Toxoplasmosis

Findings

Figure 1: Foci of vasogenic edema are appreciated in the left fronal lobe
Figure 2, Figure 3, Figure 4, and Figure 5: Diffusion, FLAIR, T2, and T1 weighted MR images respectively show vasogenic edema with an underlying ring lesion suspected.
Figure 6 and Figure 7: Post contrast T1 weighted axial and coronal images demonstrate a ring enhancing lesion with a relatively thin rim centered at the grey white junction with surrounding vasogenic edema.
Figure 8: Pulsed arterial spin labeled sequence that provides quantitative cerebral blood flow values shows decreased perfusion in the lesion.
Figure 9: Single voxel MRI spectroscopy obtained with a TE of 35 shows a dominant lipid peak at 1.3 ppm (yellow arrowhead) with depression of the normally expected metabolites including choline at 3.2 ppm, creatine at 3.0 ppm and N-Acetyl aspartate (NAA) at 2.0 ppm. This spectra is characteristic of toxoplasmosis and shows no characteristic features of a neoplastic process.

The patient was treated for toxoplasmosis and showed significant improvement in her mental status as well as interval decrease in the size and edema associated with the ring enhancing lesion.


Diagnosis: Toxoplasmosis


In the patient with HIV, a frequently encountered diagnostic dilemma is the ring enhancing lesion. On conventional MRI sequences in the immunocompromised patient, lymphoma and toxoplasmosis can have similar appearances. Historically nuclear medicine thallium or PET imaging studies have helped differentiate the two entities.

Toxoplasmosis and primary CNS lymphoma, the two most common etiologies of focally destructive brain lesions in AIDS, can share identical presentations clinically [headache, seizures, focal neurologic deficits] and radiologically [ring-enhancing lesion(s) with surrounding vasogenic edema on CT or MR]. Prognosis and treatment vary greatly; toxoplasmosis patients can respond dramatically to antibiotic therapy while lymphoma patients typically survive <1 year even when treated. Serology, polymerase chain reaction, SPECT and PET, response to treatment, and biopsy have been used to differentiate these entities. However, MR perfusion and spectroscopy are fast noninvasive ways to help differentiate these diseases and direct appropriate initial therapy in AIDS patients with acute focal neurologic deficits.

Perfusion analysis in enhancing areas of lymphoma demonstrates an increase in relative cerebral blood volume (CBV) and maximum perfusion, thought to be secondary to greater vascularity in active areas of tumor growth, while enhancing areas of toxoplasmosis demonstrate a decrease in CBV. Central areas of necrosis and surrounding vasogenic edema in both diseases typically show decreased CBV, secondary to avascularity in areas of necrosis and to vasoconstriction in surrounding edematous brain.

Spectroscopy in lymphoma demonstrates a markedly elevated choline peak in enhancing solid areas of tumor, with less pronounced increases in lactate and lipid. In contrast, spectroscopy in toxoplasmosis demonstrates marked elevation of lactate and lipid peaks with significant depression of normal brain metabolites. As with perfusion, necrotic areas of lymphoma and toxoplasmosis may demonstrate similar spectral patterns.

Historically, SPECT and PET have helped differentiate toxoplasmosis and lymphoma, demonstrating hypometabolic activity in the former and hypermetabolic activity in the latter. However, MR perfusion and spectroscopy have demonstrated congruent results with the capability of evaluating the entire brain at once with higher spatial resolution, avoiding additional contrast and radioactivity, and adding only a few minutes to the MR acquisition time.