Arab Board Radiology Part 1 cheat sheet

Photoelectric effect probability ∝ Z³/E³; Compton scattering probability ∝ electron density and decreases with increasing E.

Half-value layer (HVL) = 0.693/μ; μ is linear attenuation coefficient. HVL increases with increasing beam energy.

CT number (Hounsfield unit) = 1000 × (μ_tissue − μ_water)/μ_water; water = 0 HU, air = −1000 HU.

Nyquist theorem: sampling frequency must be ≥ 2× highest spatial frequency to avoid aliasing.

MRI Larmor frequency: ω = γB; γ for ¹H = 42.58 MHz/T. Precession frequency is proportional to field strength.

Doppler shift Δf = (2 f₀ v cosθ)/c; angle of insonation of 0° maximizes detected shift.

In digital radiography, detective quantum efficiency (DQE) = (SNR_out/SNR_in)² and is always ≤ 1.

Mammographic grid ratio typically 4:1 to 5:1; higher grid ratio increases contrast but also patient dose.

Ultrasound axial resolution ≈ spatial pulse length/2 = (n × λ)/2, where n is number of cycles per pulse.

Linear energy transfer (LET) is higher for alpha particles and neutrons than for X-rays; high-LET radiation has higher relative biological effectiveness (RBE).

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

Law of Bergonié and Tribondeau: radiosensitivity is greatest in cells that are undifferentiated, rapidly dividing, and have a long mitotic future.

Effective dose (E) = Σ w_T × H_T; tissue weighting factors sum to 1.0. Gonads and red marrow have high w_T.

Annual occupational effective dose limit (ICRP): 20 mSv averaged over 5 years, with no single year exceeding 50 mSv.

ALARA: dose optimization using time, distance, and shielding; intensity follows inverse-square law (I ∝ 1/d²).

Pregnancy: lead apron 0.5 mm Pb reduces fetal dose; typical fetal dose from a chest CT is <10 mGy.

Stochastic effects (cancer, hereditary) have no threshold; deterministic effects (skin erythema, cataract) have thresholds (e.g., ~0.5 Gy for cataracts).

Iodinated contrast osmolality: high-osmolar (~1500–2000 mOsm/kg), low-osmolar (~600–800), iso-osmolar (~290). Lower osmolality reduces osmotic nephrosis and pain.

Gadolinium-based contrast agents: macrocyclic agents are more stable than linear agents; avoid in severe renal impairment (eGFR <30) due to nephrogenic systemic fibrosis risk.

Anaphylactoid reactions are non-IgE mediated; treat with epinephrine IM 0.3–0.5 mg (1:1000) for severe reactions.

Metformin: current guidelines do not require withholding if eGFR ≥30; recheck renal function if contrast given in acute kidney injury setting.

Contrast-induced acute kidney injury (CI-AKI) risk is increased with eGFR <30, diabetes, dehydration, and high contrast volume.

Barium sulfate is contraindicated if bowel perforation or obstruction is suspected; use water-soluble iodinated contrast instead.

Ultrasound microbubble contrast agents are blood-pool agents; contraindicated in right-to-left shunt and severe pulmonary hypertension.

Liver segments are defined by the Couinaud classification based on portal and hepatic vein anatomy; segment I is the caudate lobe.

The nephron: glomerulus filters plasma; proximal tubule reabsorbs ~65% of filtered sodium and water.

Frank-Starling law: stroke volume increases with increased end-diastolic volume up to a physiologic limit.

Types of cell death: necrosis (passive, inflammatory) vs apoptosis (programmed, non-inflammatory).

Tumor suppressor genes (e.g., TP53, RB1) require loss of both alleles (two-hit hypothesis) for carcinogenesis.

Oxygen-hemoglobin dissociation curve shifts right with increased 2,3-DPG, acidosis, and hyperthermia (decreased affinity).

Cranial nerve VII (facial) exits the stylomastoid foramen; parotid tumors can cause facial nerve palsy.

The thoracic duct drains into the left subclavian vein at the venous angle; chylothorax is typically left-sided.

Silhouette sign: loss of border between two structures of same density indicates they are in contact (e.g., right middle lobe opacity obscures right heart border).

Air bronchogram indicates alveolar filling (consolidation) with patent bronchi; its absence suggests bronchial obstruction or pleural disease.

Cardiothoracic ratio >0.5 on PA erect chest radiograph suggests cardiomegaly; AP view exaggerates heart size.

CT pulmonary angiography (CTPA) is first-line for suspected pulmonary embolism in hemodynamically stable patients; high negative predictive value.

Honeycombing on HRCT is a hallmark of usual interstitial pneumonia (UIP); predominantly subpleural and basal.

Solitary pulmonary nodule: >8 mm warrants PET/CT or tissue sampling; ≤8 mm follow Fleischner Society guidelines.

Aortic dissection: Stanford type A involves ascending aorta (surgical emergency); type B does not.

Pneumothorax on supine radiograph: deep sulcus sign (lucency at costophrenic angle).

Acute appendicitis on CT: enlarged appendix >6 mm, periappendiceal fat stranding, and wall enhancement.

Small bowel obstruction: dilated loops >3 cm, air-fluid levels, and transition point on CT.

Liver hemangioma: peripheral nodular enhancement with centripetal fill-in on multiphasic CT/MRI.

Hepatocellular carcinoma (HCC): arterial hyperenhancement with venous washout in a cirrhotic liver.

Pancreatic adenocarcinoma: hypoenhancing mass with ductal dilatation (double duct sign).

Crohn disease: transmural inflammation, skip lesions, and fistulae; ulcerative colitis: continuous mucosal involvement from rectum.

Bowel wall thickening >3 mm on CT is abnormal; pneumatosis intestinalis indicates intramural gas and possible ischemia.

Barium enema ‘apple-core’ lesion suggests colorectal carcinoma.

Renal cell carcinoma: enhancing mass; clear cell subtype typically hypervascular and may contain fat (rarely).

Transitional cell carcinoma: filling defect on excretory phase CT urography; common in bladder and renal pelvis.

Ovarian torsion: enlarged ovary with peripheral follicles and decreased Doppler flow; surgical emergency.

Endometrial carcinoma: thickened endometrium (>4 mm in postmenopausal bleeding) on ultrasound; MRI for staging.

Prostate cancer: PI-RADS score on multiparametric MRI; score ≥3 warrants biopsy.

Renal stone: non-contrast CT is gold standard; stone density >1000 HU predicts poor shockwave lithotripsy response.

Hydrosalpinx: tubular fluid-filled structure with folded appearance; associated with infertility.

Testicular torsion: absent Doppler flow; radionuclide scan shows cold spot; immediate surgery required.

Osteosarcoma: aggressive bone destruction, periosteal reaction (sunburst, Codman triangle), and soft tissue mass; metaphysis of long bones.

Ewing sarcoma: permeative destruction, onion-skin periosteal reaction, diaphysis of long bones in children.

Giant cell tumor: eccentric, lytic, epiphyseal lesion in skeletally mature patients; soap-bubble appearance.

Rheumatoid arthritis: marginal erosions, periarticular osteopenia, symmetric polyarthritis; MCP and PIP joints.

Osteoarthritis: joint space narrowing, subchondral sclerosis, osteophytes, subchondral cysts; weight-bearing joints.

MRI bone marrow edema: low T1 and high T2/STIR signal; nonspecific but seen in trauma, infection, tumor.

Meniscal tear: linear high signal on T2/PD within meniscus; grade 3 indicates tear.

Bone scan: increased uptake in fractures, infection, metastases; highly sensitive but nonspecific.

Acute ischemic stroke: CT hyperdense MCA sign; MRI DWI is most sensitive early.

Hemorrhage: hyperdense on non-contrast CT; MRI signal varies with age of blood (T1/T2).

Multiple sclerosis: periventricular ovoid lesions (Dawson fingers) oriented perpendicular to ventricles; T2/FLAIR hyperintense.

Glioblastoma: ring-enhancing lesion with central necrosis and surrounding edema; crosses corpus callosum (butterfly).

Meningioma: extra-axial, dural tail, homogeneous enhancement, may cause hyperostosis.

Cerebral aneurysm: CTA or MRA for detection; subarachnoid hemorrhage on CT in basal cisterns.

Hydrocephalus: enlarged ventricles with transependymal edema (acute) or sulcal effacement.

Pituitary macroadenoma: >10 mm, may cause bitemporal hemianopia and cavernous sinus invasion.

Thyroid nodule: ultrasound features of malignancy include microcalcifications, hypoechogenicity, irregular margins, and taller-than-wide shape.

Squamous cell carcinoma of head and neck: most common malignancy; nodal metastasis often cystic in HPV-related disease.

Parotid tumors: pleomorphic adenoma (benign, well-defined) vs mucoepidermoid carcinoma (malignant).

Juvenile nasopharyngeal angiofibroma: adolescent male with epistaxis; highly vascular, enhances avidly.

Orbital cellulitis: preseptal vs postseptal; postseptal requires urgent antibiotics and imaging.

Laryngeal cancer: glottic tumors present early with hoarseness; supraglottic tumors present late with nodal spread.

Skull base tumors: chordoma (clivus, T2 hyperintense) vs chondrosarcoma (petroclival, calcified).

Facial nerve schwannoma: enhancing mass in facial nerve canal; may cause facial palsy.

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

Mammographic findings of malignancy: spiculated mass, clustered pleomorphic microcalcifications, architectural distortion.

Ductal carcinoma in situ (DCIS): linear or branching calcifications; often no mass.

Ultrasound: simple cyst (anechoic, posterior enhancement) requires no further workup.

MRI breast: high sensitivity, used for screening high-risk women (BRCA mutation) and evaluating extent of disease.

Stereotactic biopsy: for calcifications; ultrasound-guided biopsy for masses.

Male breast cancer: subareolar mass; associated with BRCA2 and Klinefelter syndrome.

Density of breast tissue on mammography affects sensitivity; dense breasts reduce sensitivity and may warrant supplemental ultrasound.

Pyloric stenosis: ultrasound shows thickened pyloric muscle (>3 mm) and elongated canal (>15 mm); target sign.

Intussusception: ultrasound shows target sign or pseudokidney; contrast enema is diagnostic and therapeutic.

Hyaline membrane disease: premature infant, ground-glass opacities, air bronchograms, low lung volumes.

Meconium aspiration: term infant, patchy infiltrates, hyperinflation, air leaks.

Neuroblastoma: abdominal mass, calcifications, crosses midline; elevated catecholamines.

Wilms tumor: renal mass, may have hematuria; associated with WAGR syndrome.

Developmental dysplasia of hip: ultrasound for screening; alpha angle <60° abnormal.

Pediatric radiation sensitivity: higher risk per unit dose; use ALARA, reduce kVp and mAs, shield gonads.

PET/CT with ¹⁸F-FDG: increased uptake in malignancy, infection, inflammation; brain and heart normally show high uptake.

Bone scan (⁹⁹mTc-MDP): sensitive for bone metastases; superscan shows diffuse uptake with absent renal uptake.

V/Q scan: high probability when multiple segmental mismatched defects; used when CTPA contraindicated.

Thyroid scan: ⁹⁹mTc-pertechnetate or ¹²³I; cold nodule requires ultrasound and possible biopsy.

Interventional radiology: Seldinger technique for vascular access; common femoral artery most common site.

TACE (transarterial chemoembolization) for HCC; Y-90 radioembolization is another option.

Nephrostomy: percutaneous drainage of obstructed kidney; contraindicated in uncorrected coagulopathy.

Radiation safety in IR: use pulsed fluoroscopy, last-image hold, and collimation to reduce dose.