Calculation of the cardiothoracic ratio from portable anteroposterior chest radiography

Cardiothoracic ratio (CTR), the ratio of cardiac diameter (CD) to thoracic diameter (TD), is a useful screening method to detect cardiomegaly, but is reliable only on posteroanterior chest radiography (chest PA). We performed this cross-sectional 3-phase study to establish reliable CTR from anteroposterior chest radiography (chest AP). First, CD(Chest PA)/CD(Chest AP) ratios were determined at different radiation distances by manipulating chest computed tomography to simulate chest PA and AP. CD(Chest PA) was inferred from multiplying CD(Chest AP) by this ratio. Incorporating this CD and substituting the most recent TD(Chest PA), we calculated the 'corrected' CTR and compared it with the conventional one in patients who took both the chest radiographies. Finally, its validity was investigated among the critically ill patients who performed portable chest AP. CD(Chest PA)/CD(Chest AP) ratio was {0.00099 × (radiation distance cm])} + 0.79 (n = 61, r = 1.00, P < 0.001). The corrected CTR was highly correlated with the conventional one (n = 34, difference: 0.00016 ± 0.029; r = 0.92, P < 0.001). It was higher in congestive than non-congestive patients (0.53 ± 0.085; n = 38 vs 0.49 ± 0.061; n = 46, P = 0.006). Its sensitivity and specificity was 61% and 54%. In summary, reliable CTR can be calculated from chest AP with an available previous chest PA. This might help physicians detect congestive cardiomegaly for patients undergoing portable chest AP.