University of Technology, Sydney. Dept. of Health Sciences.Detection and recognition of early signs of breast cancer when represented by cancer-related
types of microcalcifications, are key requirements in breast screening
mammography. Edge detection plays a vital role not only in observing detail in lesions
with microcalcifications but also for the perception of often weakly defined stellate
tumours. Mammograms are read at around 60 to 80 per hour.
We develop a neurological based functional model for edge responses of on-centre
(Cʟ) and off-centre (Cᴅ) groups of neurons in the early vision physiology, from the
retina to the striate visual cortex V1. It is shown that the responses of the resulting
differential equations at the retina and at V1 are of the Lotka-Volterra (LV) type. They
display two important properties: consistency with the retinotopic property of early vision
physiology and susceptibility to the 1/ƒ type noise for a stochastic resonance (SR) effect
on the edge responses based on Cʟ and Cᴅ interactions. Computer simulations of
repeated cycles of the LV responses when 1/ƒ noise was incorporated into the interaction
terms of the equations, showed an increased probability of occurrences of closely coupled
small values of Cʟ and Cᴅ. This indicated an underlying stochastic resonance effect.
Such occurrences suggested a method for improved edge detection for "low observables"
and improved detail in microcalcification regions in mammograms.
An experimental viewing system using a dynamic polaroid and eyepieces with colour
filters was developed to provide 1/ƒ noise to test this hypothesis. It provided for the
resulting Cʟ and Cᴅ inputs from the left and right eye of an observer to interact in the V1
laminae. Using a set of 30 microcalcification region of interest images, tests were
conducted in real-time readings with five observers, each at two different times. There
was double blind randomisation of the sequence of images in addition to the order of
presentation of "with" and "without noise". The experience levels of the observers
ranged from a low level working with technical images (not mammograms) to a high
level experienced radiologist. An initial series of 300 observations using ratings of detail
quality (1 to 5) showed significance for an improved effect (p<0.05). A second series
of 300 observations for a more stringent test with improved symmetrical experimental
design of the equipment and alternative-forced-choice for observers to reduce
subjectivity, showed a "trend to significance" (p<0.1). Observers with more experience
had better performance in the tests (p∼0.07). Intra-observer variability was
consistently good compared with inter-observer results. A parameter in the L-V
equations which is related to observer attention, coupled with a spatial search
requirement, may be part of the inter-observer variability. The findings also have
implications for the training of radiologists in reading mammograms in real-time
screening. Recent developments in spectral properties of photonics materials may
provide a simple implementation of the principles developed in the thesis