How many degrees and in which direction should the central ray be directed for an AP axial projection of the coccyx?

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Citation:

McWilliam, R., Murphy, A. Coccyx (AP view). Reference article, Radiopaedia.org. (accessed on 08 Sep 2022) //doi.org/10.53347/rID-50202

The coccyx anteroposterior (AP) view is used to demonstrate the coccyx, in conjunction with the sacrum and coccyx (lateral view). Follow departmental protocol in relation to imaging this region. 

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This projection helps to visualize pathology of the coccyx, especially fractures. To minimize superimposition of structures over the coccyx region, the urinary bladder and large colon should ideally be emptied before this examination 1.

Given that management of coccygeal fractures is nearly always non-operative, some radiology literature suggests that x-ray evaluation for coccygodynia is a waste of resources and exposes patients to unnecessary ionizing radiation, without having measurable impact on clinical outcome. Thus, in some territories (e.g. UK), the usual practice is to not perform routine imaging of the coccyx 2.

• the patient is supine, with arms placed comfortably by their side, legs extended 1

• anteroposterior view
• centering point
  • 5 cm superior to the pubic symphysis at the MSP 1
• central ray
  • angled 10° caudal 1
• collimation
  • must adhere to the ALARA principle given the radiosensitive region exposed via the primary beam
  • close collimation to the area of interest
• orientation  
  • portrait 
• detector size
  • 24 x 30 cm 
• exposure
  • 80 kVp
  • 15 mAs 
• SID
  • 110 cm
• grid
  • yes

• adequate penetration should clearly demonstrate the coccyx region
• the coccyx is free of superimposition from the pubic rami
• lateral margin of the coccyx is equidistant from the pelvic brim indicating no patient rotation

• given the proximity of this anatomy to the gonadal region, the risk versus benefit of ionizing radiation and diagnostic value should be considered before imaging occurs 2

References

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