100+ Free FC Rad Diag(SA) Part I Practice Questions

Explanation: Bremsstrahlung (braking radiation) occurs when high-speed incident electrons are decelerated by the electric field of tungsten atomic nuclei. Because incident electrons lose varying fractions of their kinetic energy in these deflections, a continuous spectrum of X-ray photon energies up to the peak kilovoltage (kVp) is produced. Bremsstrahlung constitutes approximately 80% to 90% of the X-ray beam output in standard diagnostic radiology.

Explanation: Aluminum filtration selectively absorbs low-energy ('soft') X-ray photons that would otherwise contribute to patient skin dose without penetrating to the image receptor. This process hardens the X-ray beam, effectively increasing its average energy and beam quality. As a result, the half-value layer (HVL) increases while overall beam quantity is reduced.

Explanation: The probability of photoelectric absorption per unit mass is approximately proportional to Z^3 / E^3, where Z is the atomic number of the absorbing medium and E is the incident photon energy. This strong cubic dependence on atomic number explains why bone (high Z) absorbs significantly more radiation than soft tissue (low Z) in diagnostic energy ranges, providing high subject contrast. Conversely, higher photon energies markedly decrease photoelectric probability.

Explanation: Compton (incoherent) scattering is the predominant interaction mechanism in soft tissues across most diagnostic and therapeutic energy ranges (roughly 30 keV to 30 MeV). In Compton scattering, an incident photon interacts with a loosely bound outer-shell electron, imparting part of its energy and scattering at an angle. Because it depends primarily on electron density rather than atomic number, it produces scatter radiation that degrades image contrast.

Explanation: The Hounsfield Unit scale quantifies radiodensity in CT and is defined as HU = 1000 × [(μ_tissue - μ_water) / μ_water], where μ represents the linear attenuation coefficient. By convention, water is assigned a value of 0 HU, air is assigned -1000 HU, and dense cortical bone reaches +1000 HU or higher. This standardizes grey-scale display across different CT scanners and energy spectra.

Explanation: CT pitch is defined as table travel per 360-degree rotation divided by the total beam collimation width. A pitch factor > 1.0 means the table moves further than the width of the X-ray beam during one gantry rotation, creating gaps in data sampling that are filled by interpolation algorithms. This accelerates scan coverage and reduces patient radiation dose, though it slightly broadens the section sensitivity profile.

Explanation: The Larmor equation states that f_0 = γ × B_0, where f_0 is the resonance frequency, γ is the gyromagnetic ratio (42.58 MHz/Tesla for 1H protons), and B_0 is the strength of the static magnetic field. For example, at 1.5 Tesla, the Larmor frequency of hydrogen protons is approximately 63.87 MHz, whereas at 3.0 Tesla it is 127.74 MHz. Resonance absorption and RF excitation can only occur when transmitted RF pulses match this exact frequency.

Explanation: T1 (spin-lattice) relaxation describes the exponential recovery of net longitudinal magnetization along the Z-axis (parallel to B_0) after an RF pulse. Protons transfer absorbed RF energy to the surrounding molecular lattice as thermal energy. The T1 time constant is defined as the time required for longitudinal magnetization to recover to approximately 63% of its original equilibrium value.

Explanation: A T2-weighted spin-echo image requires a long TR (> 2000 ms) to minimize T1 relaxation differences among tissues, combined with a long TE (> 80 ms) to allow significant T2 transverse dephasing differences to develop between tissues. Under these settings, fluid-filled structures (e.g., CSF) with long T2 relaxation times remain bright, while solid tissues with short T2 appear dark.

Explanation: Axial resolution (along the beam axis) is defined as half of the spatial pulse length (Axial Resolution = SPL / 2). Because SPL equals the number of cycles per pulse multiplied by wavelength, higher frequency transducers (which produce shorter wavelengths and shorter SPLs) yield superior axial resolution. Axial resolution is independent of depth and beam width.