Arab Board Radiology Part 1 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Photoelectric effect

Interaction where an incident photon is completely absorbed by an inner-shell electron, which is ejected; probability is proportional to Z³/E³ and produces high contrast.

Compton scattering

Photon interacts with an outer-shell electron, transferring part of its energy and changing direction; probability depends on electron density and decreases with increasing photon energy.

Hounsfield unit

CT number scale where water is 0 HU and air is −1000 HU, calculated as 1000 × (μ_tissue − μ_water)/μ_water; used to characterize tissue density.

Nyquist theorem

Sampling frequency must be at least twice the highest spatial frequency in the object to avoid aliasing; critical in digital radiography and CT reconstruction.

Larmor frequency

Precessional frequency of a nucleus in a magnetic field, given by ω = γB; for hydrogen-1, γ is 42.58 MHz/T, determining MRI resonance frequency.

Doppler shift

Change in frequency of reflected ultrasound from moving blood, Δf = (2 f₀ v cosθ)/c; maximum when the angle of insonation is 0 degrees.

Linear energy transfer

Average energy deposited per unit path length by ionizing radiation; high-LET radiation (alpha particles, neutrons) causes more dense ionization and greater biological damage per dose.

Law of Bergonié and Tribondeau

States that radiosensitivity is greatest in cells that are undifferentiated, rapidly dividing, and have a long mitotic future; explains high sensitivity of bone marrow and gonads.

Effective dose

Sum of tissue-equivalent doses weighted by tissue radiosensitivity factors (Σ w_T × H_T); used to compare stochastic risk across different imaging studies.

Stochastic effects

Radiation effects whose probability increases with dose but severity does not; no threshold dose exists; examples include cancer and hereditary effects.

ALARA principle

As Low As Reasonably Achievable; radiation dose optimization using time, distance, and shielding; intensity decreases with the inverse square of distance.

Iodinated contrast osmolality

High-osmolar contrast (~1500–2000 mOsm/kg) causes more pain and osmotic nephrosis; low-osmolar (~600–800) and iso-osmolar (~290) agents are preferred for safety.

Nephrogenic systemic fibrosis

Rare fibrosing disorder associated with gadolinium-based contrast in severe renal impairment (eGFR <30); macrocyclic agents carry lower risk than linear agents.

Anaphylactoid reaction

Non-IgE-mediated severe contrast reaction with hypotension, bronchospasm, and urticaria; treated with intramuscular epinephrine 0.3–0.5 mg (1:1000).

Couinaud liver segments

Eight functional liver segments based on portal and hepatic vein anatomy; segment I is the caudate lobe; used for surgical planning and lesion localization.

Two-hit hypothesis

Tumor suppressor genes require inactivation of both alleles for carcinogenesis; example is retinoblastoma (RB1 gene) with hereditary and sporadic forms.

Silhouette sign

Loss of the normal border between two structures of similar density on radiograph, indicating they are in physical contact; e.g., right middle lobe opacity obscures right heart border.

Air bronchogram

Air-filled bronchi rendered visible by surrounding alveolar consolidation; indicates patent airways and suggests pneumonia rather than obstruction or pleural effusion.

Honeycombing

Clustered cystic airspaces with thick walls on HRCT, predominantly subpleural and basal; hallmark of usual interstitial pneumonia and idiopathic pulmonary fibrosis.

Double duct sign

Simultaneous dilatation of the common bile duct and pancreatic duct on imaging; highly suggestive of a pancreatic head mass, usually adenocarcinoma.

HCC enhancement pattern

Hepatocellular carcinoma shows arterial hyperenhancement followed by venous washout on multiphasic CT or MRI in a cirrhotic liver; diagnostic without biopsy.

Ovarian torsion

Enlarged ovary with peripherally displaced follicles and decreased or absent Doppler flow; surgical emergency requiring immediate detorsion to preserve ovarian function.

PI-RADS

Prostate Imaging Reporting and Data System score (1–5) on multiparametric MRI; scores ≥3 are considered suspicious and typically warrant targeted biopsy.

Codman triangle

Periosteal reaction where the elevated periosteum forms an angle with the cortex; seen in aggressive bone tumors such as osteosarcoma.

Dawson fingers

Ovoid demyelinating lesions perpendicular to the lateral ventricles on sagittal MRI, characteristic of multiple sclerosis; best seen on T2/FLAIR sequences.

Dural tail sign

Thickening and enhancement of dura adjacent to an extra-axial mass, most commonly a meningioma; helps differentiate from other extra-axial lesions.

Juvenile nasopharyngeal angiofibroma

Highly vascular benign tumor in adolescent males presenting with epistaxis and nasal obstruction; avidly enhances on imaging and may extend into surrounding spaces.

BI-RADS category 4

Breast imaging finding suspicious for malignancy, with a 2–95% likelihood of cancer; warrants tissue sampling (biopsy).

Pyloric stenosis ultrasound

Diagnostic when pyloric muscle thickness exceeds 3 mm and canal length exceeds 15 mm; target sign on transverse view in a vomiting infant.

Hyaline membrane disease

Respiratory distress in premature infants due to surfactant deficiency; chest radiograph shows ground-glass opacities, air bronchograms, and low lung volumes.

FDG-PET

Molecular imaging using ¹⁸F-fluorodeoxyglucose to map glucose metabolism; increased uptake in malignancy, infection, and inflammation; brain and heart show physiologic high uptake.

Seldinger technique

Percutaneous vascular access using a needle, guidewire, and catheter; common femoral artery is the most frequent site for angiography and interventional procedures.