PTW NORMI 13, User Manual

Page: 21 / 43

NORMI 13 (T42023)
Operating Manual - Constancy Test
D703.131.00/03 en 21
Optical density
Using a densitometer, measure the optical den-
sity always in the same place on the test image
that is shown on a film. Choose the step in the
middle of the dynamic steps for the measurement
(
4 , Figure 2 ).
Luminance
Using a luminance meter, measure the luminance
always in the same place on the test image that is
shown on the image display device. Choose the
step in the middle of the dynamic steps for the
measurement (
4 , Figure 2 ).
Pixel value
The pixel value
P
indicates the intensity of the
exposure incident on the detector system (e. g.,
imaging plate) at a given pixel.
The pixel value is determined at the output of
the imaging system or on the images stored in
the PACS (Picture Archiving and Communication
System).
The preferred spot for determining the pixel value
is the step in the middle of the dynamic steps
(
4 , Figure 2 ). The measuring field should com-
prise no less than 1000 pixels. It has proven use-
ful to utilize the area of the steps minus a border.
The measured pixel value
P
depends on the dose
value that might be measured behind the step.
The mean of all pixel values of this measurement
area is calculated as the characteristic pixel value
and is defined as the reference value for the con-
stancy test.
3.3.4 Spatial Resolution
Determine the spatial resolution by means of the
X-ray test pattern. Use the same image process-
ing parameters for the test image as for the deter-
mination of the reference values. Visually evalu-
ate the X-ray test pattern using a magnifying lens
with a 4 to 8 x magnification. The parameter is the
number of line pairs per millimeter (Lp/mm) in the
line group which can be distinguished at the reso-
lution. Compare the parameter with the reference
value and the limit value.
3.3.5 Contrast Resolution
Determine the number of low contrast elements
that are discernible on the test image. Prerequi-
site: all steps of the dynamic range must be dis-
cernible on the same test image. Use the same
image processing parameters for the test image
as for the determination of the reference values.