Cerebral toxoplasmosis (CT) is the most common opportunistic infection of the central nervous system; it is the first AIDS-defining event in 10 to 38% of cases. This is a reactivation of latent brain cysts that occurred during childhood or adolescence and often goes unnoticed [2, 3]. Two forms can be observed on the clinical level:

-The focal abscessed form, the most frequent (80% cases) generally associating deficits and cortical irritation signs, of progressive installation.

The encephalic form with consciousness disorders and/or generalized epilepsy, often a febrile syndrome and signs of intracranial hypertension [2, 3]. Its radiological aspect, especially MRI, shows suggestive but not pathognomonic images. The most suggestive images of CT are focal lesions more often multiple in 57-85% but can be single [4], rounded, associated with a peripheral edematous reaction; after injection, contrast is usually annular (44-91%), resulting in a cocoon image (Figure 2b). The mass effect is common (Figure 1a). The lesions preferentially affect the cerebral hemispheres, particularly at the cortico-subcortical junction, the basal ganglia and more rarely the sub-tentorial level [4, 5, 6] (Figure 1). These are lesions with high signal intensity (hyper signal) or mixed signal intensity on T2 and FLAIR weighted images, and low signal intensity (hyposignal) on T1 weighted images. Diffusion-weighted imaging demonstrates an uneven high signal mainly at the peripheral part of the mass and a central decrease in ADC. In addition, the diagnosis of cerebral toxoplasmosis can be suspected by the presence of a sign. This is the "target sign" which, when present, can help make the diagnosis. The T2W target sign has a hypersignal inner enhancement core surrounded by an intermediate area of hyposignal and a hypersignal peripheral rim. The innermost core may be central or, more often, eccentric, thus describing the "asymmetric target sign" [7] (Figure 2a). The MRI spectroscopy revealed an elevated lipid peak, a drop in NAA, but above all showed a drop in the membrane proliferation marker (choline), thus going against a tumoral or demyelinating lesion in the acute phase [4].

Figure 1: axial section T2 (a, c and d) and FLAIR (b) Multiple lesions of variable size, with surrounding edema, diffuse, involving the cerebral hemispheres, brainstem and cerebellum. These lesions are hyper in T2 and FLAIR signal and concern the subcortical regions, the white matter and the basal ganglia, some of which exert a discrete mass effect (arrow).

Figure 2: T2 axial slice (a) and FAT SAT after Gadolinium injection (b) (a) T2 target sign with a central hyper signal surrounded by an intermediate hyposignal and a peripheral hypersignal border (arrow). (b) Multiple contrast with annular enhancement.

Cerebral toxoplasmosis, an opportunistic infection leading to the discovery of AIDS in half of the cases, as in our observation which was previously unknown to HIV infection. The lack of specificity on imaging makes the diagnosis difficult. It is important to know how to evoke it in front of multiple intracranial space-occupying processes (SOP) preferentially located in the basal ganglia and taking the aspect of an encephalic miliary helped also by the aspect of target image. In the presence of these types of lesions, HIV status must be systematically sought, even if a few very rare cases have been described in immunocompetent patients [1, 4].

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