Fundamental physics for probing and imaging

349 Pages·2006·5.31 MB·English

by  Allison, Wade

FUNDAMENTAL PHYSICS FOR PROBING AND IMAGING
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Fundamental Physics
for Probing and
Imaging
WADE ALLISON
DepartmentofPhysicsandKebleCollege,
UniversityofOxford
1
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For Alice, Joss and Alfie
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Preface
Fear has dominated much of the experience of the human race from
earliest times. Fear of death, fear of natural disaster, fear of human
enemies and fear of deities: these were fused together beneath a dense
shroud of the unseen and the unknown. The major impact of physics
on civilisation has been to roll back this shroud. Physics explains. It
enablesusto seeinside the Earthandinside ourownbodies. Itgivesus
ways to probe and to cure.
It has seemed to me that there are some big questions to ask, and a
dearth of books that ask them. Which aspects of physics are primarily
responsible for this revolution? How do they work and how are they
used to provide the information and images? Are the dangers that sur-
roundapplicationsofthis physicsunderstood? Are thesesafetymatters
overstatedor understated, and is the public misinformed?
This book is written to answer these questions. It is written for all
physicists who wish to understand the physics basis. Its coverage is
broad but it is also quite demanding in places, for I have a deep dislike
ofaskingthereadertotakestatementsontrust. Anyway,thereareother
booksthatdojustthat, asthey rushthroughthe fundamentalsinorder
toreachtheexcitementofthe applicationsatanearlystage. I skipover
many experimental details of particular technical realisations but give
enoughexamplesofapplicationsforusefulcomparisonsbetweendifferent
modalitiestobe made. Istronglybelievethatthe widestunderstanding
of the basic physics is essential if future advances in technology are to
exploit the possibilities to the full.
Thebookdevelopedfromashortoptionalcourseentitled‘Medicaland
Environmental Physics’ that I have given in recent years to third year
mainstream physics undergraduates at Oxford University, and assumes
some familiarity with basic mathematical methods and the core physics
ofoptics,electromagnetism,quantummechanicsandelementaryatomic
structure.
Intheintroductionweaskwhichaspectsofpurephysicshaveenabled
mankind to delve into their environment by seeing into or through oth-
erwise opaque objects. Successful solutions have centred on three areas
of fundamental physics: firstly the physics of magnetism and low fre-
quency radiation, secondly ionising radiation and the physics of nuclei,
and thirdly the mechanical properties of matter and sound. Practical
examples range from safe navigation to medical diagnosis, from finding
minerals to border security.
Theearlychaptersgiveapedagogicaldevelopmentofthepurephysics
viii Preface
of these three fundamental areas. The later chapters follow how these
ideas have been developed in applications. They are concerned not just
with imaging, but with further questions of dating, function and prove-
nance,andfinallywithinterventionandtherapy. Theapplicationsillus-
trate both the principles at work and the comparison between different
possibilities.
The pure physics concerned has changed slowly compared with the
recent rapid development of applications. The necessary understanding
ofmagnetismandelectromagneticradiationbeganinthemidnineteenth
century and was completed a century later with the theory of magnetic
resonance. Similarly the relevant ionising radiation and nuclear physics
was understood within 75 years of the discovery of radioactivity in the
1890s. The basic physics of sound is classical and the understanding
of it dates back to the work of Lord Rayleigh, more than a century
ago. In every case what has changed recently is that developments
andapplicationsusingmodernmaterials,electronicsandcomputational
powerhaveenabledthisacademicunderstandingtoescapefromthepure
physics laboratory into the everyday world.
I have avoided the temptation to follow, logically and immediately,
the discussion of each set of fundamental ideas with examples of its ap-
plication. The subject of successive chapters switches back and forth
to encourage parallel thinking about the choice of methods available.
Chapter 5 in the middle gives an overview of information and methods
of data analysis which have been used in academic physics research for
decades. In the past these were too computationally intensive to be de-
ployedin everydayanalysis. Now, as the required computational power
has become available, they are used routinely in the analysis of images
and data.
Inevitably from such a broad field, the applications are selected and
their discussion avoids experimental detail which may be found on the
Webandelsewhere. Tohavefollowedeveryidearaisedintheearlychap-
terswouldhavelengthenedthisbookbeyondwhatcouldconceivablybe
coveredin a single text. Therefore many fields of application have been
omitted entirely, or have only been mentioned in passing.
The concluding chapter takes a bird’s eye view of possible devel-
opments and the ideas that might emerge from the cupboard of pure
physics in the future. There is much in the physical world that we do
not understand, and the book ends by looking at a few such cases. For
somereadersthebookwillopenmanyquestionsthatitdoesnotanswer,
but it will not have failed in its aim if such omissions stimulate further
study. Otherreaderswill feelthe needto rebalancecompletely society’s
perception of the threats and dangers that surround it. Perhaps the
book may be a beginning to the process of turning public opinion and
decision making in the direction of a safer world.
ix
Structure of the book
The chapters are written in such a way that some may be omitted
without affecting all of those that follow, and shorter courses may be
constructed by reading them selectively, albeit with some loss of the
overview. Thus one or more of the following sub-sets of chapters might
be omitted:
(cid:1)
chapters2and7onmagnetismandmagneticresonance,and
related imaging methods;
(cid:1)
chapters3,6and8oninteractionsofionisingradiation,anal-
ysis and damage by irradiation, and medical imaging and
therapy with such radiation;
(cid:1)
chapters4and9onmechanicalwavesandpropertiesofmat-
ter, and ultrasound for imaging and therapy.
Everychapter is divided into a number of sections, each of which starts
with a summary. Some sections are more demanding and are marked
withadagger(†). Onafirstreadingofthebooksomereadersmayprefer
to study just the summary of these, returning to pick up the detail of
the derivations at a second reading.
Atthe endofeachchapterthere isashortlistofrecommendedbooks
and a list of references and searches for further material on the Web.
These should enable the reader both to keep up to date and also to
broadenprogrammes of study based on this book. With its basic intro-
duction I hope that the reader will be able to appreciate in context the
technical details of applications, galleries of images, and yet wider links
that may be found. Included is a link to the website for the book:
www.physics.ox.ac.uk/users/allison/booksite.htm
Some colour images and video related to the grey-scale material in the
book may be found there, together with later comments and news.
Because of the interdisciplinary nature of the materialsome clarity is
needed in the use of terms, abbreviations and conventions. These are
laidoutforreferenceintwoappendices. Eachofthemainchaptersends
with a short selection of questions, and the final appendix gives hints
and answers to some of these.
Acknowledgements
In writing this book I have relied heavily on others to keep my balance
and perspective in a wide landscape. Those who have read large sec-
tions of the manuscript and provided exactly the combination of crisp
commentandencouragementthatwasmosthelpfulwereRichardTuley,
Daniel McGowan, Louis Lyons, John Mulvey and Peter Jezzard. Over
the years, Peter, with Stuart Clare, Steve Smith and other members of
his group at the FMRIB at the John Radcliffe Hospital, has given me
muchtimeandencouragement. MorerecentlyIhaveenjoyedthebenefit
of discussions with Chris Gibson, Andrew Nisbett and Fares Mayia at