CT and MRI characteristics of CNS tumors

CT can usually lead to definitive diagnosis regarding brain tumors. A non-territorial localization (as opposed to arterial occlusion) of a usually ―glove‖ shaped perifocal hypodense zone is highly suspicious for a tumor.

MRI provides even more definitive proof. On T1 weighed images they are usually hypointense, on T2 weighed images their signal is strong. Although these signs are very characteristic, normally they are still insufficient for exact differential diagnostic criteria.

Contrast enhancement of tumors, specific forms of enhancement:

Intravenous contrast agents (iodinated contrast media in CT, or chelated Gadolinium in MRI) normally do not pass over the blood-brain barrier. Contrast material cannot leave the blood vessels towards the parenchyma (secondary to its strong triple layer defense).

Therefore, where contrast enhancement is seen, the blood-brain barrier is damaged. This is only possible in intra-axial brain tumors, inflammatory states, certain types of demyelinating diseases (multiplex sclerosis) and at certain states in ischemic infarcts. Low-grade

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astrocytomas typically do not enhance. A more pronounced enhancement is seen in a gliomas and it reflects their malignancy. This also means that if a low grade glioma during a follow-up study suddenly changes its enhancement pattern, the increase is regarded as a sign of

malignant transformation.

Contrast material has to be administered in required volumes and enough time has to be given for the interstitial appearance as well (late phase).

Extra-axial tumors do not have a blood-brain barrier protection, therefore meningioma, schwannoma, pituitary adenoma, pineal and choroid plexus tumor enhance differently.

Cystic lesions naturally do not show any enhancement, these include dermoid, epidermoid and arachnoid cysts.

Radiological characteristics of certain neoplasms

MRI has the greatest sensitivity in the detection of neoplastic brain lesions. The relaxation time of tumor is usually longer than that of the surrounding normal tissues. Therefore on T1W images neoplasms have slightly weaker signal intensity, while on T2W images they are more hyperintense than normal parenchyma.

This signal pattern can be very characteristic and has great diagnostic value. However,

secondary neoplastic signs, such as mass-effect of the tumor cannot be neglect either. A space occupying lesion can cause:

 the dislocation of the midline structures,

 the impression or dislocation of the ventricle,

 hydrocephalus as a sign of liquor obstruction

Besides the morphological signs, contrast enhancing properties are also characteristic.

On the other hand, although MRI is very sensitive for brain tumors, its specificity cannot be overestimated, otherwise this will eventually lead to diagnostic errors.

In order to appropriately suggest a diagnosis, besides the consideration of the clinical picture, there are other factors that need to be though of:

 the localization of the tumor

 the characteristic age group

 signal intensities (measured relaxation times)

 contrast enhancement, distribution

Tumors frequently presenting with hemorrhage are: choriocarcinoma, melanoma, metastases of renal cell carcinoma and bronchial carcinoma, pituitary adenoma, glioblastoma multiforme and medulloblastoma.

Even with these considerations the diagnosis can only be a most likely estimation. Clinicians and radiologists alike should keep in mind that pathologic diagnosis is only provided by the histologic examination of the tumor!

119 Astrocytoma:

It is essential to note that in cases of low grade astrocytomas the differentiating ability of MRI is considerably higher than that of CT examination!

Contrast enhancement in astrocytomas increases with the malignancy of the tumor.

In higher grade astrocytomas there is a very typical, extensive perifocal swelling (finger-in-glove white-matter edema).

Contrast enhancement is usually round or it resembles a garland shape.

Oligodendroglioma:

These neoplasms show an infiltrative growth and their contrast enhancement is poor.

Ependymoma:

It characteristically manifests in children and in adolescents.

There is no perifocal edema present. Due to its intra-ventricular growth this tumor can quickly lead to occlusive hydrocephalus because of the obstruction of CSF flow.

Medulloblastoma:

Clinical symptoms:

It is the most common pediatric CNS malignancy (between 5-15 years, it takes up 2-6% of all brain neoplasms).

On CT images it is mostly hyperdense.

On MRI (as opposed to CT images) the tumor can be depicted without any disturbances caused by the bony wall of the posterior fossa.

PNET:

Primitive Neuroectodermal tumor primary presents in children but it also appears in adulthood.

The tumor contains cystic and necrotic parts, at many times it is multi-centric and it shows an intense contrast enhancement.

Meningioma

Most often its symptoms present poorly and disease progression is long. It is the most

common intracranial tumor, but it is typically benign. Its complications are determined by the localization and the size of the tumor.

Meningiomas are often (but not always) surrounded by sharp edged swelling and perifocal edema. They might appear isodense compared to brain parenchyma on CT. They often contain sclerotic parts and usually they show an increased enhancement of iodinated contrast media.

MRI: Meningiomas show a good Gadolinium enhancement with a characteristic ―dural tail‖

sign (a thickening in the neighboring dura).

Tumors of the myelin sheath:

These tumors most commpnly derive from the sheath of the vestibular part of the VIII cranial nerve (vestibulocochlear nerve).

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MRI shows a substantial contrast enhancement. (MRI is preffered, since - as opposed to CT – it is able to depict the internal auditory canal and its surroundings without any artefacts.) Hemangioblastoma:

It is typically a cerebellar neoplasm.

Intravenous contrast material differentiates its markedly enhancing nidus, from the cysts that of course do not enhance at all.

Arachnoid cysts

They show liquor density on CT, they do not enhance contrast material.

Lipomas:

On CT they show a pronounced hypodensity (-100 HU) and therefore cannot be confused with anything.

On MRI they are also very characteristic, on T1W images they are markedly hyperintense.

Metastases:

The most common primary tumors that metastasize to the brain are: bronchus carcinoma, breast cancer and renal cancer. A so called early metastasis is especially typical for bronchus carcinoma, when the primary broncus carcinoma is still unknown.

Small metastases can produce very extensive edemas. Multiplicity is common. Due to the consequential blood-brain barrier disorders their contrast enhancement is very intense.

Angiomas – vascular malformations

At many times the collecting term, angioma is used for these lesions: capillary teleangiectasias, cavernosus angiomas, arteriovenosus malformations.

Vascular anomalies can be depicted reliably with MRI, even without the use of contrast medium.

Pituitary gland

Method of choice: MRI

The analysis of the sellar floor can be done with CT, if possible in the coronal plane.

==The appearance of the normal pituitary gland on MRI:===

On non contrast enhanced T1 weighted images the anterior lobe of the pituitary gland has average signal intensity, similar to brain parenchyma.

The dorsal/posterior lobe of the pituitary gland however, shows hyperintense signal.

In the anterior lobe of the gland adenomas derive from the glandular structure. They can be grouped according to their hormone producing status:

hormonally active hormonally inactive

According to their size they can be:

microadenomas (< 1 cm) macroadenomas (> 1 cm)

121 The indications for sellar examinations can be the following:

 Endocrine: due to the clinical picture or the biochemical (lab) reports.

 Ophthalmologic: large parasellar lesions can cause quadrant anopia secondary to the pressuring of the optic chiasm.

 Radiologic: a parasellar bone anomaly can be noted on radiographs The types of pituitary gland adenomas

Prolactin producing adenoma

GH (growth hormone) producing adenoma: Acromegaly ACTH producing: Cushing’s disease

Hormonally inactive,or tumors that only produce hormonal fragments do not cause clinical signs, therefore they are diagnosed due to their space occupying effect, and their symptoms are only detected in advanced states. Since patients only get to examination at this late stage the tumors can reach a large size. Expansive symptoms include bitemporal hemianopia, constantly increasing visual defects and headaches secondary to CSF obstruction.

Signal change on MRI in pituitary adenomas:

Microadenomas on T1WIs appear with much lower signal intensity compared to white matter, while compared to the grey matter they are only less intense.

On T2 weighted images they show great variability. They can be bright, isointense and hypointense as well.

Macroadenomas can show necrobiotic phenomena, thus due to hemorrhage and cystic degeneration their signal is inhomogeneous, especially on T2 weighted sequences. However homogenous macroadenomas can also be seen.

The effect of contrast agent in pituitary adenomas

The natural signal intensity difference on MRI, between the frontal and dorsal lobes of the pituitary gland ceases to exist when Gadolinium is administered (T1 weighted imaging) because of the enhancement in the frontal lobe.

Contrast enhancement is immediate in the frontal lobe because of the lack of the blood-brain barrier. In adenomas the contrast enhancement is slow.

Other pituitary tumors and tumors in the neighboring tissues Craniopharyngioma:

Originates from the remains of the epithelial cells of Rathke’s pouch.

CT can reliably differentiate its three components (calcification can already appear on

conventional X-ray images, but it is certainly detectable with CT). MRI can also differentiate its 2 or 3 components based on their characteristic signals.

Metastatic tumors:

The pituitary gland is a frequent location for metastatic lesions, especially the pituitary stalk.

Their primary cancer is breast carcinoma, lung cancer and also lymphoma.

The leading clinical symptom is diabetes insipidus and panhypopituitarism.

CT & MRI:

The contrast enhancement of metastases is greater than that of adenomas.

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Empty sella

The sellar diaphragm at the insertion of the pituitary stalk can remodel the upper portion of the gland due to the pulsation of the surrounding CSF, until the pituitary gland is pushed and compressed to the bottom of the sella. The contents of the suprasellar cistern then protrude to the sella.

Empty sella can be symptom free, but typically observed in obese women, whom present with frequent headaches around menopause, sometimes they have hypertension and slight

hyperprolactinemia.

"Balloon sella" is the extreme form of empty sella, which develops due to the prolonged, increased intracranial pressure (usually in cases of the obstruction of aquaeductus cerebri Sylvii).

Secondary empty sella usually is a result of a postoperative state. However, it can also be a possible effect of bromocriptine treatment of a (micro)adenoma or it can be the consequence of adenoma apoplexy.

14. Right parietal oligodendroglioma with finger-in-the glove edema MRI (T2W).

15. Glioblastoma multiforme in the right frontal lobe, MRI

(T1W+contrast).

16. Lymphoma in the left lateral ventricle

MRI,FLAIR

17. Solitary metastasis in the right frontal lobe, MRI (T1W

+contrast)

In document Development of an electronic educational material for the analysing of the structures and the bio-chemical processes of living organisms by imaging technology (Pldal 118-123)