Introduction to Physics in Modern Medicine 3rd Edition

Book Preface

In the third edition, we have included descriptions of recent technological innovations and new imaging techniques: optical coherence tomography using laser light to create images of, for example, retina and coronary arteries; novel matrix transducers that can be used for advanced 4D ultrasound imaging; ultrasound-mediated drug delivery, an example of a therapeutic application of ultrasound; development of new x-ray sources based on pyroelectric effects that can be used in compact portable x-ray machines; digital pixel-array detectors and nanotechnology-based contrast agents for x-ray and gamma ray imaging; x-ray phase contrast imaging, which can increase soft-tissue contrast and reduce the absorbed dose of radiation; direct ion storage dosimeters for real time measurements of radiation doses; and heavy-ion radiation therapy. We discuss how these exciting innovations in modern medicine are rooted in simple physical principles. We also have included the latest available data in the field of radiation safety and health physics.

Student Preface
Several years ago one of us (S.A.K) developed sharp, severe pains in the lower abdomen. My physician felt a lump in the painful region and immediately ordered an ultrasound exam. In spite of the uncomfortable situation, I watched in fascination as a sonographer examined the inside of my body, my organs showing up as flickering, ghostly gray outlines on a television screen. As the exam proceeded, my scientific curiosity (and a healthy sense of self-preservation) began to raise many questions.
What exactly does one see on the television screen during ultrasound? How can it distinguish between a cancerous tumor (the feared outcome) and the actual diagnosis (a common and benign [noncancerous] cyst)? How safe are ultrasound exams? Before performing the exam, the sonographer smeared a gel onto the head of the ultrasound sensor – what was its function? In an ideal world, I would have had ample opportunities to have my questions answered on the spot. Under the circumstances, I was not likely to get answers unless I sought them out myself.
Many people are curious about these devices. Many have questions about the safety of these technologies as well as how they work, but assume that extensive training is required to understand such complicated medical devices as surgical lasers or computed tomography (CT) scanners. Obviously, physicians and other healthcare workers undergo years of specialized schooling in order to understand how to diagnose and treat disease using such tools. However, there is no reason the average person cannot comprehend and benefit from a basic understanding of these technologies. This book represents an attempt to provide this necessary background. In writing it, we have focused on those physical principles necessary to understand how these technologies work. This work grew out of an introductory course on the subject that one of us (S.A.K) has taught at Haverford College for many years to college students with no prior college-level training in the natural sciences. Its writing was influenced by extensive feedback from numerous students of widely varying backgrounds.
The reason you can tackle this material without an advanced degree is that, surprisingly, the answers to many questions do not lie in a sophisticated understanding of medicine and science. Rather, the basic concepts behind many techniques derive from very simple applications of physics, biology, and chemistry. Each chapter in this book includes a short, self-contained explanation of the necessary scientific background. We have assumed at most a familiarity with these topics at the high school level. The discussion adheres to simple explanations, using examples and illustrations to convey the science. Indeed, the topics covered are exactly those most people find most appealing about elementary science: sound, the science of light, genetics, and the mysterious structure of atoms and molecules.

Our study will focus on physics, rather than biology and chemistry. Medicine owes obvious debts to biology and chemistry; these fields have yielded essential insights into the development of drugs and vaccines, and into an understanding of organisms that cause infectious diseases, physiology at the molecular level, and – more recently – the immune system and genetics. Everyone knows that physicians diagnose and treat diseases using laboratory tests and drugs. However, it is not as widely appreciated how important physics is to medicine, especially to the imaging and therapeutic methods discussed in this book. Interestingly enough, the discoverers of these technologies did not aim to solve important problems in medicine. Roentgen’s accidental discovery of x-rays touched off a medical revolution by making the body’s interior visible for the first time, but his goal was to investigate the fundamental properties of matter. Lasers were invented by physicists fascinated by the properties of atoms and light. The basic science behind magnetic resonance imaging (MRI) was discovered first in the context of understanding the fundamental structure of the atomic nucleus. The mathematics used in computed tomography was first derived for applications in astronomy – and the list goes on. These examples powerfully illustrate how useful technologies can arise unexpectedly from basic scientific research into the fundamental properties of matter. No easy dividing line exists between “curiosity-driven” research and applied research aimed at a useful biomedical outcome.
In this book you will learn enough about the science behind medical technologies to demystify them and to allow you to better understand what they can offer. However, we hope that at the same time you may also discover how the unity and wonder of physics extend beyond its unexpected benefits.