8
3. Theory of Focal Spot Reproduction
Why measure the focal spot?
RTI Slit Camera User’s Manual
1.0B
=
m+n
m
s
if the width is calculated at 0 % of the LSF, and
= 0
if the width is calculated at 50 % of the LSF.
Unfortunately the standards specify calculation depending on film blackening, which
represents roughly 10 – 20 % of the LSF. However, normally
is so small that you can
forget about it since s is much smaller than f. It (
) will though not be small for small
mammographic focal spots. The standards specify that you shall use large enlargement
factors for small focal spots, which reduces the
a bit but not as much as the reduction
of the focal spot size. The relative error will thus be larger for small focal spots.
This calculation of
is done for a square shaped focal spot, it will however do as an
approximation for a focal spot of any shape.
3.2.
Why measure the focal spot?
The main aspect of X-ray imaging is to get images of objects. A typical X-ray focal spot
is shown in Figure 3–2. As seen there are two peaks in the width direction, which
depends on the tube filament (cathode). The filament is in this case a metallic thread in
a spiral form. The tube current gives rise to an electron cloud (or space charge) around
the filament. The distribution of the electrons is roughly constant around the filament.
Since the surface of the filament is rounded in one direction there are more electrons
along the sides when viewed from the anode. See Figure 3– below. This distribution
becomes reproduced on the anode when the electrons in the cloud around the filament
accelerate towards the anode by the tube voltage. On many newer X-ray tubes,
especially with small foci, this cannot as easily be seen as new methods to focus the
electron beam and different cathode structures have been developed.
Figure 3–2 Image of the focal spot of a radiographic X-ray tube.
Length
Width