Radiographic Techniques and Positioning
Radiographic Techniques and Positioning
This domain tests your ability to select projections, centre the beam, position the patient, and mark images correctly. Expect standard views, SID choices, rotation assessment, and collimation.
Core rules
- Projection describes the beam path: PA, AP, lateral, oblique. The part closest to the detector is sharpest; the part farthest is magnified.
- Standard chest is PA erect at 180 cm SID to reduce cardiac magnification. KUB is usually AP supine at about 100 cm.
- Centre the beam to the area of interest and collimate to it. Include markers and patient identification.
- Assess rotation on a chest by comparing clavicular heads with spinous processes; on a limb, check joint spaces and cortical outlines.
- Immobilise gently and explain breathing instructions.
Worked example
A heart lies 10 cm from the detector. At 100 cm SID, magnification = 100/(100−10) = 1.11. At 180 cm, M = 180/170 = 1.06. The larger SID reduces magnification because the object-to-detector distance is a smaller fraction of SID.
Traps
- Reversing AP and PA; AP chest enlarges the heart more.
- Placing the marker after processing or omitting it.
- Centring to the cassette rather than to the anatomy.
- Ignoring rotation that mimics pathology.