There are two components of the physical universe: energy and matter. In most physical processes there is a constant interaction and exchange between the two; medical imaging is no exception. In all imaging methods, images are formed by the interaction of energy and human tissue (matter). A variety of energy types are used in medical imaging. This is, in part, what accounts for the difference in imaging methods. In this post we will review some basic energy concepts and then look in detail at radiation, which is energy on the move, and the role of electrons in energy transfer.

Images of internal body structures require a transfer of energy from an energy source to the human body and then from the body to an appropriate receptor, as shown here:

Although the types might be different, certain characteristics apply to all energy used in imaging.

A basic requirement is that the energy must be able to penetrate the human body. Visible light is the primary type of energy used to transfer image information in everyday life. However, because it usually cannot penetrate the human body, we must use other energy types for internal body imaging.
Another characteristic of any energy used for imaging is that it must interact with internal body structures in a manner that will create image information.

A common element of all imaging methods is that a large portion of the energy used is deposited in the human tissue. It does not reside in the body as the same type of energy but is converted into other energy forms such as heat and chemical change. The possibility that the deposited energy will produce an undesirable biological effect must always be considered.

As we approach the process of medical imaging, it is helpful to recognize two broad categories of energy.

Although the latter category does not require matter for its existence, it is always created within a material substance and is constantly moving and transferring energy from one location to another. This form of energy is radiation; all energy forms used for medical imaging, with the exception of ultrasound, are forms of radiation.

The significance of matter-related energy forms in medical imaging is that they supply the energy to form radiation and later recapture it when the radiation is absorbed.

Forms of Energy Involved in the Production of an X-Ray Image

A basic physical principle of the universe is that energy can be neither created nor destroyed. However, we can transform it from one form or type to another. Here we see some of the energy forms used in the production of an x-ray image. Various components of the imaging system convert the energy from one form to another.

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