One alternative to presenting images in a side-by-side format is to display the images in an animated sequence, in the same location, so that any dierences between the images will appear to ash on and o ( icker) or appear to move. Whilst there is extensive research into the sensitivity of the human visual system to temporally modulated stimuli (see section 1.5.2 for a short review), a large section of this research investigates the detection of icker as a property in its own right (where the observer is discriminating between a ickering and non- ickering stimulus) and there is considerably less research investigating the use of animated displays as an aid to the detection of signals within those displays. The intuitive notion behind animated displays is that this mode of presentation will enable the visual system to make use of the correlation between the images facilitating redundancy reduction and better signal detection performance.
The use of animation to assist in visual search and detection is not a new idea, however, and examples of its application can be found in a number of diverse applications. In astronomy the blink comparator (Mayer & Phillips, 1983) has been used since the early 1900s to study astronomical photographs and it operates by rapid alternation of the images, enabling the astronomer to detect small dierences between the images. When comparing two images of the night sky, taken several days apart, moving objects such as planets and comets will be observed to move and icker, whereas
stars will remain stationary and, in 1930, the planet Pluto was discovered by Clyde Tombaugh, using a blink comparator to study photographic plates of a region in the constellation Gemini (Fraknoi, 2009).
Security is another application, using animated displays of images of security seals, taken at the initial sealing and at a later date, typically following transport or storage. Rapidly alternating the two images will cause any dierences to ash on and o, which may be evidence of tampering (Lazerson, 1984). Research has also been carried out comparing the eectiveness of displaying two architectural drawings sequentially in the same location against displaying them side by side for the detection of dierences, and possible errors in reproduction, between the two drawings (Fleury & Jamet, 2014). Although not strictly an animated display, presenting the drawings sequentially in the same location does bear some resemblance to the animated displays used in this thesis and Fleury & Jamet (2014) demonstrated improved eectiveness for the detection of errors (seen as dierences between the two drawings) using this strategy.
A common factor across the application of animated displays in astronomy, security and ar- chitecture is the close similarity of the images being compared, with the aim of nding minor dierences. The images compared in medical imaging, and in particular in mammography, rarely have such high levels of similarity and this may be one reason why there appear to be very few examples of the use of animated displays in this eld. Notwithstanding this potential limitation, some studies have been conducted, such as the use of a blink comparator approach by Carlbom (1994), as a technique to identify mismatch between corresponding views of nerve cells to assist in registration and the construction of 3D images of the cells.
Animated displays were compared against the more traditional side by side display for the detection of tumours in brain images by Erickson et al. (2011). Erickson et al. (2011) used baseline and follow up images from 66 cases with a known malignant brain tumor and asked participants to identify changes between the baseline and follow up images. Three processing methods were tested; normal, where no processing was carried out, image subtraction and change detection using a computer algorithm. Each of these methods could be displayed in a traditional side-by-side mode or a icker mode, alternately displaying the images in the same location, thus giving a total of six presentation methods (Erickson et al., 2011). Whilst image subtraction and change detection were found to be signi cantly better than the normal images for detecting subtle changes, the option of using animated displays did not improve detection for any of the presentation methods. The study used experienced neuroradiologists, although only one had any experience of animated displays and this observer did show an improvement with icker, though it did not reach signi cance. Erickson et al. (2011) concluded that the lack of a signi cant eect for icker may have resulted from a lack
of experience and training with the technique and, despite their ndings, still believed that icker has the potential to improve detection performance.
In relation to mammography, van Engeland et al. (2003) conducted a study to investigate the use of optimised displays for the detection of temporal change between previous and current mammograms. The study used real mammogram pairs without abnormality and pasted in lesions extracted from abnormal mammograms. The extracted lesions were resized to give seven new lesion dimensions and one resized lesion was pasted into one mammogram of the pair whilst the original size lesion was pasted into the other image of the pair. Two display modes were used; the traditional side-by-side display and the optimised display which allowed the observers to toggle between the two images of the pair. Four observers, who were physicists working in the eld of mammography, were asked to select the image containing the largest lesion and a psychometric function of the fraction correct against resize factor was plotted showing a signi cant increase in performance for three out of the four observers when using the optimised display of toggling between the images (van Engeland et al., 2003). Whilst the toggle rate was manually controlled and determined by the observers and a dierent task was used, the study by van Engeland et al. (2003), nevertheless, hints at the strong potential for the eectiveness of an animated display mode such as will be tested in this thesis.
Honda et al. (2014) also conducted research into the use of an image toggle tool for the com- parison of digital mammograms. The study used the retrospective images from 12 patients with suspected breast cancer and found that when the images were well aligned the use of the image toggling tool enabled easy detection of dierences between the image pair, suggesting that the tool may be an eective aid to the radiologist (Honda et al., 2014). A similar study was conducted by Hasegawa et al. (2008), who found that with accurate registration, toggling between images was an eective method for nding subtle masses, asymmetries and architectural distortions within the breast. Whilst the research supporting the use of sequentially displayed images by toggling may be limited, the technique does have its advocates with a number of texts recommending toggling images as an aid to the detection of abnormalities (Dogan, 2012, p. 26; Kopans, 2007, p. 367).
Whilst not using sequential displays of mammogram pairs, another study investigating the eectiveness of utilising motion and icker as an aid to the detection of microcalci cations in real mammograms was undertaken by Plett et al. (2007). Plett et al. (2007) used two types of motion to create the dynamic cues; rstly, spatial motion, where each pixel oscillated sideways with the rate of oscillation determined by the pixel intensity such that lighter pixels oscillated faster than darker pixels. Secondly, they induced temporal motion by oscillating the intensity of each pixel with the rate of oscillation determined by the original intensity of the pixel and, again, lighter pixels
oscillated faster than darker pixels. Diagnostic performance for the detection of microcalci cations in real mammograms was measured for ve radiologists and ve non-radiologists and analysed using receiver operating characteristic curves (ROC). In comparison to displays without dynamic cues, the area under the ROC curve (AUC) was 20.8% and 8.4% greater for non-radiologists and radiologists, respectively (Plett et al., 2007). Although the study by Plett et al. (2007) measured performance for the detection of microcalci cations, rather than masses (as this thesis will focus on) and single mammograms, rather than prior and current mammogram pairs (again as focused on in this thesis), it does, once again, show that motion and icker are powerful cues that merit further investigation.
The evidence presented in section 1.7.3 gives an indication of the strong potential for animated displays to improve observer performance in signal detection tasks, however, the research cited is highly applied and there appears to be a dearth of research with a visual psychophysical approach that would provide the theoretical basis for an animated presentation method. One of the aims of this thesis is to to go some way to addressing this void by providing a theoretical basis for signal detection in paired noise elds and investigating this in laboratory conditions using Gaussian white noise images before addressing the applied use of animated displays to create a ickered stimulus in the eld of mammography.