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In some rare cases, demyelinating plaques appear on contrast-enhanced T1-weighted images as pseudotumoral, cyst-like lesions (hypointense, ring enhancing). Serial proton MR spectroscopy, T2 relaxometry and magnetization transfer ratios (MTR) were performed on three pseudotumoral demyelinating lesions to obtain information about their pathological basis. Baseline and 1-month MTR and T2 values were similar to those of cerebrospinal fluid, while spectra showed lactate, lipids and choline. Three-month and 1 year exams showed recovery of MTR, T2 and N-acetylaspartate, approaching the contralateral values, while creatine and choline were normal or surpassed contralateral values. Lipids and lactate gradually disappeared. These results suggest that pseudotumoral, cyst-like, ring-enhancing lesions may be characterized by an accumulation of oedema in the extracellular space with an almost complete absence of cells. Reduction of the oedema allows restoration of the tissue to its original location, indicating that cellular destruction was less important than was expected after the first exam. Thus, the evolution of this kind of lesion should be kept in mind when considering lesion volume from T1-weighted images as a marker of disability or irreversible cellular destruction in MS.

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