Criteria to Analyze by Sequence:

T1 and T1 Gd(+): Morphological Criteria

T2: Edema-lesion relationship; Signal intensity

FLAIR: Cortical infiltration

Diffusion (DWI/ADC): ADC values

Magnetic Susceptibility (SWI): Relation with medullary veins; Intratumoral susceptibility signal (ITSS)

Perfusion (RM DSC/DCE): Relative cerebral blood volume (rCBV); Percentage of signal recovery (PSR)

T1 GD (+)

Glioblastoma:

Primary central nervous system tumor

Typically irregular shape with poorly defined margins

Presence of necrosis

Metastases:

Secondary tumor (hematogenous spread)

Solitary lesion in 50% of cases; if multiple, metastasis should be the primary consideration

Generally located at the cortico-subcortical junction

Almost spherical shape with well-defined margins

Necrosis

Lymphoma:

Can be primary (typically in immunocompromised patients) or secondary

Contrast enhancement is highly variable, often intense and homogeneous; however, ring enhancement may occur (10-15%), particularly in immunocompromised patients with internal necrosis

Necrosis is rare (except in immunocompromised individuals)

Notch sign (peripheral groove)

Tumors may significantly reduce or even disappear after corticosteroid treatment

On CT, 70% of cases appear hyperdense due to high cellularity

T2 Weighted Images: Perilesional Signal Alteration

Defined as the well-demarcated area outside the solid tumor portion, which does not enhance and shows high signal intensity on T2-weighted images.

Although peritumoral hyperintensity on T2 is often considered vasogenic edema, there are histological differences:

• Glioblastoma: Infiltrative characteristics lead to peritumoral neoplastic cells along with vasogenic edema in adjacent white matter fibers.
• Metastases: Peritumoral signal alteration on T2 purely corresponds to vasogenic edema.

Lesion Signal Intensity:

• Glioblastoma and metastases: Hyperintense on T2.

• Lymphoma: Typically iso- or hypointense relative to gray matter but may appear hyperintense, especially in the presence of necrosis.

Edema-to-Lesion Ratio:

• Edema > tumor area: More common in metastases (often disproportionate edema relative to tumor size).

• Edema < tumor area: Suggestive of GBM or lymphoma (lymphoma typically has the least vasogenic edema).

FLAIR Imaging:

When a lesion is adjacent to the cortex (gray matter), signal alteration in perilesional FLAIR sequences may indicate cortical infiltration, suggesting an infiltrative lesion type (e.g., GBM).

In contrast, perilesional vasogenic edema in metastases predominantly involves the white matter while sparing the cortical gray matter.

Thus, adjacent cortical infiltration (without gadolinium enhancement) strongly suggests glioblastoma.

Diffusion (DWI/ADC):

A known inverse correlation exists between tumor cellularity and ADC values. ADC measurements help differentiate tumor types based on cellularity and peritumoral infiltration.

However, significant discrepancies exist in the literature, and ADC values alone do not reliably distinguish GBM from metastases. A major limitation is that most intracranial lesions receive corticosteroid treatment before MRI.

Nevertheless, ADC values can help differentiate lymphoma from glioblastoma and metastases. Lymphomas have the lowest ADC values due to their high cellular density. ADC values near or below 0.6 x 10³ mm² strongly suggest lymphoma.

Magnetic Susceptibility (SWI):

A highly useful technique for detecting blood products and venous vasculature.

Relationship with Medullary Veins:

• GBM and lymphoma: Medullary veins pass through the tumor due to infiltrative behavior. In lymphoma, malignant lymphocytes surround pre-existing vessels.

• Metastases: Medullary veins terminate at the tumor margin without crossing it (Medullary Vein Blockage Sign).

Intratumoral Susceptibility Signal (ITSS):

• Greater ITSS corresponds to increased hemorrhage, microhemorrhage, prominent vasculature, or intralesional calcium.

• High ITSS is suggestive of GBM, as it is the most hemorrhagic intracranial tumor with prominent internal vascularization.

• Lack of ITSS is useful in differentiating lymphoma, as microhemorrhages and calcifications are rare.

Perfusion MRI:

Relative Cerebral Blood Volume (rCBV):

• High rCBV in GBM and metastases due to increased tumor vascularization.

• Low rCBV in lymphoma due to low neoangiogenesis.

Percentage of Signal Recovery (PSR):

• PSR >1 in lymphoma

• PSR <1 in GBM and metastases