ABR DR Core cheat sheet

Attenuation follows I = I0·e^(−μx); the linear attenuation coefficient μ depends on photon energy and material density.

Half-value layer (HVL) = 0.693/μ; HVL increases with beam energy and decreases with increasing filtration.

Effective dose (mSv) ≈ DAP × conversion coefficient; CT dose index volume CTDIvol = CTDIw/pitch.

Photoelectric absorption ∝ Z³/E³; Compton scatter dominates at diagnostic energies in soft tissue and is nearly independent of Z.

MRI: Larmor frequency f = γB0; γ for ¹H = 42.58 MHz/T. T1 and T2 relaxation times lengthen with increasing field strength for most tissues.

Ultrasound axial resolution ≈ spatial pulse length/2; higher frequency gives better resolution but less penetration.

Kerma vs absorbed dose: kerma describes energy transferred to charged particles; absorbed dose describes energy deposited per unit mass (gray = J/kg).

In fluoroscopy, patient dose rate scales roughly with the square of magnification and inversely with the square of source-to-skin distance.

Linear-quadratic model: survival fraction S = e^(−αD−βD²); α/β ratio is high (~10 Gy) for early-responding tumors and low (~3 Gy) for late tissues.

Law of Bergonié and Tribondeau: radiosensitivity increases with mitotic activity and undifferentiation and decreases with maturation.

DNA double-strand breaks are the principal lethal lesion; repair occurs via nonhomologous end joining and homologous recombination.

Oxygen enhancement ratio (OER) is ~3 for sparsely ionizing radiation; hypoxic cells are relatively radioresistant.

Stochastic effects (carcinogenesis, heritable) are assumed to have no threshold; deterministic effects have a threshold and increase in severity with dose.

Fractionation spares late-responding tissues more than early-responding tissues because of their different α/β ratios and repair capacity.

Relative biologic effectiveness (RBE) increases with higher linear energy transfer; alpha particles and neutrons have higher RBE than x-rays.

Effective dose from a chest radiograph is roughly 0.1 mSv; a chest CT is roughly 5–10 mSv.

Sensitivity = TP/(TP+FN); specificity = TN/(TN+FP); positive predictive value depends on disease prevalence.

An ROC curve plots true-positive rate against false-positive rate; area under the curve of 0.5 indicates no discrimination.

Report structure: clinical indication, technique, findings, impression; critical results require documented closed-loop communication.

Informed consent requires capacity, disclosure of material risks, benefits, and alternatives, and voluntariness.

Radiation protection: ALARA and the three cardinal principles - time, distance, and shielding.

Justification and optimization are the core principles of the ICRP system of radiological protection.

Peer review and structured error reporting (e.g., RADPEER-type categories) classify discrepancies by clinical significance.

Contrast reaction categories: mild (self-limited), moderate (requires treatment), severe (anaphylactoid, requires epinephrine and airway support).

Acute ischemic stroke: noncontrast CT excludes hemorrhage; large-vessel occlusion is assessed with CTA; perfusion imaging identifies salvageable penumbra.

Pulmonary embolism: CTPA is the test of choice; right ventricular enlargement on CT correlates with severity.

Pneumothorax: upright chest radiograph shows a visible pleural line with absent lung markings; tension pneumothorax is a clinical diagnosis.

Small bowel obstruction: dilated loops >3 cm with air-fluid levels and a transition point; closed-loop obstruction is a surgical emergency.

Bowel obstruction vs ileus: ileus shows diffuse dilation without a transition point; obstruction shows a discrete transition zone.

Renal mass: Bosniak classification grades cystic renal masses I–IV by septa, calcification, and enhancement.

TI-RADS and BI-RADS use structured categories with malignancy risk estimates; BI-RADS 4 is subdivided 4A/B/C by suspicion.

Bone tumor: location (epiphysis, metaphysis, diaphysis) and margin (narrow zone of transition suggests benign) guide differential diagnosis.