Extraction of the Two-Dimensional Cardiothoracic Ratio from Digital PA Chest Radiographs: Correlation with Cardiac Function and the Traditional Cardiothoracic Ratio

The purpose of our study was to extract the two-dimensional (2D) cardiothoracic ratio from digital chest radiographs using image analysis software on a Magic View 300 system. We also wanted to investigate its correlation with cardiac function, as defined by left ventricular ejection fraction from MUGA scanning, and with the traditional one-dimensional cardiothoracic ratio. One hundred patients undergoing radionuclide ventriculography and concurrent digital PA chest radiography using a commercial selenium detector system were studied. The 2D cardiothoracic ratio was extracted by defining a region of interest around the cardiac and thoracic areas and calculating a ratio of the two pixel counts obtained. The one-dimensional cardiothoracic ratio was calculated in the traditional manner. Regression analysis was used to study the association between these ratios and the ejection fraction. The 2D ratio could be achieved on all radiographs after image manipulation. The traditional linear cardiothoracic ratio showed an inverse correlation with left ventricular ejection fraction (r = –0.45). The 2D cardiothoracic ratio showed an improved correlation (r = –0.52). Assuming that a left ventricular ejection fraction of 55% or more indicates normal cardiac function, the normal 2D cardiothoracic ratio is 0.23 or less and a ratio of 1:4 is suggested. The ratio of cardiac and thoracic area correlates better with cardiac function as assessed by left ventricular ejection fraction than the traditional linear cardiothoracic ratio. Two-dimensional cardiothoracic ratio is easily extracted and improves the data yield from digital PA chest radiographs on PACS systems.