Diffrentiate between PA and AP chest x- ray

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Normally PA view is preferred. The clavicles won’t project too high into the  apices or thrown above the apices. The heart won’t be magnified over the  mediastinum. The ribs will not appear distorted or unnaturally horizontal like  in lordotic chests. However pediatric chests normally employ AP recumbant or  upright views because the infants or toddlers can take full inspirations and  their body won’t be so thick that anatomy will matter much in PA vs AP views.


“Most differences in appearance are due to magnification differences and to changes in the alignment of the x-ray beam to the patient (mainly involving the “divergence of the beam” from its nearly “point source” to its pyramidal shape as it leaves the tube).

We usually like to perform chest x-rays with the patient in the erect or upright PA or (postero)anterior position–i.e., with the anterior surface nearest the film or image-receptor–and with the distance from the source, target or focal spot (on the anode) of the tube to the image receptor, that distance being called the “SID” (or FFD, AFD, TFD), preferably 72″ or so.

1. The erect position allows the diaphragm to descend more during (usually) suspended full inspiration, allowing the lungs to expand more, and the organs–including the heart–to be less “distorted” and “compressed” (as well as allowing gravity to help move abdominal organs downward and “out of the way”).

2. When the patient is facing the film, the heart–which is more anteriorly situated in the thorax–is nearer the image receptor, resulting in less magnification, and thus more “sharpness”, or recorded detail, than would be present were the patient in the AP position. Also, if done AP, patients tend to lean backward, causing what is called in radiology a “lordotic” appearance to the chest, with the clavicles appearing higher than normal, relative to the rib cage, and with the ribs themselves taking on a more “horizontal” appearance.

3. With a large SID, there is also less magnification (and more sharpness) of the image, compared to what would be obtained at lesser distances. In addition (or, related to this), the more central portion of the useful beam we will utilize at greater SID’s will exhibit less “divergence”
than one would note if using short SID’s (as we’d have to use more of the outer edges of the beam in order to cover the image receptor); this should mean, for the larger SID’s, there will also be less “shape distortion” than one would find with smaller distances.

The posterior ribs will appear more magnified, and the anterior less so, on a PA projection than on an AP. And the diaphragm (because the higher anterior portions are closer to the film when the patient is PA) may also appear differently than it will on an AP.

So, particularly if the patient is done not only AP but supine (lying on his back), and at a shorter SID (say, 40″ or so), the resulting image may be greatly different from that obtained with the usual PA erect CXR done at 72″:

* The chest (ribs and lungs) may appear broader and shallower
* The heart will appear larger and “flatter” (more distorted)
* The clavicles may appear higher than normally seen on a PA, and the sternoclavicular joints may appear “farther apart”
* The anterior ribs may look “wider” (and farther from the midline, due to the DOB, particularly with shorter SID’s), while the posterior ribs may look narrower and “sharper” (because they’re closer to the film), and all ribs may appear to be more horizontally directed (especially if the patient is leaning backward some)
* One might note fewer ribs visible above the diaphragm, partly due to the magnification differences and partly–for the supine patient–due to the increased difficulty in taking in a deep breath and to the “loss” of the benefits of gravity in displacing the abdominal contents downward
* In addition, when done AP, patients may tend to depress rather than to elevate the chin, which could cause it to be superimposed over the apices of the lungs (and which also may make it harder for the patient to take in a deep breath)
* The scapulae are less likely to be drawn laterally (out of the lung fields) when the patient is AP than when PA, and will also be closer to the film, so their edges will be even sharper than is normally seen”

26 thoughts on “Diffrentiate between PA and AP chest x- ray

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      Hi! I would like to know why we use SID 40″ for AP supine chest as we known that shorter SID will increase the magnification of heart. Tq

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