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Ring Enhancing Lesions on a CT scan of the Brain

Question- What is the differential diagnosis of a ring enhancing lesion on a CT scan study of a brain?

Ans- Ring enhancing lesion (REL) are non-specific and occur with several pathologies imaged by contrast CT/MRI. The common etiological entities responsible for this appearance include:

The commonest conditions encountered in India are transitional stages of neurocyticercosis and tuberculoma.

Question- On CT are there any features that can help differentiate amongst these, specially between tuberculoma and neeurocysticercosis transitional granulomas?

Ans- Location- Typically NCC granulomas are located at the cortical- sub-cortical interface in the parietal, temporal, occipital and frontal regions. Tubercolomas are often seen also at the basal regions and in the posterior fossa.

Multiple vs, single. Though tuberculomas are more often multiple and have satellite lesion this feature is also seen in NCC quite regularly.

Size: Tuberculomas typically are larger >20mm and the wall is thicker and shaggy as compared to the smooth walled small NCC. However after treatment NCC can evolve and look similar.

Calcification: NCC typically heals with calcification in the majority while this is seen in only 10% of tuberculomas.

Cystic component: in the leave state and the early transitional stage the NCC is cystic with the center having very low attenuation values (like CSF).

If an eccentric hyper density is associated along with this (the presumed scolex) the diagnosis of NCC is certain. In the later stages of the transitional granuloma, especially after treatment, these features are often obscured and disc-enhancing lesion (DEL) are common.

Question-Is MRI more helpful in the differential diagnosis? If so, should a child with focal seizures be directly subjected to an MRI scan rather than a CT scan?

Ans- MRI scan has the following advantage over the CT scan:

MRI scan could delineate small lesions that are occasionally missed by the conventional CT; a single might turn out to be multiple. RELs, which have ”disappeared” on follow up CT are often picked up on high-resolution contrast MR (personal experience).
Signal characteristics: The central portions of tuberculomas typically demonstrate hypo intensity due to the presence of central caseating material. In comparison, the central areas in NCC typically are hyper intense compared to the periphery. However after treatment and during evolution to the granular nodular stage NCC has often has central hypointensty as well.
MR spectroscopy: This technique help in identification of specific biochemical components in the granuloma. Tuberculomas typically have lipid and lactate peaks while NCC has all peaks reduced. However there are technical limitation and the technique need s further validation.
MRI is not associated with radiation exposure. Recently a higher incidence of late cancers has been reported in children several years after CT radiation exposure.

In conclusion, often CT and MRI are complementary and both may need to be done to maximize information. However, if there are costs issued involved, it may be better to use computerized topography (preferably high end) with low scan times and using lower strength currents. It must be remembered that a non contrast MRI is inferior to a contrast enhanced CT.

Question- Are there other ancillary test that can differentiate between the lesions?

Ans- Unfortunately, no ancillary test can help clear the matters.

Routine CSF in normal except in thee rare instance of associated meningitis.

Routine and CSF serology for NCC has been very intensive in providing clues in the presence of RELs.

Radiology of thigh muscles, stool examination e.t.c. Looking for a cysticercosis affecting other tissues and organs is also unhelpful.

Evidence of tuberculosis anywhere else in the body, though useful, does not confirm that the REL/DEL is a tuberculoma.

Question-When is a biopsy indicated?
Ans- A biopsy should be considered initially only if there is a strong suspicion of a tumor. Even in HIV +ve patients in whom the diagnostic possibilities are diverse and includes toxoplasma, empiric treatment is the first choice.
In case the lesion is persistent, unchanged or increasing in size despite all interventions and remains symptomatic; a biopsy may be considered after due consultation with neuro-radiologists, neurologists and neurosurgeons.

Question- Is there a role for repeat imaging?

Ans- It must be done at least once.

One of the criteria for diagnosis in granulomas especially NCC is the evolution of the neuroimaging characteristic. Initially, there is REL with edema; the edema then reduces and a DEL is then seen Finally calcification supervenes without edema / contrast enhancement. This evolution confirms the diagnosis of NCC. Atypically, recurrent edema may occur around calcified lesions.

The other important reason to repeat images is to as certain the effect of interventions. The minimum period between to two scans should be at least 3 months, as evolution takes at least that much time. Once the diagnosis is established (with as much certainty as possible) repeat imaging could be avoided due to the risks cited above.