**Spatial Relationships of MR Imaging and Positron Emission Tomography with Phenotype, Genotype and Tumor Stem Cell Generation in Glioblastoma Multiforme**

Davide Schiffer, Consuelo Valentini, Antonio Melcarne, Marta Mellai, Elena Prodi, Giovanna Carrara, Tetyana Denysenko, Carola Junemann, Cristina Casalone, Cristiano Corona, Valentina Caldera, Laura Annovazzi, Angela Piazzi, Paola Cassoni, Rebecca Senetta, Piercarlo Fania and Angelina Cistaro

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/58391

**1. Introduction**

Glioblastoma multiforme (GBM), the most malignant and frequent glioma, is a heterogene‐ ous tumor in which areas of different histological aspect, aggressiveness, genetic expres‐ sion and regressive events coexist so that one region of the tumor is not representative of the entire neoplasia. The consequences of the heterogeneity reflect on the diagnostics, prognostics and therapies. As a matter of fact, unguided surgical biopsies can lead to sampling error and to undergrade the tumor up to 30% of cases [1]. From the surgical, but also prognostic and therapeutic point of view, it is of great importance to know in advance the composition of the tumor and the biological significance of the different imaging aspects. Neuro-imaging is the only and fundamental source of information for the neurosurgeon and it has progressed today from the simple anatomic recognition of the tumor to that of functional and metabolic significance of its different regions, contributing greatly to a better approach to tumor surgery, prognosis and therapy. The detection of highly malignant regions and the definition of the tumor extent are crucial before the operation, when they are the main concern of neurosurgeons.

GBM is composed, as it is universally known, of three zones: central necrosis, proliferation and infiltration zones (Figures 1,2). Proliferation region is characterized by high indices of cell

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

density, proliferation, mitoses, vessel density or angiogenesis and circumscribed necroses. In the spectrum of the many aspects of the tumor, with the term disruption one indicates the passage from the uniform and quiescent appearance of an astrocytoma to the rupture of the structure, forms and dimensions of GBM (Figure 3A). Circumscribed necroses and angiogen‐ esis are the absolute features of GBM and their occurrence is needed for its recognition, because they are direct signs of malignancy (Figures 3B,4A-C). Angiogenesis in gliomas represents the intervened transformation, whereas it depends on the imbalance between the high prolifera‐ tion potential of tumor cells and the low reproduction capacity of endothelial cells [2]. When the diagnosis has to be carried out on small tumor samples, as for example in stereotactic biopsies, the diagnosis cannot be of certainty. When close to central necrosis, circumscribed necroses merge with it. Infiltration zone represents the invasion into the brain of tumor cells that acquire a particular phenotypic and molecular signature. It is not uniform along the tumor borders and often it is so mild that it is hardly detectable, also histologically. Frequently, it is discovered in histological sections only after counting the cells and this happens either when it affects the white matter or the cortex, where tumor cells must be distinguished from normal cells. In the latter, perineuronal satellitosis may be of help. Isolated tumor cells (ITCs) in the normal nervous tissue make the problem of the tumor delimitation very hard. They cannot be detected, of course, in the samples removed during intervention, but only in the study of the brain at autopsy and they can be found very far from the tumor borders; the classic example is the passage of normally looking corpus callosum by ITCs [3,4]. Regressive events are frequent and include haemorrhages, large necroses, vascular thrombosis, *etc* and they con‐ tribute to the so-called disruption of the tissue.

**Figure 2.** Three zones can be recognized: central necrosis, proliferation, and infiltration zone. H&E, 25x.

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**Figure 3.** A – Area of disrupture with high cell density, vessels of different size and edema dissociation of the tissue; B – Circumscribed necrosis with pseudo-palisading in an area with high cell density (H&E, 100x) and area with a high

Beside the classic T1 and T2 imaging of GBM, supplied by the anatomy based magnetic resonance (MR), physiology-based MR imaging methods, namely diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI) and proton MR spectroscopy imaging (MRSI), together with the positron emission tomography (PET), which is highly correlated with the degree of malignancy [5,6], improved the tumor characterization. Today, the advancement of the knowledge in molecular biology and cell biology, associated with new surgical procedures, radiation techniques and therapeutic possibilities ask the neuro-imaging to answer three main questions: the identification *in vivo* of the tumor sites with the highest malignancy grade, the

Ki-67/MIB.1 proliferation index (DAB, 100x).

**Figure 1.** Coronal section of a brain with GBM. The borders of the tumor show different nervous structures. H&E.

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**Figure 2.** Three zones can be recognized: central necrosis, proliferation, and infiltration zone. H&E, 25x.

density, proliferation, mitoses, vessel density or angiogenesis and circumscribed necroses. In the spectrum of the many aspects of the tumor, with the term disruption one indicates the passage from the uniform and quiescent appearance of an astrocytoma to the rupture of the structure, forms and dimensions of GBM (Figure 3A). Circumscribed necroses and angiogen‐ esis are the absolute features of GBM and their occurrence is needed for its recognition, because they are direct signs of malignancy (Figures 3B,4A-C). Angiogenesis in gliomas represents the intervened transformation, whereas it depends on the imbalance between the high prolifera‐ tion potential of tumor cells and the low reproduction capacity of endothelial cells [2]. When the diagnosis has to be carried out on small tumor samples, as for example in stereotactic biopsies, the diagnosis cannot be of certainty. When close to central necrosis, circumscribed necroses merge with it. Infiltration zone represents the invasion into the brain of tumor cells that acquire a particular phenotypic and molecular signature. It is not uniform along the tumor borders and often it is so mild that it is hardly detectable, also histologically. Frequently, it is discovered in histological sections only after counting the cells and this happens either when it affects the white matter or the cortex, where tumor cells must be distinguished from normal cells. In the latter, perineuronal satellitosis may be of help. Isolated tumor cells (ITCs) in the normal nervous tissue make the problem of the tumor delimitation very hard. They cannot be detected, of course, in the samples removed during intervention, but only in the study of the brain at autopsy and they can be found very far from the tumor borders; the classic example is the passage of normally looking corpus callosum by ITCs [3,4]. Regressive events are frequent and include haemorrhages, large necroses, vascular thrombosis, *etc* and they con‐

**Figure 1.** Coronal section of a brain with GBM. The borders of the tumor show different nervous structures. H&E.

tribute to the so-called disruption of the tissue.

64 Tumors of the Central Nervous System – Primary and Secondary

**Figure 3.** A – Area of disrupture with high cell density, vessels of different size and edema dissociation of the tissue; B – Circumscribed necrosis with pseudo-palisading in an area with high cell density (H&E, 100x) and area with a high Ki-67/MIB.1 proliferation index (DAB, 100x).

Beside the classic T1 and T2 imaging of GBM, supplied by the anatomy based magnetic resonance (MR), physiology-based MR imaging methods, namely diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI) and proton MR spectroscopy imaging (MRSI), together with the positron emission tomography (PET), which is highly correlated with the degree of malignancy [5,6], improved the tumor characterization. Today, the advancement of the knowledge in molecular biology and cell biology, associated with new surgical procedures, radiation techniques and therapeutic possibilities ask the neuro-imaging to answer three main questions: the identification *in vivo* of the tumor sites with the highest malignancy grade, the extension of tumor invasion and the sites where the capacity of the tumor to reproduce, to recur and to resist therapies resides, *i.e.* where the so-called glioblastoma stem cells (GSCs) are located.

Although several reports have shown that glioma grade inversely correlates with intra-tumor minimum ADC [7], reflecting the presence of areas with high cell density in high grade tumors [8,9], the clinical significance of ADC measurement is limited as a consequence of the tissue heterogeneity within a tumor and because of substantial overlap in ADC values among different grades of glioma [10,11]. The range of ADC values within a given glioma, therefore, can vary markedly [11] and there is no final confirmation that minimum ADC always correlates

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PWI is used to measure vascularization and perfusion of brain lesions. Different PWI techniques are available, namely dynamic susceptibility contrast (DSC) and dynamic contrast-enhanced (DCE), widely used in the clinical setting. DSC perfusion measures T2 weighted signal-intensity loss occurring dynamically over a bolus injection of contrast medium, from which relative cerebral blood volume (rCBV), a marker of tumor angiogen‐ esis, can be computed. DCE is a T1-weighted sequence that measures vascular permeabili‐ ty in tumors during a bolus injection of contrast medium; rCBV values are then calculated from DCE data. rCBV values have shown good correlation with the World Health Organization (WHO) tumor grading [12,13]; exceptions are represented by low-grade glial neoplasms with oligodendroglial features and grade I pilocytic astrocytoma, that may have

with cell density.

markedly elevated rCBV (Figure 5).

**Figure 5.** A –T1C MRI; B – T2 MRI; C – Diffusion MRI; D – Perfusion MRI.

**Figure 4.** A – High vessel density. Laminin, DAB, 100x; B – Neoformed vessels and endothelial buds. PAS, 200x; C – Glomeruloid structure. α-sm-actin, DAB, 200x.

### **2. Physiology based MRI and PET**

DWI rationale is to quantify the brownian movement of water protons within tissues that depends on the complex interaction between the intracellular and extracellular compartments, but also on cell density, cell membrane permeability and tissue structure. Water diffusivity within the extracellular compartment is inversely related to cell size and cell number. The greater the volume of the intracellular space and also the higher the cell density, the lower is the water diffusivity in the extracellular space, resulting in a low apparent diffusion coefficient (ADC), a measure of water diffusion. Diffusivity within tumors is heterogeneous due to different tumor components, being reduced in areas with high cellular density and increased in necrotic regions. Restricted ADC values in a tumor can also be related to ischemic changes, haemorragic or calcific components.

Although several reports have shown that glioma grade inversely correlates with intra-tumor minimum ADC [7], reflecting the presence of areas with high cell density in high grade tumors [8,9], the clinical significance of ADC measurement is limited as a consequence of the tissue heterogeneity within a tumor and because of substantial overlap in ADC values among different grades of glioma [10,11]. The range of ADC values within a given glioma, therefore, can vary markedly [11] and there is no final confirmation that minimum ADC always correlates with cell density.

extension of tumor invasion and the sites where the capacity of the tumor to reproduce, to recur and to resist therapies resides, *i.e.* where the so-called glioblastoma stem cells (GSCs) are

**Figure 4.** A – High vessel density. Laminin, DAB, 100x; B – Neoformed vessels and endothelial buds. PAS, 200x; C –

DWI rationale is to quantify the brownian movement of water protons within tissues that depends on the complex interaction between the intracellular and extracellular compartments, but also on cell density, cell membrane permeability and tissue structure. Water diffusivity within the extracellular compartment is inversely related to cell size and cell number. The greater the volume of the intracellular space and also the higher the cell density, the lower is the water diffusivity in the extracellular space, resulting in a low apparent diffusion coefficient (ADC), a measure of water diffusion. Diffusivity within tumors is heterogeneous due to different tumor components, being reduced in areas with high cellular density and increased in necrotic regions. Restricted ADC values in a tumor can also be related to ischemic changes,

Glomeruloid structure. α-sm-actin, DAB, 200x.

haemorragic or calcific components.

**2. Physiology based MRI and PET**

located.

66 Tumors of the Central Nervous System – Primary and Secondary

PWI is used to measure vascularization and perfusion of brain lesions. Different PWI techniques are available, namely dynamic susceptibility contrast (DSC) and dynamic contrast-enhanced (DCE), widely used in the clinical setting. DSC perfusion measures T2 weighted signal-intensity loss occurring dynamically over a bolus injection of contrast medium, from which relative cerebral blood volume (rCBV), a marker of tumor angiogen‐ esis, can be computed. DCE is a T1-weighted sequence that measures vascular permeabili‐ ty in tumors during a bolus injection of contrast medium; rCBV values are then calculated from DCE data. rCBV values have shown good correlation with the World Health Organization (WHO) tumor grading [12,13]; exceptions are represented by low-grade glial neoplasms with oligodendroglial features and grade I pilocytic astrocytoma, that may have markedly elevated rCBV (Figure 5).

**Figure 5.** A –T1C MRI; B – T2 MRI; C – Diffusion MRI; D – Perfusion MRI.

Arterial spin-labeling (ASL) is a more recent perfusion technique that uses water of the blood entering the brain as an endogenous tracer to evaluate perfusion. ASL is emerging as an alternative to gadolinium based techniques in the evaluation of tumor perfusion.

MRSI is another advanced technique that provides metabolic information of the brain tissue. The predominant metabolites are choline (Cho), N-acetylaspartate (NAA), creatine (Cr), glutamate and glutamine (Glx), myo-inositol (MI) and lactate/lipids (LL). The Cho peak contains contributions from several different choline-containing compounds, which are involved in membrane synthesis and degradation; NAA is marker of neuronal integrity; Cr is a marker of cellular energetics; MI is considered a glial cell marker; LL are markers of tissue breakdown and anaerobic glycolysis. Glx is a complex peak from glutamate (Glu), glutamine (Gln) and gamma-aminobutyric acid (GABA). Glu is an important excitatory neurotransmitter and it also plays a role in the redox cycle. In brain tumors, as malignancy increases, NAA signal decreases, as a consequence of loss, dysfunction or displacement of normal neurons, while Cho levels increase as a consequence of rapid cell membrane turnover. Malignant tumors also have reduced Cr due to high metabolic activity that depletes the energy stores; this is associated to anaerobic glycolysis leading to the appearance of lactate. Necrotic portions of tumor show the presence of lipid peaks. Elevated concentration of Gln can be found in high grade tumors.

Metabolite concentrations are usually expressed as ratios (*i.e.* Cho/Cr, Cho/NAA, NAA/Cr) rather than as absolute concentrations.

**Figure 7.** Case CTO5. Correlation among histopathology, Ki-67/MIB.1 proliferation marker, MRSI, physiologic MRI and PET values. Column 1 – ROIs on T1C MRI; Column 2 – Histopathology of a hyper-proliferating area and two areas dif‐ ferently infiltrated. H&E, 200x; Column 3 – Ki-67/MIB.1 proliferation index, DAB, 200x; Column 4 – MRSI values; Col‐

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Diffusion tensor imaging (DTI) is an advanced MRI technique that describes the movement of water molecules using two metrics, mean diffusivity (MD) and fractional anisotropy (FA), that represent the magnitude and directionality of water diffusion, respectively. FA technique measures the preferential direction of proton movement and varies among values between 0 (isotropic diffusion, *i.e.* random diffusion such as in brain gray matter) and 1 (anisotropic diffusion, such as in brain white matter where proton diffusion is constrained along myelin fibres). MD technique gives information on the whole diffusivity in the brain; the reduction of nervous fibres results in an increased MD because of a higher degree of freedom of movement of water molecules. The degree of anisotropy depends on many factors, such as fibre density and diameter, myelin sheath integrity and the intercellular space characteristics. In the presence of a structural alteration of the nervous fibre tract, the anisotropic value reduces.

Anisotropy is reduced in cerebral lesions due to the loss of structural organisation. The measurement of FA allows prediction of histological characteristics such as cellularity, vascularity, or fibre structure. This technique is useful to differentiate normal white matter from edematous brain tissue and occult white matter invasion around the enhancing portion

DTI may help to determine the white matter fibre displacement by tumor. This technique, in combination with functional neuro-imaging methods, permits to map the individual anatomo-

PET is currently the most powerful method of molecular imaging, as it has been emphasized in a recent review [19] (Figure 8). Depending on the radiotracer, various molecular processes can be visualized by PET, most of them relating to an increased cell proliferation within

functional connectivity and represents a useful tool for surgical planning [16-18].

umn 5 – PET values.

of the tumor.

Such spectra can be obtained using single voxel or multi-voxel 2D or 3D technique. Multi-voxel spectroscopy is the best to detect infiltration of malignant cells beyond the enhancing margins of tumors [14,15] (Figures 6,7).

**Figure 6.** Case CTO3. Correlation among histopathology, Ki-67/MIB.1 proliferation marker, MRSI, physiologic MRI and PET values. Column 1 – ROIs on T1C MRI; Column 2 – Histopathology of a hyper-proliferating area and two areas dif‐ ferently infiltrated. H&E, 200x; Column 3 – Ki-67/MIB.1 proliferation index, DAB, 200x; Column 4 – MRSI values; Col‐ umn 5 – PET values.

Spatial Relationships of MR Imaging and Positron Emission Tomography with Phenotype, Genotype and... http://dx.doi.org/10.5772/58391 69

Arterial spin-labeling (ASL) is a more recent perfusion technique that uses water of the blood entering the brain as an endogenous tracer to evaluate perfusion. ASL is emerging as an

MRSI is another advanced technique that provides metabolic information of the brain tissue. The predominant metabolites are choline (Cho), N-acetylaspartate (NAA), creatine (Cr), glutamate and glutamine (Glx), myo-inositol (MI) and lactate/lipids (LL). The Cho peak contains contributions from several different choline-containing compounds, which are involved in membrane synthesis and degradation; NAA is marker of neuronal integrity; Cr is a marker of cellular energetics; MI is considered a glial cell marker; LL are markers of tissue breakdown and anaerobic glycolysis. Glx is a complex peak from glutamate (Glu), glutamine (Gln) and gamma-aminobutyric acid (GABA). Glu is an important excitatory neurotransmitter and it also plays a role in the redox cycle. In brain tumors, as malignancy increases, NAA signal decreases, as a consequence of loss, dysfunction or displacement of normal neurons, while Cho levels increase as a consequence of rapid cell membrane turnover. Malignant tumors also have reduced Cr due to high metabolic activity that depletes the energy stores; this is associated to anaerobic glycolysis leading to the appearance of lactate. Necrotic portions of tumor show the presence of lipid peaks. Elevated concentration of Gln can be found in high grade tumors.

Metabolite concentrations are usually expressed as ratios (*i.e.* Cho/Cr, Cho/NAA, NAA/Cr)

Such spectra can be obtained using single voxel or multi-voxel 2D or 3D technique. Multi-voxel spectroscopy is the best to detect infiltration of malignant cells beyond the enhancing margins

**Figure 6.** Case CTO3. Correlation among histopathology, Ki-67/MIB.1 proliferation marker, MRSI, physiologic MRI and PET values. Column 1 – ROIs on T1C MRI; Column 2 – Histopathology of a hyper-proliferating area and two areas dif‐ ferently infiltrated. H&E, 200x; Column 3 – Ki-67/MIB.1 proliferation index, DAB, 200x; Column 4 – MRSI values; Col‐

rather than as absolute concentrations.

68 Tumors of the Central Nervous System – Primary and Secondary

of tumors [14,15] (Figures 6,7).

umn 5 – PET values.

alternative to gadolinium based techniques in the evaluation of tumor perfusion.

**Figure 7.** Case CTO5. Correlation among histopathology, Ki-67/MIB.1 proliferation marker, MRSI, physiologic MRI and PET values. Column 1 – ROIs on T1C MRI; Column 2 – Histopathology of a hyper-proliferating area and two areas dif‐ ferently infiltrated. H&E, 200x; Column 3 – Ki-67/MIB.1 proliferation index, DAB, 200x; Column 4 – MRSI values; Col‐ umn 5 – PET values.

Diffusion tensor imaging (DTI) is an advanced MRI technique that describes the movement of water molecules using two metrics, mean diffusivity (MD) and fractional anisotropy (FA), that represent the magnitude and directionality of water diffusion, respectively. FA technique measures the preferential direction of proton movement and varies among values between 0 (isotropic diffusion, *i.e.* random diffusion such as in brain gray matter) and 1 (anisotropic diffusion, such as in brain white matter where proton diffusion is constrained along myelin fibres). MD technique gives information on the whole diffusivity in the brain; the reduction of nervous fibres results in an increased MD because of a higher degree of freedom of movement of water molecules. The degree of anisotropy depends on many factors, such as fibre density and diameter, myelin sheath integrity and the intercellular space characteristics. In the presence of a structural alteration of the nervous fibre tract, the anisotropic value reduces.

Anisotropy is reduced in cerebral lesions due to the loss of structural organisation. The measurement of FA allows prediction of histological characteristics such as cellularity, vascularity, or fibre structure. This technique is useful to differentiate normal white matter from edematous brain tissue and occult white matter invasion around the enhancing portion of the tumor.

DTI may help to determine the white matter fibre displacement by tumor. This technique, in combination with functional neuro-imaging methods, permits to map the individual anatomofunctional connectivity and represents a useful tool for surgical planning [16-18].

PET is currently the most powerful method of molecular imaging, as it has been emphasized in a recent review [19] (Figure 8). Depending on the radiotracer, various molecular processes can be visualized by PET, most of them relating to an increased cell proliferation within gliomas. Radiolabeled 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG), methyl-[11C]-L-methio‐ nine ([11C]MET) and 3-deoxy-3-[18F]fluoro-L-thymidine ([18F]FLT) are taken up by prolifer‐ ating gliomas depending on their tumor grade as the consequence of an increased activity of membrane transporters for glucose ([18F]FDG), amino acids ([11C]MET), and nucleosides ([18F]FLT) as well as increased expression of cellular hexokinase ([18F]FDG) and thymidine kinase ([18F]FLT) genes, which specifically phosphorylate [18F]FDG and [18F]FLT, respec‐ tively [20]. Imaging of brain tumors with [18F]FDG was the first oncologic application of PET. [18F]FDG is actively transported across the blood-brain barrier (BBB) into the brain where it is phosphorylated and trapped into cells. Since 1982 [5,21], PET with [18F]FDG has been accepted and widely used in the grading of brain tumors; its uptake is generally high in highgrade tumors and it has a good prognostic value, because increased intra-tumoral glucose consumption correlates with tumor grade [22], biological aggressiveness and survival of patients in both primary and recurrent gliomas. Pathology and survival can be predicted by [18F]FDG-PET in gliomas [6]. In addition, a tumor-to-white matter ratio and tumor-to-gray matter ratio were found to increase the sensitivity of the grading evaluation [22]. Since intratumor heterogeneity of brain tumors is not adequately revealed in conventional MRI, because evaluation of the contrast enhancing lesion can either under-or overestimate the presence of active tumor, MRSI and PET are requested to gain additional information on metabolic and molecular tumor markers. In a tumor, the grading can be heterogenous with low-and highgrade areas, as it happens frequently in GBM. This may affect the choice of the site for stereotactic biopsy, which must direct towards tumor sites with the highest tumor grade. Therefore, suitable targets for biopsy will have positive contrast enhancement on T1-weighted MRI, a high choline-peak on MRSI and hypermetabolism on [18F]FDG-PET, the uptake of which is much higher in high-grade component of tumors. As a matter of fact, the [18F]FDG-PET improved the diagnostic yield of stereotactic biopsies by detecting metabolically active areas of tumor [23].

**SO)** is a nitroimidazole derivative, a PET agent used for hypoxia detection. [18F]FMISO-PET can image tumor hypoxia by increased [18F]FMISO tumor uptake, because [18F]FMISO metabolites are trapped exclusively in hypoxic cells. It accumulates in both hypo-and hyperperfused tumor regions, suggesting that hypoxia in GBM may develop irrespective of the

**Figure 8.** Female 22-year-old affected by GBM. Maximum intensity projection (MIP) fusion PET/3D spgr MRI image showing extensive lesion involving the right parietal-temporal lobe with heterogeneous increased [18F]FDG uptake, due to the lesion heterogeneity: high-grade component presenting elevated [18F]FDG activity with standardized up‐ take value (SUV)max=17 (yellow arrow), intermediate-grade component presenting SUVmax=12 (white arrow), low-grade

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Basically, three conditions can be detected with anatomy-based MRI: iso-hypo-intensity in TC1 (tumor, edema), hyper-intensity in TC2 (edema) and contrast enhancement (malignancy). The contrast enhancing regions (CERs) of untreated GBM correspond to the most histologically malignant areas of the tumor with architectural disruption, high cell density, proliferation, vessel density and angiogenesis with circumscribed necroses. Many other properties are revealed by physiology-based MRI. In CERs, in comparison with non-enhancing regions (NERs), physiologic MRI variables show higher values of rT1C, relative fast spin echo (rFSE), rCBV, relative peak height (rPH) and relative recovery factor (rRF), whereas rADC, relative fractional anisotropy (rFA) and fluid attenuated inversion recovery (rFLAIR) do not differ from NERs. All these observations have been shown and confirmed in recent studies of many cases of GBM planning pre-operatively tissue sampling sites and marking them on the anatomic images used by the surgical navigation work station. A comparison between MRI variables and histology of the samples corresponding to the MRI regions of interest (ROIs) in

**3. Biological significance of MRI variables in GBM**

magnitude of perfusion [31].

component SUVmax=4 (red arrow).

However, [18F]FDG-PET can have some diagnostic limitations, because of the high rate of physiologic glucose metabolism in normal brain tissue. In the brain cortex it is particularly high [24,25], so when a hypermetabolic lesion is close to the cortex or the subcortical white matter, the distinction of the tumor from the normal tissue may be difficult [22]. Moreover, it must be taken into account that [18F]FDG accumulation can be non-specific, because it is also observed in inflammatory or granulation tissues [26]. A later PET image acquisition [27] and a co-registration of PET images with MR images greatly improves the performance of [18F]FDG-PET [28]. Technologic advances have allowed to merge PET and MR images, combining the high resolution of MR imaging with the low-resolution functional capability of PET [23], defined as a reduction of intracellular oxygen pressure (pO2), because of decreased supply and of increased demand for oxygen. It predicts poor treatment response of malignant tumors. Two different forms of tumor hypoxia are recognized. Diffusion-limited chronic hypoxia may develop as a result of increased intercapillary distances, and acute hypoxia can result from occlusion of large tumor vessels [29]. Both forms of hypoxia have several implica‐ tions for the further evolution of tumors (induction of signaling cascades that promote angiogenesis, growth, and cell migration) [30]. Tumor hypoxia may also lead to necrosis, which is mandatory to establish the diagnosis of GBM. The **([18F]Fluoromisonidazole) ([18F]FMI‐** Spatial Relationships of MR Imaging and Positron Emission Tomography with Phenotype, Genotype and... http://dx.doi.org/10.5772/58391 71

**Figure 8.** Female 22-year-old affected by GBM. Maximum intensity projection (MIP) fusion PET/3D spgr MRI image showing extensive lesion involving the right parietal-temporal lobe with heterogeneous increased [18F]FDG uptake, due to the lesion heterogeneity: high-grade component presenting elevated [18F]FDG activity with standardized up‐ take value (SUV)max=17 (yellow arrow), intermediate-grade component presenting SUVmax=12 (white arrow), low-grade component SUVmax=4 (red arrow).

**SO)** is a nitroimidazole derivative, a PET agent used for hypoxia detection. [18F]FMISO-PET can image tumor hypoxia by increased [18F]FMISO tumor uptake, because [18F]FMISO metabolites are trapped exclusively in hypoxic cells. It accumulates in both hypo-and hyperperfused tumor regions, suggesting that hypoxia in GBM may develop irrespective of the magnitude of perfusion [31].
