Lumbar spine (AP/PA view)

The lumbar spine anteroposterior or posteroanterior view images the lumbar spine in its anatomical position. The lumbar spine generally consists of five vertebrae (see: lumbosacral transitional vertebra).

Indications

This projection is utilized in many imaging contexts including trauma, postoperatively, and for chronic conditions. Ideally, spinal imaging should be taken erect in the non-trauma setting to give a functional overview of the lumbar spine. Otherwise, patients with a suspected spinal injury must remain in the supine position without any movement.

Patient position

• the patient is erect or supine, depending on clinical history
• in the supine projection, hands are placed by the patient's side
• if performing erect, position the patient in the PA position; this has numerous advantages including reduced dose to the gonadal region and utilization of beam divergence; arms can be placed by the side, or the handlebars of the erect Bucky can be held for patient stability. The weight bearing PA view can be called the Ferguson technique. 

Technical factors

• anteroposterior projection
• suspended expiration (for a uniform density) 
• centering point
  • the level of the iliac crests at the MSP 
  • the central ray is perpendicular to the image receptor 
• collimation
  • superiorly to include the T12/L1 junction
  • inferior to include the sacral region 
  • lateral to include the transverse processes and sacroiliac joints
• orientation  
  • portrait
• detector size
  • 35 cm x 43 cm 
• exposure
  • 70-80 kVp
  • 40-60 mAs 
• SID
  • 110 cm
• grid
  • yes (ensure the correct grid is selected if using focused grids)

Image technical evaluation

• the entire lumbar spine should be visible, with demonstration of T11/T12 superiorly and the sacrum inferiorly. 
• no patient rotation as evident by central spinous processes and the symmetrical appearance of the sacroiliac joints and iliac wings
• intervertebral joints are visualized 
• adequate image penetration and image contrast is evident by clear visualization of lumbar vertebral bodies, pedicles, and facet joints, with both trabecular and cortical bone demonstrated

Practical points

• the three column concept of thoracolumbar spinal fractures is of particular importance when assessing this image for pathology
• take particular care when imaging patient on a trauma trolley that the image receptor is aligned to the central ray to prevent anatomy exclusion and grid cut-off
• ideally, the transverse processes should be visible, although demonstration is often obscured by overlying bowel gas; radiographers should ensure over exposure is not a factor contributing to the poor visualization which could mask a transverse process fracture 
• when imaging in a supine position, a triangular cushion can be placed under flexed knees to reduce lumbar lordosis, and thus aiding to open the intervertebral joints