Calculation of the cardiothoracic ratio from portable anteroposterior chest radiography

Abstract

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.

Keywords: Cardiomegaly; Dyspnea; Radiography, Thoracic.

Fig. 1

Three pitfalls which prevent antero-posterior chest radiograph from being used to measure the cardiothoracic ratio: (A) The effect of reverse position; (B) The effect of shorter distance from radiation source; (C) The effect of not fully inflated thorax.

Fig. 2

Identification of the images that include the furthest 2 cardiac borders on postero-anterior chest radiography (A) and the cross-sectional image using the navigation function intrinsic to the picture archiving system (B). Image number 32 includes right cardiac border, here.

Fig. 3

Cross-sectional images simulating posteroanterior (A) and anteroposterior chest radiographies (B-D). SO is perpendicular to thorax and SR is tangent at the right cardiac border S, radiation source; O, R, points on the imaging cassette; numbers, radiation distances (cm)).

Fig. 4

Receiver operating characteristic (ROC) curves of the corrected and non-corrected cardiothoracic ratio (CTR) from anteroposterior chest radiography to discriminate congestive conditions. AUC, area-under-curve; CI, confidence interval.