**9. Imaging features of toxoplasmic encephalitis**

Imaging systems are of great importance in revealing the pathology caused by the diseases, both in the follow-up and diagnosis.

Since hematogenous distribution has a very important place in T. gongii infections, most of the lesions are seen in the basal ganglia, corticomedullary junction, white matter, and periventricular area [100]. The most accurate modality to diagnose TE is *via* magnetic resonance imaging (MR), which may also show the full scope of the illness. Infection of the brain produces lesions with poor attenuation on CT, low or high signal on T1WI (suggesting hemorrhagic components), and high signal on T2WI with surrounding edema. Currently, several important lesions in the imaging features of TE are very helpful in the diagnosis. Foremost among these, uniform nodular or

multifocal ring-enhancing lesions are seen, particularly in the cortex and periventricular white matter [100, 101]. Lesions from toxoplasmosis exhibit uniform nodular or ring enhancement [100, 101]. If there is poor enhancement, it may be modest or nonexistent in immunocompromised individuals depending on their cellular immune response. The "concentric target sign" in T2-weighted imaging and the "eccentric target sign" in postcontrast T1-weighted sequences are significant MRI findings reported in TE [100, 102–105]. In particular, this finding is considered pathognomonic. Multiple lesions were seen in the bilateral basal ganglia, frontal, temporal, parietal, occipital, and cerebellar lobes, as well as ring enhancement on postcontrast sequence near edema and the development of "eccentric target sign" on an MRI of the brain with contrast. These lesions featured intra-lesional hemorrhagic foci and alternating hyper and hypointense zones on T2-weighted sequence, along with perilesional edema that was clearly visible and corresponded to the "concentric target sign" [105].

Susceptibility-weighted imaging (SWI) of hemorrhagic lesions commonly seen in TE was superior and more sensitive than GRE T2\*WI. It is emphasized that diffuse hemorrhage foci seen in TE and frequently detected in recent studies may also be because of the superior sensitivity of SWI in detecting hemorrhage [106].

The eccentric target sign linked histologically to a sizable necrotizing abscess. The enhancing rim histologically corresponded to a thick ring of histiocytic response with inflamed and proliferating vessels demarcated by immunostaining to Factor VIII related antigen of vascular endothelium, while the surrounding zone of necrosis caused the intermediate zone of hypointensity. On T2W images, the perilesional white matter edema with demyelination appeared as strong hyperintensities [107]. The ring-enhancing lesion's periphery occasionally contained tissue cysts containing *T. gondii* bradyzoite, while the necrotic zone's inflamed vascular zone contained diffusely dispersed tachyzoite forms that could be seen by immunohistochemistry using antibodies to tachyzoite-specific antigen (p30). Eccentric target sign, a characteristic of imaging, is thought to be strongly predictive of toxoplasmosis. Less than 30% of cases have this sign, which may be seen on postcontrast CT scans (asymmetric target sign) or T1W MRI scans [108]. There are three alternating zones in this: a centrally eccentric innermost enhancing core, a middle hypointense zone, and an outermost hyperintense enhancing rim. A leash of inflamed, leaking arteries that enter the lesion *via* a sulcus create the eccentric core [107].

Diffusion- and perfusion-weighted imaging (DWI and PWI) and spectroscopy may help distinguish toxoplasmosis from other disorders that mirror it, such as brain lymphoma and infections, especially in immunocompromised individuals. Toxoplasmosis may exhibit peripheral limited diffusion, but pyogenic abscesses often exhibit central restricted diffusion. PWI in toxoplasmosis is minimal, but high in

#### **Figure 11.**

*Ring-enhancing lesions surrounding hyperintensity, consistent with vasogenic edema, eccentric target sign, and concentric target sign.*

lymphoma and other neoplasms. Toxoplasma and pyogenic abscesses both had an increased lipid-lactate peak in spectroscopy [109, 110]. PET-CT and thallium singlephoton emission computed tomography often reveal increased uptake in lymphoma and decreased uptake in toxoplasmosis (**Figure 11**) [109, 111].
