MATHEMATICAL METHODS IN MEDICAL IMAGE PROCESSING 7
increase in availability, diversity, and res olution of medical imag ing devices over the
last 50 years threatens to overwhelm these human experts.
For image analysis, modern image process ing techniques have therefore become
indispensable. Artificial systems must be designed to analyze medical datasets
either in a partially or even a fully automatic manner. This is a challenging ap-
plication of the field known as artificial vision (see Section 4.1). Such algorithms
are based on mathematical models (see Section 4.2). In medical image analysis,
as in many practical mathematical applications, numerical simulations should be
regarded as the e nd product. The purpose of the mathematical analysis is to guar-
antee that the constructed algorithms will behave as desired.
4.1. Artificial Vision. Artificial Intelligence (AI) was initiated as a field in the
1950’s with the ambitious (and so-far unrealized) goal of creating artificial systems
with human-like intelligence
3
. Whereas classical AI had been mostly concerned with
symbolic representation and reasoning, new subfields were cr e ated a s r e searchers
embraced the complexity of the goal and realized the importance of sub-symbolic
information and perception. In particular, artificial vision [32, 44, 39, 92] emerged
in the 1970’s with the more limited goal to mimic human vision with man-made
systems (in practice, computers).
Vision is such a basic aspect of human cognition that it may superficially ap-
pear somewha t trivial, but after decades of rese arch the scientific understanding
of biological vision remains extremely fra gmentary. To date, artificial vision has
produced important applications in medical imaging [18] as well as in other fields
such as Earth observation, industrial automation, a nd robotics [92].
The human e ye-brain system evolved over tens of millions of years and at this
point no artificial system is as versatile and powerful for everyday tasks. In the
same way that a chess-playing program is not directly modelled after a human
player, many mathematical techniques a re e mployed in artificial vision that do not
pretend to simulate biological vision. Artificial vision systems will therefore not
be set within the natural limits of human per c e ptio n. For example, human vision
is inherently two dimensional
4
. To accommodate this limitation, radiologists must
resort to visualizing only 2D planar slices of 3D medical images. An a rtificial system
is free of that limitation a nd can “see” the image in its entirety. Other advantages
are that artificial systems can work on very large image datasets, are fast, do not
suffer from fatigue, and produce repeatable results. Because artificial vision system
designers have so far bee n unsuccessful in incorpora ting high level understanding
of real-life applications, artificial systems typically complement rather than replace
of human exp erts.
4.2. Algorithms and PDEs. Many mathematical approaches have been investi-
gated for applications in artificial vision (e.g., fr actals and self-similarity, wavelets,
pattern theory, stochastic point process, ra ndom graph theory; see [42]). In partic-
ular, methods based on partial differential equations (PDEs) have been extremely
popular in the pa st few years [2 0, 35]. Here we briefly outline the major concepts
involved in us ing PDEs for ima ge processing.
3
The definition of “intelligence” is still very problematic.
4
Stereoscopic vision does not allow us to see inside objects. It is sometimes described as “2.1
dimensional perception.”