While many structural and functional imaging techniques are relatively recent, the origin of structural imaging was the X-ray, developed in 1895. X-rays measure the density of tissues. X-rays use photons, a quantum of visible light that possesses energy; the photons are passed through the body and deflected and absorbed to different degrees by the person’s tissues. They are recorded as they pass out of the body onto a silver halide film. Dense structures such as bone, which block most of the photons, appear white; structures containing air appear black; and muscle, fat and fluids appear in various shades of gray. This technology was the clinician’s main imaging tool for more than half of the 20th century.
A related early technique was angiography, in which a radiopaque dye was injected through a catheter into a blood vessel to detect a blockage or narrowing or architecture of downstream vessels. The vessels were outlined on x-ray as white. Angiography was used to visualize arteries anywhere in the body, including the neck and brain.
Computerization transformed the x-ray in the 1970s, with the development of Computer Assisted Tomography (CT), and its two main developers received the Nobel Prize in Medicine or Physiology in 1979. This technology uses special x-ray equipment to obtain three-dimensional anatomical images of bone, soft tissues and air in the entire body, including the head. An x-ray emitter is rotated around a patient. It measures the rays’ intensities from different angles. For brain imaging, numerous X-ray beams are passed through the head at different angles. Special sensors measure the amount of radiation that is absorbed by different tissues. Then, a computer uses the differences in X-ray absorption to form cross-sectional images or “slices” of brain called “tomograms.”
CT imaging was the first technique, for instance, to show clear evidence, during life, of decreases in the amount of brain tissue in older compared to younger people. CT can be used with or without contrast agents (dyes, such as iodine, that make structures easier to see), but use of contrast enables CT to show bone, soft tissues and blood vessels in the same images.
Because CT can be done quickly, it is especially useful in emergency trauma situations, showing any abnormalities in brain structure including brain swelling, or bleeding arising from ruptured aneurysms, hemorrhagic stroke (a ruptured blood vessel), and head injury.
Ultrasound, another early technique developed in the 1930’s-40’s, was primarily used neurologically until the 1960s to try to identify brain tumors. Ultrasound uses sound waves to determine the locations of surfaces within tissues, and differentiates surfaces from fluids. It does so by measuring the time that passes between the production of an ultrasonic pulse and the echo created when the surface reflects the pulse. But, when scientists determined that the skull significantly distorts the signals, its use for this purpose stopped while its use in obstetrics and gynecology—to image the fetus in utero and to detect ovarian tumors—became widespread.
Fortuitously, abandonment of ultrasound to try to detect brain tumors came at the same time that new radiological technologies for brain imaging such as magnetic resonance imaging (MRI) were emerging. Beginning in the 1980s, however, new ultrasound techniques (“Laser Doppler Ultrasound”) began to evolve; these techniques employ laser technology to combine information from both light and sound and have become a vital part of intensive monitoring of cerebral blood flow in patients with severe head trauma. These technologies and their uses are described later in the section on Electrical and Doppler Ultrasonic Imaging Techniques.