Chapter 5 - X-Ray Production

Question 1

four essential requirements for the production of x-rays

Answer 1

(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.

Question 2

thermionic emission

Answer 2

As explained in Chapter 4, heat speeds up the movement of the electrons in their orbits and increases their distance from the nucleus. Electrons in the outermost orbital shells move so far from the nucleus that they are no longer held in orbit but are flung out of the atom, forming an “electron cloud” around the filament (Fig. 5.4). This process is called thermionic emission.

Question 3

space charge

Answer 3

The electron cloud is called a space charge and is the source of free (in air) electrons for x-ray production.

Question 4

filament

Answer 4

The filament consists of a small coil of tungsten wire. Tungsten (chemical symbol W) is a metal element; it is a large atom with 74 electrons in orbit around its nucleus. An electric current flows through the filament to heat it.

Question 5

target

Answer 5

At the opposite end of the tube is the anode (also referred to as the target), a hard, smooth, slanted metal surface that is also made of tungsten. The electrons are directed toward the target, which is the place where x-rays are generated. The positive, target end of the tube is called the anode; the negative, filament end is called the cathode.

Question 6

Bremsstrahlung radiation

Answer 6

Created when an incoming electron is suddenly slowed down, changes direction, and leaves the tungsten atom. The kinetic energy of the electron is converted into an x-ray photon.

Question 7

Characteristic radiation

Answer 7

Created when an incoming electron interacts with the K-shell electron and knocks it out of orbit. When the electron void is filled with an outer shell electron, an x-ray photon is created.

Question 8

Focal Spot Size

Answer 8

Actual
*Measurement of focal spot on target surface
*Affects tube heat capacity
*Bigger is better!

Effective

*Measurement of vertical projection of actual focal spot
*Affects image resolution
*Smaller is better!

Question 9

spatial resolution

Answer 9

Spatial resolution is the new digital term used to describe the sharpness of the structures recorded in the image. Generally speaking, it simply refers to the amount of detail seen on the visible x-ray image.

Question 10

focal track

Answer 10

X-ray tubes rotate at a standard speed of about 3600 revolutions per minute (rpm) during the exposure. Most x-ray tubes in use today also have a high-speed rotation at 10,000rpm.

Question 11

target angle

Answer 11

The slant of the anode surface is called the target angle (Fig. 5.15). X-ray tube target angles are between 7 and 17 degrees, with 12 degrees being most common. The target angle is built into the x-ray tube and cannot be changed. The target angle affects the tube’s heat capacity, the sharpness of the radiographic image, and the maximum size of the x-ray beam.

Question 12

Line Focus Principle

Answer 12

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

Question 13

Focal Spot Size

Answer 13

Most x-ray tubes used today contain a 0.6-mm small focal spot and a 1.2-mm large focal spot. Some x-ray tubes may be purchased with larger focal spots of 1.0mm for the small and 2.0mm for the large.

Question 14

Anode Heel Effect

Answer 14

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

Question 15

kVp

Answer 15

Therefore kVp controls the energy (wavelength) of the x-ray beam. Because x-rays with shorter wavelengths are more penetrating, the penetrating power of the x-ray beam is controlled by varying the kVp. Larger and denser body parts will require higher kVp settings than small or low-density body parts. The contrast in the radiographic image is also controlled by the kVp. High kVp creates low contrast, and low kVp creates high contrast.

Question 16

Milliamperage (mA)

Answer 16

Milliamperage (mA) is a measure of the rate of current flow across the x-ray tube, that is, the number of electrons flowing from filament to target each second. The number of available electrons is determined by the filament heat. When filament heat is increased, more electrons are available each second to cross the tube. Thus, increasing the mA increases the filament heat and decreasing the mA decreases the filament heat. When more electrons strike the target, more x-rays are produced, so mA controls the volume, or quantity, of x-ray production and thus also the rate of exposure. High mA settings produce more x-rays, and low mA settings produce fewer x-rays. Stated differently, mA controls the intensity of the x-ray beam, determining the number of photons that will strike the patient and IR. The density in the radiographic image is controlled by the mA, exposure time, or the mAs. In radiology the mA is directly proportional. If the mA is doubled, the x-rays are doubled, and if the mA is halved, the x-rays are reduced by 50%.

Question 17

Filtration

Answer 17

To prevent this unnecessary radiation dose to the patient, the primary x-ray beam is filtered. Filtration is the process of removing the long-wavelength photons from the x-ray beam. Filtration material placed between the x-ray tube and the patient absorbs these long-wavelength photons (Fig. 5.23). The primary purpose of filtration is to reduce patient dose.

Question 18

X-Ray Beam Filtration

Answer 18

*Filter material placed between the tube housing port and the patient removes the long-wavelength radiation from the primary beam.
*Because this radiation does not have sufficient energy to penetrate the patient, the cassette, and the table, it does not contribute to the image.
*Filtration lowers patient dose significantly.
*Filtration decreases the average wavelength of the x-ray beam.

Question 19

inherent filtration

Answer 19

The filtration provided by the glass of the tube and the surrounding oil is called inherent filtration (built-in) and is approximately equal to 0.5mm Al equiv. Filtration from the mirror is also considered inherent and equal to about 1.0mm Al equiv.

Question 20

Total Al equiv filtration

Answer 20

X-ray equipment capable of producing 70 kVp or more (all general-purpose equipment) is required to have total filtration of at least 2.5mm Al equiv permanently installed.