Cerebral lymphomas are hypercellular tumors consisting of large lymphoid cells. On T2-weighted images, lesions are slightly hyperintense compared to normal brain tissue with ring-shaped or diffuse enhancement. Tightly packed cells change the composition and microarchitecture of cerebral tissue leading to a decrease in extracellular water and resultant restriction in diffusion. The ADC values of the primary CNS lymphomas may be lower than the surrounding brain parenchyma, mimicking acute infarct (? Fig. 6.18). The key imaging findings to differentiate CNS lymphomas include the hypercellularity sign (low signal on T2- weighted images), large perifocal edema, contrast enhancement, and often close contact with the leptomeningeal and/or ependymal space.
Fig. 6.18 Primary central nervous system (CNS) lymphoma mimicking acute infarct. (a) Axial diffusion weighted imaging and (b) apparent diffusion coefficient map in a patient with primary CNS lymphoma show a region of restricted diffusion in the right periventricular white matter due to hypercellularity of the tumor. (arrow in (a) and (b)) (c) Contrast-enhanced T1-weighted image shows intense enhancement of the periventricular mass (arrow) with surrounding hypointense vasogenic edema (curved arrow).