Radiation Health and Safety (RHS) Exam

Ionization is the process by which an atom gains or loses electrons, thereby acquiring a net electrical charge. The resulting charged atom is called an ion. Positive ions form when electrons are removed; negative ions form when electrons are added.

X-rays possess sufficient energy to ionize atoms in the materials they encounter, including biological tissues. This ionizing capability is what makes x-rays useful for diagnostic imaging but also potentially hazardous to living organisms.

X-rays belong to the electromagnetic spectrum, which encompasses all forms of electromagnetic radiation arranged by wavelength and frequency. The spectrum includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays, and gamma rays.

X-rays have very short wavelengths (approximately 0.01 to 10 nanometers) and very high frequencies. Their high energy allows them to penetrate matter that would absorb or reflect lower-energy radiation such as visible light. This penetrating ability makes x-rays invaluable for medical and dental imaging.

X-rays and gamma rays are the only forms of electromagnetic radiation with sufficient energy to cause ionization in biological tissues. The primary difference between x-rays and gamma rays lies in their origin: x-rays are produced by electron interactions outside the nucleus, while gamma rays originate from nuclear decay processes.

X-rays exhibit several properties essential for diagnostic imaging. They travel in straight lines at the speed of light. They cannot be focused by a lens. They are invisible and undetectable by human senses. They can penetrate matter that absorbs or reflects visible light. They cause certain substances to fluoresce or emit visible light. They can produce latent images on photographic emulsions. They can ionize atoms and molecules in matter.

X-rays are differentially absorbed by various tissues based on tissue density and atomic composition. Dense structures like bone and enamel absorb more x-rays, appearing light (radiopaque) on the image. Less dense structures like soft tissue and pulp absorb fewer x-rays, appearing dark (radiolucent) on the image.

X-rays are produced within an x-ray tube, a vacuum-sealed glass or metal envelope containing two electrodes: the cathode (negative electrode) and the anode (positive electrode).

The cathode consists of a tungsten filament housed within a focusing cup. When electrical current passes through the filament, it heats up and releases electrons through thermionic emission. The focusing cup directs these electrons toward the anode.

The anode contains a tungsten target embedded in a copper stem. Tungsten is selected because of its high atomic number (74), high melting point (3,370°C), and ability to dissipate heat efficiently. The copper stem conducts heat away from the target.

When high-speed electrons from the cathode strike the tungsten target, two types of x-ray-producing interactions occur: bremsstrahlung (braking radiation) and characteristic radiation.

Bremsstrahlung radiation accounts for approximately 70–90% of x-rays produced in a dental x-ray tube. The term comes from the German words for "braking" and "radiation."

Bremsstrahlung occurs when a high-speed electron passes close to the nucleus of a target atom. The strong positive charge of the nucleus attracts and deflects the negatively charged electron, causing it to slow down and change direction. The energy lost during this deceleration is emitted as an x-ray photon.

The amount of energy converted to x-radiation depends on how close the electron passes to the nucleus. Electrons passing very close to the nucleus experience greater deceleration and produce higher-energy x-rays. Electrons passing farther from the nucleus produce lower-energy x-rays. This variation produces a continuous spectrum of x-ray energies in the beam.

The maximum energy of a bremsstrahlung x-ray equals the peak kilovoltage (kVp) applied across the tube. If the tube operates at 70 kVp, the maximum photon energy is 70 keV.

Characteristic radiation accounts for approximately 10–30% of x-rays produced in a dental x-ray tube. This type of radiation occurs when a high-speed electron from the cathode collides with and ejects an inner-shell electron from a target atom.

The collision must transfer enough energy to overcome the binding energy of the inner-shell electron. Once ejected, electrons from outer shells cascade inward to fill the vacancy. Each transition releases a photon with energy equal to the difference between the binding energies of the two shells.

Characteristic radiation produces discrete energy peaks rather than a continuous spectrum. The energies of these peaks are "characteristic" of the target element because they depend on the specific binding energies of that element's electron shells.

Beam quality refers to the penetrating ability of the x-ray beam, determined primarily by the kVp setting. Higher kVp produces higher-energy x-rays with greater penetrating power. Beam quality affects image contrast; higher kVp produces images with lower contrast (more shades of gray), while lower kVp produces images with higher contrast (more black and white differentiation).

Beam quantity refers to the number of x-ray photons in the beam, determined primarily by the milliamperage (mA) and exposure time. Increasing mA or exposure time increases the number of electrons flowing from cathode to anode, producing more x-ray photons. Beam quantity affects image density; more photons produce a darker image.

The milliampere-seconds (mAs) value represents the product of mA and exposure time in seconds. This value directly correlates with beam quantity and radiation dose to the patient.

Filtration removes low-energy x-rays from the beam before they reach the patient. Low-energy x-rays lack sufficient penetrating power to contribute to image formation; they are absorbed by superficial tissues, adding to patient dose without diagnostic benefit.

Inherent filtration results from x-rays passing through the glass envelope of the tube, the insulating oil, and the tube housing. In dental x-ray units, inherent filtration typically equals approximately 0.5–1.0 mm aluminum equivalent.

Added filtration consists of aluminum discs placed in the path of the beam, usually at the exit port of the tube housing. Federal regulations require a minimum total filtration of 1.5 mm aluminum equivalent for x-ray units operating at or below 70 kVp and 2.5 mm aluminum equivalent for units operating above 70 kVp.

Collimation restricts the size and shape of the x-ray beam to the area of clinical interest. This reduces patient exposure by eliminating unnecessary irradiation of tissues outside the target area.

In dental radiography, collimation is achieved using a lead diaphragm with a circular or rectangular aperture. The position-indicating device (PID) extends from the tube housing and helps direct the collimated beam toward the patient.

Rectangular collimation reduces patient exposure by up to 60% compared to round collimation because it limits the beam to approximately the size of the image receptor. Round collimation produces a circular beam larger than the receptor, exposing tissues that do not contribute to the diagnostic image.

Federal regulations limit the diameter of the x-ray beam to no more than 2.75 inches (7 cm) at the patient's skin surface.