MR OF TOXOPLASMA ENCEPHALITIS - SIGNAL CHARACTERISTICS ON T2-WEIGHTEDIMAGES AND PATHOLOGICAL CORRELATION

Purpose: Our goal was to determine if there are any T2-weighted MR sig nal characteristics of Toxoplasma encephalitis that might be useful in diagnosis and/or in gauging the effectiveness of medical therapy. Met hod: We retrospectively analyzed the MR, CT, thallium-201 SPECT brain scans, and medical records of 27 patients with medically proven (26) a nd biopsy proven (1) Toxoplasma encephalitis, supplemented by autopsy findings in 4 additional patients, 2 of whom had postmortem MR correla tion. The neuropathologic literature was also reviewed. Results: Among the 27 patients, we discovered three distinct imaging patterns. Ten ( 37%) patients had predominantly T2-weighted hyperintense lesions and h ad been on medical therapy an average of 3 days (excluding one outlier ). Ten (37%) patients had T2-weighted isointense lesions and had recei ved medical therapy an average of 61 days. Seven (26%) patients had le sions with mixed signal on T2-weighted images and had been on treatmen t an average of 6 days, Analysis of autopsy material from the four add itional patients revealed the presence of organizing abscesses in thre e and necrotizing encephalitis in one, while the patient who had a bra in biopsy demonstrated both types of pathologic lesions. In both cases having postmortem MRI, organizing abscesses appeared isointense to hy pointense on T2-weighted images. Conclusion: There is a definite varia tion in the appearance of lesions of Toxoplasma encephalitis on T2-wei ghted images that precludes a definitive diagnosis based on signal cha racteristics alone. Pathologically, our data suggest that T2-weighted hyperintensity correlates with necrotizing encephalitis and T2-weighte d isointensity with organizing abscesses. Furthermore, in patients on medical therapy the T2-weighted MR appearance may be a transition from hyperintensity to isointensity as a function of a positive response t o antibiotic treatment, indicating that the signal change might be use d to gauge the effectiveness of medical therapy.