Assessment of cardiac size on portable chest films.

Using both anthropomorphic phantoms and human patients, the authors have shown that the effects on cardiothoracic (CT) ratio of changing from the posteroanterior (PA) to the anteroposterior (AP) projection are much greater than the effect of reducing the anode-to-film distance (AFD). This is because, in the AP projection, not only is the transverse diameter (TD) of the heart much farther from the film (and its image is therefore enlarged), but the TD of the chest is usually closer to the film and its image is reduced in size. These factors, operating in different directions, cause a major change in the CT ratio. The authors have shown that small hearts are magnified more than large hearts by changing from PA to AP, and that the position of the TD of the thorax (whether it is farther anterior or farther posterior) markedly affects what happens to the CT ratio when the AFD is changed. Because of this, it is impossible to predict accurately what effects a given AFD and projection will have on the CT ratio. However, using approximate correction factors (-12.5% of CT ratio for a 40 in AFD, AP film and -10% of CT ratio for a 72 in AFD, AP film) a clinically useful determination can be rapidly made as to whether the heart is enlarged or not.