| Standard chest technique is subject to local variations with
the principal two areas for difference in practice being
centering and exposure, over which there has been much
discussion in the last 20 years.
The debate on the "correct" centering is generally
concerned with using a horizontal central ray and centering to
T7 or a caudally angled central ray centered at T6.In practice
the variation in patient kyphosis is generally of greater
influence than the small variations in centering point.
With regard to exposure there are a great many variations in
technique, possibilities include.
1) Low Kv around 65 Kv
2) Medium Kv around 80 Kv
3) High Kv around 120 Kv
4) High Kv with an air gap
5) High Kv with a grid
Note generator waveform will influence the image also.
My personal preference is using 85 Kv (medium frequency
generator) no grid and Low contrast film screen combination
Imation T8 Screens and Kodak (Imation) XLA film
Images demonstrating changes in exposure Kv
 65
Kv 8 mAS (High Contrast)(Under penetrated)
 85
Kv 3 mAS (Medium Contrast)
 95
Kv 2 mAS (Low Contrast)
The effects of poor Positioning
There are 3 principal positioning errors
1) Rotation the median saggital plane is not at 90
degrees to the film, shown by asymmetry in the distance of the
medial ends of clavicles from the midline
 Rotated
2) Failure to rotate the shoulders forward and down,
shown by the imposition the scapulae on the lung fields.
 Scapula
projected over lungfield
3) Failure to ensure that the coronal plane is parallel to
the film, either a) the patient leans back or b) leans
forward,
a) Lordotic
b) Clavicles
projected too low
Another important aspect to remember is to make the
exposure on full inspiration
Comparison of Inspiration and Expiration films on the same
patient.
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