Phantom positioning
A fixed source-to-image-distance (SID) of 115 cm was used
together with automatic exposure control (AEC) using the central
chamber. A tube potential of 75 kVp was selected and when combined
with the above factors allowed the production of a reference
image that was consistent with typical clinical imaging parameters.
These exposure parameters were established following a brief
consultation with four local departments and after reviewing recommendations
in the EC guidelines.7
For all exposures the collimated field was adjusted to include
the twelfth thoracic vertebra superiorly and the sacro-iliac joints
inferiorly. The use of fixed collimation was essential in order to
ensure it did not impact on phantom radiation dose or image
quality, as the amount of scattered radiation varies when different
volumes of tissue are irradiated.11 Anatomical markers were purposefully
omitted from the imaging process to avoid bias as this
could enable observers to determine the orientation of the
projection.
For AP projections the phantom was positioned supine in
accordance with standard radiographic technique,12 ensuring that
the median sagittal plane was coincident with, and at right angles
to the midline of the tabletop and bucky. The vertical central ray
was centred towards the midline of L3 at the level of the lower
costal margin.
For PA projections the phantom was positioned prone. In order
to ensure the centring point was replicated, masking tape was
applied to and wrapped around the torso of the phantom with its
superior border directly at the level of the horizontal line of the AP
centring point. The diameter to the left and right of the vertical line
in the AP projectionwas measured using a ruler and then replicated
in the PA orientation. Collimation was once again fixed and
consistent with the AP projection.