Medical Imaging

Preface

Recall how, after several years of college French, or Spanish, or Urdu, you ventured abroad and discovered that you could actually get by? You could order more or less what you wanted for dinner, you began to find your way around town. And when people asked you, slowly and clearly, where you were from and what you were doing there, you could both make out much of what they were saying and respond reasonably coherently. It might take you another year or so before you began sounding like a native, but you definitely could survive.

The objective of this book is to help and encourage you to acquire survival-level medical imaging. As such, it is designed to provide a technically solid, clear, largely non-mathematical understanding of the most significant ideas underlying the field, and yet be as short as possible (but no shorter). We include references to several much longer texts that provide reams of factual details, which are important for those who wish to go farther. The idea here, however, is to get you up and going, with a reasonable degree of competence and confidence, and to do so quickly.

Medical Imaging: Essentials for Physicians is a survival-level introduction to the extraordinary instruments and processes that create medical images, and to the ways in which radiologists, cardiologists, orthopedists, neurosurgeons, and most other physicians employ them to assist in resolving medical problems. While intended primarily for attending physicians, radiology and other residents, and medical students with limited direct familiarity with imaging, it should also be accessible to other senior medical professionals, and to biological and physical scientists as well.

This is not a textbook, nor is it written in a formal style. Hopefully you will find the manner to be relaxed and engaging, more or less like the way people speak. (We sometimes even split infinitives and end sentences with prepositions, if there's reason to.) But it is nonetheless a very serious and non-trivial book written for professionals who wish to acquire enough command of the imaging tools of their trade to be able to explain the general ideas accurately to students, nurses and technologists, to discuss them self-confidently with medical and technical colleagues and vendors, and to describe them effectively to concerned patients and loved ones. While there are no prerequisites, the read will be much more pleasurable if you happen to be the curious type. You will come to grasp a good deal about what and how imaging things happen, in any case, but it should be a more rewarding experience if you, like the authors, tend to frequently ask yourself why, as well. We like to explain things, not just describe them.

The reward for your perseverance will be a firm and technically rigorous qualitative understanding of the basic ideas of how computed tomography (CT), magnetic resonance imaging (MRI), digital subtraction angiography (DSA), and the other principal modalities really work – along with the strengths and risks of each, and the reasons that the knowledgeable physician finds one technology to be preferable to the others in a particular clinical situation. You will come to see the reasons that digital radiography (DR) and single photon emission computed tomography (SPECT) may be able to detect hairline bone fractures, say (although nuclear medicine is only rarely employed for that purpose), while ultrasound and fluoroscopy cannot. And why positron emission tomography (PET) is so adept at seeking out and searching out neoplasms, while few of the other modalities are.

You will also learn the essential role that quality assurance and safety programs play in determining that devices are delivering images with clinically optimal contrast and resolution, and at the same time with as little risk as possible from radiation or other hazards. Distressingly large numbers of physicians misread images and overdose patients with radiation, in part because they do not realize that their equipment is producing less than adequate pictures, or how to check on that routinely. Nor are they aware of just how great, or not, are the risks and benefits from routine screening mammograms, or from CT of a neonate – or that it may be possible to reduce the doses to kids considerably without compromising clinical efficacy.

In most situations, the issue of greatest importance in a diagnostic image is the visual contrast that allows the viewer to distinguish among and examine organs and other tissues, both normal and pathological. For X-ray technologies, including screen-film and digital radiography, digital subtraction angiography, and CT, contrast arises because X-ray probes collide with the atoms of different tissues in different ways and by different amounts; the shadows of bone emerge in a radiograph because that material is much more adept at absorbing and scattering away X-rays than are the surrounding soft tissues, so fewer of them make it through to expose the film or digital image receptor. There is contrast in nuclear medicine images, including SPECT and PET, because various radiopharmaceuticals concentrate preferentially in certain specific biological compartments, resulting in the differential emission of high-energy photons that can be detected and imaged from outside the body. High-frequency vibrational waves of ultrasound radiation reflect at boundaries between tissues that differ in density or elasticity, and processing the echoes can give rise to those cute pre-baby pictures of such extraordinary clarity and detail. And with MRI, in all its glory, one can generate contrast among tissues in multiple ways, based on differences in their proton density; the proton spin relaxation times T1 and T2; the flow of blood through vessels; the diffusion of water along the tracts of neural axons in the brain; the motion of relatively well-oxygenated blood to parts of the brain that require it; the list goes on.

These approaches give rise to contrast among the tissues in remarkably dissimilar ways. The resulting various kinds of contrast naturally accentuate different aspects of anatomy or physiology, and one of them may provide exceptionally valuable kinds of clinical information in a given medical situation. Much of the discussion herein will therefore focus on the ways in which the diverse methods of imaging exploit distinct biophysical processes to create contrast, on how the operator can draw it out and enhance it most effectively, on the relative risks, costs, and availability of the technologies, and ultimately on the medical reasons that a particular modality may be especially diagnostically useful in a given case.

The authorship of the book reflects its intent. The first draft was prepared by a medical physicist who has written a widely used text for radiology residents on imaging science and technology. It was then edited by a practicing interventional radiologist, who cut out some of the stuff unnecessary or uninteresting for most doctors; translated the rest into proper medicalese; and added a large number of cases to illustrate important points. Then an emergency-room physician with much experience but no special training in imaging went through it carefully to ensure the clarity of presentation and its intelligibility to non-radiologists, and to add further cases. Finally, each completed chapter on a modality was sent for review to radiologists, medical physicists, and other radiological specialists. As in the Acknowledgments section, the authors wish to express here again their great and sincere sense of indebtedness to these extremely busy professionals who took time out of their packed schedules to critique the chapters and to offer ideas for improvements.

A challenge to the authors has been, of course, to determine what topics to discuss, and in how much depth and breadth. We have attempted to cover enough to satisfy readers searching for a rapid, practical understanding of the whole field, yet also to whet the appetites of others to pursue more. Some will brush through the central topics quickly, just to pick up the main points, at least on the first pass, while others will wish to dig in a bit deeper. The presentation is therefore on two levels: the main story line appears in normal type, but paragraphs containing more detail or explanation than might be needed for a first encounter will be presented in this manner. We welcome suggestions from readers on what should be added in future editions of the book, or cut back or deleted, or changed, bearing in mind that we are making a conscious effort to keep it as short as possible while it can still do its job.

A comment on equations: in some places it clarifies ideas to express them in the language of elementary algebra. Nothing is more mathematically demanding in the text than linear equations (y = mx + b), or sine waves (sin x), or the occasional exponential (e− μx). The equations will serve to summarize and elucidate notions by re-phrasing them in another form, not to complicate them, and they are guaranteed not to cause tachycardia or severe gastric distress.

Another comment, about repetition: we use it, very intentionally so, because it is well known to aid in learning the central points in an extensive and sometimes challenging new subject. We feel that it will be helpful for many readers – but if you got the idea the first time around, please be patient if it comes up again. In any case, additional information is almost always added each time a topic is revisited – material that might have...