Radiography Exam Radiographic Equipment and Maintenance 1 — Questions and Answers
Question 1: Which component of the x-ray tube is responsible for producing electrons through thermionic emission?
- Filament (cathode) (Correct answer)
- Anode target
- Focusing cup
- Glass envelope
Correct answer: Filament (cathode)
The filament at the cathode is heated by a low-voltage current, causing thermionic emission — the release of electrons from the wire.
The filament is a coiled tungsten wire within the cathode assembly. When heated by the filament current (3–6 A), its temperature rises to about 2200°C, causing thermionic emission of electrons. The focusing cup (molybdenum) shapes the electron beam toward the anode target. Tungsten is used for the filament due to its high melting point and low vapor pressure.
Question 2: The primary purpose of the rotating anode in an x-ray tube is to:
- Distribute heat over a larger target area to prevent damage (Correct answer)
- Increase the number of x-rays produced
- Reduce the size of the focal spot
- Improve the efficiency of x-ray production
Correct answer: Distribute heat over a larger target area to prevent damage
The rotating anode distributes the heat generated during x-ray production over a larger area (the focal track), preventing localized overheating and extending tube life.
During x-ray production, only about 1% of electron kinetic energy is converted to x-rays; the remaining 99% becomes heat at the anode. The rotating anode (typically 3,000–10,000 RPM) spreads this heat over a circular track rather than concentrating it at a single point. Anode diameter, rotation speed, and target angle all affect heat loading capacity. Tungsten-rhenium alloys are used for the target material.
Question 3: The line-focus principle in x-ray tube design allows for:
- A large actual focal spot that produces a small effective focal spot for improved image sharpness (Correct answer)
- A small actual focal spot that creates a larger effective focal spot
- Equal actual and effective focal spot sizes
- Maximum heat dissipation without affecting image quality
Correct answer: A large actual focal spot that produces a small effective focal spot for improved image sharpness
The line-focus principle uses an angled anode target so that a large actual focal spot (better heat loading) projects as a small effective focal spot (better image sharpness).
The line-focus principle exploits the angle of the anode target (typically 7–20°). The actual focal spot is a rectangle on the angled target, but viewed from below (the effective focal spot), it appears much smaller due to foreshortening. This allows larger heat loading capacity while maintaining small effective focal spots (better spatial resolution). Steeper angles give smaller effective focal spots but reduce the usable field size.
Question 4: Which of the following is a quality control test that evaluates the accuracy of the x-ray beam's alignment with the light field?
- Beam-light field congruence test (Correct answer)
- Half-value layer measurement
- Focal spot size measurement
- Automatic exposure control (AEC) calibration
Correct answer: Beam-light field congruence test
The beam-light field congruence test verifies that the visible light field from the collimator accurately represents the x-ray field, ensuring proper collimation.
The beam-light field congruence test is required by federal performance standards (21 CFR 1020.31). Acceptance criteria require that the sum of misalignments on all four sides does not exceed 2% of the SID. This test is performed using a graduated test tool placed in the beam and exposing an image receptor. Proper alignment is critical for accurate collimation, which affects both patient dose and image quality.
Question 5: The heel effect in radiography refers to:
- Reduced x-ray intensity on the anode side of the beam compared to the cathode side (Correct answer)
- Increased x-ray intensity at the center of the beam
- Loss of image sharpness at the edges of the field
- Differential absorption of the beam by varying tissue thicknesses
Correct answer: Reduced x-ray intensity on the anode side of the beam compared to the cathode side
The heel effect occurs because x-rays produced at depth within the anode must travel through more anode material when exiting toward the anode side, reducing their intensity on that side.
The heel effect results from self-absorption by the anode target. X-rays produced beneath the anode surface must traverse more target material to exit toward the anode end, causing attenuation and lower intensity on the anode side. The cathode side has greater intensity. Clinically, this effect is used to advantage by placing the thicker body part under the cathode side (e.g., proximal femur or thick end of the AP spine) for more uniform film density.
Question 6: During a quality control assessment, kVp accuracy is typically evaluated using a:
- Digital kVp meter (non-invasive kVp meter) (Correct answer)
- Densitometer
- Sensitometer
- Penetrometer
Correct answer: Digital kVp meter (non-invasive kVp meter)
Non-invasive digital kVp meters measure the actual peak kilovoltage output of the x-ray unit and are used to verify kVp accuracy during quality control.
Digital (non-invasive) kVp meters use semiconductor detectors or filtered ionization chambers to measure the peak kilovoltage from outside the x-ray beam. AAPM and ACR quality control protocols require that kVp accuracy be within ±5% of the indicated value. Inaccurate kVp affects image contrast and patient dose. Sensitometers and densitometers are used for film/screen quality control, while penetrometers (step wedges) evaluate beam quality visually.
Radiography Exam Radiographic Equipment and Maintenance 1
Which component of the x-ray tube is responsible for producing electrons through thermionic emission?