For softcopy-reading of mammograms, a room illuminance of 10 lx is recommended in standard procedures.
Room illuminance affects both the maximal monitor contrast and the global luminance adaptation of the visual
system. A radiologist observer has to adapt to low luminance levels, when entering the reading room. Since the
observer's sensitivity to low-contrast patterns depends on adaptation state and processes, it would be expected
that the contrast sensitivity is lower at the beginning of a reading session. We investigated the effect of an initial
time of dark adaptation on the contrast sensitivity. A study with eight observers was conducted in the context of
mammographic softcopy-reading. Using Gabor patterns with varying spatial frequency, orientation, and contrast
level as stimuli in an orientation discrimination task, the intra-observer contrast sensitivity was determined for
foveal vision. Before performing the discrimination task, the observers adapted for two minutes to an average
illuminance of 450 lx. Thereafter, contrast thresholds were repeatedly measured at 10 lx room illuminance over
a course of 15 minutes. The results show no significant variations in contrast sensitivity during the 15 minutes
period. Thus, it can be concluded that taking an initial adaptation time does not affect the perception of lowcontrast
objects in mammographic images presented in the typical softcopy-reading environment. Therefore, the
reading performance would not be negatively influenced when the observer started immediately with reading of
mammograms. The results can be used to optimize the workflow in the radiology reading room.
For quality control in mammographic softcopy reading (SCR) a number of recommendations exists. Among them is a room illuminance of 10 lx. Moreover, the use of masks on the image seems to be advantageous, due to a reduction of scattered light in the focus of view. Room illuminance affects the global luminance adaptation and the maximal monitor contrast; masking decreases the luminance in the central and near-peripheral region. We investigated the effects of masking and illuminance on foveal contrast sensitivity. A study with eight observers was conducted in the context of mammographic softcopy reading. Using Gabor patterns with varying spatial frequencies, orientations and contrast levels as stimuli and an orientation discrimination task, the intraobserver contrast sensitivity was determined for foveal vision. Tested illuminances for a non-masked image were 10, 30, 50 and 90 lx, and for a masked image 10 lx. Major findings are: (1) Masking does not lead to improved contrast sensitivity. Instead, all observers reported a strong fatigue effect during the presentation of the masked image. (2) Among the illuminances tested, only half of the observers showed the best contrast sensitivity at 10 lx. For the other observers best results were achieved at illuminance levels of 50 or 90 lx, respectively. The results can be used to appraise the effects of viewing conditions with the aim of drawing conclusions for mammographic SCR, and to initiate further studies.
In radiological practice the term recognition of detail is widely used. We examined how the term can be defined
and interpreted, and how recognition of detail relates to radiological phantoms such as CDMAM (Contrast
Detail Mammography). For tasks in visual perception a processing hierarchy can be assumed: The perception of
a structure can occur at different processing levels, such as required in detection, discrimination, identification
and recognition, with an ascending order of hierarchical relation. It is not always possible to predict from results
at one hierarchy level those at another level. If an observer detects a structure, there is no prediction whether
the observer will be able to discriminate the structure from another or whether he or she is even able to interpret
the structure. Furthermore, the perceptibility of a detail is influenced by surrounding or overlapping anatomical
noise. The presence of noise elevates visual thresholds and may change the overall perceptual behavior with
regard to the examined parameter. Thus, perceptibility of structures (details) is strongly bound to the type of
perceptual task and the image background used. Speaking of recognition of detail should not liberally extended
to evaluating performance parameters of a technical system. If the term is applied, it needs to be specified how
detail is characterized and which perceptual task is used for operationalizing recognition.
In mammographic softcopy reading, assessment of contrast resolution is mainly performed with phantoms, including
detection tasks with a homogeneous image background. For tasks in visual perception a processing
hierarchy is assumed, where detection tasks represent the base level. The results of investigations based on
detection tasks might not allow predictions on the sensitivity for recognizing low-contrast patterns in a situation
with complex images. We introduce the MCS method (Mammographic Contrast Sensitivity) for determining
the contrast sensitivity function (CSF) in mammograms. Gabor patterns and digits are used as visual targets.
The observers have to cope with an orientation discrimination task for the Gabor patterns and an identification
task for the digits. The contrast thresholds are measured by a psychophysical staircase procedure at six spatial
frequencies up to 16 cycles per degree. A study with eight observers was performed to show the applicability of
the MCS method. The results of the observer study with several mammographic cases show that the approach
is applicable independent of the chosen images. The results for Gabor pattern targets were different from those
with digits, both in overall sensitivity and in the shape of the contrast sensitivity function. Sensitivity to pattern
recognition is thus not reliably predicted from the Gabor CSF, and a more complex target like a digit or a
character should be preferred. The measurement of a contrast sensitivity function does not take more than 4
minutes. The results can be used to appraise the effects of viewing conditions with an aim of drawing conclusions
for mammographic softcopy reading.
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