Dusting off this old chestnut again as I feel I have some different perspectives.
1. For plain film radiography, palpation of the symph is not needed. Ever.
2. If you absolutely must know the location of the pubic symphisis, it is located at the level of the greater trochanters in the mid sagittal plane. Specifically, the midpoint between the inferior margins of the greater trochanters is a strong approximation of the location of the inferior aspect of the pubic symphisis - particularly with internally rotated hips such as for demonstration of femoral neck.
3. Centering points using unit distances assume that all patients have the same physical dimensions and have developed in the same way. This is not the case. As patients can in a large variety of sizes, techniques using measured distances would also need to come in a large number of measured distances per projection. Use geometrical descriptors and ratios instead. I don't know how many radiographers told me that "a lateral knee will be perfect if you can put 3 fingers between the patella and the cassette." Some said 2 fingers, some said 3 fingers, I said what if they don't have a patella, what if its a 3 year old, what if its Andre the giant, what if I happen to have massive fingers. A lateral knee is lateral when the central ray passes through the centre of each femoral condyle, perpendicular to the image receptor.
4. Where do I centre for a pelvis? When students are around, at the level of the inferior 1/3 interval between greater trochanter and ASIS in median sagittal plane. When students aren't around, eyeball it, then palpate crests and trochanters to assess coverage/collimation.
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