The Tube & Beam Restriction
The diameter of the lower rim of a cone is 5 inches. The bottom of the cone is placed at a distance of 15 inches from the focal spot. What will be the size of the projected image at a 40-inch SID?
13-inch circle
A conscientious technologist will always handle the x-ray tube assembly with care, particularly when the x-ray tube has been used for high mAs and kVp exposures over a short period of time. Technologists are cautioned about moving the x-ray tube assembly quickly and with "jerking" movements. Explain why this is important to remember.
Hot anode assemblies are subject to metal fatigue and the stress placed upon them by gyroscopic inertia. Moving the tube quickly from one orientation to another places undue fatigue on the anode stem and disc and may cause it to become unbalanced or misaligned. Many manufacturers will place an "anode brake" or reverse induction force on the stator to slow the anode in just a few seconds. This feature of extra technology will extend the life of the tube but does cost additional money in the x-ray tube purchase.
The diameter of the lower rim of a cone is 4 inches. The bottom of the cone is placed at a distance of 10 inches from the focal spot. What will be the size of the projected image at a 40-inch SID?
16-inch circle
How many heat units are generated by an exposure of 70 kVp, 300 mA, and 0.1 second on a 1f rectified unit?
2,100 HU
A diaphragm is placed 4 inches from the focal spot. The diameter of the opening in the aperture is a 2-inch circle. What would be the projected image size if the image receptor distance (SID) were 40 inches?
20-inch circle
A diaphragm is placed 6 inches from the focal spot. The diameter of the opening in the aperture is a 3-inch circle. What would be the projected image size if the SID were 40 inches?
20-inch circle
How many heat units are generated by an exposure of 70 kVp, 400 mA, and 0.1 second on a 3f rectified unit?
3,780 HU
How many heat units are generated by an exposure of 100 kVp, 400 mA, and 0.01 second on a high-frequency unit?
400 HU
List the three primary functions of the x-ray tube's protective housing.
The x-ray tube's protective housing controls scatter and leakage radiation, isolates the high voltages, and provides a means to cool the tube.
Which of the following is placed between the patient and the image receptor to absorb scatter radiation? a. radiographic grid b. AEC detector c. collimator d. cylinder
a. radiographic grid
Penumbra is ____ when the beam restrictor is at a/an ____ distance from the port. a. reduced; increased b. reduced; decreased c. unchanged; increased d. increased; increased
a. reduced; increased
The only portion of the anode assembly placed outside the vacuum of the glass envelope is the a. stator. b. rotor. c. anode stem. d. molybdenum neck.
a. stator.
A factor that affects the amount of scatter produced is a. the irradiated material. b. milliamperage. c. time. d. anode composition.
a. the irradiated material.
All of the following are related except one. Select the choice that is NOT RELATED. a. a source of electrons b. a vacuum c. thermionic emission d. a large potential difference
c. thermionic emission
Cracking of the anode typically occurs a. with extended use. b. when the anode is off-focus. c. when a cold anode is hit with a high kVp exposure. d. when the cathode cup touches the anode, causing a short.
c. when a cold anode is hit with a high kVp exposure.
Pitting of the anode commonly occurs a. when the unit is not warmed up properly. b. when the anode is off-focus. c. with extended use. d. when the kilovoltage is too high.
c. with extended use.
How many heat units are generated by three (3) exposures of 100 kVp, 400 mA, and 0.01 second using a high-frequency generator?
1,680 HU
Using the following exposures factors on a high-frequency generator, how many joules of electrical energy are being created? 400 mA 95 kVp 26 ms
1,773 J
The use of stress-relief anodes is effective at dealing with what inherent characteristic(s) of x-ray production?
A stress-relief anode is specially designed to permit rapid anode thermal expansion without anode cracking. These stress-relief slits are similar to the stress lines in the disc brakes of motorcycles and automobiles. Generally, stress-relief anodes do not require an elaborate tube warm-up procedure.
In which direction should the anode be placed for an AP projection of the femur?
Because the inferior upper leg is less dense, the anode's less-intense beam should be positioned toward the knee to help decrease the film density in that region.
T/F: Bone absorbs more radiation and produces more scatter than soft tissue.
False (bone absorbed more, but produces less)
List the three functions of the x-ray tube anode.
It serves as the target surface for the high-voltage electrons from the filament (x-ray photon source), it conducts the high voltage from the cathode back into the x-ray generator circuitry, and it serves as the primary thermal conductor.
Collimators that offer positive beam limitation (PBL) are very popular with technologists for a variety of reasons. Even though the federal requirement for PBL has been repealed, their popularity has continued and most department managers consider PBL as a "standard equipment" feature. PBL is electronically and mechanically highly engineered technology and subject to abuse by the inattentive and unprofessional radiographer. In the event of a PBL failure during an examination, what circumstances will be evident to the radiographer and what can he/she do to overcome the PBL error and continue with the procedure.
Malfunctioning PBL will not put the collimation into the "Positive" mode and therefore the exposure will be prevented. With PBL, the receptor size and the x-ray field size must match each other in dimension to be in the positive mode. Your field size can be smaller than the receptor size and still be in the positive mode. Under this circumstance, exposure will be permitted. If the PBL is not in positive mode due to malfunction, a "key" may be turned to bypass the PBL sensing technology. This will allow the radiographer to operate the collimation in the manual mode. If this occurs, collimation may be done manually and should not exceed the dimension of the receptor. The exposure will be permitted in the manual mode. Following the examination, the malfunction should be reported to department leadership for repair.
Metal envelope x-ray tubes are gaining in popularity. Explain some key advantages to a metal housing x-ray tube.
Metal envelope tubes place a ground on the metal casing. This feature minimizes the buildup of tungsten deposits on the inside of the envelope, as the envelope never builds up a positive charge to attract the electrons as they travel from cathode to anode. This is not possible with a glass envelope tube. Because of this feature, x-ray tubes last longer and remain more efficient for the life of the tube. Additionally, metal envelope tubes prevent electrons from exiting the tube and striking the patient if the SID is extremely close.
What is the function of the filament?
The function of the filament is to provide sufficient resistance to the flow of electrons so the heat produced will cause thermionic emission to occur.
Why is a vacuum created in the x-ray tube?
The removal of air permits electrons to flow from cathode to anode without encountering the gas atoms in the air. This greatly increases the efficiency of the tube's operation and prevents tube arcing.
As you perform an AP abdomen radiograph and review the image on your control display monitor, it appears as though the right lateral aspect of the patient's abdomen is not on your image. When you check the patient positioning, you confirm that the positioning is correct and the light field correctly shows the edges of the expected exposure field. When you repeat the study, the same results occur. What is the likely problem?
This situation demonstrates a non-congruence of the x-ray exposure and positioning light field. Often times this is due to a shifting of the mirror in the collimator which reflects light down onto the patient, showing the exposure field. You can perform a simple x-ray-to-light congruency check using four coins to identify the problem. If that is the case, a service engineer should be contacted for realignment.
The most common diagnostic radiography x-ray tube target angle is ____ degrees. a. 12 b. 24 c. 45 d. 54
a. 12
An effective method for reducing scatter radiation production is the a. 15% rule of kVp change. b. use of x-ray beam angulation. c. use of an increased exposure time with mA compensation. d. selection of small focal spot.
a. 15% rule of kVp change.
Scatter radiation in radiography is produced during a. Compton interactions. b. photoelectric effect interactions. c. coherent or classical interactions. d. an annihilation interaction.
a. Compton interactions.
X-ray photon production occurs at the a. anode. b. cathode. c. filament. d. stator windings.
a. anode.
When a beam-restricting device is used to limit field size, technical factors may need to ____ to compensate for the change in image receptor exposure. a. be increased b. be decreased c. remain unchanged d. none of the above
a. be increased
To increase the thermal loading capacity of the anode, x-ray tube manufacturers a. bond graphite to the back of the anode. b. decrease the anode angle. c. use a thicker cathode filament. d. use an anode alloy of tungsten and copper.
a. bond graphite to the back of the anode.
All of the following are preferred cathode filament materials EXCEPT a. copper. b. tungsten. c. rhenium. d. molybdenum.
a. copper.
The area of the focal spot that is projected out of the tube toward the object being radiographed is the a. effective focal spot. b. line focus. c. actual focal spot. d. thermionic stream.
a. effective focal spot.
The collimation assembly of a radiographic unit a. employs an upper and lower set of shutter blades. b. allows the radiographer to adjust the field size in a circular dimension. c. removes the mirror assembly for exposures. d. all of the above.
a. employs an upper and lower set of shutter blades.
The volume of the irradiated tissue increases as a. field size and patient thickness increase. b. patient density increases. c. patient orientation to the beam changes. d. tissue composition increases.
a. field size and patient thickness increase.
Tungsten is the material of choice for the source of x-ray photons primarily because of its a. high atomic number. b. low melting point. c. low mass number. d. G-force tolerance.
a. high atomic number.
As the actual focal spot size increases, the effective focal spot size a. increases. b. decreases. c. remains the same. d. doubles proportionally.
a. increases.
As the angle of the anode increases and the incident electron beam size remains the same, the effective focal spot size a. increases. b. decreases. c. remains the same. d. cannot be determined.
a. increases.
As the volume of irradiated tissue ____, the amount of scatter produced ____. a. increases; increases b. increases; decreases c. decreases; increases d. decreases; decreases
a. increases; increases
A key factor that affects the amount of scatter produced is a. kilovoltage. b. milliamperage. c. time. d. anode composition.
a. kilovoltage.
When collimating down from a 14"x17" field size to a 8"x10" field size, a. mAs must be increased to maintain image receptor exposure. b. kVp must be increased to maintain image quality. c. off-focus radiation production is increased. d. the mAs should be reduced by 50 percent.
a. mAs must be increased to maintain image receptor exposure.
As the kilovoltage increases, the percentage of photons that undergo a ____ interaction will decrease. a. photoelectric b. coherent c. Compton d. characteristic
a. photoelectric
In order to time the x-ray exposure with rapid inspiration on an infant, a. three-phase, high-frequency generators are preferred. b. the exposure button and prep are pressed simultaneously. c. exposure may be initiated as fast as 0.10 sec. d. pulsating DC current should pass through the x-ray tube.
a. three-phase, high-frequency generators are preferred.
When radiographing a wedge-shaped anatomical structure such as the femur, the thicker portion of the anatomy should be positioned a. under the cathode end of the tube. b. in the exact center of the beam, as it is most intense in the center. c. off center laterally to negate the heel effect. d. under the anode side of the tube.
a. under the cathode end of the tube.
If an increase in kilovoltage is accompanied by a reduction in mAs, to maintain the same exit dose the overall result will be a. a decrease in the amount of radiation produced. b. a decrease in the amount of scatter produced. c. an increase in the amount of radiation produced. d. an increase in the amount of scatter produced.
b. a decrease in the amount of scatter produced.
All of the following are considered beam-restricting devices EXCEPT a. aperture diaphragm. b. beam filter. c. positive beam limitation (PBL) device. d. collimator.
b. beam filter.
The exact point on the anode where the x-ray beam emanates is the a. focusing cup. b. focal spot. c. anode heel. d. rotor.
b. focal spot.
An effective technology used in pulsed fluoroscopy to produce precise electron streams from the cathode is a. stress-relief anodes. b. grid-biased x-ray tubes. c. solid-state diodes. d. intermittent filtration.
b. grid-biased x-ray tubes.
Filament materials are chosen primarily for their a. atomic number. b. high melting points. c. ability to vaporize easily. d. voltage tolerance.
b. high melting points.
The ____ the atomic number of the irradiated material, the ____ the number of photoelectric absorption interactions that occur. a. higher; lower b. higher; higher c. lower; higher d. lower; unaffected
b. higher; higher
The ____ the atomic number of the irradiated material, the ____ the number of photoelectric absorption interactions will be, and the ____ the scatter produced. a. higher; lower; less b. higher; higher; less c. lower; higher; more d. lower; lower; less
b. higher; higher; less
The actual focal spot is controlled by the a. thickness of the anode disc. b. length of the filament. c. anode rotation speed. d. angle of the cathode focusing cup.
b. length of the filament.
The function of the stator and rotor assembly is to a. change the anode angle. b. rotate the anode. c. insulate the anode. d. disrupt the electromagnets of the induction motor.
b. rotate the anode.
All of the following are part of the cathode assembly EXCEPT a. filament. b. stator. c. focusing cup. d. associated wiring.
b. stator.
Placing a lead blocker adjacent to an obese patient's anatomy during a lateral lumbar spine radiograph a. is unnecessary with today's digital receptors. b. will improve image quality. c. will reduce exposure to the patient. d. will reduce exposure to the radiographer.
b. will improve image quality.
As the kilovoltage increases, the percentage of x-rays that undergo ____ interactions will increase. a. photoelectric b. coherent c. Compton d. Bremsstrahlung
c. Compton
All of the following are true of positive beam limitation (PBL) EXCEPT: a. PBL automatically collimates the beam. b. PBL senses the size and orientation of the receptor. c. PBL is possible with portable radiography. d. PBL may be overridden by the radiographer in select instances.
c. PBL is possible with portable radiography.
The function of the cathode is to produce a. a focal point. b. an electrical connection to the rectifier. c. a thermionic cloud. d. a grounding effect.
c. a thermionic cloud.
The protective housing controls leakage and scatter radiation through a. evaporation. b. vaporization. c. absorption. d. condensation.
c. absorption.
The physical area of a rotating anode focal track, affected by the cathode electron beam, is the a. effective focal spot. b. effective focal track. c. actual focal spot. d. projected focal track.
c. actual focal spot.
To determine the allowable combination of exposures factors, thus preventing x-ray tube damage, a. a tube rating chart is referenced after exposure to check correctness. b. the automatic exposure circuit is activated. c. an x-ray tube rating chart is checked prior to exposure. d. an anode cooling chart is referenced prior to exposure.
c. an x-ray tube rating chart is checked prior to exposure.
The use of the line-focus principle causes a problem known as the ____ effect. a. space charge b. quantum mottle c. anode heel d. saturation current
c. anode heel
What type of radiation is composed of photons not produced at the actual focal spot? a. off-focus b. extra-focal c. both a and b d. remnant
c. both a and b
As more photons pass through the irradiated material unattenuated, the resulting image receptor exposure will a. demonstrate greater image distortion. b. be unchanged. c. increase. d. decrease.
c. increase.
As more and more electrons build up in the area of the filament, their negative charges oppose the emission of additional electrons. This phenomenon is known as a. thermionic emission. b. biased cloud formation. c. space charge effect. d. current saturation.
c. space charge effect.
Leakage radiation from the lead-lined housing must not exceed a. 10 mR/hr measured at 40 inches. b. 100 mR/min measured at the underside of the collimator. c. 10 mR/min measured at the tube head. d. 100 mR/hr measured at 1 meter.
d. 100 mR/hr measured at 1 meter.
To cover a 14 in by 17 ft field at 40 ft, a ____-degree target angle is required. a. 3 b. 5 c. 7 d. 12
d. 12
Cones and cylinders would be commonly employed for the following examinations EXCEPT a. orbits. b. L5-S1 spot. c. skull. d. AP thoracic spine.
d. AP thoracic spine.
Driving the thermionic cloud from the cathode to the anode requires a. a grid biased anode. b. a small voltage. c. anode rotation. d. a small amperage.
d. a small amperage.
Which of the following would affect the production of a thermionic cloud? a. filament diameter b. filament length c. filament temperature d. all of the above
d. all of the above
A radiographer can best reduce the amount of scatter radiation from reaching the receptor by a. collimating. b. beam restriction. c. utilizing a grid. d. all of the above.
d. all of the above.
As the x-ray tube anode target angle increases from 10 degrees to 15 degrees: a. higher exposure values (mA, time, kVp) may be used. b. the size of the actual focal spot increases. c. image resolution decreases. d. all of the above.
d. all of the above.
Scattered photons will a. impair image quality. b. add to image receptor exposure. c. not be considered part of the useful beam. d. all of the above.
d. all of the above.
The shutters of a collimator a. regulate field size. b. reduce penumbra. c. reduce stem radiation. d. all of the above.
d. all of the above.
Insulation of the high-voltage components of the tube and absorption of the heat produced by x-ray production are achieved through the use of a. a small fan. b. a vacuum. c. a stator. d. dielectric oil.
d. dielectric oil.
The cathode focusing cup has a/an ____ charge. a. neutral b. isomeric c. positive d. negative
d. negative
When more photons are absorbed photoelectrically, the image receptor will a. receive increased exposure due to the increase in secondary radiation production. b. increase its sensitivity automatically. c. not operate due to lack of photons. d. receive less exposure.
d. receive less exposure.
All of the following would lengthen x-ray tube life EXCEPT: a. minimize anode rotation times and prepping. b. avoid sudden movements of the tube head assembly. c. use low-mA selections whenever clinically possible. d. use high-speed anode rotation on distal extremity studies.
d. use high-speed anode rotation on distal extremity studies.
A filament may break due to a. evaporation. b. deposition. c. ablation. d. vaporization.
d. vaporization.
Melting of the anode commonly occurs a. when the unit is not warmed up properly. b. with extended use. c. when the kilovoltage is too high. d. when the stator fails and the rotor ceases to turn.
d. when the stator fails and the rotor ceases to turn.
T/F: The principal factors that affect the amount of scatter produced are focal spot size and the type of irradiated material.
false
T/F: When kilovoltage is increased with no other changes in technical factors, fewer scattered photons will result.
false
T/F: Soft tissue absorbs more radiation and produces more scatter than bone.
false (soft tissue absorbs less but produces more)
List two other names for the anode target.
target, focus, focal point, focal spot, or focal track
List the three parts of the anode assembly.
the anode, the stator, and the rotor
T/F: A radiographer can best minimize the amount of scatter radiation reaching the image receptor by restricting the beam.
true
T/F: Automatic collimators are known as positive beam limitation (PBL) devices.
true
T/F: Image quality is improved when scatter reaching the image receptor is reduced.
true
T/F: Lead masks may serve as beam restrictors.
true
T/F: PBLs may also filter the x-ray beam.
true
T/F: Penumbra is a geometric unsharpness around the periphery of an image.
true
T/F: Scattered photons from Compton interactions are of no use in demonstrating radiographic structures of interest.
true
