The susceptibility ring

To the Editor,

We read with interest the study by Parry and colleagues titled “Evaluation of intracranial tuberculomas using DWI, MRS and SWI” recently published in BJR¹; as the conundrum of effectively discriminating ring enhancing tuberculomas from metastases and neurocysticercosis remains a significant radiological challenge in the Indian subcontinent and abroad owing to the large Indian diaspora.

The authors report the presence of a complete peripheral hypointense rim on SWI in 58.3% of tuberculomas & none in 26 metastatic lesions; indicating a high specificity (90.91%) but low sensitivity of this finding in discriminating between the two. In our experience, complete peripheral rims of susceptibility are not uncommon in metastatic nodules, albeit these are more common following radiation therapy. Furthermore, it is our observation that when present; the rims of metastatic lesions show more profound susceptibility & are thicker (Figure 1) as compared to tuberculoma, wherein the rim is faint and thin. We believe the difference in susceptibility may reflect the variation in paramagnetic content of the lesions. The free radicals postulated to cause susceptibility in tuberculomas may have lesser paramagnetic effects than deoxyhemoglobin and hemosiderin deposited within metastases. Intralesional hemorrhage in brain metastases is related to hypoxia from rapid cell turnover leading to expression of vascular endothelial growth factor (VEGF) and formation of abnormal vessels with increased permeability. Thus differences on SWI suggest that the magnetic susceptibility at lesion margins are of different sources and signify diverse histologic components.²

Figure 1.

Metastatic breast cancer. (A) Axial MIP (maximum intensity projection) of contrast enhanced 3D T1 sequence shows multiple metastases. (B) Axial SWI shows a complete peripheral rim of profound susceptibility in the right frontal metastasis. (C) Post processed filtered phase SW image shows the peripheral hemosiderin rim.

Metastatic brain nodules with a high propensity for intralesional hemorrhage include thyroid, choriocarcinoma, melanoma, renal cell, breast and lung cancer.³ Radbruch et al⁴ have postulated morphologic variability of ITSS (intratumoral susceptibility signals) to enable differentiation between different entities of cerebral metastases which may thus improve differential diagnosis in cases of unknown primaries.

The authors report a significant difference in presence of a complete hypointense peripheral rim of susceptibility between tuberculomas and metastatic brain lesions (p < 0.001). They report presence of a complete and regular hypointense peripheral ring in 42 cases of tuberculoma and none of the 26 cases of metastases. The authors also observe that the susceptibility signal of the ring in tuberculomas varied from slightly hypointense to profoundly hypointense. Contrary to this, our anecdotal experiences from clinical reporting suggests that a complete, regular rim of susceptibility is not uncommon in metastases. However, unlike tuberculomas, the rim in metastases (when present) is more profoundly hypointense on SWI and is thicker. Therefore, it would be instructive and interesting for the authors to comment if any of the metastases from their study demonstrated susceptibility and describe the morphology of the susceptibility artifacts in those cases.

We agree with the authors that the differentiation of tuberculomas from other ring enhancing lesions using susceptibility weighted imaging is a novel approach and merits future studies with larger numbers that further stratify ring enhancing lesions based on morphology and intensity of intralesional susceptibility.

Response to the “Letter to the editor: The susceptibility ring”

¹Arshed Hussain Parry MD, ¹Abdul Haseeb Wani, MD ²Feroze A. Shaheen, MD and ¹Mohd Ilyas, MD

¹Senior Resident, Department of Radiodiagnosis, Sher-i-Kashmir Institute of Medical Sciences Srinagar, Jammu & Kashmir, India

²Professor, Department of Radiodiagnosis, Sher-i-Kashmir Institute of Medical Sciences, Srinagar, Jammu & Kashmir, India

We read with enormous relish the response of Darshana Sanghvi et al. entitled “Letter to the editor: The susceptibility ring” wherein they have recounted their anecdotal experiences from clinical reporting. Their observation of presence of a complete peripheral hypointense rim in metastatic brain lesions on SWI is something we didn’t encounter in any of our 26 cases, although we did observe intralesional hemorrhage in many lesions. Additionally they have mentioned that such a finding was more frequently observed in metastatic lesions post radiotherapy. In our study we had already excluded known cases of malignancy and as such we did not study appearance of metastatic brain lesions post radiotherapy. We had postulated that it is the presence of paramagnetic free radicals within the peripheral capsules of tuberculomas that accounts for the presence of a complete regular peripheral hypointense rim on SWI. Though unsubstantiated, we believe, that hypoxia induced by rapid cell turnover leading to heightened expression of vascular endothelial growth factor (VEGF) within a metastatic tumor resulting in formation of fragile and leaky microvessels (neoangiogenesis) will not necessarily lead to a set pattern of uniform peripheral hemorrhage around the metastatic brain lesions. Rather such hemorrhages within a metastatic tumor are expected to be more random and haphazard in distribution unlike tuberculomas which are surrounded by free radical rich capsules and are thus expected to produce a complete and regular hypointense rim on SWI more consistently. However, we do not, based on our limited data of 26 patients rule out the possibility of a seemingly less probable phenomenon of uniform peripheral hemorrhagic staining to occur in metastatic brain lesions and thus yield a complete and regular hypointense rim on SWI. We furthermore reiterate that our study should serve the purpose of idea stimulation and thus pave way for further studies on this topic of immense importance.