A Tour around Imaging Techniques

(Date of publication 26 October 2005)

Pity the poor souls who required a head X-ray in the early part of the last century; the procedure could take up to 11 minutes, during which the unlucky patients had to hold themselves (and the not insubstantial film cassette!) motionless. They also received up to 50 times as much radiation as that produced by today's digital systems, which generate pictures with much greater definition in a matter of milliseconds. This page documents the history of medical imaging from the days of Roentgen onwards, giving a brief outline of how each method works.

We have all seen X-rays, such as these of the chest and spine. They are essentially shadow pictures, in which dense structures – such as bone – block the X-rays and appear white, while the less dense muscle, fat and fluid appear as shades of grey. Medline Plus provides a simple outline of the procedure from the patient's perspective, while there is a surprisingly understandable description of the underlying physics at Absolute Astronomy.

Computed axial tomography, or CAT scanning, is a form of X-ray examination in which the X-ray source and detector rotate around the patient's body to obtain data from different angles. This data is then processed by computer to give well-defined cross-sectional pictures of organs and tissues – essentially slices through the body at different levels. Here is a section through the abdomen from the same web site. Unlike conventional X-rays, this technique enables soft tissues to be outlined with great clarity. A new development is spiral or helical computed tomography, in which the patient, supine on an examination table, advances at a constant rate through the continuously rotating X-ray apparatus, which therefore traces a spiral path through his or her body. One advantage is that the entire procedure can often be completed during a single breath hold; with conventional CAT, small tumours may sometimes be missed because the patient breathes differently on consecutive 'slices' and causes them to be unequally spaced. A video recording the progress of the scan over the chest and abdomen is available for both low speed and high speed connections.

Magnetic resonance imaging (MRI) works by placing the patient within a giant circular magnet to align the protons of hydrogen atoms and then exposing him or her to radio waves. This causes the protons to spin, producing a faint signal which is detected by a receiver and processed by computer to give detailed, high resolution images, such as these of the head and knee. The patient is not exposed to ionising radiation and there are no known side effects. Although patients with metallic material in their bodies (such as pacemakers, artificial joints or bullet fragments!) are generally regarded as unsuitable for MRI scanning, research published earlier this year indicates that patients fitted with recent pacemakers and defibrillators may safely undergo this procedure, providing the machine is set to a relatively low level of energy output.

Individuals who undergo ultrasound imaging are similarly not exposed to ionising radiation, because this technique uses high frequency sound waves to capture real time visual images, and thus show the movement of tissues and organs. Most images, such as this one of an aortic aneurysm, are much more easily interpreted than those of foetuses which have baffled generations of expectant parents. One specialised technique is Doppler ultrasound, used to evaluate bloodflow and identify impediments such as clots and constriction of vessels.

Nuclear medicine imaging, or radionuclide scanning, involves the introduction of very low level radioactive tracer chemicals into the body. These are taken up by the various organs, which then emit faint gamma ray signals which can be detected by a gamma camera and reconstructed into an image by computer. The image may be in grey scale or colour-coded to show functional activity.

One type of nuclear medicine imaging is positron emission tomography (PET) scanning. As the radioactive tracer decays it releases positrons, the antimatter equivalent of electrons, and when these encounter electrons the particles annihilate each other and release gamma rays, which are detected and converted into a digital picture. Unlike other imaging techniques, PET scans can be used to examine physiological processes such as oxygen and sugar metabolism in the brain, although most are conducted to detect soft tissue tumours. Longer-established and more widely used is Single Photon Emission Computer Tomography (SPECT), a similar technology which constructs an image from the photons emitted by the patient. There are many indications for SPECT scans, but around half are carried out to evaluate coronary artery disease. The images have a lower resolution than PET scans, but cost only one third as much to produce.

Medical imaging is evolving rapidly; in the last fourteen months research has produced one new technique for measuring the relative stiffness of soft tissue in order to diagnose injury and disease and another (magnetic particle imaging) which promises both high spatial resolution and high sensitivity, as well as a method of creating 2- and 3-dimensional images of soft tissue from X-rays. However, the ever-increasing amount of data generated, particularly as different imaging methods and techniques are combined, threatens to swamp hospital computer networks, creating delays before doctors can view the (admittedly much-improved) results.

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