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 utilised 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 utilisation 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) 

• centring 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 visualised 

• adequate image penetration and image contrast is evident by clear visualisation 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 visualisation 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