Introduction

The cardiothoracic ratio (CTR) represents a fundamental radiological parameter in cardiovascular assessment, offering a straightforward and non-invasive approach to evaluate cardiac dimensions and monitor therapeutic responses . This radiographic index is obtained from postero-anterior (PA) chest radiographs by calculating the proportion between the maximal horizontal cardiac diameter and the internal thoracic width . The cardiac measurement comprises the sum of the maximum right and left heart border distances from the midline, while the thoracic component is measured between the inner rib margins .

Technical Aspects and Measurement

The accuracy and reliability of cardiothoracic ratio measurements are fundamentally dependent on standardized radiographic technique and precise measurement methodology. Proper image acquisition requires postero-anterior (PA) chest radiographs obtained at a focus-to-detector distance of 1.8m, with the X-ray tube set at 120 kV for optimal imaging quality . Patient positioning is crucial, demanding a standing position with full inspiration during exposure, while minimizing rotational artifacts that could compromise measurement accuracy .

The measurement protocol involves two primary components: the maximal horizontal cardiac diameter (MHCD) and the maximal horizontal thoracic diameter (MHTD). The MHCD is determined by summing the widest distances from the cardiac borders to the midline of the spine , while the MHTD is measured at the widest point of the inner thoracic cage at the level of the right hemidiaphragm . The CTR is then calculated as the ratio of MHCD to MHTD .

Several technical limitations warrant consideration during interpretation. Notably, supine and anteroposterior projections can lead to cardiac size overestimation due to magnification effects . Additionally, respiratory phase significantly influences measurements, with the cardiac silhouette appearing larger during exhalation . Digital image quality factors, including various types of noise, may also impact measurement accuracy .

Values and Interpretation

Standard reference ranges have been established through extensive research, with values below 0.42 considered below normal and warranting careful clinical attention . The normal range spans from 0.42 to 0.50, while measurements between 0.50 and 0.55 indicate mild cardiomegaly. Values exceeding 0.55 suggest moderate to severe cardiomegaly, necessitating further clinical evaluation .

Demographic factors significantly influence CTR interpretation. Women typically demonstrate higher mean CTR values (0.467) compared to men (0.459) . Age-related changes are particularly noteworthy, as CTR tends to increase with advancing age in both genders. This increase correlates with changes in chest width, which typically expands until the sixth decade before beginning to decrease .

Population variations warrant consideration in CTR interpretation. Research has indicated that Black and Asian populations may have higher normal thresholds, with ratios up to 0.55 potentially falling within normal limits . Despite these variations, studies across different geographical regions, including Ghana, have validated the universal applicability of the 0.5 threshold as a general reference point .

Pediatric CTR interpretation presents unique challenges. Measurement accuracy can be compromised by children's inability to maintain breath-hold during examination . While CTR has limited diagnostic value for congenital heart defects in young children, it becomes increasingly relevant in adult populations with congenital heart disease, particularly in monitoring post-repair outcomes . Notably, neonates may present with normal CTR values up to 0.60 .

In clinical practice, CTR serves as a valuable first-line screening tool for cardiopulmonary diseases, offering a cost-effective and low-radiation method for detecting cardiomegaly . Its utility extends beyond basic screening to specific disease states, where it holds significant prognostic value. In hemodialysis patients, an elevated CTR (>0.55) emerges as a crucial independent predictor of two-year all-cause mortality . Similarly, in rheumatic heart disease patients undergoing valve replacement surgery, a baseline CTR exceeding 0.6 independently predicts increased perioperative and first-year mortality .

Heart failure patients with CTR values above 0.5 demonstrate higher mortality and hospitalization rates, making CTR a valuable tool for cardiovascular risk stratification in this population . However, clinicians must remember that CTR measurements have inherent limitations, including variations due to radiographic technique and positioning. Therefore, while CTR provides valuable diagnostic and prognostic information, it should be interpreted within the broader clinical context rather than in isolation .

Correlation with Other Imaging Modalities

The diagnostic value of cardiothoracic ratio (CTR) should be considered in relation to more advanced cardiac imaging modalities. In comparison with echocardiography, CTR demonstrates moderate sensitivity (83.3%) but limited specificity (45.4%) in identifying increased left ventricular volume . Similar patterns of moderate sensitivity and specificity are observed in detecting both left and right ventricular systolic dysfunction when compared to echocardiographic findings .

Cardiac Magnetic Resonance Imaging (MRI), widely recognized as the gold standard for cardiac volumetry and functional assessment , shows a weak but statistically significant correlation with CTR measurements. However, substantial overlap exists in cardiac MRI parameters between patients with normal and increased CTR values, highlighting the limitations of CTR as a standalone diagnostic tool .

In the context of computed tomography (CT), research suggests that simple axial left ventricular area-based measurements may provide more accurate heart size assessment compared to traditional CTR measurements . This finding underscores the evolving role of CTR in modern cardiac imaging, where it serves as an initial screening tool that often requires correlation with more sophisticated imaging modalities for definitive diagnosis.

In conclusion, the cardiothoracic ratio (CTR) is a valuable, accessible screening tool for assessing cardiac size on PA chest radiographs, but its accuracy is affected by technical factors, patient demographics, and physiological variables. While it can help detect cardiomegaly and other cardiac conditions, CTR often requires confirmation with more precise imaging modalities like echocardiography, cardiac MRI, or CT for a definitive diagnosis.