Radiology and Medical Imaging

Summary

Radiology is the field of medicine concerned with imaging patients for the purposes of diagnosing and treating diseases and other medical conditions. Advances in medical imaging have helped revolutionize the contemporary practice of medicine. Through medical imaging, doctors are able to obtain a detailed view of the inside of the human body and obtain relevant qualitative and quantitative information regarding body tissues. Common uses of medical imaging include detecting broken bones using X-ray imaging, following fetus development using ultrasound imaging, diagnosing cancer through computed tomography or magnetic resonance imaging, and measuring the body's metabolic activity using positron emission tomography.

Definition and Basic Principles

Medical imaging, which uses a wide diversity of imaging modalities and procedures, relies on the use of radiation—ionizing and nonionizing—to obtain information regarding the imaged subject. Electromagnetic radiation covers a wide range of frequencies. The electromagnetic spectrum can be divided into categories based on wave frequencies. From low to high frequency waves, the categories are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

Radiation can be broadly categorized as ionizing and nonionizing. Ionizing radiation is electromagnetic radiation that has sufficiently high energy to remove electrons on interaction with specific atoms, thereby producing ionization in the substance through which it passes. Nonionizing radiation cannot change the atomic structure of the materials with which it interacts. Gamma rays and X-rays are ionizing radiation; ultraviolet, visible light, infrared, microwaves, and radio waves are nonionizing types. Both ionizing and nonionizing forms of radiation are used in medical imaging. In addition to electromagnetic radiation, sound waves are commonly used in medical imaging.

Background and History

Historically, people regarded light as something they could see with their eyes. Invisible radiation was not a scientifically proven idea until the early part of the seventeenth century. In 1800, British astronomer William Herschel experimented with a prism that split sunlight into a color spectrum. By measuring the temperature of each color, he realized that the temperature increased past the red light. Herschel interpreted the results by proposing the presence of an invisible form of radiation past the red part of the spectrum. This was eventually termed infrared radiation.

Other types of radiation were discovered in a similar manner. By observing physical effects on various materials, scientists were able to trace the cause to different forms of invisible radiation. The relationship between electricity and magnetism was discovered by Scottish scientist James Clerk Maxwell, who concluded that light is an electromagnetic disturbance that propagates through an electromagnetic field. Maxwell laid down much of the theoretical foundation that was used in subsequent discoveries of other forms of radiation in the electromagnetic spectrum.

The first type of ionizing radiation to be discovered was X-rays. In 1895, when experimenting with a cathode-ray tube, German physicist Wilhelm Conrad Röntgen noticed a form of light that was capable of penetrating through most materials, including body tissues, and could form an image on a special form of phosphorescent plates. Shortly after the discovery of X-rays, French physicist Antoine-Henri Becquerel discovered another form of ionizing radiation. Becquerel discovered natural radioactivity by experimenting with fluorescent minerals, including uranium. Radioactive materials could disintegrate by emitting specific forms of ionizing radiation. In 1898, Marie Curie discovered another radioactive element, radium.

When ionizing radiation was discovered, it was not known that it could harm people, and many scientists and others who were exposed to the rays suffered forms of radiation sickness and cancer. The field of health physics, which is most concerned with the safety of radiation, began to develop in the early part of the twentieth century. By the 2000s, the benefits and risks of radiation had come to be clearly understood by physicians, nuclear energy workers, and most of the public. Federal laws were created to regulate the use of ionizing radiation in medicine and other industries.

How It Works

The basis of all medical imaging modalities is radiation, which is coupled with detectors that acquire the information collected after the radiation has interacted with matter. Image processing and viewing can be performed entirely with computer software and algorithms. The end result of any imaging modality is a qualitative image that can be used to assess the body's parts based on prior knowledge of anatomy and physiology. Quantitative information can often be obtained from the images, and the quantitative parameters are characteristic of the imaging modality used.

Ultrasound Imaging. In ultrasound imaging, sound waves are generated and propagated through tissues at frequencies beyond the frequency of sound that can be heard by the human ear. The intercepting tissues either scatter (reflect) or absorb the sound waves. Absorbed wave energy is dissipated as heat and cannot be recovered, whereas scattered sound waves are acquired by the detector. How sound waves are reflected depends on the tissue type, with different types producing varying signal intensities on the final image. Ultrasonic transducers are used to generate and receive the ultrasound signals. The transducer converts the acquired acoustic energy into electric signals that are then manipulated using computer algorithms to produce an intensity image that can be interpreted by a radiologist.

Magnetic Resonance Imaging (MRI). The type of radiation used in magnetic resonance imaging lies in the radio frequency (RF) range of the electromagnetic spectrum. RF waves have frequencies ranging from 3 hertz to 300 billion hertz and are widely used in many broadcasting and electronic devices. In MRI, magnets with field strengths ranging from 3,000 to 30,000 times the Earth's magnetic field are used. RF waves and strong magnetic fields produce a specific RF frequency, termed the resonance frequency, which causes a change in the alignment of specific atoms within the specified magnetic field. The resonant frequency for hydrogen atoms—which are abundant in the human body—at a magnetic field strength of 1.0 by-prod is 42.6 megahertz. (By-prod is a unit of magnetic flux density, equal to one weber per square meter.) The Earth's magnetic field strength is 0.00005 by-prod. Most clinical MRI scanners have field strengths of 1.5 by-prod. Magnetic fields and RF signals are manipulated in many ways using computer algorithms. A specific set of signal manipulations is called a pulse sequence. Different pulse sequences can provide images with varying contrast, resolution, and intensity, depending on the type of information required.

X-Ray Imaging. Planar X-ray imaging and computed tomography (CT) imaging rely on the use of ionizing radiation in the X-ray part of the electromagnetic spectrum. X-ray photons are generated when high-energy electrons bombard a target material, such as tungsten, that is placed in an X-ray tube. The resulting X-ray beam has a continuous energy spectrum ranging from low-energy photons to the highest-energy photons, which corresponds with the X-ray tube potential. However, since low-energy photons increase the dose to the body and do not contribute to image quality, they are filtered from the X-ray beam. After a useful filtered X-ray beam is generated, the beam is directed toward the subject, while an image receptor (film or detector) is placed in the beam direction past the subject to collect the X-ray signal and provide an image of the subject. As X-rays interact with the body's tissues, the X-ray beam is attenuated to different degrees, depending on the density of the material. High-density materials such as bone attenuate the beam drastically and result in a bright signal on the X-ray image. Low-density materials such as lungs cause minimal attenuation of the X-ray beam and are darkened on the X-ray image because most of the X-rays strike the detector.

In CT, X-ray images of the subject are taken from many angles and reconstructed into a three-dimensional image that provides exquisite contrast of the scanned subject. At each angle, X-ray detectors measure the intensities of the X-ray beam, which are characteristic of the attenuation coefficients of the material through which the X-ray beam passes. Generating an image from the acquired detector measurements involves determining the attenuation coefficients of each pixel within the image matrix and using mathematical algorithms to reconstruct the raw image data into cross-sectional CT image data.

Nuclear Imaging. Nuclear imaging involves the use of radioactive substances to obtain functional information regarding various processes in the body. Specific radioactive compounds, called radionuclides, are manufactured using nuclear reactors and packaged in a safe way for administration to a patient. The radionuclides are biodegradable and do not pose a significant risk to the patient. However, because the emitted radiation is ionizing in nature, there is a slight risk of cancer induction. The most commonly used type of radionuclide for nuclear imaging is fluorodeoxyglucose (FDG), which is analogous to glucose and is metabolized by the body in the same manner as glucose.

Radionuclides can be administered orally or intravenously. Each radionuclide has a characteristic rate of decay, and therefore, a limited amount of time is available between uptake of the radionuclide and imaging of the patient. The products of radioactivity are detected using gamma cameras. The process of acquiring the signal from the radionuclide can be performed using either positron emission tomography (PET) or single photon emission computed tomography (SPECT). PET provides higher resolution images but uses more expensive and sophisticated equipment relative to SPECT. The level of intensity of the signal on PET or SPECT images is representative of the activity of the radionuclide. Radiologists can infer from PET/SPECT images any unusual activity in the body and request further examination based on the findings.

Applications and Products

Ultrasound. The most common application for ultrasound is fetal imaging during pregnancy. Ultrasound is used to detect defects in the development of the fetus and to determine the sex of the fetus at later stages of development. Ultrasound is also used to detect abnormalities in muscles, tendons, and internal organs and to obtain information regarding the size and functionality of the organs. Cardiologists often use ultrasound to detect heart defects and plaque development in major body vessels. Patients with high cholesterol levels often undergo ultrasound imaging on a regular basis to measure plaque development in blood vessels. The goal is to detect atherosclerosis at an early stage and provide treatment to prevent devastating consequences such as heart attacks.

Magnetic Resonance Imaging. Applications of MRI exist in numerous branches of medicine in which detailed anatomical views of the patient are required. MRI provides soft-tissue contrast and high-resolution images without the use of ionizing radiation. MRI is commonly used to diagnose cancer, hemorrhage, stroke, musculoskeletal disorders, infections, inflammatory disorders (multiple sclerosis), and degenerative disorders (Alzheimer disease). MRI can also be used to obtain measurements of blood flow in the body. Such measurements provide doctors with pertinent information on the condition of the body's blood vessels, thereby allowing early intervention when abnormal conditions are found. Furthermore, developments in MRI technology have shown the feasibility of using it as a functional brain-mapping tool. Functional magnetic resonance imaging (fMRI) can detect which parts of the brain are activated in response to various external stimuli. Functional magnetic resonance imaging has found applications in early detection of Alzheimer's disease and in assessing cognitive and intelligence capabilities in children and adults.

Nuclear Imaging. The power of nuclear imaging lies in its ability to provide functional information on cellular processes within the body. PET imaging using the radionuclide fluorodeoxyglucose (FDG), or FDG-PET, is used to assess the body's metabolic activity and infer information regarding the body's condition. The most common use for FDG-PET imaging is detecting the spread of cancer throughout the body. Cancer patients have a high risk of developing secondary cancers in the body; through routine nuclear imaging tests, physicians are able to monitor cancer patients and detect the spread of cancer at an early stage and offer effective treatment or palliation of the disease.

X-Rays. The most common application of planar X-ray imaging is in orthopedics, where it is used to assess bone integrity and detect fractures and breaks. Bone absorbs X-ray photons readily and therefore appears bright on X-ray film. In addition to planar X-ray imaging, there are a number of commonly used X-ray-based imaging modalities. Fluoroscopy is an imaging procedure that allows doctors to obtain real time information on the relevant area in the patient. By obtaining sequential X-ray images through the use of an X-ray source and a fluorescent screen, fluoroscopic imaging can provide an interior view of the patient. Fluoroscopy has found applications in gastrointestinal (GI) imaging, where it is commonly used with a fluorescent contrast agent to detect abnormalities in the GI tract; in imaging of blood vessels (angiography), where it is used to detect blood clots and plaque development; in surgical procedures, where it can be used to guide surgeons to the target; and in numerous other areas of medicine. Because fluoroscopic procedures are performed over relatively long periods of time, the amount of radiation that the patient and the operating radiologist receive is concerning. Reducing exposure to radiation from fluoroscopy is of high importance to achieve good medical care.

Mammography is another X-ray-based imaging procedure. Mammography equipment is customized for breast imaging in women. Mammography uses a slightly different type of X-ray because of the differing density and composition of breast tissue. The most common application for mammography is early screening for breast cancer in high-risk groups of women, including women with a family history of cancer and women above the age of forty. In some countries, including Canada, women above the age of forty are required to undergo routine mammography for screening of breast cancer. Early detection and treatment of breast tumors has been shown to prevent recurrence in late life and has been associated with long-term survival. As with all ionizing radiation, mammography carries a slight risk of cancer induction. In addition, the rate of false-positive results in mammography is slightly higher than for other imaging modalities. This can be distressing to patients undergoing screening for breast cancer.

Computed tomography (CT) is used as an initial procedure for the evaluation of specific types of patients and assessment of surgical or treatment options. From CT images, radiologists are able to locate the presence of foreign bodies such as stones, cancers, and fluid-filled cavities. Radiologists can also analyze CT images for size and volume of body organs and infer diagnosis of diseases and medical conditions such as pancreatitis, bowel disease, aneurysms, blood clots, abnormal narrowing of vessels, infections, injury to organs, tuberculosis, abnormal bone density, and diseases involving changes in tissue density or size.

In addition to the use of CT for disease diagnosis—which is based on a patient's complaint of a specific symptom or visual detection of an abnormality—CT has been used in private radiological clinics where patients may choose to obtain a CT image of their entire body to ensure the absence of abnormalities. However, the use of CT imaging for screening of apparently healthy people is controversial, because CT uses X-ray radiation, and patients who undergo X-ray imaging have an associated risk of cancer induction. CT imaging deposits between fifty and two hundred times the dose deposited by a conventional X-ray image. Although the association between CT imaging and cancer induction is not well established, its unjustified use remains an area of considerable debate.

Advances in computed tomography have allowed doctors to easily perform tissue biopsies that otherwise would have been invasive and time-consuming. Doctors insert a needle into the patient under the guidance of real-time CT imaging. By observing the location of the needle within the patient in real time, doctors are able to accurately obtain a tissue biopsy in a relatively short time and without the need for invasive procedures.

Careers and Course Work

Careers in radiology and medical imaging are primarily in the health care sector. Medical imaging equipment exists in every major hospital and in most small hospitals in the developed world. X-rays are by far the most common and widely distributed imaging modality across the globe. More sophisticated imaging modalities, such as computed tomography and magnetic resonance imaging, are also widely distributed in hospitals in the developing world, and experts in the operation and maintenance of these scanners have frequently been imported from the West.

Many routes exist for working with medical imaging. From the academic side, several roads can lead to medical imaging careers. A degree in physics or engineering with specialization in biomedical or electrical engineering provides a solid ground in the electronics and hardware of medical imaging equipment. Engineers can find work in hospitals or in industries that design and manufacture scanners. Degrees in mathematics and computer science provide the necessary background for working with image reconstruction algorithms and advancing software-related operation of the various imaging modalities. Computer programmers can find work in the research and development sector of most imaging industries. In particular, MRI and CT software and computer algorithms are always being developed and upgraded. A graduate degree in medical physics provides theoretical and practical experience in medical imaging from data acquisition to image reconstruction and troubleshooting. Medical physicists often work in imaging facilities and hospitals, where they usually supervise the personnel operating imaging equipment.

A degree in medicine with specialization in radiology provides theoretical and practical experience in understanding human anatomy, pathology, and physiology, and in interpreting medical images and providing diagnosis of specific disease conditions. Radiologists undergo residency training for a period of four to six years after graduating from medical school.

Technical colleges can provide education in the operation of the various medical scanners. Graduates with a technical degree in medical imaging often work as technologists in hospitals, where they are responsible for patient scheduling and machine operation. Technical colleges provide a broad overview of all the imaging modalities, while allowing the student to specialize and gain experience in one or more imaging procedures.

Medical imaging, with all of its tools and procedures, is an invaluable field in the medical sector. Career prospects involving any of the medical imaging modalities are very promising, and there has been a continual demand for experienced radiology workers. In particular, MRI is increasingly becoming the modality of choice for doctors because of its high-quality images and use of nonionizing radiation. In some countries, patient waiting lists for MRI procedures can be as long as six months because of the heavy demand on the scanners. The number of MRI scanners is expected to increase to meet this demand.

Social Context and Future Prospects

Medical imaging has grown substantially since the fourth quarter of the twentieth century and has become a multibillion-dollar global industry. As each imaging modality emerged, it was heralded by medical professionals, but the focus has shifted from new developments to further enhancing the efficiency, quality, and safety of each modality. In addition, emphasis has been placed on combining imaging modalities to obtain a comprehensive understanding of disease pathology. In the 2000s, combined PET/CT scanners and MRI/PET scanners emerged, providing an enhanced understanding of the anatomical and functional aspects of disease. In addition, portable imaging modalities such as ultrasound have commonly been merged with CT and MRI to provide a detailed portrait of a patient's condition.

Concerns regarding the use of ionizing radiation continue to exist, and the issue has often been debated by the medical community, the media, and regulatory bodies. MRI is the preferred imaging modality for acquiring high-quality anatomical information because of its use of nonionizing radiation, but its high cost and lower availability make CT a more practical imaging modality in many situations. In addition, CT provides different quantitative parameters than MRI.

The dependency of PET and SPECT imaging on radionuclides makes them vulnerable to the limitations of nuclear reactors. The 2009 closure of the Chalk River reactor, which supplied more than a third of the world's supply of medical radionuclides, had a significant impact on nuclear imaging in many clinics. Such incidents can have a negative impact on health care if nuclear imaging is the sole provider of imaging data in a clinic.

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