User manual
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At this point the analysis is complete although small adjustments could be made
by refining the default settings as described in the following steps 7 and 8.
The diameter of the analysis circles: In general, the larger circle should
extend almost to the end of the beam lines, but be within fiducial marks. In the
gantry starshot the beam lines extend the film edge so the larger circle could
be 15 cm, or more, in diameter. The smaller circle should be ¼ to ½ the
diameter of the large circle. Smaller is generally better, but if too small the
maxima along the circular profile become less distinct as the dose to the film
“fills in” the decreasing space between the beam lines.
The width of the analysis circles: To a point the method benefits from
having wider analysis lines because image pixel noise is dampened through
an averaging process. But for the larger analysis circle the wider lines must
not extend past the end of the beam lines or be wide enough for the fiducial
marks to interfere. For the smaller circle the analysis line should not be broad
enough to be close to the area where the beam lines come together. A path
width of about 10 – 20 pixels at scan resolution of 72 dpi is ideal.
The minimum angle between the beam lines: In general, the error in
determining the intersection of two lines is in inverse proportion to the angle
between them. So the analysis can benefit by restricting the analysis to only
those lines crossing at more than a specified angle. The nine beam lines in
the gantry starshot are at 20° intervals so restriction to an intersection angle
>25° leaves 27 of the 36 beam intersection points in the analysis. In the
collimator starshot the six beam lines are at 30° intervals and restricting the
analysis to beam lines crossing at >30° leaves 9 of 15 intersection points for
the analysis.
7. In the following gantry starshot example, the analysis circles have radii of
about 250 pixels (approx. 88 mm at 72 dpi) and 125 pixels (approx. 44
mm) and an analysis path width of about 20 pixels (approx. 7 mm). The
result, depicted in Figure 9, shows the smallest circle encompassing all