Milne E N, Burnett K, Aufrichtig D, McMillan J, Imray T J
Department of Radiological Sciences, University of California, Irvine Medical Center, Orange 92668.
J Thorac Imaging. 1988 Apr;3(2):64-72. doi: 10.1097/00005382-198804000-00008.
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.
作者使用拟人化体模和人类患者进行研究，结果表明，从后前位（PA）投照改为前后位（AP）投照对心胸比（CT）的影响远大于减小阳极到胶片距离（AFD）的影响。这是因为，在AP投照中，不仅心脏的横径（TD）离胶片远得多（因此其影像被放大），而且胸部的TD通常离胶片更近，其影像尺寸减小。这些作用方向相反的因素导致CT比发生重大变化。作者表明，从PA投照改为AP投照时，小心脏比大心脏放大得更多，并且胸部TD的位置（无论是更靠前还是更靠后）显著影响改变AFD时CT比的变化情况。因此，无法准确预测给定的AFD和投照对CT比会产生何种影响。然而，使用近似校正因子（对于40英寸AFD、AP胶片，为CT比的-12.5%；对于72英寸AFD、AP胶片，为CT比的-10%），可以快速做出关于心脏是否增大的具有临床意义的判断。
