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II./3.2.: Imaging of brain tumors
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CT can demonstrate brain tumors or their indirect, eye-catching signs. Sometimes the tumor itself is not visible on the unenhanced CT image but perifocal edema refers to its presence. This edema is non-territorial (such as in case of cerebral infarctions, necroses due to vessel occlusion) but „glove” shaped. Most of brain tumors enhance the contrast agent, however, we must note that non-enhancing tumors also exist.
MRI examination can demonstrate also even those tumors on the unenhanced scan which ones were not visible on the unenhanced CT images. It is characteristic that primary brain tumors have low signal on T1 weighting in general, and high signal on T2 weighted images (this feature, as known, depends on the high fluid content). Although it is characteristic but not enough differential diagnostic criteria concerning the primary brain tumors. Hypercellular tumors, especially those with a cell content of large nuclei, have low signal on T2 weighted images. These type of tumors – PNET, lymphoma – have also restricted diffusion (see there) and are hyperdense on CT.
There are morphologic signs besides density/signal intensity which might be of our help in the judgment of benign or malignant nature. Such signs are e.g. irregularity and bluntness of tumor margin, and signs referring to infiltration of the adjacent parenchyma. Both imaging modalities, especially after contrast material administration, are capable to adjudge the signs of tumor necrosis. VEGF expression, signs of neovascularisation are also demonstratable by these imaging modalities. General features of malignancy include inhomogeneicity, pleomorphism and, in advanced stages, destruction of the adjacent bony structures.
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To determinate the nature of tumor it is helpful if we know that there are certain, well depictable tissue components which are characteristic for certain tumors, such as for example the oligodendroglioma's tendency to calcify. However, astrocytomas, which are more common as oligodendrogliomas, can be also calcified, therefore, in case of a Ca-containing cerebral tumor we should think of astrocytoma. Additional calcareous tumors: ependymoma, plexus papilloma. Meningeomas are also calcified, but extraaxial, such as the intra/parasellar craniopharyngeomas by dint of their localisation and consistency of three components. Differentiation of these tumors usually does not present some difficulty. Clivus chondromas and clivus chordomas can also contain calcification. Besides the pineal tumor's, calcification of corpus pineale can be excessive.
CT demonstrates the calcifications well (as known, MR cannot), but we must be careful e.g. in the differentiation from the acute bleedings (HU value helps). Lymphomas can be also – moderately – hyperdense. On MR T2 weighting, especially on T2*, calcification is signal free (such as the hemosiderin), but we must note that pending calcification can have also high signal on T1 weighting. Blood flowing with an appropriate velocity is also signal free.
Fat content is not characteristic for intraaxial cerebral tumors (such as e.g. extraaxial midline – „balken” – lipomas or dermoid cysts and teratomas). On CT, fat is strongly hypodense (even -100 HU). On MRI scans, signal intensity of the fat is high on T1 weighting, but acute bleeding (methemoglobin), melanine and high protein content can be also such. Fat suppression sequences can help in this differentiation.
Cysts or cystic formations can develop inside the tumors. In these cases, restricted and free water must be distinguished (by PD and FLAIR sequences).
Inside the fluids, the Brown motion related diffusion is free, therefore, there is no signal increase on DWI images. Neither signal intensity nor internal diffusion of the intratumoral necroses does not reach a degree equal to the liquor.
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Since the perfusion (tissue devolution) depends on the vascularisation of the tumor (neoangiogenesis), it is a better indicator of malignancy as contrast enhancement. We must know that the perfusion does not depend on the intactness or the permeability of the blood-brain barrier (e.g. meningeomas, see there).
Diffusion phenomena: Protons move freely in the extracellular space (Brown motion) actuated by their thermal energy in the „normal” tissues. In most tumors the diffusion is not restricted, especially not in the necrotic and cystic areas. Consequently, the intratumoral signal can be similar to one of the normal parenchyma on DWI images, i.e. weak signal (black) is visible. However, if the diffusion is restricted (e.g. hypercellular tumor with narrow extracellular spaces), protons remain in their spaces, i.e. protons do not leave the voxel and relaxate on the spot which indicates a high signal in black background, since the protons, which do free diffusion movement in the normal tissue, move from their locations. The from DWI calculated ADC map (see there) is more useful since a T2 high signal intense structure does not „shine through” and it helps in the typisation of the tumorous tissue of a certain extent. Application of DWI can be useful in the differential diagnostics (tumor or abscess).
Diffusion tensor imaging (DTI) based tractography demonstrates the relationship of the tumor and the white matter tracts of high concern. It is helpful for the surgeon to plan and carry out the operation precisely so that the penetration and the resection is amount to the least functional deficit (e.g. motion and speech ability).
fMRI (functional MRI) examination can help to clarify the relationship of the cerebral areas of accentuated importance (e.g. primary motor cortex, speech centers) and the tumor.
Contrast enhancement features of brain tumors:
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Applied CT and MRI contrast agents (see there)
If an area of an intraaxial lesion (tumor, inflammation) enhances the contrast, the blood-brain barrier became permeable there. Awhile ischemic cerebral infarcts can also enhance, which can be illusive (however, whilst it is territorial, till then the ones of venous origin are not)???. Accordingly, only on the morphology based differentiation from the glioblastoma and the abscess (both demonstrate annular enhancement) or the metastases (both can be multiple, however, liquor findings might help) can be difficult. Shape of the edema can be helpful (tumor: glove shaped, infarction: territorial). Enhancement around the subacute hematomas can be annular because of the breakthrough of blood-brain barrier, organisation (demarcation) and granulation processes. A large inflammatory nodule producing space occupying MS (multiple sclerosis) lesion (tumefactive) can enhance the contrast either in toto or annularly.
However, e.g. low grade astrocytomas do not enhance. In case of gliomas, the stronger the enhancement is the more malignant is the tumor. If a low grade, barely or non enhancing glioma starts to enhance during the follow-up, it should be evaluated as the sign of malignisation (however, we must know that contrast enhancement or its dynamics is also not an unequivocal, obligate sign of malignancy).
Certain tumors demonstrate inhomogeneous / heterogeneous enhancement: characteristically such a tumor is the multiform glioblastoma, which is a highly malignant (high grade) tumor, including cysts, necroses interspersing the contrast enhancement pattern. Due to its infiltrative growth, tumor cell groups can be present in the parenchyma also far away – though MR does not detect them, FLAIR images can refer to them!
We must be aware of the appropriate quantity of the contrast agent and that appropriate time must be spared for the interstitial penetration as well (late phase).
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