typical meningiomas and distinguishing typical from atypical/malignant meningiomas may not be possible from the conventional MRI sequences, however. Also, distinguishing meningiomas from other dural-based extra-axial masses, such as in a patient with a known primary malignancy that has a finding of a dural-based extra-axial mass intracranially, would be difficult with conventional T1-weighted and T2-weighted sequences alone. Distinguishing cystic meningiomas from other rim-enhancing/necrotic neoplasms, such as glioblastoma multiforme, also can be difficult, especially when the mass is large and there is difficulty identifying whether or not the mass is intra-axial or extra-axial.
Diffusion-weighted imaging/diffusion tensor imaging (DWI/DTI), perfusion imaging, and magnetic resonance spectroscopy (MRS) can add additional information that can help refine the diagnostic considerations of a dural-based mass and has the potential to characterize the different subtypes of meningiomas. This article reviews the imaging findings of the techniques as they apply to meningiomas.
Background
Meningiomas arise from the arachnoid meningothelial cells. Intracranially, they are extra-axial masses that are typically T1 isointense to slightly hypointense and T2 hyperintense to cortex, demonstrating avid and often homogeneous postcontrast enhancement. Differentiating meningiomas from other intracranial masses typically involve identifying the extra-axial nature of the mass, with evidence of hyperostosis of the adjacent osseous structures, as well as presence of calcification within the mass helpful for confirming the diagnosis of meningioma. The imaging appearance of meningiomas on MRI is typically characteristic enough that routine imaging sequences of T1-weighted and T2-weighted sequences, as well as postcontrast T1-weighted sequences are sufficient in making the diagnosis.
Differentiating different subtypes of typical meningiomas and distinguishing typical from atypical/malignant meningiomas may not be possible from the conventional MRI sequences, however. Also, distinguishing meningiomas from other dural-based extra-axial masses, such as in a patient with a known primary malignancy that has a finding of a dural-based extra-axial mass intracranially, would be difficult with conventional T1-weighted and T2-weighted sequences alone. Distinguishing cystic meningiomas from other rim-enhancing/necrotic neoplasms, such as glioblastoma multiforme, also can be difficult, especially when the mass is large and there is difficulty identifying whether or not the mass is intra-axial or extra-axial.
Diffusion-weighted imaging/diffusion tensor imaging (DWI/DTI), perfusion imaging, and magnetic resonance spectroscopy (MRS) can add additional information that can help refine the diagnostic considerations of a dural-based mass and has the potential to characterize the different subtypes of meningiomas. This article reviews the imaging findings of the techniques as they apply to meningiomas.
Perfusion imaging
Although routine diagnostic MRI of the brain without and with intravenous contrast is typically able to diagnose a meningioma, it may sometimes be difficult to differentiate a mass as intra-axial or extra-axial in location because of the location or size of the mass. Magnetic resonance perfusion (MR perfusion) can potentially help differentiate a primary glial neoplastic process from an extra-axial mass. MR perfusion can be performed either with a dynamic susceptibility contrast (DSC) technique or a dynamic contrast enhancement (DCE) technique, both of which require the administration of intravenous gadolinium. The two techniques differ in the image acquisition sequence used, with DSC using an echo planar imaging technique and DCE using a gradient echo imaging technique. Both techniques require rapid administration of intravenous gadolinium in a quick bolus, with rapid imaging of the area of interest performed, typically approximately 40 volumes in a 2-minute to 5-minute period.
DSC MR perfusion, the more typically used of the MR perfusion techniques, measures relative cerebral blood volume. Neoplastic processes typically have elevated relative cerebral blood volume compared with the contralateral white matter, and MR perfusion can identify masses with elevated relative cerebral blood volume. The time course of DSC MR perfusion also differs for primarily glial neoplastic processes as compared with intracranial metastases/extra-axial masses. The DSC signal time curve of a primary glial neoplastic process typically demonstrates greater than 50% return to baseline, whereas the DSC signal time curve for metastases/extra-axial masses typically demonstrates less than 50% return to base line due to the increased breakdown in blood-brain barrier as well as the presence of dural-based blood supply for metastases/extra-axial masses.
Fig. 1 illustrates the utility of MR perfusion in characterizing meningiomas. A patient from our institution with a history of multiple meningiomas and resection of a large right frontal meningioma in the past presented for follow-up imaging. On the postcontrast axial T1 sequence (see Fig. 1 A), there was a left frontal dural-based homogeneously enhancing extra-axial mass compatible with a meningioma, along with a smaller meningioma along the falx. MR perfusion was performed ( Fig. 2 B ), with a region of interest (ROI) placed on the normal, contralateral white matter (ROI #1, green) and another placed on the left frontal meningioma (ROI #2, purple). The contrast enhancement time curve of the left frontal meningioma (curve #2, purple) demonstrates elevated relative cerebral blood volume when compared with the contrast-enhancement time curve of the contralateral white matter (curve #1, green), with the contrast-enhancement time curve of the meningioma demonstrating less than 50% return to baseline, which is the typical perfusion behavior of meningiomas as described in the literature ( Fig. 2 C).
MR perfusion also has been used to differentiate subtypes of meningiomas as well as differentiating typical from atypical meningiomas. Angiomatous meningiomas have been shown to have significantly higher tumoral relative cerebral blood volume compared with meningothelial, fibrous, or anaplastic meningiomas, and anaplastic meningiomas have higher peritumoral relative cerebral blood volume compared with the other types of meningiomas. Peritumoral edema surrounding malignant meningiomas (World Health Organization [WHO] grade III) also has been shown to have increased relative cerebral blood volume compared with benign meningiomas (WHO grade I) via MR perfusion imaging. The volume transfer constant, K trans , which is a measurement of vascular permeability, for atypical meningiomas (WHO grade II) has been shown to be higher than that for typical (benign) meningiomas.
MR perfusion also can be performed to evaluate the contributions of the blood supply to the meningioma. Intra-arterial injection of gadolinium via catheter selection of the internal and external carotid arteries, in combination with intraoperative MR perfusion, can differentiate which portions of the tumors are supplied by which arterial supply. MR perfusion also can be performed after the embolization to assess for residual perfusion to the treated meningioma.
Arterial spin labeling (ASL) is an MRI technique in which a radiofrequency pulse is applied to the arteries proximal to the ROI in such a way that the protons in the inflowing blood have a detectable signal. Cerebral blood flow can therefore be calculated in the ROI as a function of increased amount of “tagged” blood that has flowed into the ROI. ASL does not require the use of intravenous gadolinium, and therefore has the advantage of being usable even in patients with impaired renal function who otherwise may not be able to receive intravenous gadolinium. ASL has been shown to be able to detect increased cerebral blood flow within meningiomas, with the technique also demonstrating statistically significant increased cerebral blood flow in angiomatous meningiomas compared with fibrous and meningothelial meningiomas (see Fig. 2 B).
Although not typically used to diagnose meningiomas, computed tomography (CT) perfusion, like ASL, can more conspicuously identify intracranial meningiomas due to the increased cerebral blood flow and cerebral blood volume within the meningiomas ( Fig. 3 ). Although the meningioma may be less distinct in a noncontrast head CT, blending into the adjacent brain parenchyma (see Fig. 3 A), CT perfusion, performed due to suspicion of stroke in this case, demonstrates a left frontal mass with elevated cerebral blood volume (see Fig. 3 B) as well as elevated cerebral blood flow (see Fig. 3 C) when compared with the adjacent as well as contralateral brain parenchyma. A contrast-enhanced head CT also clearly delineates the left frontal meningioma from the brain parenchyma (see Fig. 3 D).
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