Lesson 1, Topic 1
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X-Ray Production

April 11, 2024

X-Ray Production

Learning Objective: Examine the components of x-ray production.

This section discusses x-ray tube structure and function of how these factors affect the primary beam. The electric factors that control x-ray production are introduced. Roentgen accidentally discovered x-rays while experimenting with a Crookes cathode ray tube. In the late 19th century, these tubes were used in laboratory experiments. In 1913, a prototype for the modern x-ray tube was introduced by the General Electric Company.

X-Ray Tube

Learning Objective: Describe the x-ray tube.

There are four essential requirements for the production of x-rays: (1) a vacuum, (2) a source of electrons, (3) a target, and (4) a high potential difference (voltage) between the electron source and the target.
The x-ray tube is contained within a glass structure made of Pyrex. The glass envelope contains the source of electrons and the target. The air is removed from the glass envelope to form a vacuum. The source of electrons is a filament at one end of the tube. The filament consists of a small coil of tungsten wire. An electric current flows through the filament to heat it. As the filament is heated, it begins to “boil off” electrons; this process is called thermionic emission . The electrons form an electron cloud called a space charge and are the source of free electrons for x-ray production.
At the opposite end of the tube is the anode (also referred to as the target), a slanted metal surface made of tungsten. The electrons are directed toward the target, which is where x-rays are generated. A high-voltage electric source provides acceleration of the electrons. A step-up transformer supplies the voltage required for x-ray production. The two ends of the x-ray tube are connected in the transformer circuit so that the filament end is positive during exposure. The positive end of the tube is the anode, and the negative end is the cathode. The high positive electric potential at the target attracts the negatively charged electrons of the space charge, which move rapidly across the tube, forming an electron stream. When these fast-moving electrons collide with the target, the kinetic energy of their motion is converted into heat (>99%), and only a small amount is converted into x-rays.

Bremsstrahlung and Characteristic Radiation

Learning Objective: Differentiate between Bremsstrahlung radiation and characteristic radiation.

X-rays are produced at the target due to either a sudden deceleration (called bremsstrahlung radiation ) or absorption of the electron stream (called characteristic radiation ).

Bremsstrahlung Radiation

X-rays are produced when an incoming electron misses all the electrons in the tungsten atom, gets very close to the nucleus, and then suddenly slows down and abruptly changes direction. As a result, the electron loses energy and is converted into an x-ray photon. The new photon is a small bundle of electromagnetic energy. X-rays that are created by this interaction are bremsstrahlung radiation. Bremsstrahlung is a German word that means braking or slowing. Every x-ray exposure will contain photons produced from bremsstrahlung interactions in the anode.

Characteristic Radiation

Another type of radiation, characteristic radiation, is created when an incoming electron strikes the K-shell electron and ejects it out of orbit. The initial electron and the K-shell electron are eliminated. This creates a void in the K-shell which is filled with an electron from another orbit. The primary x-ray beam is made up of both bremsstrahlung and characteristic radiation. The wavelength and energy of the beam are known to be heterogeneous, which means that the beam is made up of many different wavelengths and energies. X-ray energy is measured in kiloelectron volts (keV).

Characteristics of the Cathode and Anode

Learning Objective: Describe the characteristics of the cathode and anode.

The cathode, the negative side of the x-ray tube, contains two filaments, one large and one small (FIGURE 37.10). Only one filament is used at a time. Each filament is located in the cathode in the focusing cup. The focusing cup has a slightly negative charge and causes the electrons to be repelled in the direction of the target at the focal spot. Spatial resolution, the sharpness or detail seen in the image, is best with a small filament and small focal spot. For small-to-average sized patients, the small focal spot should be used on all body parts except the chest and abdomen. The chest and abdomen, because they are larger body parts require the use of the large focal spot.
Again, the majority of energy in the electron stream is converted to heat. This takes place at the anode, the positive side of the x-ray tube, which is made of tungsten. Tungsten is used because it has a high melting point and efficiently conducts heat away from the anode. The anode rotates during the exposure, so the heat is distributed all around the circumference of the disk. When the operator pushes the exposure button, there is a short delay before the exposure is made. This delay allows the rotor to accelerate and rotate, and the filament is heated. An audible sound is made during the exposure. When the sound ends, the exposure ends, and the operator releases the button. When the exposure is completed, the rotating anode slows down and stops. The anode is slanted or angled around the edges, forming a bevel. The slant, or angle, is typically between 7 and 17 degrees, with 12 degrees being the most common. The angle affects the tube’s heat capacity, the sharpness of the image, and the maximum size of the x-ray beam.

Line Focus Principle

Learning Objective: Describe the line focus principle.

The term actual focal spot refers to the area on the target surface struck by the electron stream. A larger focal spot is best for heat capacity. The effective focal spot refers to the vertical projection of the actual focal spot onto the patient and IR. A smaller focal spot produces a smaller radiation field, which reduces the patient’s radiation exposure.
The size of the effective focal spot influences the resolution of the image. This is known as the line focus principle (FIGURE 37.11). When vertical lines are drawn from each corner of the angled focal spot, these lines define an image of the focal spot as viewed from the IR. The effective focal spot is always smaller than the actual focal spot. A smaller effective focal spot will result in greater resolution in the image, and that a larger effective focal spot will have the opposite effect. The smaller the target angle, the greater the size difference between the actual and effective focal spots.

Line Focus Principle

• The size of the effective focal spot determines image resolution.
• The target angle determines the relative size of the effective focal spot.
• The steeper the target angle, the greater the difference between the actual and effective focal spot sizes.

Anode Heel Effect

Learning Objective: Describe the anode heel effect.

Most x-rays are not produced on the surface of the target. Electrons from the beam may penetrate through multiple layers of atoms before interacting with any material. The interaction creates the x-ray and the x-ray will pass through, or exit, the target and travel toward the IR to create the image. Some of the x-rays will be absorbed by the target during this process. Because of the angle of the target, some x-rays will pass through more target material than others. The x-rays that are directed away from the cathode are more likely to be absorbed. This results in an uneven distribution of radiation intensity in the x-ray beam, called the anode heel effect (FIGURE 37.12). If the anode heel effect is not used correctly, the thinner portions of the anatomy will appear too dark on the IR, and the thicker portions will be too light.

FIGURE 37.10  Principal parts of an x-ray tube. From Bushong SC: Radiologic science for technologists: physics, biology, and protection, ed 12, St. Louis, 2021, Mosby.

FIGURE 37.11  The line focus principle allows high anode heating with small effective focal spots As the target angle decreases, so does the effective focal spot size. From Bushong SC: Radiologic science for technologists: physics, biology, and protection, ed 12, St. Louis, 2021, Mosby.

Anode Heel Effect

• Variation in radiation intensity across the length of the radiation field.
• Greater radiation intensity toward the cathode end of the field.
• Only significant with using the entire beam (14 × 17 IR at 40 inches or full spine at 72 inches).
• Place the thinner portion of the body part toward the anode end of the tube.

FIGURE 37.12  Anode heel effect. The anode heel effect is simply caused by photons produced on the anode side, which must penetrate the heel of the target before exiting the tube. This causes some photons to be weakened and some to be absorbed. The net effect is reduced intensity of the beam on the anode side. From Johnston J, Fauber TL: Essentials of radiographic physics and imaging, ed 3, St. Louis, 2020, Mosby.

Electric Control of X-ray Production

Learning Objective: Describe the components of the electric control of x-ray production.

Three terms describe electricity: amperes, volts, and watts (wattage). Amperes (also called amps) are simply the amount of electricity used by the item. Volts are the measure of the force of electricity. Amps multiplied by volts give you the total wattage (workload). In radiography, we are concerned with amperes and volts. However, to create an x-ray, the volts must be increased to kilovolts, and the amperes are decreased to milliamps.

Kilovoltage

Voltage is measured at the peak of the electric cycle, and the unit is stated as kilovolts peak (kVp). The voltage applied to the x-ray tube controls the speed and power of electrons in the electron stream. The electrons move faster when the voltage is increased, producing x-ray with shorter wavelengths and greater energy. Therefore, kVp controls the energy (wavelength) of the x-ray beam. The kVp controls the penetrating power of the x-ray beam. The kVp also controls the contrast in the image. Contrast is the difference between light and dark areas in an x-ray, which allows for detail to be seen. Low kVp, as used for extremities, creates low contrast on the image, while high kVp, as used for the chest and abdomen, creates low contrast.

Milliamperage

Milliamperage (mA) measures the rate of current flow across the x-ray tube, that is, the number of electrons flowing from the filament to target each second. The filament heat determines the number of available electrons. When filament heat is increased, more electrons are available each second to cross the tube. Therefore, increasing the mA increases the filament heat, and decreasing the mA decreases filament heat. When more electrons strike the target, more x-rays are produced, so mA controls the number of x-rays produced and the exposure rate. High mA settings produce more x-rays, and low mA settings produce fewer x-rays.

Exposure Time

Exposure time refers to the amount of time that x-rays are being produced. Most exposure times are measured in milliseconds (ms) because they are less than 1 second. The operator adjusts the time and a timer will end the exposure at the preset time.

Milliampere-Seconds

The total quantity of x-ray photons in an exposure cannot be determined by either mA or time alone. The quantity, or number, of x-rays created is determined by mA. However, mA does not dictate the length of the exposure. To establish the total quantity of radiation created during an exposure, the mA and the time are considered. Millliampere-seconds (mAs) is the unit used to indicate the quantity of exposure and mAs if the product of exposure time and mA: mAs = Time (seconds) × mA
In the radiology department, most generators are designed so that the operator sets the kVp and the mAs. The operator can adjust the mA and time separately if needed.

X-Ray Beam Filtration

Learning Objective: Describe x-ray beam filtration.

The x-ray beam contains photons of varying energies. This is known as heterogenous. Photons with a long wavelength are readily absorbed by the body and are less likely to penetrate through the body part and travel to the IR. The photons are absorbed by the patient and contribute to patient dose. To reduce radiation dose, the x-ray beam is filtered. Filtration removes the long-wavelength photons. Aluminum is placed between the x-ray tube and the patient and it will absorb unnecessary radiation and decrease patient dose. Filtration provided by the glass of the tube is called inherent filtration (built-in) and is approximately 0.5 mm Al equivalent. Filtration from the collimator is also considered inherent and equals about 1.0 mm Al. Additional filtration can be added by installing aluminum plates between the tube port and the collimator. The total filtration is equal to the inherent filtration plus the added filtration. The federal law requires a total filtration of at least 2.5 mm Al for machines producing 70 kVp or more.