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Elefante et al. Mini-invasive Surg 2021;5:7 I http://dx.doi.org/10.20517/2574-1225.2020.102 Page 7 of 11

meningioma subtypes. In particular, angiomatous meningioma has demonstrated higher tumor rCBV
[45]
compared with meningothelial, fibrous, or anaplastic subtypes .

Diffusion tensor imaging
Given the possibility to assess magnitude and directionality of water diffusion, diffusion tensor imaging
(DTI) has been applied to differentiate meningioma grades. Although in most studies high-grade
meningiomas have demonstrated low ADC values when compared with low-grade ones, controversial results
have been obtained especially for the other DTI parameters [49-51] . DTI has shown promising potential in
terms of preoperative consistency prediction. Besides some contrasting findings, most studies have shown
higher fractional anisotropy (FA) values in hard meningiomas compared to soft ones [52-54] . Signal intensity
on FA and mean diffusivity maps have also been found to be predictive of meningioma consistency [52,53,55] .
Tractography, derived from DTI data, may give additional information for treatment planning of skull
base meningiomas, but it is usually not necessary: resolving the course of cranial nerves with CSF sensitive
[12]
sequences is technically easier and less sensitive to artifacts .

MR elastography
MR elastography (MRe) is a promising emerging technique that may have the potential to define tumor
consistency and its relationship with adjacent structures. It provides a measurement of tissue stiffness,
determined by the assessment of share wave movement through that given tissue. Recent studies have
demonstrated a significant correlation between the MRe measurements and intraoperative qualitative
[33]
assessment of tumor consistency . Furthermore, differing stiffness on both sides of a tissue boundary
allow defining the measurement of freedom of the adjacent tissue planes, thus evaluating the marginal
[56]
invasiveness .
Molecular imaging
The most used molecular imaging technique in oncological field is 2- [18F] -fluoro-2-deoxy-D-glucose
(18F-FDG)-PET, which uses a glucose analog to identify metabolically active cells, but it does not have a
primary role in intracranial tumors diagnosis due to high physiological FDG uptake in cerebral cortex and
FDG accumulation in inflammatory processes. The ability of FDG-PET to differentiate meningioma grades
has shown contrasting results. Although some studies have demonstrated its ability to differentiate benign
meningioma from atypical/malignant ones and to distinguish recurrent/growing meningiomas from static
ones, there is a lack of correlation between FDG uptake and WHO grading, MIB-1 labeling index, and
[12]
tumor doubling time . On the other hand, a high meningioma-to-background contrast can be obtained
using radiolabeled somatostatin receptors II (SSTR II) ligands due to the increased expression of SSTR II
in meningiomas compared to the very low expression in bone and brain tissue [57,58] . PET with gallium-68-
labeled SSTR-ligands, such as 68Ga-DOTATOC (DOTA-(Tyr3)-octreotid) and 68Ga DOTATATE (DOTA-
DPhe1-Tyr3-octreotate), has demonstrated a higher sensitivity in detecting meningiomas when compared
to contrast enhanced MRI . SSTR-PET is also useful for differential diagnosis, for example when studying
[59]
[60]
optic sheath meningioma . This technique also allows a detailed meningioma extent delineation,
necessary for treatment planning but challenging in the case of complex localization (skull base, orbit, falx
cerebri, sagittal, and cavernous sinuses), trans-osseous growth, or in pre-treated meningiomas, when MR
contrast results are limited [12,61] . Integration of SSTR-PET imaging increases the precision of resection and
target radiation. Furthermore, SSTR-PET can differentiate viable tumor and scar tissue using a semi-qua
ntitative data analysis, since semi-quantitative uptake values (SUV) correlate significantly with SSTR II
expression assessed by immunostaining. Patient treatment stratification can take advantage of SSTR-PET
since SUV measurements have also demonstrated a correlation with tumor growth rate in WHO Grades I
and II meningiomas (not in Grade III). Furthermore, SSTR-PET has been demonstrated to be more specific
for detecting residual meningioma and may be considered in the case of equivocal MRI findings [62-64] .
Recently, the RANO-PET taskforce has proposed an evidence-based recommendation for the use of