Central nervous system disorders

VI./3.3.: Magnetic resonance imaging – MR

MRI is the first choice method for detecting metastases. T2 weighted and T1 weighted unenhanced and contrast enhanced series are usually performed.

On T1 weighted images, most tumors and metastases have a moderate or low signal intensity in the cerebrum (certain tumor types can be exceptions). Particular tissue factors reduce T1 relaxation time (e.g. paramagnetic effect). Calcification has low signal on T1 weighted image, but high signal can be observed on the course of calcification process. Flow phenomenon can be also „illusive” – especially the high signal intensity on gradient echo images should not be confounded by enhancentment (this dubiosity occurs if no unenhanced T1 weighted images have been performed).

Signal intensity of tumors /metastases is high on T2 weighted images due to high water content. If a very dense stromal tissue characteristic tumor with low water content is concerned, its signal intensity will not be high on T2 but dark – such as e.g. hypercellular tumors, especially if high nucleus/cytoplasm ratio is present. Lymphoma and PNET are such tumors – that is why they are hyperdense on CT. Calcifications are rather hypointense on T2 weighted images compared to T1.

If hemosiderin (due to previous bleeding) is present in a metastasis, its paramagnetic effect reduces the signal significantly. Sometimes differentiation of a tumorous bleeding from a hemorrhagic stroke is not easy. Whilst hemorrhage of vascular origin contains hemoglobin decomposition products which are usually produced at the same time, this spectrum is elongated in tumorous hemorrhage containing decomposition product of different ages. High protein-content cysts (colloid cysts) have low signal intensity on T2 weighted images. If a black „hole” is senn in the tumor, this sign can indicate a high vascularisation and dilated vessels (flow void). However, similar sign can be observed in non-tumorous vascular malformations and in primary hemangioblastomas as well.

In contrast to CT, which modality demonstrates metastases similarly with high contrast enhancement, advantage of MRI is that tiny and peripherial – e.g. on the convexity located – metastases are also well detectable.

In case of melanoma metastases and mucin producing tumors this hypointensity is very conspicuous because of the paramagnetic effect of melanin and mucin (calcification is frequent in mucin producing tumors). Since this „paradoxical” signal intensity is caused by melanin, this is no longer the case in amelanotic melanoma metastases. The situation is further complicated by the bleeding, which is common in melanomas, and the complex signal course of hemoglobin decomposition products.

Following contrast agent administration the enhancement can be homogeneous, annular, nodular or mixed. Additional tiny metastases often can appear also in virtue of their enhancement (even in the previous large edematous zone).

Metastases of gastrointestinal tumor origin can be hypointense on T1 and T2 weighted images (or isointense on T1) which is referred to the included high protein content of mucin.

Differentiation of metastasis from ischemic laesion and demyelinisation can be problematic on the unenhanced MR scan. Since metastases usually enhance, contrast agent administration supports the differentiation from other lesions. Contrast administration is also important to exclude the presence of additional metastases since singular metastases are treated yet by surgery/irradiation (gamma knife) nowadays.

Carcinomatous encephalitis (miliary) is a very rare form of metastatic tumor deposits. Rapid course and fatal outcome is characteristic. Very tiny perivascular tumor deposits are visible in the gray matter (high signal on T2).

Tumor metastases spreading on meninges (arachnoidea, dura) are less frequent compared to parenchymal ones (primary leptomeningeal tumor is extremely rare). At this time, tumor cells can get onto the cerebral surface via liquor / thin walled leptomeningeal vessels. Tumor „breaking out” from the parenchyma is rare. Neurological symptoms are caused by certain large tumors or if the tumor mass is located in a critical localisation.

Most frequently the following tumors give leptomeningeal metastasis: lymphoma (non-Hodgkin), leukemia, melanoma, breast tumor, lung cancer and neuroblastoma (children). Do not forget about neurogenic tumors which spread onto the meninges also very often. These tumors are: glioblastoma, medulloblastoma, ependymoma, corpus pineale tumors, retinoblastoma (children).

On the unenhanced CT basal cisterns and sulci (sulcus Sylvii) is filled in this case, and enhancement appears here following contrast administration which can spread on the convexity cranially. Contrast enhancement is not necessarily continous, it can be intermittent and flares can be also visible inside. Tentorium enhances the contrast normally as well, metastasis can be suspected only if this enhancement is irregular, nodular. Differentiation from inflammatory origin does not belong to the task of imaging (clinical signs+lab tests). Enhancement can spread further intraventricularly, ependymally / subependymally. In this case, differentiation from ventriculitis is necessary (also clinical signs + lumbal punction, lab). Leptomeningeal tumor spread might lead to liquor absorption disturbance: communicating hydrocephalus can be observed on both CT and MRI scans.

Leptomeningeal metastases can be imaged on the course of MRI examination (with contrast agent administration) most demonstratively. Unenhanced MRI cannot be demonstrative since both liquor and tumor deposition has high signal on T2 weighted images (both have low signal on T1), thus differentiation is impossible from each other. However, the enhancing tumor, which fills the subarachnoid space and sulci, can be prominent if T1 weighting + i.v. paramagnetic contrast agent (Gadolinium) is applied besides the liquor which has low signal by this time. In addition, it can be also observed that whilst inflammations (meningitis) usually have regular contours and uniform widths, metastases are knotty, gnarled, asymmetrically nodular (subependymal metastases as well) according to the above mentioned characteristics.

However, it must be also known that not every meningeal enhancement indicates a tumor or meningitis. Sarcoidosis, vasculitis can also cause meningeal enhancement, furthermore, it can also develop following chemotherapy and radiotherapy, and tuberculous meningitis must be also excluded. An even more important knowledge is that a taint for leptomeningeal enhancement can exist not only following surgeries (craniotomy or shunt implantation) but also for several days following a „simple”, uncomplicated lumbal punction. However, not always /every leptomeningeally spreading tumor enhances: e.g. sometimes recurrence of medulloblastoma can be detected by liquor sampling only.

Dural metastases often can stem from prostatic cancers, but in case of lung and breast carcinomas as well (neuroblastoma in children). These metastases are asymmetric, enhance the contrast. The differentiation from meningeomas is not easy (especially if the primary tumor is still unknown) (meningeomas are rather irregular). Dural metastases can appear on both sides of falx or tentorium, but do not darm them, more often appear by Pacchioni’s granulations, and can cause (chronic, developing in multistaged manner) subdural hematoma.

Agressive head and neck tumors (squamous cell, basal cell carcinoma) can spread onto the skull base directly. Moreover, passing on they can appear in the cranium as well, but scrambling there via basal foramina (perineural) is even more often. Adenoid cystic carcinoma spreads onto the skull base perineurally. „Swarming up” is not always continous, „skipping” form is also observable. Although this propagation can be also diagnosed by CT (dilation of the foramen is also demonstratable), in order to image it appropriately, MRI is the choosable imaging method – as known, CT imaging of head and neck transition is limited due to bony artefacts. MR can also visualize the perineural thickening and its homogeneous enhancement well. Fat saturated imaging is preferable in this case.

Imaging of bone metastases is the role of CT principally. Largest part of bone metastases is lytic, but prostatic carcinoma is known to tend to give osteoplastic metastases in bones. However, mixed / miscellaneous is also known (such as neuroblastoma in children). Metastasis can exceed the bony borders, „rubbing out” the internal / external lamina in both intra- and extracranial directions.

MRI imaging indicating an inverse change in the signal course of diploe demonstrates the presence of a metastasis. Fat, which has high signal on T1 weighted images, loses this feature spotty, whilst metastasis is „lighted up” with its high signal on the fat saturated, T2 weighted images. Diploe metastases enhance the contrast.