CT and pet scanners

Type of physical science: Nuclear physics

Field of study: Nuclear techniques

CT and PET scanners are related machines that use X rays and positrons, respectively, along with a computer, to produce detailed images of the brain and other body parts for use in the diagnosis of disease processes. These radiological techniques are essential to modern medicine.

Overview

The 1979 Nobel Prize in Physiology or Medicine was shared by Godfrey N. Hounsfeld, a British electronics engineer, and Alan M. Cormack, an American physicist, for their contributions to the development of the first CT (or CAT) scanner. A CT scanner carries out the procedure called computerized axial tomography in which X-ray data are collected and assembled by a sophisticated computer into a three-dimensional image--tomogram--of an opaque, solid mass (for example, part of the human body). CT scanners are used most widely for diagnostic medical imaging.

CT scanning did not become possible until 1972, when a merger between the use of X rays and modern computer technology was engineered by Hounsfeld. The procedure may be viewed as a centrally important aspect of medical radiology, an area of medicine that began in 1885 when Wilhelm Conrad Rontgen discovered the high-energy electromagnetic radiations that he named "X rays." Rontgen and others soon discovered that medically useful X-ray images could be produced and used as diagnostic aids. The medical use of X-ray analysis, however, developed slowly and for many years was confined to a method in which broad, high-energy X-ray beams were passed through a body part. These beams then hit a covered sheet of photographic film and developed the film because of their energy, producing conventional X-ray pictures.

When a conventional X ray is taken, bones absorb much more X-ray energy than do the "soft tissues" that surround them. Therefore, when an X-ray film is developed, bones appear as predominant white areas. This conventional procedure is valuable for identifying bone fractures and has some use in identifying health problems in the soft tissues. The distinction among the various soft tissues is poor, however, and comparative examination of diseased and healthy tissue in any particular organ is not achieved well by this method.

Starting in the late 1950's, Cormack pioneered a basic mathematical modeling method usable to obtain detailed X-ray images of biological samples. He used narrow X-ray beams to probe test samples from many different reference points, leading to the ability--with the use of appropriate mathematics--to construct a picture of the interior structure of thin slices of the sample studied. Cormack published his data but received almost no recognition because computers able to analyze the data adequately had not yet been developed. Yet, X-ray tomography--the ability to utilize X rays to prepare detailed images of thin sections of solid objects (from the Greek tomos, or section)--had been born.

Next, Hounsfeld independently--reportedly with no knowledge of Cormack's endeavors--designed the first CT scanner. The procedure that Hounsfeld developed utilized a mathematical approach quite different from that of Cormack. His method merged with modern computers to allow for a computerized reconstruction of thin body sections that was more than one hundred times as sensitive as conventional X-ray methodology. This process also allowed researchers and physicians to distinguish between normal and diseased tissues. In 1973, Hounsfeld published a detailed description of his first CT scanner. At the same time, his clinical collaborator, James Ambrose, described the clinical aspects of the technique.

Hounsfeld and Ambrose's first CT scanner consisted of an X-ray generator and a scanner unit composed of an X-ray tube and a detector in a circular chamber, around which could rotate a computer to process all the obtained data and a cathode-ray tube (CRT) on which the tomograms were viewed. To produce the tomograms of the brain, the patient to be tested was placed on a couch, with his or her head inside the scanner chamber, and the X-ray emitter-detector was rotated one degree at a time. At each position, 160 readings were taken and input to the computer. In all, 28,800 readings were taken and processed. The computer then converted the data into a tomogram (cross-sectional representation) of a thin slice of the brain that showed differences in tissue density. A Polaroid picture of the tomogram was taken and interpreted by the physicians involved. In the two decades following the design of the original CT scanner, the several generations of its progeny have become faster, more sophisticated, and capable of whole-body tomography, where necessary.

The data obtained with CT scanners are limited to the delineation of the anatomy of the body part examined. In the study of disease, it is also desirable to have methodology that allows the specific examination of individual biochemical processes in particular organs. In such methodology, other tissues and processes unrelated to the disease entity of interest must not interfere with diagnostic procedures. These needs led to the development of positron emission tomography (PET, or PETT) in the middle of the 1970's.

PET utilizes a machine similar to the CT scanner, but modified so that positrons (antimatter particles produced by the decay of specific radionuclides into γ radiation) replace nonspecific X rays. Detection is made via scintillation detectors that quantify the photons of γ radiation produced by positron annihilation. The advantages of PET include the fact that specific pharmaceuticals that react with individual tissues can be prepared for use and, when such pharmaceuticals are administered, they accumulate in the tissue of interest. Therefore, examination by PET scanning allows for the identification of areas of excess or subnormal accumulation, which are valuable in explaining the basis for disease.

Of the radionuclides that produce positrons, carbon 11, oxygen 15, and fluorine 18 are most commonly used. These radionuclides, produced in a cyclotron, are parts of the pharmaceutical to be introduced to the patient under study. In the tissue to be studied, an emitted positron travels a few millimeters and encounters a free electron, leading to the mutual annihilation that produces γ radiation. Then, the γ radiation photons are detected and used to produce a tomogram.

Applications

A primary, widespread use of CT scanners is in the facilitation of the diagnosis and handling of brain disorders. Even the very first of these machines could distinguish between brain tumors and blood clots, aiding physicians to diagnose better a wide variety of brain-related birth defects. Furthermore, CT scanners have also reputedly saved many lives by enabling physicians to avoid the use of dangerous techniques such as having to open the brain cavity for diagnostic purposes before proceeding with the corrective procedures that were then indicated.

The more modern versions of the CT scanner not only allow clearer and better imaging of anatomic structures but also are much faster to operate, exposing the patient to lower radiation doses. In addition, the more current instruments are full-body CT scanners that can be utilized for diagnosing diseases in other parts of the head (for example, the sinuses), the neck, the chest, the abdomen and pelvis, and the spine.

The versatility of CT scanning can be made clear by describing some of its uses in the examination of the lung. One aspect of such an examination is the identification of abnormalities in lung blood vessels and bronchi. Furthermore, the basis for lung trauma can be identified easily, as can the presence and localization of cancers and benign tumors. Benign and malignant tumors are also differentiated by this methodology. The methodology of CT scanning also can facilitate the identification of diseases of the pleura, which is the membrane that surrounds the lungs.

One inherent deficiency of diagnosis via CT scanner is the fact that the X-ray examination made possible by this procedure is relatively nonspecific, identifying tissue morphology without the ability to examine specific elements of an organ (such as the ability of various parts of an organ to interact with specific hormones or various types of pharmaceutical drugs). The development of the PET scanner has gone a long way toward offering solutions to such problems.

For PET scanning, the patient to be examined can be treated with a positron-emitting form of a hormone, a neuroreceptor, a neurotransmitter, or some other drug that localizes in a specific tissue area. Then, when the PET scan is taken, it becomes possible to identify--more specifically--disease entities that relate to a wide variety of highly individualized causative agents. For example, such methods can be used on the brain in order to study opiate receptors, hormone receptors, tranquilizers, and other chemicals related to the fine control of mental problems and cognitive processes. It should be remembered, however, that the use of PET scanning is not limited to the brain and that other organs are under similar study. Some examples include the use of PET scanning in the study of cardiovascular problems.

Context

Many neurologists have proposed that CT scanning is the most important medical method developed in the twentieth century for the facilitation of the diagnosis of brain disorders.

The first CT scanners were able to differentiate between brain tumors and blood clots. The ongoing development of these machines led to much better, and faster, scanning procedures. This diminished the time required to carry out tomography. The decreased time frame has been very useful because it lowers the amount of radiation to which a patient is exposed in the course of studying a medical problem.

Also, the more advanced CT scanners evolved to become full-body scanners, making it possible to examine other parts of the body--including the lungs, the cardiovascular system, and the abdominal organs--for tumors and other diseases that could be identified by examining the anatomical features of the sites investigated. Indeed, CT scans have become so widely used that countless radiology departments have changed their names to departments of medical imaging.

A basic fundamental shortcoming of CT scanner use has been that it shows only the anatomical features of the organ components being viewed. The necessity for the identification of defects in specific biochemical processes (for example, in brain action as the result of neurotransmitters and other chemicals that affect thought processes) led to modifications that produced PET scanners. These machines, designed on the basic conceptual framework of computerized axial tomography but using positron-emitting chemicals, have added much insight into brain action and many other biochemical processes. They are also evolving into exceptionally useful additions to the overall armamentarium of diagnostic medicine.

Another technique, cross-fertilized from the methodology of CT and PET scanners, is the use of nuclear magnetic resonance imaging, often called MRI. In a process similar to CT and PET scans, magnetic fields are used to examine the body by computerized tomography in a process that does away with the use of radiation. All three methods--CT, PET, and MRI--have different degrees of utility in various situations, although often their use can be complementary.

Consequently, these methods, all traceable back to the pioneering work of Cormack and Hounsfeld, have expanded the ability of the medical community to carry out diagnoses and enriched or saved the lives of countless people with myriad medical problems.

Principal terms

CATHODE-RAY TUBE: a vacuum tube for television or computer image production in which a hot cathode emits an electron beam accelerated through a high-voltage anode, which is focused and made to fall on a fluorescent screen

COMPUTERIZED TOMOGRAPHY: a radiological technique that collects X-ray (CT) data or γ radiation data from positron annihilation (PET, PETT) and uses it to assemble three-dimensional images of opaque, solid masses such as parts of the human body

CT SCANNER: a sophisticated X-ray imager capable of taking detailed X-ray pictures of thin slices of opaque objects, usually body parts, via a rotating X-ray source-detector assembly and a computer that deciphers the obtained data

GAMMA RAYS: electromagnetic radiation emitted by certain radionuclides and having energies similar to those of X rays

PET SCANNER: a device conceptually similar to the CT scanner but using positron-emitting pharmaceuticals, hormones, or other chemicals to produce images of body parts that identify diseases and other health problems related to specific biochemical processes

POSITRON: an antimatter particle of the same mass--but opposite charge--as an electron, produced when certain radionuclides undergo radioactive decay; an encounter between an electron and a positron causes mutual annihilation and produces γ rays

RADIOLOGY: the area of medicine that uses ionizing radiation (such as X rays and γ rays) for diagnostic purposes

RADIONUCLIDE: an atom that breaks down spontaneously to produce ionizing radiation (for example, X rays or positrons)

TOMOGRAM: a three-dimensional image of a thin slice of an opaque object, usually a body part, obtained by use of a CT scanner or a PET scanner

X RAY: a short-wavelength, high-energy photon of electromagnetic radiation used in radiology because of its high tissue-penetrating power

Bibliography

Abrams, Herbert L., et al. ASSESSING COMPUTED TOMOGRAPHY. Rockville, Md.: Public Health Service, Office of Health Research, Statistics, and Technology, 1981. This volume is part of a monograph series that explores the societal implications of health care technology. Its component articles address computed body tomography, brain imaging, computed tomography of the head and spine, neurologic overview of the techniques, and the contributions of CT to neurosurgery.

Ambrose, James. "Computerized Transverse Axial Scanning (Tomography). Part 2: Clinical Application." BRITISH JOURNAL OF RADIOLOGY 46 (October, 1980): 1023-1047. Describes CT scanner applications that indicate that data are used to construct images for qualitative and quantitative examinations. It is noted that tomograms are examined in the same way as conventional radiographs and that lesions appear as altered density in soft tissues, interpreted in the light of known pathological changes.

Chiu, Lee C., James D. Lipcamon, and Victoria S. Yui-Chiu. CLINICAL COMPUTED TOMOGRAPHY: ILLUSTRATED PROCEDURAL GUIDE. Rockville, Md.: Aspen Systems, 1985. This well-illustrated book covers many aspects of CT scanning. Although designed for technologists, radiology residents, and medical engineers, this book is valuable to any reader desiring advanced coverage and useful illustrations. Topics covered include the principles and instruments of CT, a description of its use throughout the body, and 179 references for specific, in-depth coverage.

Di Chiro, Giovanni, and Rodney A. Brooks. "The 1979 Nobel Prize in Medicine." SCIENCE 206 (November 30, 1979): 1060-1062. This brief article describes the development, fundamental operation, and evolution of the CT scanner. It also identifies the preeminence of Godfrey Hounsfeld's contributions without overlooking other contributors to the field. Identifies some factors contributing to the success of Hounsfeld's endeavors. Bibliographical material is included.

Frost, J. James, and Henry N. Wagner, Jr. QUANTITATIVE IMAGING: NEURORECEPTORS, NEUROTRANSMITTERS, AND ENZYMES. New York: Raven Press, 1990. A book primarily dedicated to describing the use of PET to study receptors in the central nervous system; it does an excellent job. Chapter 4 thoroughly explains the physics and instrumentation of PET. Recommended for the reader who wants in-depth information on receptors and on PET.

Greenberg, Mark, Brent M. Greenberg, and Irving M. Greenberg, eds. ESSENTIALS OF BODY COMPUTED TOMOGRAPHY. Philadelphia: W. B. Saunders, 1983. This technical compilation of articles by many authorities in the field contains a nice introductory chapter on the principles of CT, followed by authoritative descriptions of CT use in organs including the lung, liver, spleen, pancreas, kidneys, pelvis, digestive tract, and spine. Interested readers will be able to explore many widely divergent uses of CT.

Hounsfeld, Godfrey N. "Computed Medical Imaging: Nobel Lecture, December 8, 1979." JOURNAL OF COMPUTER ASSISTED TOMOGRAPHY 4 (October, 1980): 665-674. This Nobel acceptance lecture describes aspects of development of the CT scanner, focusing on early tests, the principles of the technique, the improvement of the system, its accuracy, and the relationship between resolution and picture noise. Future improvements are foreseen and important diagrams and tomograms are included.

Hounsfeld, Godfrey N."Computerized Transverse Axial Scanning (Tomography). Part 1: Description of System." BRITISH JOURNAL OF RADIOLOGY 46 (October, 1980): 1016-1022. This article describes the original CAT scanner. Explains the system, wherein multiple X-ray readings are converted, by computer, to a series of pictures of cranial slices. The system is noted to be one hundred times as sensitive as conventional X-ray systems and able to display variations in soft tissues of similar density.

Phelps, Michael E., John C. Mazziotta, and Heinrich R. Schelbert. POSITRON EMISSION TOMOGRAPHY AND AUTORADIOGRAPHY: PRINCIPLES AND APPLICATIONS FOR THE BRAIN AND HEART. New York: Raven Press, 1986. This book includes a description of basic principles of PET and explains its use in the brain and heart. Describes aspects of the biochemistry, pharmacology, and physiology of these organs; strategies for preparation of positron-labeled test chemicals; and their applications. Much information will be found by the tenacious reader wishing in-depth coverage.

Robb, Richard A. THREE-DIMENSIONAL BIOMEDICAL IMAGING. Vol. 1. Boca Raton, Fla.: CRC Press, 1985. This technical book provides useful information and important illustrations for the interested reader. Chapters 3 to 5 cover many aspects of CAT scanning, renamed X-ray computed tomography here. They include technical descriptions of the basic principles of the method, the implementation and expansion of its applications, and some advanced CT systems and their use.

Ter-Pogossian, Michel M., Michael E. Phelps, Edward J. Hoffman, and Nizar A. Mullani. "A Positron-Emission Transaxial Tomograph for Nuclear Imaging (PETT)." RADIOLOGY 4 (January, 1975): 89-98. This article describes a prototype PET scanner and its use in obtaining transaxial images of sections of organs containing positron-emitting radiopharmaceuticals. The methodology of PET scanning and the construction of the instrument are discussed. This landmark publication gives useful detail to the interested reader.

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