Collimator Centering

This test determines the center of of the collimator's rotation. The images below are an example of those analyzed, with edges and center annotated.

Calculation of Collimator Center

To determine the X and Y center, an average is taken of the four vertical and four horizontal edges respectively. Edges are measured by finding the level at which they are at 50% intensity.

X = (Y1:90 + Y2:90 + Y1:270 + Y2:270) / 4

Y = (X1:90 + X2:90 + X1:270 + X2:270) / 4

Using the values below as an example calculation would be:

X = (-50.286 + 49.524 + -49.265 + 50.510) / 4 =  0.121

Y = (-50.286 + 50.410 + -50.600 + 50.062) / 4 = -0.104

For the examples, the following values are used:

Inputs

Description	Value
X collimator center	0.121
Y collimator center	-0.104
X1 90 degrees	-50.286
X2 90 degrees	50.410
Y1 90 degrees	-50.286
Y2 90 degrees	49.524
X1 270 degrees	-50.600
X2 270 degrees	50.062
Y1 270 degrees	-49.265
Y2 270 degrees	50.510