X-rays pass from the anterior to the posterior of the patient - hence Anterior-Posterior (AP) projection. The image is still viewed as if looking at the patient face-to-face.
AP v PA - Heart size
The heart, being an anterior structure within the chest, is magnified by an AP view. Magnification is exaggerated further by the shorter distance between the X-ray source and the patient, often required when acquiring an AP image. This leads to a more divergent beam to cover the same anatomical field.
As a rule of thumb, you should never consider the heart size to be enlarged if the projection used is AP. If however the heart size is normal on an AP view, then you can say it is not enlarged.
The upper diagram shows an AP projection. Heart size is exaggerated because the heart is relatively farther from the detector, and also because the X-ray beam is more divergent as the source is nearer the patient.
The lower diagram shows a conventional PA projection. The apparent heart size is nearer to the real size, as the heart is relatively nearer the detector. Magnification of the heart is also minimised by use of a narrower beam, produced by the increased distance between the source and the patient.
AP v PA - Scapular edges
Radiographers will often label a chest X-ray as either PA or AP. If the image is not labelled, it is usually fair to assume it is a standard PA view. If, however, you are not sure, then look at the medial edges of each scapula.
AP projection images are of lower quality than PA images. Compare this image with the PA view below.
The image has been acquired by a mobile X-ray unit in the resuscitation room. Note the AP SITTING label.
The scapulae are not retracted laterally and they remain projected over each lung.
Heart size is exaggerated (cardiothoracic ratio approximately 50%). If seen on a PA image this would be at the borderline for cardiac enlargement.
The radiograph was repeated - see below.
In order to take a PA view the patient places his or her arms around the side of the detector plate, or stands with hands on hips. This ensures the scapulae are rotated laterally and no longer overlap the lungs.