Medical Imaging

Essentials for Physicians

Anthony B. Wolbarst, Patrizio Capasso, Andrew R. Wyant

• Publisher: Wiley-Blackwell
• Year: 2013
• ISBN:
  9781118480243
• Edition: 1
• Topic: Medicine
• Subtopic: Radiology, Radiotherapy & Nuclear Medicine

"An excellent primer on medical imaging for all members of the medical profession... including non-radiological specialists. It is technically solid and filled with diagrams and clinical images illustrating important points, but it is also easily readable... So many outstanding chapters... The book uses little mathematics beyond simple algebra [and] presents complex ideas in very understandable terms."
— Melvin E. Clouse, MD, Vice Chairman Emeritus, Department of Radiology, Beth Israel Deaconess Medical Center and Deaconess Professor of Radiology, Harvard Medical School

A well-known medical physicist and author, an interventional radiologist, and an emergency room physician with no special training in radiology have collaborated to write, in the language familiar to physicians, an introduction to the technology and clinical applications of medical imaging. It is intentionally brief and not overly detailed, intended to help clinicians with very little free time rapidly gain enough command of the critically important imaging tools of their trade to be able to discuss them confidently with medical and technical colleagues; to explain the general ideas accurately to students, nurses, and technologists; and to describe them effectively to concerned patients and loved ones. Chapter coverage includes:

• Introduction: Dr. Doe's Headaches
• Sketches of the Standard Imaging Modalities
• Image Quality and Dose
• Creating Subject Contrast in the Primary X-Ray Image
• Twentieth-Century (Analog) Radiography and Fluoroscopy
• Radiation Dose and Radiogenic Cancer Risk
• Twenty-First-Century (Digital) Imaging
• Digital Planar Imaging
• Computed Tomography
• Nuclear Medicine (Including SPECT and PET)
• Diagnostic Ultrasound (Including Doppler)
• MRI in One Dimension and with No Relaxation
• Mapping T1 and T2 Proton Spin Relaxation in 3D
• Evolving and Experimental Modalities

CHAPTER 1

Sketches of the Standard Imaging Modalities

Different Ways of Creating Visible Contrast Among Tissues

“Roentgen has surely gone crazy!”

Different imaging probes interact with different tissues in different ways and yield different kinds of medical information

Twentieth-century (analog) radiography and fluoroscopy: contrast from differential attenuation of X-rays by tissues

X-ray film of a cracked phalange

Generating the beam at the anode of the X-ray tube

Contrast from differential attenuation of the beam within the body

Exposure of a screen-film image receptor

Image intensifier-based fluoroscopy with a CCD/CMOS electronic optical camera

Twenty-first century (digital) images and digital planar imaging: computer-based images and solid-state image receptors

Digital images

Computed tomography: three-dimensional mapping of X-ray attenuation by tissues

Helical, multi-slice CT

Nuclear medicine, including SPECT and PET: contrast from the differential uptake of a radiopharmaceutical by tissues

Radiopharmaceutical = radionucleus + organ-specific agent

Creating contrast through differential uptake of photon-generating radiopharmaceuticals

SPECT and PET

Diagnostic ultrasound: contrast from differences in tissue elasticity or density

B-mode anatomic imaging

Doppler imaging of blood flow

Magnetic resonance imaging: mapping the spatial distribution of spin-relaxation times of hydrogen nuclei in tissue water and lipids

Spin-relaxation times of protons in water and lipids in a strong magnetic field

Mapping the spatial distribution of proton T1 and T2

Appendix: selection of imaging modalities to assist in medical diagnosis

Cardiac versus non-cardiac chest pain

Non-cardiogenic chest pain

Cardiogenic chest pain

Abdominal/pelvis imaging

Abdominal calcifications: non-palpable but seen on radiograph

Head and neck imaging

Musculoskeletal imaging

Vascular imaging

References

The principal job of a medical imaging modality is to provide clear maps of anatomy, or to make it possible to identify irregularities in physiology, or both (Figure 1.1). It does so by creating contrast among tissues, and the various modalities do this in biophysically diverse ways.

Figure 1.1 Breast imaging for a patient with a biopsy-proven lobular carcinoma. (a) When a woman has her routine annual digital mammographic examination after the age of 40, two nearly orthogonal views are obtained of each breast. Mammography demands high soft-tissue contrast to detect neoplasms and fine resolution to examine microcalcifications but, at the same time, very low dose deposition. (b) B-mode ultrasound is often able to distinguish quickly, reliably, and inexpensively between a fluid-filled cyst and a solid tumor detected earlier with mammography. Here, the acoustic attenuation confirms the presence of a suspicious solid lesion. (c) MRI is often the screening tool of choice for patients at high risk for breast cancer. (d) PET with its standard radiopharmaceutical fluorine-18 deoxyglucose (18FDG) is highly sensitive to tissues that, like many tumors, consume an excessive amount of glucose. These modalities produce contrast through radically different biophysical mechanisms, and provide complementary kinds of medical information.

This chapter provides brief sketches of the major imaging technologies that are employed routinely in modern diagnostic clinics to examine the structure and functioning of the body. It begins with modalities slowly developed over the first three quarters of the twentieth century, like screen-film radiography and mammography, image-intensifier tube fluoroscopy, and analog nuclear medicine (NM) and ultrasound (US) imaging. Then came twenty-first century technologies that have flourished only with the advent of high-speed and powerful, but small and affordable, computers – digital planar imaging like computed radiography (CR); digital radiography (DR); digital mammography (DM); digital fluoroscopy (DF), including digital subtraction angiography (DSA); computed tomography (CT), culminating in helical, multi-detector ring CT (MDCT); single photon emission computed tomography (SPECT) and positron emission tomography (PET); advanced forms of B-mode and Doppler US; and magnetic resonance imaging (MRI) in all its glory, with T1-, T2-, and proton density-weighted imaging, functional MRI (fMRI), MR angiography (MRA), diffusion tensor imaging (DTI), and many other variants. All of these modalities will be discussed further in the forthcoming chapters.

“Roentgen has surely gone crazy!”

Although no one realized it at the time, the discovery of X-rays in 1895 foreshadowed the quantum upheavals that would turn the physical sciences upside down in the first quarter of the twentieth century. More immediately and spectacularly, however, it flung open a door that led into a new and completely unanticipated dimension in the practice of medicine − the ability to look non-invasively within a patient's body, without having to cut into it.

A century ago, medical diagnosis was as much art as science. The doctor could measure body temperature, blood pressure, pulse rate, and a few simple chemical attributes of blood and urine, but not much else. Odors and subtle aspects of a patient's appearance during a physical examination often provided equally important clues. But medicine lacked any means to view the interior of the body directly, apart from surgery, to reach critically important diagnoses.

That abiding problem ceased to exist, literally overnight, on the evening of November 8, 1895, when the German physicist Wilhelm Conrad Roentgen chanced upon X-rays (Figure 1.2a). Roentgen, a respectable but little known professor at the University of Würzburg, had been experimenting with an apparatus of widespread scientific interest at the time that is now called a cathode ray tube – a partially evacuated glass tube containing two metal electrodes at its opposite ends that were attached to the outside world by means of a pair of wires passing through the glass. Scientists had been intrigued by what happens when a high voltage is applied between the electrodes: the thin gas within would glow, as would the glass itself in the area near the anode (the electrode attached to the positive pole of the voltage source). It was argued that the agent responsible for this phenomenon was some sort of wave or particle, perhaps negatively charged, that emerged from the cathode (the negative electrode) and that was attracted toward the anode. These so-called “cathode rays” presumably excited the gas and, on striking glass, caused it to fluoresce as well. The nature of cathode rays themselves, now understood to be ordinary electrons, remained obscure for several more years after Roentgen's discovery.

Figure 1.2 In the beginning … (a) An engraving of Wilhelm Conrad Roentgen, from Something About X-rays for Everyone, which was published in 1896, less than a year after his discovery. Reproduced from Trevert E, Something about X-rays for Everyone, 1896. Reprinted by Medical Physics Publishing Company, Madison, WI, 1988. Soon thereafter, one visitor described him as “a very tall man, with a scholarly stoop, his face somewhat pockmarked, stern but kindly, and very modest in his remarks upon his achievements” (Mould RE, A Century of X-Rays and Radioactivity in Medicine, Institute of Physics Publishing, London, 1993). (b) The earliest extant X-ray record, of Roentgen's wife Bertha's hand and signet ring, taken by her husband on December 22, 1895. Courtesy of the Deutsches Roentgen-Museum, Remscheid-Lennep, Germany.

It is not clear what Roentgen was attempting on November 8, since his will stipulated that all of his laboratory notes be burned unread upon his death. In any case, as he worked in a darkened room late in the evening, something unusual caught his eye: when an electric discharge occurred in his tube, a nearby piece of paper that happened to be coated with a chemical compound of barium, platinum, and cyanide produced a glow. With his glass tube completely enveloped in black cardboard, there was no way that visible light from the tube could be reaching the coated paper. So something invisible had to be passing through the cardboard and reaching the barium platinum cyanide, inducing it to give off light. Roentgen had, in fact, discovered X-ray radiation by observing X-ray fluorescence (the emission of light caused by an X-ray stimulus) in a nearby material that was fluorescent. (Patton [1–3] provides a fascinating and detailed accounting of Roentgen's discovery.)

Roentgen was aware that he might have stumbled onto something altogether new, and he was excited and shaken by the remarkable thing he was seeing. But as he explored this totally unexpected phenomenon, he worried that perhaps there might be a simple, obvious explanation that he was overlooking. Far more disturbing was the possibility that perhaps he could not trust his own senses – after all, this appeared to be a physical process that was trivially easy to produce, and undeniably of extraordinary significance, so why had no-one else already seen it and reported it? He knew the physics literature well, and was quite certain that nothing like this had been described before. But were his observations genuine, or might they possibly be the creation of his own mind?

“I believed, ” he later recalled, “that I was the victim of deception when I observed the phenomenon of the ray” [4]. He wrote to his longtime friend, physicist Ludwig Zehnder: “I had spoken to no one about my work. To my wife I merely mentioned that I was working on something about which people would say, when they found out about it, ‘Roentgen has surely gone crazy.’”

But Roentgen persevered. Placing various objects between the tube and the fluorescent screen, he learned that they affected the brightness of the emitted light by different amounts. A few pieces of paper or cardboard had little impact, but a thick sheet of metal quenched the light completely. And when he held his hand in the path of the beam, he could make out the bones of his fingers projected in silhouette upon the screen. A short while later, Roentgen produced the first X-ray record, permanently capturing his wife Bertha's hand and signet ring on a glass photographic plate (Figure 1.2b). Bertha, regrettably, was not overly impressed by the medical significance of the discovery − she had long harbored a terrifying premonition of an early death, and seeing the resemblance of her hand to a skeleton gave her a most unpleasant shock. She ran screaming from her husband's laboratory and never went near it again.

On December 28, Roentgen submitted a paper describing his findings, “On a new kind of ray, ” to a local scientific journal. Within days, news of the discovery was excitedly picked up by the press and spread like wildfire throughout the world, along with the instantly famous picture of Bertha's hand. People found the experiment easy to reproduce, and within months physicians everywhere were using the pictures it produced to set broken bones and to remove bullets and shrapnel. Over the single year following the discovery, more than a thousand technical and medical papers were published on the subject. With his new kind of rays, Roentgen had discovered a splendid window for looking within the living body and painlessly examining organs and bones.

For his work, Roentgen was offered a title, which he refused, and received numerous awards, including the first Nobel Prize in Physics in 1901. Soon af...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Epigraph
• Preface
• Acknowledgments
• Introduction: Dr. Doe's Headaches: An Imaging Case Study
• Chapter 1: Sketches of the Standard Imaging Modalities: Different Ways of Creating Visible Contrast Among Tissues
• Chapter 2: Image Quality and Dose: What Constitutes a “Good” Medical Image?
• Chapter 3: Creating Subject Contrast in the Primary X-ray Image: Projection Maps of the Body from Differential Attenuation of X-rays by Tissues
• Chapter 4: Twentieth-century (Analog) Radiography and Fluoroscopy: Capturing the X-ray Shadow with a Film Cassette or an Image Intensifier Tube plus Electronic Optical Camera Combination
• Chapter 5: Radiation Dose and Radiogenic Risk: Ionization-Induced Damage to DNA can cause Stochastic, Deterministic, and Teratogenic Health Effects – And How To Protect Against Them
• Chapter 6: Twenty-first Century (Digital) Imaging: Computer-Based Representation, Acquisition, Processing, Storage, Transmission, and Analysis of Images
• Chapter 7: Digital Planar Imaging: Replacing Film and Image Intensifiers with Solid State, Electronic Image Receptors
• Chapter 8: Computed Tomography: Superior Contrast in Three-Dimensional X-Ray Attenuation Maps
• Chapter 9: Nuclear Medicine: Contrast from Differential Uptake of a Radiopharmaceutical by Tissues
• Chapter 10: Diagnostic Ultrasound: Contrast from Differences in Tissue Elasticity or Density Across Boundaries
• Chapter 11: MRI in One Dimension and with No Relaxation: A Gentle Introduction to a Challenging Subject
• Chapter 12: Mapping T1 and T2 Relaxation in Three Dimensions
• Chapter 13: Evolving and Experimental Modalities
• Suggested Further Reading
• Index