Knee (skyline Merchant view)

Mr Andrew Murphy et al.

The knee skyline Merchant view is a superior-inferior projection of the patella it is one of many different methods to obtain an axial projection of the patella. This is an ideal projection for patients that are better suited in the supine postion.

  • patient is supine on the table with both knees flexed at roughly 45°
  • patient's feet should be very close to the detector end of the bed (see technical factors)
  • superior-inferior axial projection
  • centring point
    • the central ray will be angled 160° from the vertical axis( or 30° from horizontal), shooting superior-inferior towards the base of the patella. This will require the tube to sit, close to the patient's upper body.
    • the detector will be angled at roughly 30° at the foot end of the bed in line with the tube, ensuring the toes are not in the way
  • collimation
    • laterally to include the skin margins of the knee 
    • inferior to include the femoropatellar joint space 
    • superior to include skin margin
  • orientation  
    • landscape
  • detector size
    • 18 cm x 24 cm
  • exposure
    • 60-70 kVp
    • 7-10 mAs
  • SID
    • 100-120 cm
  • grid
    • no
  • patella should be free from superimposition of all bony structures
  • clear visualisation of the patellofemoral joint space 

This projection is one of the more technically demanding projections of, the lower limb. Hence it being one of seven techniques (that the author can find) to achieve it. 

This particular method has a high yield if your patients can tolerate the position. Some points to consider when performing this projection: 

  • dose
    • this projection often requires more dose than conventional knee radiographs due to tube angulation and, more often than not, a larger FFD 
  • tube angle 
    • 30°  from horizontal is the academically acceptable angle for this technique, however, by assessing the lateral projection and working out the optimal angle from the inferior-superior approach can be beneficial
  • patients feet
    • the patient's feet will be at the end of the table and often if not careful; can get in the way of the projection, if the patient cannot move their feet to be flat (this is common) you may adjust the tube angle to compensate (within reason)
  • detector 
    • often this projection requires the use of either an angled wall detector or a free standing detector in a holder, If this isn't possible see the Laurine method
  • the pen test 
    • turning the collimator light on, hold the other end of a pen and place it on the lateral border of the patella, if the patient is positioned correctly, the pen will cast a shadow on the detector
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Article information

rID: 50666
Section: Radiography
Tags: cases, snippet
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