Biomedicine and Health: Medical Imaging and Non–invasive Medicine
Biomedicine and Health: Medical Imaging and Non-invasive Medicine
By the end of the twentieth century, high-technology diagnostic imaging machines had become powerful medical tools that allowed physicians to explore bodily structures and functions with minimum invasion of the patient's body. Since the late 1800s, x rays have allowed some imaging of bones, but later technologies made it possible to image a variety of soft tissues in two and sometimes three dimensions rather than merely projecting shadows onto a photographic plate. Advances in diagnostic technology also allowed doctors to evaluate processes and events in real time as they occur in the living body.
The second great leap in imaging technology (after the invention of conventional x rays) occurred fairly suddenly in the 1970s, when small, affordable computers became powerful enough to allow difficult equations to be solved in medically realistic time-frames, namely minutes or seconds rather than days or hours. Although relying on various physical principles—radio waves, high-energy photons emitted by particle-antiparticle annihilations, x rays, and sound waves—all the new high-tech methods relied on computers to construct visual images from a raw data. These tools were added to the array of other non-invasive tests long in use, including conventional projective x rays, electroencephalograms (brain-wave recordings), and electrocardiograms. All these techniques allow physicians to gather information about the health of internal organs without invading the body surgically and have revolutionized the treatment of some diseases.
Historical Background and Scientific Foundations
Non-invasive medical imaging traces its roots to nineteenth-century advances in the understanding of electromagnetism. In 1885, German physicist Wilhelm Konrad Roentgen (1845–1923) discovered x rays (a form of high-energy electromagnetic radiation, akin to light but not visible to the eye). He instantly recognized their potential medical benefits, but the difficulty involved in confirming Roentgen's work and designing, manufacturing, and installing equipment delayed the availability of x rays in most hospitals until after the turn of the century. The first x-ray facilities in the United States were located in Boston, where physicians were soon overwhelmed with patients. By the end of World War I (1914–1918), x-ray machinery in hospitals throughout the United States helped physicians to visualize fractures and disorders of some internal structures. Roentgen was awarded the 1901 Nobel Prize in medicine for his discovery. Developments in radiology progressed throughout the first half of the twentieth century, finding extensive use in the treatment of soldiers during World War II (1939–1945).
After the war, non-invasive medical imaging evolved on a number of fronts at once. The development of diagnostic technologies using ultrasound—sound vibrations too rapid for the human ear to detect—began in the 1940s. In the early 1950s, the use of fluorescent screens combined with x rays allowed physicians their first real-time look into the body: previously, only snapshots of stationary structures had been possible. Used in connection with improved contrast mediums (dyes that cause vessels and internal organs to stand out in images), real-time diagnosis became a way for physicians to watch the dynamics of certain diseases.
The injection of small amounts of radioactive material into the body to allow better imaging is one aspect of the field termed nuclear medicine. Although nuclear medicine traces its origins to the 1930s, the invention of the scintillation camera by American engineer Hal Anger (1920–2005) in the 1950s brought nuclear medical imaging to the forefront of diagnostics. Nuclear studies involve the introduction of low-level radioactive chemicals that are consequently transported throughout the body to target organs and tissues. The scintillation camera created images from the gamma rays emitted from these radioactive chemicals, enabling physicians to detect tumors or other disease processes. The basic techniques of the scintillation or Anger camera are still used in nuclear medicine today: the PET (positron emission tomography) scanner, described further below, is essentially an advanced development of nuclear scintillation technology.
In the 1960s and 1970s, cheaper, more powerful computers opened the way for the initial development of ultrasonic imaging and magnetic resonance imaging (MRI). Better equipment and procedures made diagnostic procedures—including those already developed—more accurate, less invasive, and safer for patients. By the end of the twentieth century, x-ray images that once took minutes of exposure could be formed in milliseconds with doses less than one-fiftieth of those used at in early techniques. Increased resolution (sharpness of detail) was also available, allowing physicians to see more clearly and with greater detail and enabling earlier diagnosis of serious pathology (e.g., tumors). Earlier diagnosis often translates to a higher likelihood of patient survival.
Below, the most important technologies for gathering information about internal conditions without invading the body surgically are briefly reviewed. Conventional x rays have been discussed above.
ECG and EEG
The electrocardiogram (ECG or EKG), first introduced in 1901, provides valuable information to physicians on the performance of the heart. Dutch physician Willem Einthoven (1860–1927) developed the electrocardiogram based on the previous work of British physiologist Augustus D. Waller (1856–1922). Einthoven modified a galvanometer (voltage-measuring device) to record the voltage changes on the surface of the skin that are generated by the contractions of the chambers of the heart. Minute amounts of electrical current created by the heart caused deflections of the galvanometer's needle, which could be recorded as pen tracings on a moving scroll of paper.
Einthoven's first electrocardiogram machine weighed over six hundred pounds. By 1950, electrocardiogram machines had become small, portable, and capable of pinpointing irregularities in heart rhythm. They aid in the diagnosis of heart enlargement, thyroid disease, and myocardial infarction (heart attack). For his invention of the electrocardiogram and further research on the electrical activity of the heart, Einthoven was awarded the 1924 Nobel Prize in medicine. ECG machines are universally used today by cardiologists.
Much like the heart, the brain produces slight voltage changes on the surface of the body as it functions. These voltages can be detected on the scalp and graphed to produce an electroencephalogram (EEG), which gives some insight into brain activity. When first introduced in the 1920s, EEG interpretation was likened to a scientific art requiring many years of comparative experience. Thus, EEGs were not widely used, other than by a few pioneering experts, until the early 1940s. Today it is in widespread use. The EEG is particularly helpful in the diagnosis of epilepsy, and is also used to aid in the diagnosis of brain tumors and brain dysfunction.
Principles of radar and sonar imaging found clinical diagnostic use starting in the 1960s. In medical ultrasound imaging, a sound-producing device termed a transducer is placed against the skin of a patient (or, in some cases, is inserted into the body using an existing passageway such as the throat or vagina). Pulses of high-frequency sound produced by the transducer travel into the body and are reflected off of internal structures. The echoes return to the transducer, which acts as a microphone and converts them into electrical signals. These are recorded and then another sound pulse is sent out. By scanning the aim of successive pulses from side to side as if waving a flashlight, then piecing together the resulting echoes using a computer, an ultrasound scanner can produce a fan-shaped image of an internal slice of the body. By the 1980s, ultrasound examinations had become commonplace in the examination of fetal development and in the diagnosis of heart disease. Ultrasonic Doppler techniques allowed color-coded mapping of the direction and velocity of blood flow in real-time moving pictures of the circulatory system, greatly aiding the identification of abnormal flows such as those caused by heart-valve defects.
Ultrasound, like MRI, is safer than any form of x-ray imaging because it relies not on electromagnetic radiation but rather on pressure waves that are non-ionizing. While unable to image through bone or air (e.g., inside the skull or lungs) and producing fuzzier images than MRI, it is far cheaper, more portable, and quicker to use. As of the early 2000s, ultrasound was still the only medical imaging technique that could cheaply yield motion pictures of body processes, such as the heart beating, and the image sharpness of state-of-the-art ultrasound machines had dramatically improved.
CAT and PET
During the early 1970s, enhanced digital capabilities spurred the development of computed tomography (CT) imaging. The term “tomography” is derived from the Greek word tomos, meaning slice, because this class of methods produces images of the body that simulate what would be seen if the body were sliced. CT, which is also termed computed axial tomography (CAT), was invented by English physician Godfrey Hounsfield (1919–2004). It uses advanced computer methods to combine into a slice-like image different readings or views of the x-ray shadows cast by a patient's internal structures. Hounsfield's innovative use of a sensitive detector mounted on a rotating frame and of digital computing to create images earned him a Nobel prize in 1979. As with x-ray imaging, CT technology has gradually progressed to allow the use of less energetic beams and decreased exposure times. CT increased the scope and safety of x-ray imaging procedures, allowing physicians to view the arrangement and functioning of the body's internal structures with high resolution.
The work of American chemist Peter Alfred Wolf (1923–1998) on positron emission tomography (PET) led to the clinical diagnostic use of the PET scan, which allows physicians to measure cell activity non-invasively in the living body. PET scans use rings of detectors that surround the patient to track the movements and concentrations of radioactive tracer elements injected into the blood. The detectors measure gamma radiation produced when positrons emitted by tracer atoms are annihilated during collisions with electrons. PET scans, along with other techniques, are used by neurologists to study the underlying metabolic changes associated with mental diseases such as schizophrenia and depression. Starting in the 1990s, PET scans found clinical usage in the diagnosis and characterizations of certain cancers and heart disease.
Magnetic Resonance Imaging (MRI)
The development of MRI in the late 1970s (the first clinical MRI image was acquired in 1980) depended on fundamental advances in nuclear physics, especially the properties of energetically excited atomic nuclei in
magnetic fields. MRI, which is widely used in medical practice and research today, depends on the phenomenon known as nuclear magnetic resonance. In this process, a single proton—such as the nucleus of a typical hydrogen atom—preferentially absorbs (is resonant with) certain frequencies of radio radiation when placed in a strong magnetic field. In MRI, the patient is placed in such a field, causing the nuclei of the hydrogen atoms in the body's water molecules to line up in a limited number of quantum states. These lined-up hydrogen nuclei are then hit with a radio pulse that is crafted to be absorbed by them. The protons then re-radiate this absorbed energy as radio pulses that are picked up by antennas surrounding the patient. The signals are recorded and processed by a computer to produce images of the body's tissues.
The absorption and radiation properties of hydrogen nuclei vary slightly according to what kinds of molecules they are bound to. MRI thus yields information about the structure of organic molecules, allowing the imaging of contrast between tissue types. The result is imaging of startling clarity. MRI has not displaced other imaging methods, however, because MRI machines are extremely expensive and, because of the intense magnetic field involved, cannot be used on persons who have magnetic metal embedded in their bodies (e.g., cardiac pacemakers).
Versions of MRI and PET scanning can be used to produce functional imaging: that is, in addition to detailing structures, they can provide a view of dynamic chemical processes. These functional imaging methods are used increasingly in research and medicine. Functional scans are used to measure reactions of the brain when challenged with sensory input (e.g., hearing, sight, smell), activities associated with processing information (e.g., reasoning tasks), physiological reactions to addiction, and metabolic processes associated with osteoporosis and atherosclerosis. They are also used to shed light on pathological conditions such as Parkinson disease, Alzheimer's disease, and attention-deficit hyperactivity disorder.
Mammography, x-ray visualization of the breast, became a common diagnostic procedure in the 1960s, when physicians showed its usefulness in the diagnosis of breast cancer. At first, mammograms were used only to aid in diagnosis of tumors already discovered by other means, and the x-ray dose to the tissue was relatively high. In 1973, the Breast Cancer Detection Demonstration Project, a five-year study of over 250,000 women, helped to establish mammography as an effective screening tool for breast cancer detection. Modern mammography machines allow greater precision of breast tissue imaging with less radiation exposure to the patient. Mammography is today recommended as a standard screening procedure for women over 50, and had been credited with reducing mortality from breast cancer in women over 50. Whether women between 40 and 50 should have regular mammograms is medically controversial: a clear, significant benefit for women in this group had not, as of 2008, been demonstrated by scientific studies.
Modern Cultural Connections
Late in the twentieth century, various non-invasive imaging tools moved into the operating room, where image-guided surgical methods have allowed surgeons to more accurately determine the locations of tumors, lesions, and a host of vascular abnormalities. One benefit of these techniques is that they allow surgeons to track the positions of surgical instruments. Rapidly advancing computer technology and imaging, when used in conjunction with optical, electromagnetic, or ultrasound sensors, allows physicians to make real-time diagnosis a part of surgical procedures. During the 1990s and early 2000s, the explosive development of information technologies and the Internet also allowed physicians to begin making some diagnoses from remote locations and to teleconference with colleagues over real-time data.
The explosion of medical imaging has not, however, been entirely uncontroversial. For example, x rays, which are the basis of both traditional x-ray imaging and CT, are a form of ionizing radiation: that is, they can knock electrons away from atoms or molecules, ionizing (electrically charging) them and damaging chemical structures in living cells, including DNA molecules. The body can often repair such damage, but sometimes ionization of DNA causes cancer or harmful mutations in offspring. The more ionizing radiation any living thing is exposed to, the greater the chances of cancer or mutation. Medical imaging that uses ionizing radiation—x rays and CT, including mammograms and dental x rays—therefore not only allows the discovery of cancers and some other diseases, but increases the likelihood that patients will develop cancer. The more x rays a patient is exposed to, the more likely they are to contract cancer at some later time.
From 1980 to 2006, the greatly increased use of medical imaging, especially CT, caused U.S. per capita average exposure to medical radiation (not counting radiotherapy for cancer) to increase six-fold. By 2006, medical diagnostic radiation had surpassed natural background radiation (from radioactive atoms naturally present in the environment) as the largest single source of radiation exposure for U.S. citizens. Since background radiation continued unabated, this meant that medical radiation had approximately doubled the amount of radiation exposure for the average citizen compared to the pre-x-ray period. During 1980–2006, the U.S. population grew by less than 1% per year but the number of CT scans increased by 10% per year, from 3 million in 1980 to 62 million in 2006. Similar trends occurred in other industrialized societies, although the exact figures varied from country to country.
Even ultrasound, which is cheap, portable, and depends on non-ionizing sound waves that do not cause cancer, has been used in ethically controversial ways. Ultrasound scans of human fetuses can reveal gender early in development by demonstrating the presence or absence of a penis. In India, China, and some other countries where male children tend to be favored over female children, this information has been used to selectively abort tens of millions of female fetuses, causing an increasing imbalance in the number of men and women. In India, it has been illegal since 1994 for an ultrasound operator to reveal fetal gender to prospective parents, but the law is widely ignored. According to a study published in 2006 in the journal The Lancet, about 500,000 female fetuses are aborted every year in India based on ultrasound imaging results, with a total of 10 million female fetuses aborted in India from 1978 to 1998. By 2006, in the city of Mumbai, among children under the age of 6 there were only 710 girls for every 1,000 boys. In India, one motive for aborting female fetuses is that dowry payments must traditionally be made when female children marry, but not when male children marry: it is therefore cheaper to have sons than daughters.
Another widely discussed, disturbing possibility is that technologies that reveal brain function in real-time detail may be used to invade the ultimate human privacy—thought itself. For example, functional MRI has been proposed, and is being researched, as a tool for determining the truthfulness of verbal statements by direct examination of brain activity. Such methods could potentially be substituted for, or combined with, physical torture to produce near-infallible interrogation techniques for use by governments or other organizations.
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Brenda Wilmoth Lerner