Discussion

Our scoping review highlights the diverse use of fat suppression techniques in MRI for visualizing TEBPI and shows temporal trends in published fat suppression techniques over the last 30 years. We found that STIR was the most commonly reported 2D imaging technique, appearing in 37 studies, while among advanced 3D methods, 3D STIR SPACE was the most frequently utilized sequence. As MRI is a principal non-invasive imaging modality for evaluating these injuries, selecting the appropriate fat suppression technique remains critical for obtaining accurate diagnostic information [27], [28], [29]. Effective fat suppression may improve visualization of the affected nerve roots and the extent of injury, thereby supporting decisions regarding conservative versus surgical treatment and preoperative planning.

STIR, the most reported 2D technique, offers effective fat suppression while providing high contrast between nerve tissue and surrounding structures [30]. While comparative studies in the context of the brachial plexus are limited, Dixon has demonstrated robust and homogeneous fat–water separation in several anatomical regions [29]. We found that while novel methods such as Dixon are gaining acceptance due to their enhanced diagnostic capabilities, traditional techniques such as STIR remain prevalent.

The adoption of 3D MRI sequences used in brachial plexus imaging were also analyzed, with 3D STIR SPACE being the most frequently reported sequence (8 studies), followed by 3D T2-weighted STIR SPACE and 3D FIESTA. Even though 3D T2 STIR SPACE is known for its reliable nerve visualization due to varying T2 relaxation times, it was not the most reported. This may be attributed to inconsistent reporting of whether 3D sequences were acquired with T1- versus T2-weighted imaging. Isotropic 3D sequences enable multiplanar and curved-planar reconstructions, although spatial resolution, signal-to-noise ratio, and acquisition time vary according to the applied protocol. Nonetheless, 3D sequences offer enhanced through-plane resolution, which significantly improves the visualization of complex brachial plexus injuries, although their in-plane resolution is arguably inferior compared to high resolution 2D techniques [30], [31].

Next, our findings depict trends seen in the use of magnetic field strengths with various 2D and 3D techniques. Overall, amongst 2D techniques the 1.5 T was used more often than the 3 T magnetic field strength. The STIR technique showed a preference for using 1.5 T magnetic field strength over 3 T. The CHESS and Dixon techniques were equally divided between the two field strengths. The SPAIR technique, though less commonly reported, favored 3 T. Other techniques predominantly utilized 1.5 T, with fewer papers mentioning the use of 3 T. In contrast, the 3D techniques had similar reporting frequencies of 1.5 T and 3 T across all papers, except in the T2 STIR group, where both papers reported using 3 T, and in the FIESTA group, where all six papers reported using 1.5 T. Although 1.5 T remains widely used, 3 T has been shown to provide superior signal-to-noise and contrast-to-noise ratios. These benefits, however, are counterbalanced by higher specific absorption rate (SAR) limitations and greater susceptibility to magnetic-field inhomogeneity artifacts at 3 T [32]. Despite its documented advantages, the impact of 3 T versus 1.5 T on clinical outcomes for brachial plexus injuries remains uncertain [33].

Lastly, we assessed the use of advanced MRI techniques such as diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI) which provide insights into the microstructural organization of the brachial plexus. DTI enables nerve fiber tractography and the calculation of in-vivo biomarkers such as fractional anisotropy and mean diffusivity [34], [35]. Their limited reporting in our study likely reflects technical complexity, acquisition and post-processing requirements, and limited availability [36]. Further studies are needed to establish their diagnostic value in mechanical brachial plexus injuries. Variability in their application across studies highlights evolving technology and differing radiologist familiarity. Despite this, these methods hold promise for improving MRI diagnostic accuracy in mechanical brachial plexus injuries.

Benefits and drawbacks vary across fat suppression techniques. Fat saturation methods such as CHESS offer high signal-to-noise ratio (SNR) and relatively short acquisition times; however, they are sensitive to B0 inhomogeneity, especially in the brachial plexus region [17]. STIR, using a 180° RF pre-pulse, offers more reliable suppression but adds T1-weighting, which limits its use for tissues with similar T1 properties to fat. STIR has also been associated with poor signal-to-noise ratio and a susceptibility to pulsation artifacts [37]. SPIR and SPAIR enhance fat suppression through spectral pre-saturation; however, SPIR remains sensitive to B1 inhomogeneities, whereas SPAIR is generally less sensitive to B1 inhomogeneity than SPIR because it uses an adiabatic inversion pulse. Variations in utilization amongst various fat suppression techniques may be attributable to differences in diagnostic benefits versus limitations of these techniques relative to one another.

Our review findings align with and expand upon existing literature in this domain. STIR has been the most frequently reported 2D fat suppression technique in the literature for visualizing mechanical brachial plexus injuries, largely due to its longstanding status as the gold standard for fat suppression in MRI protocols [37], [38]. We hypothesize that radiologist familiarity has likely driven its continued and comparatively higher use over Dixon or CHESS [39]. The introduction of a new technique does not automatically lead to its clinical adoption. There is a tendency to rely on established methods until the efficacy of the new technique is sufficiently demonstrated through research. A significant barrier to the adoption of new techniques is the need for their implementation and optimization of existing machines, which demands both time and financial investment. However, as newer protocols offering superior visualization become more widely adopted, the reliance on STIR is likely to decrease in favor of these advanced fat suppression modalities. In the context of surgical planning for complex cases, STIR and Dixon both offer accentuated delineation of the neural plexus segments that are distinguishable from surrounding soft tissue. Our review suggests a shift towards the adoption of newer techniques. The increasing reporting of Dixon suggests growing interest in its technical advantages; however, direct comparative studies are needed to establish its diagnostic and clinical value relative to STIR [14], [40]. The prominence of 3D STIR SPACE in our review aligns with its reported advantages in the literature, particularly its ability to produce high-contrast images with effective fat suppression, making it ideal for assessing complex nerve injuries, such as in traumatic settings. Given the challenges in assessing the severity of postganglionic nerve damage (i.e. rupture versus contusion), 3D STIR SPACE’s ability to provide detailed imaging of both preganglionic and postganglionic segments of the brachial plexus makes it a valuable tool for comprehensive injury evaluation [41], [42], [43]. Furthermore, its high spatial resolution, its ability to minimize artifacts and capture subtle caliber changes are important for accurately assessing the intricate anatomy of the brachial plexus [44]. Additionally, the adaptability of STIR sequences across different MRI environments likely contributes to the widespread use of 3D STIR SPACE. Studies consistently highlight 3D STIR SPACE’s superior diagnostic accuracy in detecting nerve thickening, neuromas, and other pathological changes, explaining its frequent use in our review [28]. However, lack of flow compensation is a known limitation, particularly in detecting nerve root avulsions, where optimized acquisition planes or flow compensation may be required. While these adjustments improve visualization, they also extend scan time, potentially limiting clinical feasibility [43].

Limitations

Our study is not without its limitations. The variability in study designs may limit the generalizability of our findings. The included studies did not describe protocols and application areas in sufficient detail to differentiate between sequences used for diagnosis of traumatic brachial plexus injuries versus surgical planning. This is an important knowledge gap as an assessment of the volume of viable nerve tissue in proximal stump is of crucial importance for patients with more extensive injuries, especially viewed through the lens of trends in surgical techniques for treatment of TEBPI. Future studies should correlate MRI findings obtained with different fat suppression techniques with intraoperative findings to assess their diagnostic accuracy and value in surgical planning. Additionally, this review did not identify any studies utilizing high-field MRI systems (>3 T), limiting our ability to assess their potential advantages in brachial plexus imaging. Future research should explore whether these systems improve diagnostic accuracy or clinical outcomes. Our analysis of correlations between fat suppression techniques and magnetic field strengths does not account for the time of publication or regional variations in funding, both of which may influence the adoption of specific fat suppression techniques and MRI field strengths over time. Furthermore, many clinical protocols likely incorporate a combination of 2D and 3D imaging techniques rather than using them in isolation, which was not consistently specified in the included studies. Another limitation is the inherent subjectivity in scoping reviews, although we took measures to minimize reviewer bias by implementing standardized criteria for study selection and data extraction. Furthermore, variations in patient populations (e.g., pediatric vs. adult studies), and operator expertise could have influenced the reported findings. Lastly, the exclusion of non-English studies could have resulted in the omission of relevant data from other regions. Despite these limitations, the study provides valuable insights into current imaging practices and offers a comprehensive foundation that can guide future research and clinical decision-making.

Conclusion

A variety of fat suppression techniques are utilized in the imaging of traumatic and entrapment-related brachial plexus injuries, with STIR being the most frequently reported 2D technique. Dixon and advanced 3D sequences, particularly 3D STIR SPACE, appeared increasingly in more recent publications. Future research should compare these techniques using standardized protocols and clinically relevant reference standards, including intraoperative findings, to determine their diagnostic accuracy and value for treatment planning.