Scientific discoveries and advancement in scientific knowledge have led to the evolution of imaging techniques in the medical area.
The first imaging technique used in the medical field were x-rays. This came after the discovery of x-rays by Rntgen. An x-ray is light (an electromagnetic wave) with very high energy and it is not visible to human eye. A greater amount of the x-ray is absorbed by materials with higher density. For example, when x-rays are passing through a human body, bones, which are much denser than the soft tissues in the body, will absorb a greater amount of x-rays than the soft tissues around the bones. This difference in absorption creates an image with a clear contrast between bones and tissues.
By injecting or taking x-ray absorbing drugs (radiopaque contrast agent), x-ray imaging can also show the image of blood vessels and the digestive system. This technique is called angiography. A three-dimensional x-ray image can be produced by taking two-dimensional x-ray images at different directions and combining them with a computer. This technique is called Computed Tomography commonly know as CT scan.
Magnetic Resonance Imaging, known as MRI, is an excellent technique to create images of soft tissues such as brain, muscle, ligaments, and tendons. Since it uses magnetic field and radio frequency wave with low energy, it is safer than an x-ray. It also gives much clearer images of soft tissues than an x-ray. MRI takes advantage of the way protons in hydrogen atoms react to a magnetic field and a radio frequency wave. A proton is a subatomic particle in the nucleus of an atom. A proton has a quantum mechanical property called, "Spin". The proton's spin generates a magnetic field around it, therefore, a proton acts like a tiny magnet. These tiny magnets are pointing in a random direction if no magnetic field is applied.
To obtain an image of the body, a strong magnetic field is applied around the body. Most of the tiny proton magnets align themselves to the same direction of the applied magnetic field because this is the lower energy state. Second, pulses of radio frequency wave are sent through a body. Some of the protons absorb the energy of radio frequency wave and tiny proton magnets align themselves in the opposite direction of the applied magnetic field, which is the higher energy state. Next, when the radio frequency wave is turned off, tiny proton magnets realign themselves to the same direction of the applied magnetic field by releasing energy. It is this release of energy that is used to generate the image. Protons in different local environments have different response times to realign themselves. For example, protons in the fat have a much faster response time than protons in water. Therefore, the different response times of protons in different soft tissues creates an image with a clear contrast between a variety of soft tissues.
Neutron imaging is a new imaging technique that complements x-ray imaging. Neutron's interaction with material provides details of structure of materials. Unlike x-rays, neutrons interact with the nuclei in the material. Consequently, they do not depend on the density of materials. Neutrons can be absorbed by some of the light elements such as hydrogen and they can penetrate some of the heavy elements such as lead. Neutron imaging technique provides very detailed images of materials due to the neutron's ability to distinguish many different types of elements. Therefore, neutron imaging has a wide range of applications in a variety of areas, such as, medical, archeology, agriculture, auto industry, aviation industry, and many areas of science. A few neutron images are shown in the figure below.
In the medical area, neutron imaging has shown great potential in the detection of cancers. Active research is in progress on the use of neutrons for detecting and treating cancers.