Can MRI Provide a Bosniak Score?
Yes, MRI can definitively provide a Bosniak score for cystic renal masses, and the updated Bosniak Classification version 2019 now formally incorporates MRI as an equivalent imaging modality to CT. 1, 2
Evidence Supporting MRI for Bosniak Classification
Guideline-Based Recommendations
The American College of Radiology explicitly states that the Bosniak classification system can be applied using either CT or MRI with contrast, with both modalities requiring assessment of enhancement patterns to evaluate enhancing nodules, walls, or thick septa within cystic masses. 1
The European Association of Urology guidelines confirm that the Bosniak classification is based on CT or MRI diagnostic criteria, not conventional ultrasound, establishing MRI as a valid modality for this classification system. 1
MRI without and with IV contrast is recommended as an appropriate alternative to CT for renal lesion characterization and Bosniak classification, with the American College of Radiology noting that MRI demonstrates higher specificity than CT (68.1% vs 27.7%) for distinguishing benign from malignant masses. 3, 4
Comparative Performance: CT vs MRI
Agreement between modalities is substantial but not perfect:
In a study of 69 cystic renal masses, CT and MRI showed agreement in 81% of cases using the Bosniak classification, with disagreement in 19% of lesions. 3
Using Bosniak version 2019, overall interrater agreement was substantial for both CT (κ = 0.745) and MRI (κ = 0.655), with no statistically significant systematic category upgrades by either modality. 2
Another study demonstrated 72.3% agreement (kappa = 0.63) between CT and MRI for Bosniak version 2019 classification. 5
Key Differences in MRI Findings
MRI may detect additional features that can affect classification:
MRI frequently depicts more septa than CT (p < 0.001), which can lead to classification upgrades in some cases. 2, 5
MRI may show increased septal or wall thickness and enhancement that is not visible on CT, potentially upgrading lesions from category II to IIF, IIF to III, or III to IV. 3, 6
MRI identifies more protrusions (p = 0.034) compared to CT, which can influence categorization. 5
The new Bosniak version 2019 category of heterogeneously T1-weighted hyperintensity is unique to MRI and accounts for some classification differences. 2
Diagnostic Performance
MRI demonstrates excellent sensitivity and specificity:
Using Bosniak version 2019, MRI achieved 100% sensitivity and 86% specificity for identifying renal cell carcinoma in cystic masses, compared to 95% sensitivity and 81% specificity with the original classification system. 7
Bosniak version 2019 improved specificity on MRI (70%) compared to the original Bosniak classification (66%) while maintaining sensitivity. 5
Optimal MRI Protocol for Bosniak Classification
The American College of Radiology recommends a comprehensive MRI protocol including:
- T2-weighted images for characterizing cystic components 4
- Chemical shift T1-weighted images for detecting fat and hemorrhage 4
- Contrast-enhanced T1-weighted images (pre- and post-contrast phases) for assessing enhancement 4
- Diffusion-weighted images for additional characterization 4
Important Caveats
Limitations of MRI compared to CT:
MRI has limited ability to detect calcifications, though calcifications no longer have a significant role in the updated Bosniak version 2019 classification system. 3
Small cysts (<1.5 cm) remain challenging to evaluate even with MRI, though MRI may be superior to CT for these lesions due to higher specificity and absence of pseudoenhancement artifacts. 3, 1
Clinical implications of modality differences:
When MRI and CT disagree on classification, MRI more commonly leads to upgrades due to detection of additional septa, protrusions, or enhancement features. 2, 6
Neither modality is systematically superior; the choice should be based on patient factors (renal function, contrast allergies) and institutional expertise. 2