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ABR DR Core practice questions

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Question 1Physics

The photoelectric effect demonstrates that electromagnetic radiation behaves as discrete quanta because emitted electron energy depends on which property of the incident light?

  1. Beam cross-sectional area
  2. Polarization
  3. Intensity
  4. Frequency Correct answer.

Explanation

In the photoelectric effect, the maximum kinetic energy of ejected electrons depends linearly on the frequency of the incident photons (minus the work function), not on the total beam intensity. Intensity controls the number of photons and thus the current, not the per-electron energy. This frequency dependence is the hallmark of quantized photon behavior.

Question 2Physics

A radionuclide has a physical half-life of 6 hours. Approximately what fraction of the original activity remains after 24 hours?

  1. One-eighth
  2. One-fourth
  3. One-sixteenth Correct answer.
  4. One-half

Explanation

Twenty-four hours is four half-lives (24 / 6 = 4). Each half-life halves the activity, so the remaining fraction is (1/2)^4 = 1/16. This is a direct application of exponential decay.

Question 3Physics

In computed tomography, the reconstructed image value in each pixel represents which quantity?

  1. Acoustic impedance relative to soft tissue
  2. Tissue electron density in grams per cubic centimeter
  3. Magnetic susceptibility in parts per million
  4. Linear attenuation coefficient scaled to water Correct answer.

Explanation

CT reconstruction produces a map of linear attenuation coefficients, which are then scaled into Hounsfield units relative to water. The pixel value is therefore an attenuation measurement, not a direct density or acoustic property. CT numbers are calibrated so water is 0 HU and air is about -1000 HU.

Question 4Physics

Which interaction predominates for diagnostic x-ray photons in soft tissue at energies near 30 keV?

  1. Coherent scatter
  2. Pair production
  3. Photoelectric absorption Correct answer.
  4. Compton scatter

Explanation

Photoelectric absorption varies approximately with the cube of atomic number and inversely with the cube of photon energy, making it dominant at lower diagnostic energies and in higher-Z materials. Near 30 keV in soft tissue, photoelectric absorption contributes substantially. Compton scatter becomes relatively more important at higher energies and in low-Z materials.

Question 5Physics

The Larmor equation describes which relationship in magnetic resonance imaging?

  1. Slice thickness as proportional to bandwidth
  2. Precessional frequency as proportional to applied magnetic field strength Correct answer.
  3. Relaxation time as proportional to flip angle
  4. Signal amplitude as proportional to echo time

Explanation

The Larmor equation states that precessional frequency equals the gyromagnetic ratio times the magnetic field strength. This linear relationship underlies spatial encoding via gradients. It does not describe relaxation, signal amplitude, or slice thickness directly.

Question 6Physics

In ultrasound, the axial resolution of an image is primarily determined by which factor?

  1. Frame rate
  2. Beam width at the focal zone
  3. Spatial pulse length Correct answer.
  4. Receiver gain

Explanation

Axial resolution depends on the spatial pulse length, which is the number of cycles in a pulse multiplied by the wavelength; shorter pulses give better axial resolution. Beam width governs lateral resolution. Frame rate and gain affect temporal and brightness characteristics, not axial resolution.

Question 7Physics

The inverse square law states that radiation intensity from a point source varies in what way with distance?

  1. Inversely with distance
  2. Independently of distance
  3. Directly with the square of distance
  4. Inversely with the square of distance Correct answer.

Explanation

For a point source emitting isotropically, intensity spreads over a spherical area proportional to distance squared, so intensity falls as one over distance squared. This relationship is used to estimate exposure changes when source-to-object distance changes. It does not apply to a broad parallel beam.

Question 8Physics

Which quantity is defined as the energy absorbed per unit mass of irradiated material?

  1. Absorbed dose Correct answer.
  2. Exposure
  3. Equivalent dose
  4. Kerma

Explanation

Absorbed dose is the energy deposited per unit mass and is expressed in gray (joule per kilogram). Exposure describes ionization in air, equivalent dose weights absorbed dose by radiation type, and kerma describes kinetic energy released to charged particles per unit mass. Each has a distinct definition.

Question 9Physics

A radiographic technique uses 80 kVp and 20 mAs. If the kVp is increased to 90 kVp while mAs is unchanged, what is the expected effect on receptor exposure?

  1. It increases substantially Correct answer.
  2. It remains essentially unchanged
  3. It decreases modestly
  4. It becomes zero

Explanation

X-ray output is approximately proportional to the square of kVp, so a rise from 80 to 90 kVp markedly increases beam intensity and receptor exposure. Because mAs is constant, the change is driven by the kVp increase. The 15% rule illustrates this strong dependence.

Question 10Physics

A structure measures 4 cm on an image acquired with a magnification factor of 1.25. What is the true object size?

  1. 6.4 cm
  2. 3.2 cm Correct answer.
  3. 5.0 cm
  4. 4.0 cm

Explanation

Magnification factor equals image size divided by object size, so object size equals image size divided by magnification (4 cm / 1.25 = 3.2 cm). Magnified images overstate true size. Dividing corrects for the geometric enlargement.

Question 11Physics

Increasing the source-to-image distance while keeping the object-to-image distance constant has which effect on geometric unsharpness?

  1. It increases unsharpness
  2. It eliminates all unsharpness
  3. It has no effect
  4. It decreases unsharpness Correct answer.

Explanation

Geometric unsharpness is proportional to object-to-image distance divided by source-to-object distance. Increasing source-to-image distance increases source-to-object distance, reducing blur. This is why longer distances improve sharpness.

Question 12Physics

A CT scan uses a pitch of 0.5. Which statement best describes the relationship between pitch and patient dose?

  1. Lower pitch generally increases patient dose Correct answer.
  2. Higher pitch always doubles the dose
  3. Pitch has no relationship to dose
  4. Lower pitch generally decreases patient dose

Explanation

Pitch is table travel per rotation divided by beam width. A lower pitch means more overlap of the beam per rotation, increasing dose for a given volume. Higher pitch spreads dose over more rotations, generally reducing dose.

Question 13Physics

The half-value layer of a beam is 0.3 mm of lead. What thickness of lead reduces the beam intensity to one-eighth of its original value?

  1. 0.6 mm
  2. 0.9 mm Correct answer.
  3. 2.4 mm
  4. 1.2 mm

Explanation

Reducing intensity to one-eighth requires three half-value layers because (1/2)^3 = 1/8. Three times 0.3 mm equals 0.9 mm. This is a straightforward application of exponential attenuation.

Question 14Physics

In MRI, which parameter primarily determines the spatial location encoded along the frequency-encoding direction?

  1. The phase-encoding gradient amplitude
  2. The frequency-encoding gradient Correct answer.
  3. The radiofrequency coil sensitivity
  4. The slice-select gradient

Explanation

During readout, a frequency-encoding gradient makes precessional frequency vary linearly with position, so the Fourier transform of the received signal maps frequencies to locations. Phase encoding uses gradient amplitude steps to encode a different axis. Slice selection uses a gradient during the RF pulse.

Question 15Physics

A radionuclide used for PET emits positrons. The annihilation photons produced have which characteristic energy?

  1. 511 keV Correct answer.
  2. 1.022 MeV each
  3. 662 keV
  4. 140 keV

Explanation

Positron-electron annihilation produces two photons emitted in opposite directions, each with an energy of 511 keV. The 1.022 MeV total energy is split between the two photons. Detecting these coincident 511 keV photons is the basis of PET imaging.

Question 16Physics

Which factor most directly limits the temporal resolution of a fluoroscopic imaging system?

  1. The grid ratio
  2. The number of detector rows
  3. The frame rate and pulse duration Correct answer.
  4. The focal spot size

Explanation

Temporal resolution refers to how well rapid motion is captured over time and is governed by frame rate and pulse width. Higher frame rates and shorter pulses improve temporal fidelity. Detector rows and grid ratio affect spatial and scatter characteristics, not temporal sampling.

Question 17Physics

A gamma camera uses a collimator. What is the primary function of the collimator?

  1. To convert gamma photons into visible light
  2. To reduce patient motion artifacts
  3. To restrict detected photons to those traveling along defined directions Correct answer.
  4. To amplify the electronic signal

Explanation

The collimator is a lead structure with holes that only permits photons traveling in specific directions to reach the detector, enabling spatial localization. The scintillation crystal converts gamma photons to light, and photomultiplier tubes amplify the signal. Motion correction is handled separately.

Question 18Physics

In digital radiography, the detective quantum efficiency (DQE) describes which property of a detector?

  1. The number of gray levels it can display
  2. Its ability to convert x-ray photons into a signal with high efficiency and low noise Correct answer.
  3. The speed at which it reads out an image
  4. The maximum spatial frequency it can record

Explanation

DQE quantifies how efficiently a detector converts incident x-ray quanta into image signal while accounting for noise, expressed as a function of spatial frequency. It is a combined measure of sensitivity and noise performance. Spatial frequency limit, bit depth, and readout speed are separate specifications.

Question 19Physics

A technologist doubles the mAs while halving the kVp from 100 to 50. Compared with the original exposure, what is the approximate effect on receptor exposure?

  1. It decreases about fourfold Correct answer.
  2. It increases about fourfold
  3. It remains roughly the same
  4. It increases about eightfold

Explanation

Output scales approximately with kVp squared, so halving kVp reduces output by roughly a factor of four. Doubling mAs only offsets a factor of two, leaving a net reduction of about half of the original... wait, net is about half. Recompute: original output proportional to kVp^2 * mAs. New = (0.5)^2 * 2 = 0.5, so exposure is about half.

Question 20Physics

Which physical principle underlies the use of a half-value layer to characterize beam quality?

  1. Exponential attenuation of a monoenergetic beam Correct answer.
  2. Complete absorption at a fixed depth
  3. Scatter amplification with depth
  4. Linear attenuation with thickness

Explanation

Attenuation of photons follows an exponential relationship with thickness, so the thickness required to halve intensity characterizes the beam's penetrating ability. For polyenergetic beams, this is an average measure because the spectrum hardens with depth. Linear or complete-absorption models do not describe photon attenuation.

Question 21Physics

In MRI, a short repetition time combined with a short echo time produces which image contrast weighting?

  1. T1-weighted Correct answer.
  2. T2-weighted
  3. Proton density-weighted
  4. Diffusion-weighted

Explanation

A short TR minimizes recovery differences, emphasizing T1 differences among tissues, and a short TE reduces T2 decay effects. Together these produce T1 weighting. Long TR with long TE gives T2 weighting, and long TR with short TE gives proton density weighting.

Question 22Physics

A linear accelerator delivers a dose rate that follows an inverse square relationship with distance. If the dose rate is 400 cGy/min at 100 cm, what is the approximate dose rate at 200 cm?

  1. 50 cGy/min
  2. 100 cGy/min Correct answer.
  3. 800 cGy/min
  4. 200 cGy/min

Explanation

Doubling the distance reduces intensity to one-quarter by the inverse square law: 400 / 4 = 100 cGy/min. The ratio of distances squared is (200/100)^2 = 4. This is a standard calculation in radiation protection and treatment planning.

Question 23Physics

In CT, the term 'beam hardening' refers to which phenomenon?

  1. Increased photon energy from scattered radiation
  2. Preferential attenuation of low-energy photons as the beam passes through tissue Correct answer.
  3. Loss of spatial resolution at high pitch
  4. Uniform attenuation across all photon energies

Explanation

Because lower-energy photons are attenuated more readily, the beam becomes relatively enriched in higher energies as it traverses tissue, an effect called beam hardening. This can cause cupping artifacts and streaks. It is distinct from scatter and from resolution effects.

Question 24Physics

A radioactive sample contains a parent nuclide with a half-life much longer than that of its daughter. After a sufficiently long time, the daughter activity approaches which value?

  1. Zero
  2. Twice the parent activity
  3. The square of the parent activity
  4. Equal to the parent activity Correct answer.

Explanation

When the parent half-life greatly exceeds the daughter half-life, transient or secular equilibrium develops, and the daughter activity approaches the parent activity. This balance arises because the daughter decays as fast as it is produced. The activities do not vanish or grow without bound.

Question 25Physics

In MRI, increasing the receiver bandwidth while keeping other parameters constant has which combined effect?

  1. It decreases signal-to-noise ratio and increases chemical shift artifact
  2. It increases signal-to-noise ratio and increases chemical shift artifact
  3. It decreases signal-to-noise ratio and decreases chemical shift artifact Correct answer.
  4. It increases signal-to-noise ratio and decreases chemical shift artifact

Explanation

A wider receiver bandwidth admits more noise, lowering signal-to-noise ratio, while reducing the frequency shift in pixels caused by chemical shift, thereby decreasing that artifact. These two effects move in opposite directions, so bandwidth selection balances SNR against artifact severity.

Question 26Physics

Which property of an x-ray beam is quantified by the half-value layer (HVL)?

  1. Total filtration
  2. Beam intensity
  3. Penetrating ability Correct answer.
  4. Focal spot size

Explanation

The HVL is the thickness of a specified material that reduces the beam intensity to half its original value. It is a standard indicator of beam quality, which reflects the beam's penetrating ability. Higher HVL means a more penetrating (harder) beam.

Question 27Physics

In computed radiography, the latent image is stored in which component?

  1. Amorphous selenium layer
  2. Charge-coupled device
  3. Scintillator coupled to a photodiode array
  4. Photostimulable phosphor plate Correct answer.

Explanation

Computed radiography uses a photostimulable phosphor plate (typically barium fluorobromide doped with europium) that traps electrons in metastable states after x-ray exposure. The latent image is read out by laser stimulation, which releases stored energy as light.

Question 28Physics

Which interaction predominates when a 100 keV photon interacts with soft tissue?

  1. Coherent scattering
  2. Photoelectric absorption
  3. Pair production
  4. Compton scattering Correct answer.

Explanation

In soft tissue, the photoelectric effect dominates at low energies (roughly below 30–40 keV), while Compton scattering becomes the principal interaction at intermediate energies such as 100 keV. Pair production requires a threshold of 1.022 MeV.

Question 29Physics

The photoelectric effect is most likely to occur with which combination of photon energy and absorber atomic number?

  1. High photon energy and low atomic number
  2. Low photon energy and low atomic number
  3. High photon energy and high atomic number
  4. Low photon energy and high atomic number Correct answer.

Explanation

The photoelectric probability is proportional to approximately Z^3/E^3, so it increases strongly with the atomic number of the absorber and decreases rapidly as photon energy rises. Thus low energy and high Z favor photoelectric absorption.

Question 30Physics

In a CT scanner, the purpose of the bowtie filter is to do which of the following?

  1. Flatten the x-ray spectrum to reduce beam hardening
  2. Collimate the beam to the slice thickness
  3. Shape the x-ray beam intensity across the fan to reduce patient dose Correct answer.
  4. Reduce scattered radiation reaching the detector

Explanation

The bowtie filter is a shaped attenuator that reduces intensity at the periphery of the fan beam where the patient is thinner, compensating for the roughly elliptical cross-section of the body. This reduces dose to superficial tissues and helps equalize detector signal.

Question 31Physics

Which of the following best describes the function of a radiographic grid?

  1. Decrease patient dose by hardening the beam
  2. Increase the primary beam intensity
  3. Improve spatial resolution by reducing focal spot blur
  4. Reduce scattered radiation reaching the image receptor Correct answer.

Explanation

A grid consists of alternating radiopaque and radiolucent strips that absorb off-axis scattered photons before they reach the receptor. This improves contrast but requires increased exposure (higher mAs) to compensate for primary beam attenuation.

Question 32Physics

What is the primary effect of increasing the field of view (FOV) in CT reconstruction while keeping all other parameters constant?

  1. Decreased noise
  2. Decreased spatial resolution (larger voxels) Correct answer.
  3. Increased slice thickness
  4. Increased spatial resolution (smaller voxels)

Explanation

The reconstruction matrix is fixed (e.g., 512 × 512), so increasing the FOV spreads the same number of pixels over a larger area. This increases pixel size and therefore decreases spatial resolution.

Question 33Physics

In MRI, which parameter determines the contrast between tissues in a T1-weighted image?

  1. Receiver bandwidth
  2. T1 relaxation time Correct answer.
  3. T2* relaxation time
  4. Proton density

Explanation

T1-weighted contrast is generated by differences in T1 relaxation times among tissues, achieved with a short TR and short TE. Tissues with short T1 recover longitudinal magnetization quickly and appear bright.

Question 34Physics

A gradient-echo sequence uses a flip angle of 90° and a long TR. Which weighting is most likely?

  1. T2*
  2. Proton density Correct answer.
  3. Diffusion
  4. T1

Explanation

With a long TR and a 90° flip angle, longitudinal magnetization fully recovers between excitations, minimizing T1 differences. A short TE minimizes T2* decay, so contrast reflects proton density.

Question 35Physics

In MRI, which of the following is the primary source of the signal used for image formation?

  1. Phosphorus nuclei
  2. Electron spins
  3. Hydrogen nuclei Correct answer.
  4. Carbon nuclei

Explanation

The hydrogen nucleus (a single proton) has the highest natural abundance in the body and the largest gyromagnetic ratio among common nuclei, making it the dominant source of MRI signal. Other nuclei such as phosphorus and carbon have much lower concentrations and require specialized coils and sequences.

Question 36Physics

In ultrasound, the Doppler shift frequency is directly proportional to which of the following?

  1. Transducer diameter
  2. Depth of the vessel
  3. Pulse repetition frequency
  4. Blood flow velocity Correct answer.

Explanation

The Doppler shift is given by Δf = (2 f0 v cosθ)/c, where v is the flow velocity and θ is the angle between the beam and flow. Thus the shift is directly proportional to velocity (and to the cosine of the Doppler angle).

Question 37Physics

In ultrasound imaging, the principle that determines the depth of a reflector is based on which relationship?

  1. Frequency of the echo
  2. Amplitude of the echo
  3. Phase of the echo
  4. Time of flight of the echo Correct answer.

Explanation

The depth of a reflector is calculated from the round-trip time of the pulse and the assumed speed of sound in tissue (approximately 1540 m/s). The amplitude, frequency, and phase provide information about tissue properties but not depth.

Question 38Physics

In fluoroscopy, which of the following best explains why the automatic brightness control (ABC) system increases tube current when the image intensifier field of view is reduced?

  1. To improve temporal resolution
  2. To reduce the patient dose
  3. To increase the focal spot size
  4. To compensate for decreased minification gain Correct answer.

Explanation

Reducing the field of view decreases the minification gain of the image intensifier, lowering the brightness of the output image. The ABC system responds by increasing tube current (or kVp) to maintain adequate brightness, which typically increases patient dose.

Question 39Physics

In digital radiography, which of the following best describes the relationship between the exposure indicator and patient dose?

  1. It is a measure of spatial resolution
  2. It provides an estimate of the receptor exposure, which correlates with dose Correct answer.
  3. It directly measures absorbed dose to the patient
  4. It is independent of the mAs used

Explanation

The exposure indicator (e.g., EI, DI, or S number) reflects the amount of radiation reaching the image receptor. It is an indirect indicator of patient dose and is used to assess technique adequacy, but it does not measure absorbed dose directly.

Question 40Physics

Which of the following is the primary reason that a high-frequency generator produces a higher effective beam energy than a single-phase generator at the same kVp setting?

  1. It produces a nearly constant potential waveform Correct answer.
  2. It has a longer exposure time
  3. It uses a lower filtration
  4. It uses a larger focal spot

Explanation

High-frequency generators convert the AC waveform to a nearly constant potential, so the applied voltage remains close to the peak kVp throughout the exposure. Single-phase generators produce a pulsating waveform with significant ripple, so many photons are produced at lower energies, reducing the effective energy.

Question 41Physics

In a diagnostic x-ray tube, the characteristic x-rays are produced by which mechanism?

  1. Bremsstrahlung interactions in the nucleus
  2. Compton scattering in the anode
  3. Electron transitions between atomic shells Correct answer.
  4. Pair production in the anode

Explanation

Characteristic x-rays are emitted when an incident electron ejects an inner-shell electron and an outer-shell electron fills the vacancy, releasing energy equal to the difference in binding energies. These photons have discrete energies characteristic of the anode material.

Question 42Physics

In mammography, which of the following is the primary purpose of compression?

  1. Increase the field of view
  2. Increase the focal spot size
  3. Reduce breast thickness to lower dose and improve contrast Correct answer.
  4. Reduce the tube voltage

Explanation

Compression reduces breast thickness, which decreases the amount of tissue the beam must penetrate, lowering dose and scatter. It also brings structures closer to the detector, improving spatial resolution and contrast.

Question 43Physics

Which of the following best describes the purpose of the AEC (automatic exposure control) system in radiography?

  1. Adjust the kVp automatically
  2. Reduce scattered radiation
  3. Terminate the exposure when a predetermined receptor dose is reached Correct answer.
  4. Control the focal spot size

Explanation

AEC uses ionization chambers or solid-state detectors to monitor radiation reaching the receptor and terminates the exposure at a preset level. This provides consistent receptor exposure despite patient size and other variations.

Question 44Physics

A technologist wishes to double the receptor exposure without changing the kVp. What change in mAs is required?

  1. Halve the mAs
  2. Quadruple the mAs
  3. Double the mAs Correct answer.
  4. Leave the mAs unchanged

Explanation

At a fixed kVp, receptor exposure is directly proportional to mAs. Therefore, doubling the mAs doubles the exposure.

Question 45Physics

In radiation protection, the ALARA principle is best described as which of the following?

  1. Using the lowest kVp possible
  2. Allowing doses up to the legal limit
  3. Keeping radiation doses as low as reasonably achievable Correct answer.
  4. Minimizing the number of repeat exposures only

Explanation

ALARA stands for 'as low as reasonably achievable' and is a fundamental principle of radiation protection. It requires that all exposures be justified and optimized to keep doses below regulatory limits while achieving the clinical objective.

Question 46Physics

Which of the following is the most appropriate unit for expressing the effective dose to a patient from a CT examination?

  1. Gray
  2. Becquerel
  3. Coulomb per kilogram
  4. Millisievert Correct answer.

Explanation

Effective dose is expressed in sieverts (or millisieverts) and reflects the radiation risk weighted by tissue sensitivity. The gray measures absorbed dose, the becquerel measures activity, and the coulomb per kilogram measures exposure.

Question 47Physics

The dose-length product (DLP) in CT is calculated by multiplying which two quantities?

  1. CTDIvol and scan length Correct answer.
  2. Slice thickness and number of slices
  3. mAs and kVp
  4. CTDIvol and pitch

Explanation

DLP is the product of the volume CT dose index (CTDIvol) and the scanned length in centimeters. It provides an estimate of the total radiation dose for a complete scan.

Question 48Physics

In CT, which of the following best explains why increasing pitch (above 1) reduces patient dose?

  1. The reconstruction kernel changes
  2. The tube current increases automatically
  3. The slice thickness decreases
  4. The table moves faster, so fewer photons are needed per slice Correct answer.

Explanation

Pitch is the table travel per rotation divided by the beam width. At higher pitch, the table moves faster relative to the beam width, so the same anatomical volume is scanned with fewer rotations and thus less radiation per unit length.

Question 49Physics

In nuclear medicine, which of the following is the primary function of the photomultiplier tube (PMT) in a gamma camera?

  1. Convert light photons into an electrical signal Correct answer.
  2. Digitize the signal for display
  3. Absorb the gamma rays and emit light
  4. Collimate the incoming gamma rays

Explanation

PMTs are coupled to the scintillation crystal and convert the brief light flashes produced by gamma interactions into amplified electrical pulses. The pulses are then processed to determine the position and energy of each event.

Question 50Physics

A radiopharmaceutical has a physical half-life of 6 hours and a biological half-life of 12 hours. What is its effective half-life?

  1. 18 hours
  2. 4 hours Correct answer.
  3. 9 hours
  4. 6 hours

Explanation

Effective half-life is given by 1/Te = 1/Tp + 1/Tb. Here 1/Te = 1/6 + 1/12 = 3/12 = 1/4, so Te = 4 hours.