Systematic review and comparison of measurement methods for determining the cardiothoracic ratio (CTR) on chest X-rays with subsequent validation using chest CT

Background: The cardiothoracic ratio (CTR) is a widely used measurement on chest x-rays (CXR), yet its accuracy and consistency remain limited due to the lack of a standardized measurement method. This study aims to systematically identify and compare established CTR measurement methods from the literature and validate them against computed tomography (CT)-based measurements, which serve as a precise anatomical reference. Methods: A systematic review was conducted for studies that reported CTR measurement methods on adult CXRs. Eight distinct measurement methods were identified. Five reproducible methods were selected and retrospectively applied to CXRs of 10 patients who also had chest CT scans. The CTR$_{CXR}$ values were compared with CTR$_{CT}$ measurements. Statistical analysis was performed with Friedman test and Dunn–Bonferroni post hoc comparisons. Results: Systematic literature search yielded in 251 studies of which 38 were included. A total of 8 different measurement methods were identified. Comparative analysis showed significant differences between CTR$_{CXR}$ methods (Friedman test: χ$^{2}$ = 27.104, df = 5, P <.001). Method 1 showed a statistically significant underestimation of CTR values compared to CT (adjusted P =.004), while Methods 4 and 6 demonstrated the smallest average deviation from CT measurements (3.64% and 4.42%, respectively) and no significant difference (adjusted P > .999). Conclusion: CTR$_{CXR}$ estimation is highly technique-dependent, with considerable variability across published methods. Methods using a midline reference for cardiac measurements and clearly defined anatomical landmarks for assessing thoracic width showed the strongest agreement with CT-based values. The results highlight the need for a standardized and reproducible CTR measurement protocol to ensure consistency and diagnostic accuracy in clinical practice. Based on our analysis, we strongly recommend using a midline reference for cardiac diameter measurements and suggest measuring the thoracic diameter along the internal rib margins at the level of either the left or right diaphragm dome.