Fluke Biomedical 07-645 - Manual (Page 7)

Instructions
Discussion
1
1-3
1.3 Discussion
The test object is compact and the dimensions of the target plates are consistent with the commonly used
Center of Devices and Radiological Health test stand. The target arrangement is designed to reduce the
ambiguity and difficulty associated with employing a sequential array of targets with small differences in
contrast between adjacent targets. With the Fluoro-Test target plates the observer focuses on a subset of
three targets at a given time. In each subset or row, large differences (≅3%) are present between
adjacent targets and it is easy to decide if a target is visualized or not. As noted above the threshold
contrast for a plate is the lowest of the values observed for the three rows of targets, and a threshold
contrast resolution precision of 0.5% is obtained by the sequential use of the two plates. Of practical
importance is that the effect of fluoroscopic tube potential can be taken into account employing Table 1
which lists the percent contrast for the Fluoro-Test targets with 2 mm of Cu attenuator as a function of
tube potential from 50 to 130 kVp in 10 kV increments. If in the event that a 1 mm or 3 mm Cu attenuator
is employed rather than a 2 mm attenuator, the percent contrast of the test plate targets for these
thicknesses of Cu attenuators are listed in Tables 2 and 3. If one needs the contrast values of the targets
for a 1.2 or 2.4 mm Cu attenuator, the two decimal place accuracy of the Tables 1-3 permit one to
develop an interpolated table for the desired in-between copper thickness.
The Fluoro-Test is designed to yield a quantitative assessment of fluoroscopic threshold contrast
resolution. This index along with limiting high contrast resolution and image intensifier input phosphor
exposure rate should be measured during acceptance testing and routine quality control audits. High
contrast spatial resolution and image intensifier input phosphor exposure rate should be measured for
each image intensifier mode i.e., 9", 6" and 4.5". However, it is only necessary to determine threshold
contrast resolution in the 9" image intensifier mode. If one has a 12", 9", and 6" image intensifier,
threshold contrast should also be assessed in the 9" mode.
Factors that affect fluoroscopic contrast resolution are the quality of the imaging chain, the image
intensifier input phosphor exposure rate and, on fluoroscopic units equipped with digital image processing
and averaging, the degree of digital averaging. For a given image intensifier input phosphor exposure
rate, the better the imaging chain, the lower the contrast resolution. If the input phosphor exposure rate is
increased, the fluoroscopic image contrast resolution will decrease and therefore improve. Likewise, if
digital averaging is activated, contrast resolution will decrease and therefore improve.
For a typical image intensifier input phosphor exposure rate (3.6 mR/min with the grid out and 2.0 mm of
copper in the beam), the low contrast resolution of a reasonable imaging chain will range from 3 to 4%. A
good imaging chain will achieve a low contrast resolution of less than 3.0% and the low contrast
resolution of a poor imaging chain will be as high as 5 or 6%. Often on a poor imaging chain, the input
phosphor exposure rate is increased to achieve acceptable low contrast resolution and image quality. For
additional information and further discussion, the user is referred to the paper by Wagner, Barnes and Wu
[1], It is anticipated that tracking the threshold contrast resolution of a fluoroscopic imaging chain following
acceptance testing will allow one to quantify its performance over time and determine when adjustments
and maintenance need to be performed or when imaging chain components need to be replaced.
1. A.J. Wagner, G.T. Barnes and X. Wu, "Assessing fluoroscopic contrast resolution: A practical and
quantitative test tool," Med. Phys. 18, 894-899 (1991).