Applications of Modern Physics in Medicine

Mark Strikman, Kevork Spartalian, Milton W. Cole

• Éditeur: Princeton University Press
• Année: 2014
• ISBN:
  9781400865437
• Sujet: Technology & Engineering
• Sous-sujet: Applied Sciences

The connections between modern physics and medical technology Many remarkable medical technologies, diagnostic tools, and treatment methods have emerged as a result of modern physics discoveries in the last century—including X-rays, radiation treatment, laser surgery, high-resolution ultrasound scans, computerized tomography (CT) scans, and magnetic resonance imaging. This undergraduate-level textbook describes the fundamental physical principles underlying these technological advances, emphasizing their applications to the practice of modern medicine.Intended for science and engineering students with one year of introductory physics background, this textbook presents the medical applications of fundamental principles of physics to students who are considering careers in medical physics, biophysics, medicine, or nuclear engineering. It also serves as an excellent reference for advanced students, as well as medical and health researchers, practitioners, and technicians who are interested in developing the background required to understand the changing landscape of medical science. Practice exercises are included and solutions are available separately in an instructor's manual.

• Complete discussion of the fundamental physical principles underlying modern medicine
• Accessible exploration of the physics encountered in a typical visit to a doctor
• Practice exercises are included and solutions are provided in a separate instructor's manual (available to professors)
• A companion website (modernphysicsinmedicine.com) presents supplementary materials

CHAPTER 1

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Introduction

1.1 OVERVIEW

Ours is an increasingly technological world. We take for granted the rapid pace of scientific progress and its consequences for our daily lives, exemplified by Moore’s law, which says that computer speeds double about every 18 months [¹]. This remarkable fact is familiar to many of us because most educated people in the western world use computers, to some extent, so that the speed increase with each successive computer purchase is evident. Less well known is the equally remarkable speed at which modern medicine advances. The situation is different, partly because medical progress of a patient is often not quantifiable and partly because most people are not aware of incremental developments in the medical field. For those individuals who are interested in understanding this progress, like this book’s readers, it is fortunate that the underlying principles of modern medicine change much more slowly than the applications themselves. Since these principles and their connection to modern applications are the focus of this book, we are optimistic about its value lasting longer than the lifetime of most computers.

The reader of this text will become capable of understanding the fundamental basis of many of the most important medical tools used today. While space considerations alone constrain the extent to which we can provide detailed descriptions of these devices and the principles underlying them, there are several other reasons for this limitation. One is that two of the three authors (Cole and Strikman) are theoretical physicists, who are not researchers in this field. The third (Spartalian) is an experimental physicist familiar with the underlying scientific principles, but he, too, is not a researcher in the field of medical physics.

Another reason for the book’s focus on general principles is related to the pace of technical advance; tomorrow’s technology will not be that of today, so the details of the most recent developments are not crucially important for most readers to understand—except in those few instances when the progress is based on the application of new or different principles. Finally, we think that it is important for “students” (including the authors!) to appreciate how these developments have occurred; this means that some discussion of the history of these various developments is provided. For example, by learning that some of the most famous pioneers of nuclear physics (e.g., three Nobel Prize winners: Pierre and Marie Curie and their daughter, Irène Joliot-Curie) suffered from radiation damage, one is reminded of the importance of laboratory precautions. By learning that António Egas Moniz won the 1949 Nobel Prize for the invention of frontal lobotomy, a now discredited technique, we are reminded of the ephemeral nature of some scientific “discoveries.” Figure 1.1 provides another kind of learning experience, taken from an era when the perceived benefits of radiation received more attention than concern about its potential harm. The advertisement, shown in full color in plate 1, attributes to the radioactive skin product the glow emanating from the woman’s face, exuding beauty and health. The subtitle “Embellissantes parce que curatives” translates as [these products are] “beautifying because (they are) healing.”

Figure 1.1. Advertisement for Tho-Radia cream and powder from 1934. It describes this product as beautifying because it is healing, made “according to the formula of” Dr. Alfred Curie, no relation to Pierre or Marie Curie. Figure provided by the Marie Curie Museum, Paris, where the advertisement’s description labels this product as “very costly, but with such a small dose of radiation as to be harmless.”

Monographs and numerous scientific publications describe each of the tools of modern medicine. We will refer only to some of the most useful of these in the appropriate sections of this book. A reader interested in a complete, detailed survey of either the principles or their applications will have to search beyond this book. One place to find some relevant articles is the Web site accompanying this book, http:modernphysicsinmedicine.com. At that site is a bibliography of books and papers selected from the literature of medical physics, emphasizing articles at the level intended for readers of this book. Also found there are some of the classic articles important to the history of this field as well as a current version of the lectures accompanying this course.

The term modern physics appears in this book’s title. Its meaning, which is conventional in physics, includes the numerous revisions of our scientific understanding that began with the discovery of X-rays by Wilhelm Roentgen in 1895 and that of radioactivity by Henri Becquerel in 1896. The flood of subsequent discoveries includes that of the particle nature of matter (atoms and the zoo of subatomic particles) as well as the laws of relativity and quantum mechanics. Many of these revolutionary findings are critically important for the subsequent progress in medical technology. In describing these relationships, we will do our best to explain the basic principles underlying them.

“Think like a physicist” is an expression often quoted in our community and an attribute that we hope to foster in our readers and students. This expression refers to a thinking person’s ability to use whatever information is known to estimate a quantity of some interest. Such a strategy has immense practical value, even in the age of the Internet, when many alleged “answers” are readily obtained. A famous example of this approach is attributed to the Italian-American physicist, Enrico Fermi, who tried to imbue in his students an appreciation of this approach to both physics and thinking, in general. His classic question to his students was more or less this:

How many piano tuners work in the city of Chicago?

Obviously, this question has very little to do with physics, but it can be answered approximately with Fermi’s approach to estimating quantities. For the sake of this book, we propose that you (the reader) address a more relevant question about medicine:

How many obstetricians are there in the United States?

Before reading the following “solution,” try thinking about the problem patiently and then answering it as best you can. As you will see here, the Fermi way of thinking yielded different “solutions” for the three authors of this book, who addressed it independently. We will call our answers N₁, N₂, and N₃.

Here is one author’s method of answering the question. His calculation (yielding N₁) assumes “steady state,” meaning no net population growth. It also oversimplifies many other variables. Assume that there are 300 million residents of the United States. Of these, about half are female and, of those, perhaps a fourth are of childbearing age. This means that there are about 40 million women in the cohort who are capable, in principle, of bearing children. Let us suppose that each of these women has two children during her 20-year, childbearing interval. Hence, there are 2/20 = 0.1 children per year borne by each woman. This results in a total of 0.1 × 40 = 4 million births per year. Now, let us suppose (and this is a completely uneducated guess!) that the average obstetrician delivers 3 babies per day for each of 300 working days of the year, or a total of 900 babies per year. By dividing this number into the total number of births, one concludes that there are about N ₁ = 4 × 10⁶/900 ~ 4000 obstetricians in the United States. Note the rounding present at each step of the calculation, in recognition of the uncertainty present in each variable.

This “solution” to the problem can be written as a formula:

This very approximate formula is based on several crude estimates as well as rounding and hand-waving assumptions. We could provide uncertainties of each quantity, resulting in an estimate of the overall uncertainty in the calculation. Suppose, for example, that each of the four factors appearing in the preceding equation has an uncertainty of ±15%. Then, the final answer has an uncertainty of 4 × 15% = 60%. Thus, the actual answer could reasonably be expected to lie within the interval 1600 to 6400. That is quite an extended range of values, but perhaps even this uncertainty is an understimate.

The answer N₂ found by a second author of this book is much higher than these values: N₂ = 40,000 obstetricians, based on a set of alternative assumptions, equally naïve and equally valid. Start with a population of 4 million babies born in a year, estimated as before, which is equivalent to a birth rate of 13 babies per 1000 residents, that is, (4 million babies per year)/(300 million residents).

Assumptions of author 2:

(a) The mothers of each of these babies see an obstetrician for prenatal care.

(b) All pregnancies last 36 weeks.

(c) Including delivery, a mother is seen by her obstetrician 10 times altogether.

(d) An obstetrician sees 20 prospective mothers in a week and works 48 weeks per year.

From the last assumption, the number of visits per year to one obstetrician is 20 × 48 = 960 visits. The number of obstetricians is then given as follows:

This answer is a factor of 10 higher than the previous estimate, N₁, even lying outside of the range of uncertainty mentioned before. The second calculation (made independently of the first) evidently relies on alternative assumptions. It is not obvious where either estimate made one or more fundamental errors. However, a careful examination will reveal that the different approaches correspond to a number of visits in the first case that is lower by a factor of 10 than in the second.

What about N₃, the third author’s estimate? Without going into detail, we report that author 3’s answer was N₃ = 4000 obstetricians, consistent with the first estimate. Does the agreement between authors 1 and 3 suggest that their answer is correct? No! Even unanimous agreement would ensure nothing; after all, most people thought the earth’s surface was flat just a few centuries ago. In point of fact, according to the American College of Obstetricians and Gynecologists, there are more than 30,000 board-registered obstetricians or gynecologists in the United State at present. Hence the low estimate made by authors 1 and 3 is much less reliable than that made by author 2.

What is the lesson here for medicine and science in general? First, to reiterate, even if everyone obtains similar answers, such a consensus does not guarantee that the consensus answer is close to correct! We may all have made one or more unwarranted approximations. This happens often in science when new discoveries are made that contradict existing assumptions, however confidently these are held. The need to revise knowledge in the light of new evidence is especially important in the field of medicine; this is one aspect that makes the field so fascinating. However, remembering that human lives may be at stake, uncertainties have serious consequences. We must all be aware, at all times, of the provisional nature of human knowledge.

1.2 THE MEANING OF THE TERM MODERN PHYSICS

The understanding of physics achieved during the nineteenth century entailed numerous discoveries that gave the scientific community a well-justified sense of accomplishment. One of the most remarkable of these discoveries was the realization that work and heat transfer are closely related properties, the reason being that both involve the fundamental concept of energy. This realization led to a fundamental conclusion, embodied in the first law of thermodynamics:

Energy is conserved at the macroscopic scale just as it is at the microscopic scale.

Arguably of equal consequence is an apparently unrelated discovery involving a connection between two other physical properties:

Electricity and magnetism are “unified.”

The term unified means that one must take into account simultaneously the presence of both electric and magnetic fields rather than considering either one in isolation. A change in one field necessaril...

Table des matières

• Cover Page
• Title Page
• Copyright Page
• Contents
• Preface and Guide to Using This Book
• Technical Abbreviations
• Timeline of Seminal Discoveries in Modern Physics
• Timeline of Discoveries and Inventions in Modern Medical Physics
• Chapter 1 Introduction
• Chapter 2 When You Visit Your Doctor: The Physics of the “Vital Signs”
• Chapter 3 Particles, Waves, and the Laws that Govern Them
• Chapter 4 Photon and Charged-Particle Interactions with a Medium
• Chapter 5 Interactions of Radiation with Living Tissue
• Chapter 6 Diagnostic Applications I: Photons and Radionuclides
• Chapter 7 Diagnostic Applications II: MRI and Ultrasound
• Chapter 8 Applications in Treatment
• Appendix A Constants, Powers of 10, and Conversions Mentioned in the Text
• Appendix B Mortality Modeling
• Appendix C Evaluation of the Sound Field from One Transducer
• Notes
• Index