Philips Azurion - Source-to-Image Distance; Table Height; Influence of Oblique Projections

Using the wedges reduces the r
adiaon intensity in a user-dened area and improves the image
quality. The wedges also reduce the dose area product and the sta dose.
The amount of radiaon that is reduced by the wedges depends, for example, on the amount of the
image coverage by the wedges.
Source-to-Image Distance
According to the inverse square law, beam intensity increases proporonally with the square of the
distance.
When the source-to-image distance is increased by a factor x, the system increases the skin dose by a
factor x
2
to maintain the requested detector dose.
Hence, the source-to-image distance should be kept to a minimum (for a given source skin distance), so
the requested detector dose is reached with as low as possible skin dose. This implies that the source-
to-image distance should be reduced so that the distance between the paent and the detector is as
small as possible.
Table Height
The table height at a constant source-to-image distance does not inuence the reference air kerma
(rate), and the indicated air kerma (rate) value, as these are only applicable at the paent entrance
reference point.
It does however, inuence the paent skin dose through the inverse square law. For more informaon
on the inverse square law, see Source‐to‐Image Distance (page 293).
To minimize the skin dose (rate), the X-ray source must be as far from the skin as possible.
Inuence of Oblique Projecons
Due to the absorpon of radiaon in human ssue, the X-ray eld strength is reduced by a factor 2,
approximately every 3 cm.
For example, if the paent thickness is 27 cm, the X-ray beam loses intensity within the body by a factor
of 512 (2
(27/3)
). This shows that a thicker paent requires a larger entrance dose than a thin paent, to
obtain the same detector dose.
The same applies to oblique projecons of the X-ray beam since an oblique view generally increases the
perceived paent thickness. This can be seen in the gure below where distance 2 (oblique) is
considerably larger than distance 1.
Figure 132 P
aent thickness
The following example shows that the resulng air kerma is larger for a 30 cm PMMA than for a 20 cm
PMMA paent thickness, when measured at the same system sengs for three typical exposure
procedures.
Technical In
formaon System Sengs Inuencing the Radiaon Dose
Azurion Release 1.2 Ins
trucons for Use 293 Philips Healthcare 4522 203 52421