6. Appendix A
19
A.1. Nominal Focal Spot Size - IEC 60336
1.0B
RTI Slit Camera User’s Manual
6.
Appendix A
This appendix shows the tables for the nominal focal spot values for the different
standards.
6.1.
A.1. Nominal Focal Spot Size - IEC 60336
The publication IEC 60336 describes the different methods to measure the
characteristics of the focal spot. In IEC 60336 the slit method is recommended to get
accurate determinations of the focal spot dimensions. The other methods, pinhole and
star pattern, are also described in IEC 60336 because they are often used by
manufacturers and for clinical testing of X-ray tubes.
The nominal focal spot value (f) is a dimensionless quantity defined by IEC and the
permissible values of focal spot dimensions measured with the slit method is shown in
table A-1. For instance, if the dimensions of the focal spot size measured with a slit is
0.31
0.60 mm then, according to table A-1, the nominal focal spot size is 0.3.
When the dimensions of the focal spot are measured with the slit method the length
shall be multiplied with a factor of 0.7 if the nominal focal spot size is larger than 0.3. In
IEC 60336 the motivation for this is that the distribution in radiant intensity over the
length has relatively shallow shoulders, and the linear dimensions for length of the focal
spot become larger compared to the width even though the MTFs for both width and
length may be approximately equal. The reason for not using the multiplier 0.7 on tubes
with nominal focal spot sizes below 0.3 is said to be that in that range the dimensions
of the distribution of radiant intensity are more rectangular over both length and width.
The reasons given above for multiplying the length of the focal spot with 0.7 for focal
spot sizes larger than 0.3 implies special tube constructions which are out of date when
today's X-ray tubes are concerned, especially modern mammography X-ray tubes.
To get accurate results of the focal spot size measurements with slit and film they shall
be performed with an enlargement factor described in table A-2. This will not be a
problem when the manufacturers do the measurements because they have the
opportunity to dismantle parts of the X-ray unit (for instance the breast support).
However, when the slit method is used in a clinical environment to measure the focal
spot size on mammography X-ray tubes it may be impossible to get the magnification
of the focal spot required. The nominal focal spot value is often less than 0.4 and the
distance between the collimator and the breast support is not large enough to get an
enlargement factor as large as three.
When the slit method is used to estimate the focal spot size only (not MTF calculations)
it is accepted, according to IEC 60336, to examine the slit images by eye trough a
magnifying lens. The magnifying lens shall have a built-in graticule with divisions of 0.1
mm and a magnification of 10.
