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Basic Physics for Radiography

April 11, 2024

Basic Physics for Radiography

Learning Objective: Examine fundamental physics for radiography.

Limited operators do not require an extensive background in physics, but some basic principles of physical science are essential to an understanding of x-rays and their use. This section covers the basic concepts of matter, energy, and electricity and relates these principles to radiography. Everything of a physical nature in the universe can be classified as either matter or energy. Both matter and energy can exist in several forms.

Matter

Learning Objective: Explain matter.

Matter is defined as anything that occupies space and has shape or form. The primary forms of matter are solids, liquids, and gases. All matter is composed of building blocks called atoms. The fundamental particles that constitute atoms are neutrons, protons, and electrons. The neutrons and protons form the nucleus of the atom, its center. The electrons circle the nucleus in orbits called shells.
The characteristic of an atom that determines its type is the number of protons in the nucleus. An element comprises only one kind of atom; all atoms of an element have the same atomic number. The atomic number of a material determines how it will absorb radiation and appear on an image.
When a neutral atom gains or loses an electron, the atom is said to be ionized. Ionization produces an atom with an electric charge. If an electron is added to a neutral atom, electrons will outnumber the protons, and the atom will have a negative charge. If an electron is removed, there will be more protons than electrons, so the atom will have a positive charge. Applying a small amount of energy to an outer orbital electron will remove the electron from the atom. Ionization is significant in radiology because x-rays cause ionization in the atoms of the human body.

Energy

Learning Objective: Explain energy.

Energy is the ability to do work. It occurs in several forms and can be changed from one form to another. Electromagnetic energy is the energy we deal with in radiology. The energy consists of light, x-rays, radio waves, microwaves, and other forms of energy. These energies have both electric and magnetic properties. Electromagnetic energy occurs in the form of a sine wave. The distance between the crest and the trough of the wave is the amplitude. The distance from one crest to another is the wavelength, and the frequency is the number of times per second that a crest passes a given point. X-rays with greater energy have shorter wavelengths but higher frequencies and are more penetrating. The smallest possible unit of electromagnetic energy is the photon, which can be thought of as a tiny bundle of energy. Photons come out of the x-ray tube as discrete bundles of energy.

Characteristics of X-Rays

Learning Objective: Describe the characteristics of x-rays.

X-rays and visible light are both forms of electromagnetic energy. They have some similar characteristics. Both travel in straight lines at the same velocity. X-rays and light can cause changes in living organisms (e.g., sunburn). X-rays are also different than visible light. X-rays are capable of producing more harmful effects than light because of their greater energy. X-rays can penetrate matter, dependent on the mass, atomic number, and thickness of the matter. X-rays come out of the x-ray tube and into space until they are absorbed or go into the human body.

Unique Characteristics of X-rays

X-rays exhibit the following characteristics:

• Have no mass
• Are highly penetrating and invisible
• Are electrically neutral
• Produce over a wide range of energies and wavelengths
• Travel in a straight line at the speed of light
• Can ionize matter
• Produce biological changes in tissue
• Produce secondary and scatter radiation

Electricity

Learning Objective: Define electricity.

X-ray energy is human made and is produced electrically. Electric current will flow in a vacuum, in particular liquids, and through certain metals called conductors. Copper wire is a common conductor. It is connected to form a circuit. Current will flow in the circuit when there is a difference in electric charge, known as a potential difference, between two points in the circuit.

Electric Circuits

Three electric factors are part of a circuit:

• Resistance: Hinders the flow of current.
• Current: The quantity of electrons flowing in a circuit. The ampere (A) is the unit of measure for current.
• Potential difference: The difference in electric potential between two points in an electric circuit. The volt (V) is the unit for potential difference. In radiology, we use milliamperes (mA) and kilovolts peak (kVp) .

An electric circuit is a continuous path for the flow of electric charges from the power source through one or more electrical devices and back to the source. The electricity provided by the utility company is in the form of an alternating current (AC). This means the current will flow in one direction and then alternate to another direction. Therefore, the current is constantly changing. As the voltage changes, the current flow increases, peaks, and decreases. AC can be converted so that it flows in one direction only. This process is called rectification. (Rectification will be discussed in more detail in a later section.) The x-ray tube cannot produce x-rays unless the current is rectified. Rectified AC is sometimes referred to as direct current (DC). The transformer of the x-ray machine will change AC to DC.

Electromagnetic Induction and Transformers

Learning Objective: Explain electromagnetic induction and transformers.

Magnetic fields and electric energy are interrelated. Magnetic fields can be used to produce electricity and, conversely, electric currents create magnetic fields. When a conductor is placed in a magnetic field, an electric current will flow in the conductor. This process is electromagnetic induction. When the direction of the movement changes, the direction of the current flow changes. Electromagnetic induction is the basis for the transformer, the device used to produce the high voltage required for x-ray production. A transformer consists of primary and secondary coils, usually surrounding an iron core. Current always flows from the primary to the secondary coils. When there are more turns, or “windings,” in the secondary coil than in the primary coil, the voltage on the secondary side is greater, and the transformer is called a step-up transformer. On the other hand, if the secondary side has fewer turns, the secondary voltage and the transformer will be a step-down transformer.
A transformer can increase or decrease voltage, the change is based on the number of turns in the coil. This is known as the transformer ratio which is the ratio between turns on each side of the coil. The first number in the ratio is the number of windings on the secondary side. It is helpful to remember that both voltage and amperage flow in an electric circuit. Thus, amperage also flows through the transformer; however, kilovoltage and amperage are inversely proportional while flowing through the transformer. It is important to remember that as a step-up transformer increases the voltage from primary to secondary, amperage decreases. An x-ray machine uses both step-up and step-down transformers.