which the tumor will usually be low in signal, consistent with lack of restricted diffusion,
whereas an abscess usually does exhibit increased intensity due to restricted diffusion. The
enhancement pattern is also different, as residual foci of viable tumor within a necrotic center
will continue to enhance, resulting in a heterogeneous enhancement pattern. The center of an
abscess does not enhance (28,30).
Differentiation of tuberculoma from tumor can be difficult. Imaging characteristics on
MRI can be nearly indistinguishable. MR spectroscopy is one potential technique that has been
utilized to successfully differentiate an unusual presentation of extra-axial tuberculoma from
meningioma. The high lipid and lactate peaks and lack of amino acid resonances may prove
useful for distinguishing tuberculoma from other entities in the correct clinical context,
potentially sparing unnecessary biopsy (34).
CNS Lymphoma
Primary CNS lymphoma is a B-cell lymphoma that originates from and generally remains
within the brain, spinal cord, optic tract, or leptomeninges. Disease incidence in both
immunocompetent and immunocompromised patients has been increasing for as yet
undetermined reasons. Differential diagnoses differ between immune competent and
compromised patients, with primary or metastatic tumor considered for the former and
opportunistic infection, such as toxoplasmosis, for the latter. The enhancement pattern of
lymphoma on imaging studies is usually heterogeneous on both CT and MR. However, in the
immunocompromised population, enhancement can be heterogeneous or ring enhancing
(Fig. 16A and B). Lesions are isointense to hypointense on T1-weighted images and
hyperintense on T2-weighted images. There is often leptomeningeal or periventricular/
intraventricular extension (28,30).
Toxoplasmosis is difficult to differentiate from primary CNS lymphoma. Both affect gray
and white matter, particularly the basal ganglia, and affect immunocompromised patients.
Lesion multiplicity can be observed in both conditions. Lymphoma may demonstrate
ependymal spread, which is not characteristic of toxoplasmosis. Positron emission
tomography (PET) findings do differ, as toxoplasmosis is usually hypometabolic, whereas
CNS lymphoma is usually hypermetabolic (28).
Figure 16 (A) Contrast-enhanced axial CT image of the brain in an HIV-positive male demonstrates a gyriform-
enhancing mass in the right occipital lobe associated with vasogenic edema (arrow). (B) Contrast-enhanced T1-
weighted axial MR image demonstrates intense right occipital lobe enhancement as well as a second small right
frontal cortical focus of enhancement in this patient with lymphoma.
Radiology of Infectious Diseases and Their Mimics in Critical Care 89