AP CHEST X-RAY (Supine or Semierect in Department or as Beside Portable)
Pathology Demonstrated:
This projection demonstrated pathology involving the lungs, diaphragm, and mediastinum. Determining air-fluid levels (plueral effusion) requires a completely erect position with a horizontal CR, as in a PA or decubitus chest projection.
Technical Factors:
IR size - 35 x 43 cm (14 x 17 inches) crosswise (average to large)IR with or without stationary grid (IR without grid sometimes is used in mobile imaging at 70 to 80 kV)
90 to110 kV range with grid (large patient)Shielding:
Place lead shield to shield gonads.
Patient Position:
AP Supine Chest or Portable Xray
Patient is supine on cart; if possible, the head end of the cart or bed should be raised into a semierect position (see note). 
Roll patient's shoulders forward by rotating arms medially or internally.
Part Position:
Place IR under or behind patient; align center of IR to CR (top of IR about 1 1 /2 inches [4 to 5 cm] above shoulders).Center patient to CR and to IR; check by viewing patient from the top, near the tube position.
Central Ray and Collimation:
CR angled caudad to be perpendicular to long axis of sternum (generally requires more than 5degress angle, to prevent clavicles from obscuring the apices)CR to level of T7, 3 to 4 inches (8 to 10cm) below jugular notch.
Minimum SID of 40 inches (100cm) for supine
Collimate to area of lung fields.
Respiration:
Make exposure at the end of second full inspiration.
Remember: Crosswise IR placement is recommended for large or hypersthenic type patients to minimize chance of lateral cutoff.
This requires accurate CR alignment with center of IR with only minimal caudal angle to prevent grid cutoff is grid is in used.
Semi-Erect or Semi-Sitting Position
Use 72-inches (180cm) SID if this is possible.Always place markers on the IR or label the image to indicate the SID used; also indicate those projection obtained, such as AP supine or AP semierect.
Radiographic Criteria:
Criteria for chest x- ray radiographs taken in supine or semierect positions should be similar to those for PA chest projection.Three Exemptions of AP Chest Projection:
The heart will appear larger as a result of increased magnification from a shorter SID and increase OID of the heart.Possible pleural effusion for this type of patient often will obscure vascular lung markings when compared with a fully erect PA chest x -ray.
Usually, inspiration will not be as full, and only eight or nine posterior ribs will be visualized above diaghragm. Thus the lung will appear more dense beacause they are not as fully aerated.
