ABR Core Exam cheat sheet

Pulmonary embolism on CTPA: acute thrombus is central with a polo-mint sign; chronic thrombus is eccentric, calcified, and forms webs/bands.

Bowel obstruction: small bowel >3 cm, colon >6 cm, cecum >9 cm define dilation; air-fluid levels and transition point localize obstruction.

Acute cholecystitis: sonographic triad of wall thickening >3 mm, pericholecystic fluid, and sonographic Murphy sign; HIDA shows nonvisualization after 4 h.

BIRADS: 0 incomplete, 1 negative, 2 benign, 3 probably benign (<2% malignancy), 4 suspicious, 5 highly suggestive (>=95%), 6 known biopsy-proven malignancy.

LI-RADS 5 requires arterial phase hyperenhancement plus washout, capsule, or threshold growth in an at-risk liver; LR-M denotes probable malignancy not HCC-specific.

Stroke: DWI positive within minutes; ADC pseudonormalizes around 7-10 days; large-vessel occlusion shows susceptibility vessel sign on GRE/SWI.

Adrenal adenoma: unenhanced CT <10 HU or absolute washout >60% at 15 min; pheochromocytoma enhances avidly and is T2-hyperintense.

Pediatric: intussusception shows target sign on ultrasound; midgut volvulus shows whirlpool sign and needs emergent surgical evaluation.

CT number (HU) = 1000 x (mu_tissue - mu_water)/mu_water; water = 0 HU, air = -1000 HU, dense bone >400 HU.

Half-value layer: HVL = 0.693/mu; mean energy of a 120 kVp beam is roughly one-third to one-half of peak kVp.

Photoelectric effect dominates at low energy and high Z, varying as Z^3/E^3; Compton scatter dominates in soft tissue at diagnostic energies.

MRI Larmor frequency = gamma x B0; at 1.5 T proton frequency is ~63.9 MHz, at 3 T ~127.7 MHz.

MRI safety: 5 gauss line bounds the controlled area; projectile risk scales with field strength; SAR rises with B0 squared.

Ultrasound axial resolution = spatial pulse length/2 = (wavelength x cycles)/2; higher frequency improves resolution but reduces penetration.

Radiation units: 1 Gy = 1 J/kg; 1 Sv = 1 Gy x radiation weighting factor; effective dose = sum of tissue doses x tissue weighting factors.

Mammography: mean glandular dose ~1.2-2 mGy per view; grid improves contrast by removing scatter at cost of dose.

ALARA and Image Gently/Image Wisely: justify every study, use size-based pediatric protocols, and limit multiphase CT to clinically necessary phases.

ACR Appropriateness Criteria and Choosing Wisely support evidence-based exam selection and reduce low-value imaging.

Informed consent requires decision-making capacity, disclosure of material risks/benefits/alternatives, and voluntary agreement.

Contrast reactions: mild (hives) -> observe; moderate (vomiting, urticaria) -> antihistamine; severe (anaphylaxis) -> epinephrine IM 0.3-0.5 mg of 1:1000.

Metformin and iodinated contrast: hold only if eGFR <30 or AKI; otherwise no routine hold is required per ACR guidance.

Gadolinium-associated nephrogenic systemic fibrosis risk is highest with older linear agents and eGFR <30; macrocyclic agents are preferred in renal impairment.

Peer review and RADPEER use a 1-4 scale; error classification separates perceptual from cognitive misses and supports system improvement.

Quality improvement uses Plan-Do-Study-Act cycles and root cause analysis; reporting should follow structured templates for consistency.

Brain herniation: subfalcine shifts cingulate gyrus; uncal herniation compresses CN III and PCA; tonsillar herniation compresses medulla.

Multiple sclerosis: periventricular ovoid T2 lesions perpendicular to ventricles (Dawson fingers), juxtacortical, infratentorial, and spinal cord involvement.

Liver: segmental anatomy follows hepatic veins; segment IV lies between the middle and left hepatic veins; Couinaud has eight segments.

Renal cell carcinoma: clear cell subtype is hypervascular and T1-hyperintense; papillary subtype is hypovascular and T2-hypointense.

Bone tumor: osteosarcoma shows sunburst periosteal reaction and Codman triangle; Ewing sarcoma is diaphyseal with onion-skin periosteum.

Chest: sarcoidosis shows bilateral hilar lymphadenopathy with perilymphatic nodules; usual interstitial pneumonia shows basal honeycombing.

Thyroid nodule: TI-RADS uses composition, echogenicity, shape, margin, and echogenic foci; taller-than-wide is suspicious.

Prostate: PI-RADS 4-5 lesions in the peripheral zone show focal DWI hyperintensity with ADC hypointensity and early enhancement.

X-ray tube: bremsstrahlung produces a continuous spectrum; characteristic peaks depend on anode material (tungsten K-shell ~69 keV).

Increasing mAs increases photon quantity linearly; increasing kVp raises both quantity and quality (penetration).

Beam filtration removes low-energy photons, raising mean energy and lowering patient dose while preserving contrast.

CT pitch = table travel per rotation / collimator width; pitch >1 lowers dose but may increase noise or artifacts.

MRI: TR controls T1 weighting, TE controls T2 weighting; short TE/short TR = T1, long TE/long TR = T2.

MRI artifacts: chemical shift (fat-water at frequency-encode), susceptibility blooming, and motion ghosting along phase-encode direction.

Ultrasound Doppler: aliasing occurs when velocity exceeds Nyquist limit = PRF/2; increase PRF, use continuous wave, or shift baseline.

PET: FDG SUV is normalized to body weight and injected dose; resolution is limited by positron range and detector size.

Communication of critical results should be timely, documented, and closed-loop with the ordering provider per ACR guidelines.

Radiation risk: effective dose of a chest radiograph ~0.1 mSv, chest CT ~7 mSv, and background annual exposure ~3 mSv in the US.

Patient safety: time-out before procedures verifies patient identity, site, consent, and team roles.

Contrast extravasation: most are managed conservatively with elevation and ice; large volumes or blistering warrant surgical consultation.

Malpractice elements: duty, breach, causation, and damages must all be established for negligence.

Reimbursement: Medicare uses RVU = (work + practice expense + malpractice) x GPCI; documentation drives coding accuracy.

Informed refusal should be documented when a competent patient declines a recommended study after risks are explained.

ACR accreditation and MIPS quality reporting tie imaging practice to measurable safety and outcome metrics.