From:
G.S. Al-Balool, D.L. Newman,
The relationships between kV, mAs and thickness in film-based radiography: 25% and 15% rules. OK?,
Radiography,
Volume 4, Issue 2,
1998,
Pages 129-134,
ISSN 1078-8174,
https://doi.org/10.1016/S1078-8174(98)90009-8.(
https://www.sciencedirect.com/science/article/pii/S1078817498900098)
Abstract: Purpose: To investigate the validity of the 25% and 15% rules in manual exposure selection. Methods: The mAs required to produce a constant film density of 1.5 was determined over the X-ray tube potential range 50–150 kV for different thicknesses of water, under grid and no grid conditions. Results: The mAs change is an exponential function with depth and is dependent upon the attenuation coefficient. The 25% rule assumes a constant attenuation coefficient with a half value layer (HVL) equal to 3 cm. The results indicate that for non-grid work at low X-ray tube potentials and for grid work at higher tube potentials the 25% rule is applicable. The 15% rule implies an inverse power relationship between mAs and tube potential, the power term being 5. The results indicate that for both grid and no grid conditions the 15% rule is obeyed only over the tube potential range 50–100 kV and for thicknesses up to 15 cm. Discussion: The 25% and 15% rules have provided useful practical guidelines for exposure factor selection. It is shown that they are applicable to some but not all radiographic situations.
Keywords: exposure factors; selection; 25% rule; 15% rule
Simplified Conclusion of the paper: 15% rule holds for tissue of less then 13 cm thickness and Voltages less then 100 kV.
For other more drastic conditions the 20$% rule holds.
I quote:
" In fixed kV techniques, the 25% rule is widely
applied [1, 2]. This states that for every I cm
change in patient thickness a 25% mAs change is
required. This rule derives from the exponential
attenuation relationship in that the required mAs
change is proportional to e ^ ±Hx where His the
attenuation coefficient and x the thickness, the sign
depending on whether an increase or decrease in
mAs is required, i.e. whether there is an increase or
decrease in patient thickness. Therefore, as the
thickness changes the mAs needs to be changed
exponentially to maintain constant film blackening.
For the 25% per cm rule to apply over the whole
range of diagnostic tube potentials,/a is assumed to
remain constant and equal to 0.23I c m - ~ i.e. a half
value layer (HVL) for soft tissue of 3 cm. This, of
course is a simplification since 1~and hence HVL are
energy dependent. However, the mAs stations
on many X-ray units today follow this rule in that
each consecutive mAs step represents a change of
25%. Thus, in fixed kV techniques, thickness
changes can be compensated by appropriate step
changes in mAs."
For the 15% rule:
I quote
"In variable kV techniques the situation is not as
straightforward because there are a number of kV
dependent changes that take place when the tube
potential is varied. These include changes in tube
output, X-ray attenuation (absorption and scatter)
and film screen speed. At a given patient thickness,
adjustment of the tube potential requires a compen-
satory adjustment in mAs so that the resultant
exposure produces the same level of film blacken-
ing. The so-called 15% rule [3-6] has been used as
a guide to the change in mAs that is required. This
rule states that to achieve the same film blackening,
a 15% change in kV requires a compensatory
change in mAs of a factor of 2. If an inverse power
relationship exists between mAs and tube poten-
tial, this rule assumes that the power term is 5,
since 1.15s=2, i.e. mAscz kV -5. This rule results
from the relationships between mAs and tube
potential in determining X-ray output, attenuation
and film screen response. With the introduction of
high frequency generators and energy-dependent
intensifying screens it is possible that these rela-
tionships have changed and hence the rules may
now not be applicable or at least need to be
amended"
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