ABR Practice Test Video Answers

1. B
The primary purpose of a linear accelerator (linac) in radiation therapy is to accelerate electrons to high energies, which then either hit a target to generate high-energy photons (x-rays) or are used directly for electron therapy.

2. B
In MRI physics, the Larmor frequency (the precession frequency of a proton) is directly proportional to the magnetic field strength and the gyromagnetic ratio of the nucleus.

3. B
The half-value layer (HVL) is the thickness of a specific material required to reduce the intensity of an X-ray or gamma-ray beam to 50% of its original value.

4. C
Kilovoltage peak (kVp) is the parameter most responsible for controlling image contrast in radiography. Higher kVp settings result in lower contrast images, while lower kVp settings produce higher contrast.

5. A
The Modulation Transfer Function (MTF) is used to quantify the spatial resolution of an imaging system by measuring its ability to transfer contrast from the object to the image at different spatial frequencies.

6. B
In radiation protection, the effective dose accounts for both the radiation type (by using a radiation weighting factor) and the tissue sensitivity (by using a tissue weighting factor).

7. B
At diagnostic X-ray energies around 30 keV, the photoelectric absorption interaction dominates when X-rays interact with soft tissue, as it is highly dependent on the atomic number of the absorbing material.

8. B
A flattening filter in a medical linear accelerator is used to reduce the off-axis beam intensity variation, creating a more uniform dose distribution across the treatment field.

9. C
In CT imaging, the detector element size is the primary factor that determines the in-plane spatial resolution, as a smaller detector can resolve finer details.

10. B
An advantage of megavoltage photon beams is a reduced skin dose due to the skin-sparing effect, which is caused by the dose buildup that occurs below the skin surface.

11. C
In nuclear medicine, the purpose of a collimator in a gamma camera is to select photons by their direction, ensuring that only photons traveling perpendicular to the detector face are counted, which is crucial for forming an image.

12. B
When calibrating a therapy beam using an ionization chamber, the quantity most directly measured is the exposure in air, which can then be converted to absorbed dose to water.

13. B
In MRI, T1 relaxation time represents the recovery of longitudinal magnetization as the nuclei release energy to the surrounding lattice (spin-lattice interactions).

14. A
A wedge filter is a device used in radiation therapy to modify the dose distribution across the treatment field, creating a uniform dose in a specific region of the patient.

15. C
Bragg-Gray theory is applied in dosimetry to relate ionization measurements in a small gas-filled cavity (like an ionization chamber) to the absorbed dose in the surrounding medium.

16. A
Reverberation is an ultrasound artifact that results from multiple reflections between two strong interfaces, appearing as a series of evenly spaced, parallel lines.

17. B
In PET imaging, the positron from a radioactive decay annihilates with an electron, producing two 511 keV photons that are emitted at approximately 180 degrees to each other.

18. B
The purpose of quality assurance (QA) in diagnostic radiology is to maintain image quality and minimize patient exposure by ensuring that equipment is performing optimally.

19. C
A key advantage of Intensity-Modulated Radiation Therapy (IMRT) is the better sparing of critical structures by modulating the intensity of the radiation beam.

20. A
A Thermoluminescent Dosimeter (TLD) is the most suitable dosimeter for measuring skin dose during fluoroscopic procedures due to its small size and high sensitivity.

21. A
The Tissue-Air Ratio (TAR) is primarily a function of beam quality (energy) and depth within the tissue.

22. A
The main advantage of digital radiography over screen-film is its higher dose efficiency (allowing for lower patient dose) and extensive post-processing capability for image enhancement.

23. B
Geometric unsharpness in radiographic imaging is primarily affected by the focal spot size of the source and the object-detector distance, as these factors influence the clarity of the image.

24. B
The photoelectric effect is most significant in tissues with a high atomic number (like bone) and is enhanced by using low-photon energy beams.

25. C
A scintillation detector is the component of a CT scanner that converts incoming X-ray photons into light, which is then converted into an electronic signal.

26. B
Lead is the most effective shielding material for high-energy gamma rays due to its high density and high atomic number, which promotes interactions that absorb the photons.

27. B
A bolus is a tissue-equivalent material used in radiation therapy to improve dose buildup at the skin surface, ensuring that the maximum dose is delivered to the target at or near the skin.

28. B
A compression force check is a specific quality control test for mammography to ensure that the proper amount of force is applied to the breast, which improves image quality and reduces dose.

29. B
The energy range of diagnostic X-rays typically lies between 30–150 keV.

30. B
The most appropriate method for verifying the calibration of a brachytherapy source is an ionization chamber measurement in a well chamber, which provides an accurate measure of the source’s output.

31. A
Kilovoltage and field size are the factors that most strongly influence scatter radiation in diagnostic imaging. Higher kVp and larger field sizes lead to more scatter.

32. B
The purpose of the isocenter in a linear accelerator is to ensure that all rotational axes (gantry, collimator, and treatment couch) intersect at a fixed point to guarantee accurate and precise radiation delivery.

33. A
Skin erythema is an example of a deterministic radiation effect, which is characterized by a threshold dose below which the effect does not occur, and the severity of the effect increases with dose.

34. B
In SPECT imaging, attenuation correction is used to compensate for photon absorption within the patient’s body, which can distort the image and lead to inaccurate activity quantification.