Rad caharacteristics

Radiation characteristics include x-ray beam quality, quantity, and intensity. Variations in the character of the x-ray beam influence the quality of the resulting images.

The dental radiographer must have a working knowledge of radiation characteristics and exposure factors. The purpose of this chapter is to detail the concepts of x-ray beam quality and quantity, define the concept of beam intensity, and discuss how exposure factors influence these characteristics.

Current dental x-ray units have control panels with preset, predetermined exposure factors (kV, mA, time) for the various anatomic areas of the maxilla and mandible, so no manual operator adjustment choices are needed (Figure 5-1). On older x-ray units, the exposure factors could be manually adjusted by the operator. Whereas all current dental x-ray units allow for the adjustment of time, the ability to adjust kilovoltage and milliamperage varies from model to model. On many current dental x-ray units, manual adjustments of kilovoltage and milliamperage are not an option. Although today’s x-ray units allow only for limited operator adjustments, the dental radiographer still needs a working knowledge of how changing exposure factors affects the appearance of resultant images.

FIG 5-1 Kilovoltage (kV) control and milliamperage (mA) of the unit are located on the dental x-ray machine. (Courtesy KaVo Imaging, Brea, CA.)

Photograph of a dental x-ray machine’s control panel highlighting kilovoltage (kV) and milliamperage (mA) controls. These controls are used to regulate the quality and quantity of x-rays produced. The kilovoltage determines the energy and penetrating ability of the x-ray beam, while the milliamperage controls the number of x-rays generated by adjusting the current through the cathode filament. This image illustrates the importance of exposure factor settings in achieving diagnostic-quality radiographs.

X-Ray Beam Quality

Wavelength determines the energy and penetrating power of radiation. X-rays with shorter wavelengths have more penetrating power, whereas those with longer wavelengths are less penetrating and more likely to be absorbed by matter. In dental imaging, the term quality is used to describe the mean energy or penetrating ability of the x-ray beam. The quality, or wavelength and energy, of the x-ray beam is controlled by kilovoltage.

Voltage and Kilovoltage

Voltage is a measurement of force that refers to the potential difference between two electrical charges. Inside the dental x-ray tubehead, voltage is the measurement of electrical force that causes electrons to move from the negative cathode to the positive anode. Voltage determines the speed with which they move. When voltage is increased, the speed of the electrons is increased. The electrons strike the target with greater force and energy, resulting in a penetrating x-ray beam with a short wavelength.

Voltage is measured in volts or kilovolts. The volt (V) is the unit of measurement used to describe the potential that drives an electrical current through a circuit. Dental x-ray equipment requires the use of high voltages. Most radiographic units operate using kilovolts; 1 kilovolt (kV) is equal to 1000 volts. It is common to refer to tube voltage as “kilovoltage,” which is abbreviated as kV, the same as its unit, the kilovolt. The term kilovoltage (kV) is the maximum voltage, or peak voltage of an alternating current (AC) (Figure 5-2). In older units, the kilovoltage fluctuated depending on the voltage waveform applied to the tube. In current dental x-ray units, this fluctuation is so very small that the kilovoltage can be considered as a fixed value during exposure.

FIG 5-2 Kilovoltage (kV) controls the quality of the x-ray beam and measures the peak voltage of the current.

Diagram depicting alternating current waveforms at three kilovoltage levels: 50, 70, and 90 kV. The vertical axis represents kilovoltage, and the waveform shows the fluctuations over time across one cycle. As kilovoltage increases, the peak amplitude of the wave also increases, indicating a stronger electrical potential. This peak voltage affects the speed and energy of electrons, thereby influencing the x-ray beam’s penetrating power and diagnostic image quality.

In the past, dental x-ray units were available with adjustable settings ranging from 65 to 100 kV. With these units, the kilovoltage could be adjusted according to the individual diagnostic needs of patients. For example, a higher kilovoltage setting was used when the area to be examined was dense or thick. The use of higher kV produces more penetrating dental x-rays with greater energy, whereas the use of lower kV produces less penetrating dental x-rays with less energy. Current intraoral x-ray units include adjustable settings that range from 60 to 70 kV, or else a fixed setting of 70 kV. If the unit has a fixed kV, that kV number is found imprinted on the face of the control panel. On units that allow for the adjustment of kilovoltage, a kV button is found on the control panel.

The quality, or wavelength and energy, of the x-ray beam is controlled by the kilovoltage. The kilovoltage regulates the speed and energy of the electrons and determines the penetrating ability of the x-ray beam. Increasing the kilovoltage results in a higher energy x-ray beam with increased penetrating ability.

Density and Kilovoltage

Density is the overall darkness or blackness of an image. An adjustment in kilovoltage results in a change in the density of a dental 44image. If the kilovoltage is increased while other exposure factors (milliamperage, exposure time) remain constant, the resultant image exhibits an increased density and appears darker (Figure 5-3). If the kilovoltage is decreased, the resultant image exhibits a decreased density and appears lighter (Figure 5-4). Table 5-1 summarizes the effect of kilovoltage on density (also see Chapter 6). With digital imaging, special image enhancement software can be used to change the density by adjusting the brightness. For example, if a digital image is too dark or too light, the brightness can be adjusted so that the image is readable. In comparison, if film is used and the density is nondiagnostic, the image must be retaken with adjusted exposure factors.

Table 5-1

Effect of Kilovoltage (kV) on Image Density and Contrast

Adjustment Density Contrast

↑ kV ↑ (Darker) Low

↓ kV ↓ (Lighter) High

↑, Increase; ↓, decrease.

FIG 5-3 A, Diagnostic radiograph. B, Increase in kilovoltage results in an image that exhibits increased density; the image appears darker.

Pair of dental radiographs illustrating the impact of kilovoltage on image density. Part A presents a standard diagnostic radiograph with appropriate exposure and contrast. Part B demonstrates a radiograph taken with increased kilovoltage, resulting in a darker image due to greater x-ray beam penetration and energy. The increase in kilovoltage produces x-rays with shorter wavelengths and higher energy, leading to an overall increase in image density. This example emphasizes the role of kilovoltage in determining image quality during dental imaging.

FIG 5-4 A, Diagnostic radiograph. B, Decrease in kilovoltage results in an image that exhibits decreased density; the image appears lighter.

This figure presents a pair of dental radiographs to demonstrate the effect of decreasing kilovoltage on image density. Part A displays a properly exposed diagnostic image. Part B, taken at a lower kilovoltage, shows a noticeably lighter image due to reduced beam energy and decreased penetrating power of the x-rays. The lower kilovoltage results in x-rays with longer wavelengths, which are less capable of penetrating dense oral structures. This comparison highlights how kilovoltage adjustment influences image darkness and diagnostic quality.

Contrast and Kilovoltage

Contrast refers to how sharply dark and light areas are differentiated or separated on an image. An adjustment in kilovoltage results in a change in the contrast of a dental image. When lower kilovoltage settings are used, a high-contrast image will result. An image with “high” contrast has many black areas, many white areas, and few shades of gray (Figure 5-5). An image with high contrast is useful for detecting and determining the progression of dental caries.

FIG 5-5 Image produced with lower kilovoltage exhibits high contrast; many light and dark areas are seen, as demonstrated by the use of the stepwedge.

Radiographic image of a stepwedge demonstrating the effect of low kilovoltage on contrast. The image reveals distinct black and white regions with few intermediate shades of gray, indicating high contrast. Lower kilovoltage settings produce x-rays with reduced penetrating power, which enhances contrast by exaggerating differences between dense and less dense tissues. This high-contrast result is especially useful for detecting dental caries, where clear differentiation between enamel and decay is needed. The stepwedge serves as a visual guide to contrast response.

With higher kilovoltage settings, low contrast results. An image with “low” contrast has many shades of gray instead of areas that are predominantly black and white (Figure 5-6). An image with low contrast is useful for the detection of periodontal or periapical disease. In dental imaging, a compromise between high and low contrast is desirable. See Table 5-1 for a summary of the effect of kilovoltage on contrast (also see Chapter 6).

FIG 5-6 Image produced with higher kilovoltage exhibits low contrast; many shades of gray are seen instead of black and white.

This stepwedge radiograph illustrates the effect of high kilovoltage on image contrast. The image shows a broad range of gray tones with reduced black-and-white separation, indicating low contrast. Higher kilovoltage settings increase x-ray beam energy and penetration, which minimizes differences in tissue absorption and creates more uniform exposure across structures. This low-contrast appearance is advantageous for detecting subtle changes in bone structure, such as periodontal or periapical disease. The image underscores the diagnostic trade-off between contrast and beam energy.

With digital imaging, special image enhancement software can be used to change the contrast by altering the distribution of the gray levels seen in the image. A digital image can be adjusted so that the contrast is higher, which is desirable in caries interpretation, or so that the contrast is lower, which is desirable in evaluating periodontal disease. In comparison, if film is used and the contrast is incorrect, the image must be retaken with adjusted exposure factors.

Exposure Time and Kilovoltage

Exposure time refers to the interval of time during which x-rays are produced. The timer controls the length of exposure time and determines how long the x-rays will be emitted from the machine. The longer the exposure time, the more x-rays are delivered, and a darker image results. Every x-ray machine has a timer. The timer is the exposure factor that is recommended to adjust in order to lighten or darken an image. For example, to get the same end result, a larger patient may require more x-ray exposure time, whereas a smaller patient may require less x-ray exposure time. The timer may be calibrated in either seconds or impulses, depending on when the unit was manufactured. 45On older units, exposure time may be indicated as “pulses” or “impulses.” An impulse is a term of measurement that refers to the fact that x-rays are created in a series of bursts or pulses rather than in a continuous stream. One impulse occurs every 1/60 of a second; therefore 60 impulses occur in 1 second. Newer x-ray units designed to be used with digital imaging use exposure times measured in hundredths of a second, instead of impulses of 1/60 of a second.

Kilovoltage and exposure time are inversely related. On older x-ray units, if the kilovoltage was changed, the exposure time needed to be adjusted in order to maintain the diagnostic density of an image. When kilovoltage was increased, the exposure time was decreased in order to compensate for the penetrating power of the x-ray beam. When kilovoltage was decreased, the exposure time was increased.

X-Ray Beam Quantity

Quantity of the x-ray beam refers to the number of x-rays produced in the dental x-ray unit.

Amperage and Milliamperage

Amperage determines the amount of electrons passing through the cathode filament. An increase in the number of electrons available to travel from the cathode to the anode results in production of an increased number of x-rays. The quantity of the x-rays produced is controlled by milliamperage.

The ampere (A) is the unit of measure used to describe the number of electrons, or current, flowing through the cathode filament. The number of amperes needed to operate a dental x-ray unit is small; therefore amperage is measured in milliamperes. One milliampere (mA) is equal to 1/1000 of an ampere. It is common to abbreviate milliamperage as mA, the same as its unit, the milliampere. Some dental x-ray units have a fixed milliamperage setting, whereas others have a milliamperage adjustment on the control panel (see Figure 5-1). In the past, dental x-ray units were available with adjustable setting choices of 7 mA or 15 mA. With these units, the mA setting could be chosen according to the individual diagnostic needs of patients. For example, the higher mA setting was used when the area to be examined was dense or thick. The use of 15 mA produced more dental x-rays, whereas the use of 7 mA produced fewer dental x-rays. Current intraoral x-ray units may include adjustable settings that range from 6 to 8 mA, or else a fixed setting of 7 mA. If the unit has a fixed mA, that mA number is found imprinted on the face of the control panel.

Milliamperage regulates the temperature of the cathode filament. A higher milliampere setting increases the temperature of the cathode filament and consequently increases the number of electrons produced. An increase in the number of electrons that strike the anode increases the number of x-rays emitted from the tube.

The quantity, or number of x-rays emitted from the tubehead, is controlled by milliamperage. Milliamperage controls the amperage of the filament current and the amount of electrons that pass through the filament. As the milliamperage is increased, more electrons pass through the filament, and more x-rays are produced. For example, if the milliamperage is increased from 7 to 15 mA, approximately twice as many electrons travel from the cathode to the anode, and approximately twice as many x-rays are produced.46

Density and Milliamperage

Milliamperage, as with kilovoltage, has an effect on the density of a dental image. An increase in milliamperage increases the overall density and results in a darker image. Conversely, a decrease in milliamperage decreases the overall density and results in a lighter image. Table 5-2 summarizes the effect of milliamperage on density.

Table 5-2

Effect of Milliamperage (mA) on Image Density

Adjustment Density

↑ mA ↑ (Darker)

↓ mA ↓ (Lighter)

↑, Increase; ↓, decrease.

As previously described, with digital imaging, special image enhancement software can be used to change the density by adjusting the brightness. In comparison, if film is used and the density is nondiagnostic, the image will need to be retaken with adjusted exposure factors.

Exposure Time and Milliamperage

Milliamperage and exposure time are inversely related. On older units, if the milliamperage was changed, the exposure time also needed adjustment in order to maintain the diagnostic density of an image. When milliamperage was increased, the exposure time was decreased. When milliamperage was decreased, the exposure time was increased.

Table 5-3 lists guidelines for adjusting kilovoltage, milliamperage, and exposure time.

Table 5-3

Guidelines for Adjusting Kilovoltage (kV), Milliamperage (mA), and Exposure Time

Adjustment Exposure*

↑ kV ↓ Exposure time

↓ kV ↑ Exposure time

↑ mA ↓ Exposure time

↓ mA ↑ Exposure time

↑, Increase; ↓, decrease.

*Adjustment in exposure time needed to maintain diagnostic density of image.

Helpful Hint

How to Remember Quality and Quantity

Modified from istock.com/stevanovicigorIllustration of a chalkboard-style alphabet with the letters K, L, M, and N highlighted using a red oval. This mnemonic visual aids in remembering the relationship between kilovoltage and image quality, and milliamperage and x-ray quantity. The positioning of the letters in sequence reinforces the connection: “kV” for kilovoltage and quality, and “mA” for milliamperage and quantity. This teaching tool supports dental radiography students in distinguishing which exposure setting influences which characteristic of the x-ray beam, offering an easy alphabetical cue for memory retention.

Exposure Factor Tips

All dental x-ray machines have three exposure factor settings: kV, mA, and time. As described in this chapter, changing any one of these three exposure factors changes the appearance of the resultant image.

On dental x-ray units, only the exposure time setting is always adjustable. Most manufacturers recommend that once an x-ray unit has been installed, calibrated, and inspected, it is best not to adjust the kV and mA. Instead, adjust only the exposure time to make any needed changes. There is less potential for confusion, errors, and retakes when only the exposure time is adjusted. The exposure time adjustment is based on patient size; it should be decreased with small children and increased with adults with large jaws.

X-Ray Beam Intensity

Quality refers to the energy or penetrating ability of the x-ray beam; quantity refers to the number of x-ray photons in the beam. Quality and quantity are described together in a concept known as intensity. Intensity is defined as the product of the quantity (number of x-ray photons) and quality (energy of each photon) per unit of area per unit of time of exposure, as follows:

Intensity of the x-ray beam is affected by a number of factors, including kilovoltage, milliamperage, exposure time, and distance.

Kilovoltage

Kilovoltage regulates the penetrating power of the x-ray beam by controlling the speed of the electrons traveling between the cathode and the anode. Higher kilovoltage settings produce an x-ray beam with more energy and shorter wavelengths; higher kilovoltage levels increase the intensity of the x-ray beam.

Milliamperage

Milliamperage controls the penetrating power of the x-ray beam by controlling the number of electrons produced in the x-ray tube and the number of x-rays produced. Higher milliampere settings produce a beam with more energy, increasing the intensity of the x-ray beam.

Exposure Time

Exposure time, as with milliamperage, affects the number of x-rays produced. A longer exposure time produces more x-rays. An increase in exposure time produces a more intense x-ray beam.

Distance

The distance traveled by the x-ray beam affects the intensity of the beam. Distances that must be considered when exposing a dental image include the following (Figure 5-7):

FIG 5-7 Distances to consider when exposing dental radiographs: target-surface, target-object, and target-receptor distance.

Diagram illustrating key distances in dental radiography: target-surface distance (from x-ray source to patient’s skin), target-object distance (from source to the tooth), and target-receptor distance (from source to image receptor). All three measurements originate from the tungsten target in the anode of the x-ray tube. These distances affect the intensity and sharpness of the x-ray beam, with increased distance resulting in reduced beam intensity due to divergence. Accurate distance measurement is essential to ensure optimal image quality and correct radiation dosage.

• Target-surface distance: The distance from the source of radiation (tungsten target in anode) to the patient’s skin

• Target-object distance: The distance from the source of radiation (tungsten target in anode) to the tooth

• Target-receptor distance: The distance from the source of radiation (tungsten target in anode) to the receptor

The distance between the source of radiation and the receptor has a marked effect on the intensity of the x-ray beam. As x-rays travel from their point of origin or away from the target anode, they diverge like waves of light and spread out to cover a larger surface area.47

Helpful Hint

How to Remember Distance and Intensity

Modified from istock.com/connect11Illustration featuring a flashlight projecting a concentrated beam of light. As the beam extends outward, it becomes wider and less intense. A red bar close to the flashlight indicates the region of higher intensity, while a second red bar farther from the source marks a region of lower intensity. This visual analogy demonstrates the inverse square law in radiography: as the distance from the source increases, the intensity of radiation decreases. The image simplifies the concept by using a flashlight to illustrate how beam energy spreads and weakens with increased distance, mirroring how x-ray intensity behaves during dental imaging.

As x-rays travel away from their source of origin, the intensity of the beam lessens. Unless a corresponding change is made in one of the other exposure factors (kilovoltage), the intensity of the x-ray beam is reduced as the distance increases. The inverse square law is used to explain how distance affects the intensity of the x-ray beam.

Inverse Square Law

The inverse square law is stated as follows:

The intensity of radiation is inversely proportional to the square of the distance from the source of radiation.

“Inversely proportional” means that as one variable increases, the other decreases. When the source-to-receptor distance is increased, the intensity of the beam is decreased.

According to the inverse square law, when the target-receptor distance is doubled, the resultant beam is one-fourth as intense (Figure 5-8). When the target-receptor distance is reduced by half, the resultant beam is four times as intense.

FIG 5-8 The inverse square law states that the intensity of radiation is inversely proportional to the square of the distance from the source. Note that as the source-to-receptor distance is doubled, the radiation is one fourth as intense.

Diagram visually explaining the inverse square law in dental radiography. It shows that as the distance between the x-ray source and the receptor doubles, the intensity of the radiation becomes one-fourth as strong. The diagram includes a central source emitting beams at distances labeled 1x, 2x, 3x, and 4x, with corresponding intensity reductions of 1, 1/4, 1/9, and 1/16 respectively. This law is fundamental in understanding how increasing the distance affects exposure levels and image brightness, emphasizing the need for consistent positioning during radiographic procedures.

The following mathematical formula is used to calculate the inverse square law.

Example

If the target-receptor distance is changed from 8 inches to 16 inches, how does this increase in source-to-receptor distance affect the intensity of the beam?

48

This mathematical formula reveals that the beam will be one-fourth as intense if the target-receptor distance is changed from 8 to 16 inches (assuming that kilovoltage and milliamperage remain constant). In this example, the inverse square law reveals that doubling the distance from the source of radiation to the receptor results in a beam that is one-fourth as intense.

Helpful Hint

How to Remember Inverse Square Numbers

Diagram illustrating the inverse square law using a triangular beam projection. A red arrow at the beam’s origin represents the source of radiation, positioned above the number 0 on a horizontal scale marked from 0 to 10. Along the beam path, intensity values are labeled: 1 at the origin, followed by ¼, ¹⁄₉, ¹⁄₁₆, ¹⁄₂₅, ¹⁄₃₆, and so on up to ¹⁄₁₀₀ at position 10. The diagram visually demonstrates how radiation intensity decreases sharply as distance from the source increases. This visual supports the principle that intensity is inversely proportional to the square of the distance—an important concept in radiography and lighting physics.

Remember: The intensity of the radiation is inversely proportional to the square of the distance. This is also true for lighting used in professional photography.

Half-Value Layer

To reduce the intensity of the x-ray beam, aluminum filters are placed in the path of the beam inside the dental x-ray tubehead. Aluminum filters are used to remove the low-energy, less penetrating, longer-wavelength x-rays. Aluminum filters increase the mean penetrating capability of the x-ray beam while reducing the intensity. When placed in the path of the x-ray beam, the thickness of a specified material (e.g., aluminum) that reduces the intensity by half is termed the half-value layer (HVL).

For example, if an x-ray beam has an HVL of 4 mm, a thickness of 4 mm of aluminum would be necessary to decrease its intensity by half. Measuring the HVL determines the penetrating quality of the beam. The higher the half-value layer, the more penetrating the beam. (Filtration of the x-ray beam was discussed in Chapter 4.)