AbstractID: 8239 Title: The Independence of Blur Radius on Magnification
When determining high contrast resolution of an imaging system such as a fluoroscopic chain, it is customary to express the results in
line pairs per unit length. Of course, the result is dependent upon magnification and so the resultant data is often presented as a table
with the line pairs per unit length for each magnification available. An alternative method of specifying the resolution which is
independent of magnification provides a quick and convenient method for observing and expressing the resolution. This method takes
advantage of the readily available radial (wedge shaped) line pair phantoms or, equivalently, a star phantom such as is used for
determining focal spot size.
When viewing such a phantom on the monitor, the blur radius observed on the monitor is readily seen to be independent of
magnification. By invoking the Nyquist Theorem the formula for blur radius is
2H
R=
NK A
where R is the blur radius measured on the monitor, H is the height of the monitor viewing screen, N is the number of TV lines or
rows of pixels, K is the Kell factor and A is the angle per line pair on the phantom. For an ideal Kell factor of unity, 1024 rows of
pixels and a line pair angle of 0.0134 radians, the blur radius is 4.4 cm. In practice, the blur radius often remains fixed while stepping
through the magnifications indicating an independence of Kell factor on magnification.