In this study, we have defined the T1-dark rim sign in the MRI exam of patients with MS. This sign, found in 3D T1 gradient-echo imaging, is highly sensitive and can detect nearly all (97%) PRLs observed in SWIp phase. However, with a 74% PPV, over a quarter the of lesions with a T1-dark rim did not have identifiable PRLs on SWIp phase. The clinical significance of these T1-dark rim lesions with no identifiable paramagnetic rim and their possible histopathological relationship with smoldering lesions remain unexplored.
The recent shift in MS patient management underscores the importance of accurate prognostic biomarkers. Early high-efficiency disease-modifying treatments can delay disability in patients with high inflammatory activity [3, 4]. Additionally, new treatments are effective in non-active progressive MS forms, which previously had no treatment [5]. Therefore, markers aiding the early identification of suitable patients are essential [2].
PRLs, visible in SWI, indicate smoldering inflammation and have been proposed as disability progression biomarkers [14, 15]. However, SWI is not routinely used in MS MRI follow-up [16]. Furthermore, these lesions have been primarily characterized using high-field scanners, and lower-field scanners have been less commonly used [11, 12, 27, 28]. Lastly, the identification of PRLs is complex and requires a detailed examination of images, which presents a challenge for their quantification in clinical practice [21].
However, previous studies have described the characteristic T1-hypointensity of PRLs [10, 18–20]. Recently, in one study, normalized-intensity 3DT1TFE was used to compare the intensity profiles of PRLs versus non-PRL white-matter lesions, and it was found that a semi-quantifiable deep hypointensity was highly specific to PRLs and, thus, could be studied as a surrogate marker of smoldering inflammation [21]. In the current study, we validated that these pronounced hypointense foci are a PRL hallmark and are that they have a characteristic organization as peripheral rims, easily spotted on 3D gradient echo T1-weighted imaging like 3DT1TFE.
Building on these observations, we postulated that these intensely hypointense voxels might signify active inflammation, unlike "shadow" and "chronic inactive" lesions which are not as hypointense [21]. So, while in acute active lesions, the whole lesion appears hypointense on T1WI [29], our study demonstrates that in smoldering lesions, it is the lesion rim that is hypointense, possibly indicating the location of the inflammatory front or active inflammation zone.
While 74% of T1-dark rim lesions matched with true positive phase-rim lesions, the clinical significance of the remaining 26% is intriguing. Are they indicative of smoldering inflammation, and do they share the prognostic value of PRLs on SWI? The high sensitivity of T1-dark rims suggests they could be a more sensitive measure for smoldering inflammation than SWI's paramagnetic rims. Contemplating the broader picture of PRLs, it could be suggested that we are witnessing the “tip of the iceberg” regarding smoldering inflammation. For example, slowly-expanding lesions defined on consecutive MRIs are also considered smoldering inflammatory lesions, and they do not always have an accompanying paramagnetic rim on SWI [30, 31]. Also, as MRI field strength escalates, PRL detection sensitivity on SWI increases; more lesions surface on 7T than on 3T or 1.5T [11, 12, 27, 28]. This situation leaves us in the dark about the amount of unseen inflammation based on different imaging parameters.
Our research has several inherent strengths, foremost of which is the introduction of the T1-dark rim sign. Indeed, this novel imaging sign serves as a practical alternative to the more advanced SWI techniques for detecting PRLs. Secondly, the image review performed by two neuroradiologists with substantial experience in MS neuroimaging ensured a robust interpretation of the findings. Thirdly, by viewing multiple imaging modalities concurrently in a lesion-centric approach, we ensured a comprehensive and nuanced understanding of the lesion morphologies to enhance the accuracy of our classifications. Finally, the transparent sharing of our imaging data not only solidifies the robustness of our findings but also invites collaboration and scrutiny within the scientific community.
However, the study has limitations. Although we incorporated a second imaging reviewer, we did not execute inter-reviewer correlation. This decision was driven by the aim to be as comprehensive and precise as possible across all sequences, ensuring proximity to the ground truth. Secondly, SWI is not a standardized sequence, and the characteristics of SWI vary between vendors. This may particularly affect aspects such as the presence of the T1-shine-through effect, which might not necessarily occur when using scanners from other vendors. Lastly, the relatively small size of our subject sample, which was not balanced in terms of disease, prognosis, or demographic characteristics, is another limitation, which prevented us from conducting more comprehensive subject-wise analyses.
Overall, our study introduces the T1-dark rim sign as a highly sensitive and accessible imaging marker for the detection of iron-rim lesions. As SWI is not part of standard clinical MS imaging protocols, the T1-dark rim could serve as an alternative marker to identify these lesions, thus potentially enhancing early detection and treatment adjustments for patients with MS at higher risk of disease progression. The findings of this study pave the way for further research to validate the clinical significance and prognostic value of T1-dark rims, which could substantially influence clinical decision-making and patient outcomes in MS.