Patient Undergoing a CT Scan

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Patient Undergoing a CT Scan

Medical Imaging: PET Scanner, CT, and MRI Replace Many Exploratory Surgeries

Photograph

By: Anonymous

Date: April 9, 1978

Source: Corbis Corporation

About the Photographer: An unknown photographer contributed this photograph to Corbis, a media agency headquartered in Seattle, Washington.

INTRODUCTION

Medical imaging—the ability to look inside the human body without cutting into it—began with the discovery of x rays by Wilhelm Roentgen (1845–1923) in 1895. X rays pass through skin, fat, blood and muscle more easily than through bone. The shadow cast by the bones can be captured upon film and such x-ray images have long been used for diagnostic purposes—to assess a bone fracture, for instance. However, conventional x rays reveal little of the anatomy of soft tissue, such as tendons, blood vessels, and nerves. It is the air in the lungs which allows detail to be seen in a chest x ray, which is still a useful way of diagnosing lung cancer or tuberculosis.

Another limitation of conventional x rays is that a three-dimensional structure is being compressed onto a two-dimensional film which can make interpretation difficult. Nor can conventional x rays distinguish between variations in the density of different tissues. The South African researcher Allan Cormack (1924–1998) addressed this last issue by working out the mathematics of the variation in tissue density, which he published in 1963 and 1964. But there was no practical way of putting this to use until the British engineer Godfrey Hounsfield (1919–2004) developed the first computerized tomography (CT) scanner in 1972, known as the EMI scanner.

As shown in the photograph below, the scanner directs an x ray from a tube on a rotating ring into the patient's body to be picked up by a detector at the other side of the ring. In this way, the x-ray beam can, with computer assistance, create a cross-sectional image of the human body. For the first time, the brain could be seen clearly with white and gray matter, and cavities, all being revealed. Later developments allowed visualization of other parts of the body. The CT scanner proved a milestone in the non-invasive diagnosis of conditions such as cancer and head injury. In 1979, shortly after the photograph below was taken, Hounsfield and Cormack shared the Nobel Prize for Medicine for the invention of the CT scanner.

PRIMARY SOURCE

PATIENT UNDERGOING A CT SCAN

See primary source image.

SIGNIFICANCE

CT scanning has eliminated the need for most exploratory surgery by allowing doctors to examine most parts of the body to pinpoint signs of disease. Conventional CT scanning is, however, limited by the need for the rotating ring to return to its original position once an image of a one centimeter slice of the patient's body has been created. The patient is moved gradually into the scanner while many such slice images are gathered. In spiral CT, which was developed in the late 1980s, the scanner can move continuously over the patient, scanning a volume of tissue rather than a series of slices. This is faster and gives clearer images. Small lesions are thus less likely to be missed. In many hospitals, conventional scanners are used for routine examinations of the head, for stroke or head injury, while the spiral scanner is reserved for more complex diagnostic work.

An even more recent advance is the multislice scanner—a spiral CT scanner which uses multiple rows of detectors, rather than a single row. It is fast, taking less than a second to image a volume of tissue and gives an excellent quality of image. New applications such as vascular, cardiac, and colonic imaging have been made possible with the advent of multislice spiral scanners. But they are costly and generate a great deal of data, raising implications for storage and interpretation.

The patient receives a higher dose of x rays from a multislice scanner, and even a conventional CT scan exposes the patient to some radiation. There has been concern that the over use of CT scans may, therefore, increase a patient's risk of cancer. For instance, some patients opt to have a whole-body CT scan as a form of medical check-up to detect problems early, even when they have no symptoms. The health benefit of the whole-body CT scan has not been proven to outweigh the risks. Similarly, the value of screening smokers for early signs of lung cancer by spiral CT is also not yet established.

Magnetic resonance imaging (MRI) does much the same job as CT, but without any exposure to radiation. In MRI, the patient is exposed to a magnetic field that interacts with hydrogen atoms within the tissues of the body. Signals are thus generated which can be used to build an image of the tissue. CT is thought better at imaging organs in motion—the lungs and bowel—while MRI is often preferred for imaging the pelvis and diagnosing neurological disorders like multiple sclerosis. For solid organs such as the liver, spleen, pancreas, and kidneys, either CT or MRI can be used.