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imaging techniques

imaging techniques In recent decades, technological advances in imaging of the body have been phenomenal. X-ray photographic images (radiographs), with or without the use of contrast medium, became of major importance in medical diagnosis, from their invention at the end of the nineteenth century onwards. But traditional X-ray images have their limitations: there is no clue to ‘depth’ from a single image; the shape, site, and size of, say, a growth or infection showing a ‘shadow’ in a lung can only be assessed approximately by taking radiographs from different angles or by ‘screening’ the subjects as they move around.

Whilst plain radiographs remain the cornerstone of modern radiology, new imaging techniques have developed along with advances in physics — nuclear and otherwise — and in computer technology. Some utilize X-rays and others gamma rays from radioisotopes, with exposures to radiation too small and too brief to cause damage. Ultrasound uses high frequency sound waves and magnetic resonance imaging uses radio waves. All these are utilized for ‘scans’ in which a moving source provides information to be picked up by multiple detectors, computed to yield detailed 2-dimensional images, or 3-dimensional reconstructions.

CT scans

A new form of X-ray imaging, computed tomography (CT scan), was developed in 1972 by Sir Godfrey Hounsfield in the UK and Allan Cormack in the US. A narrow X-ray beam traverses the body in an axial plane (the anatomical term for a vertical plane dividing the body from front to back) and multiple detectors, surrounding the body, record the strength of the exiting X-ray. This data is analysed, integrated, and reconstructed by computer to produce images of cross-sections or ‘slices’ of an organ or region of the body. Using sophisticated software, 3-dimensional reconstruction images can be created to demonstrate specific organs.

The introduction of CT was one of the greatest advances particularly in neurological diagnosis, giving 2-dimensional thin section images or ‘slices’ of the brain or spinal cord, and allowing precise localization of ‘space occupying lesions’ such as blood clots or tumours. CT was also a major advance in imaging tumours and other abnormalities in almost any part of the body, often allowing their detection without resorting to exploratory surgery. The advanced technique of Electron Beam Computed Tomography (EBCT) has more recently become useful for cardiac imaging and identification of coronary artery calcification. Spiral CT is another new technique which gives very rapid multi-slice scans with fast image reconstruction, providing greatly improved anatomical definition of organs and blood vessels.

Digital subtraction angiography (DSA)

is a sophisticated method which improves on traditional X-rays for imaging blood vessels following injection of a soluble radio-opaque substance. Using digitized images, computer analysis subtracts overlying and unwanted shadows of bones and soft tissues, giving greatly improved definition of blood vessels whilst minimizing the radiation dose and the amount of contrast medium required.

Isotope scanning

Together with the application of radioactive isotopes in treatment (radiotherapy), diagnostic isotope scanning now constitutes the specialty of nuclear medicine. Following an intravenous injection, an isotope is taken up by different organs in varying amounts. The radiation emitted is detected by a scintillation counter (a gamma camera, invented in 1961) and the image recorded. Having a very short half-life, the radioactive isotope decays completely before it can cause damage. Different isotopes tend to concentrate in particular organs. Technetium-99m is most widely used, especially to investigate lungs and bone, whereas iodine-131 shows thyroid function.

Positron emission tomography (PET scanning)

involves the emission of particles of antimatter (positrons) from an isotope incorporated in a tracer substance given by injection or inhalation; the positrons are neutralized by electrons within the body tissues, releasing energy in the form of radiation, which is detected and analysed to produce an image. This is used in neurological studies since it can map out blood flow and metabolic activity (oxygen or glucose usage) in the different areas of the brain in health or disease. For example oxygen-15 (15O) is a positron emitter, and can be used to label oxygen in the inspired air. The labelled oxygen, is transferred to the circulating blood and then passes into water when it is used in metabolism, multiple detectors around the head can provide computed tomographic images showing the relative degrees of oxygen uptake in all regions of the brain — and hence their level of function compared to the normal. Such methods can show localized increases in blood flow and metabolism accompanying specific tasks, such as speech, or finger movements, and can trace the pathways of cerebral activation. PET scans are available only where there is a cyclotron close by, because the emitters have a very short life: oxygen-15, for example, has a half-life of only two minutes.

Single photon emission tomography (SPECT)

is an alternative, less expensive but less quantitative method for obtaining two- and three-dimensional reconstructions of the distribution of brain blood flow, involving injection of a radionuclide and detection by a gamma camera rotating around the head.

Ultrasound scans (ultrasonography)

Originally developed from underwater sonar (used to detect fish and submarines in World War II), ultrasound moved from sonar and non-destructive industrial research into medical imaging in the 1950s and 1960s. Ultrasound waves are of a frequency at least a hundred-fold greater than that of the sound waves at the top of the range of human hearing. The waves are reflected from any barrier they meet in proportion to the ‘acoustic impedance’ of the substance. Pulses of high-frequency ultrasound, usually greater than 1 MHz, are emitted from transducers and traverse the body. Different tissue layers cause reflections. Analysis and digitization of the signals provides a greatly improved and clearly defined image. Many pioneers worldwide contributed to this new technology, the growth of its medical applications, and its involvement with manufacturers, leading to its present sophistication. Douglas Howry, a trainee radiologist, and Joseph Holmes, a kidney specialist, of Denver Colorado, along with Ian Donald, Professor of Obstetrics in Glasgow, were among the earliest pioneers in clinical application.

Being quick, relatively inexpensive, and non-ionizing, with no known biological hazard when used within the diagnostic range of frequency, ultrasound scans are a prime diagnostic method in obstetrics — monitoring pregnancy and imaging fetal development and the newborn brain. Ultrasound is also widely used to image stones in the gall bladder, suspected abdominal tumours, or the thyroid gland, and for evaluating the heart by echocardiography. It is also used for biopsy guidance almost anywhere in the body.

Doppler ultrasound scan

Christian Doppler was an Austrian physicist who first described the Doppler Effect: that the apparent frequency of light or sound waves is affected by the relative motion of the source and the detector. This effect is applied in the detection and measurement of the velocity of flow in blood vessels. An ultrasonic beam with a frequency of 4–5 MHz is deflected by the flowing blood, causing a Doppler shift proportional to the flow rate; the resulting small modification of frequency can be detected and analysed. The combination of ultrasound images and Doppler blood flow studies is known as duplex scanning and is of considerable clinical value in diagnosing problems of blood flow, especially arterial disease and venous thrombosis.

Magnetic resonance imaging (MRI scan)

The patient is placed within a magnetic coil and radio-frequency energy is applied to the body. The harmless radio waves excite protons that form the nuclei of hydrogen atoms in the body. The protons give off measureable electric energy which with the aid of computation create an image. MRI provides a hazard-free, non-invasive way of generating images without using X-rays or gamma rays. In the late 1980s MRI proved to be superior to other imaging methods — and more expensive — giving outstanding definition especially in the brain and the skeletal system, as well as most other organs and tissues. MRI angiography is a unique non-invasive method of imaging blood vessels without using contrast medium. As in CT, three-dimensional reconstruction images can be created to demonstrate specific organs.

J. K. Davidson

See also radioactivity; radiology; X-rays.

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Cormack, Allan MacLeod

Allan MacLeod Cormack (məkloud´, côr´mək), 1924–98, American physicist, b. Johannesburg, South Africa. After studying at the Univ. of Cape Town (B.S. physics, 1944, M.S. crystallography, 1945), Cambridge, and Harvard, Cormack became a professor at Tufts Univ. in 1958. His interest in X-ray technology led him to develop the theoretical foundations that made computerized axial tomography (CAT) scanning possible. He published his results in two papers in 1963–64, but these generated little interest until the first CAT scan machine, built under the leadership of Godfrey Hounsfield, was introduced in 1972. For their independent efforts, Cormack and Hounsfield shared the 1979 Nobel Prize in Physiology or Medicine.

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