Iraqi Board Radiology cheat sheet

Hounsfield unit: HU = 1000 × (μ_tissue − μ_water) / μ_water; water = 0 HU, air ≈ −1000 HU, dense bone ≈ +1000 to +3000 HU.

X-ray beam attenuation: I = I₀e^(−μx); half-value layer (HVL) = 0.693/μ; HVL increases with increasing kVp (beam hardening).

Photoelectric effect dominates at low kVp and high atomic number (∝ Z³/E³); Compton scatter dominates at higher energies and in soft tissue (∝ electron density).

ALARA: occupational dose limit 20 mSv/year averaged over 5 years (max 50 mSv in any single year); public limit 1 mSv/year; lens of eye 20 mSv/year (ICRP).

CTDIvol = CTDIw / pitch; pitch = table feed per rotation / beam collimation; DLP = CTDIvol × scan length (mGy·cm).

MRI safety zones: Zone I (public), II (screened), III (restricted, supervised), IV (magnet room); 5-gauss line marks the boundary of the static field hazard.

Gadolinium: macrocyclic agents are more stable than linear agents; avoid in eGFR <30 mL/min/1.73 m² due to nephrogenic systemic fibrosis risk.

Iodinated contrast: use low-osmolar or iso-osmolar agents; premedication for prior reaction is not required for mild reactions; metformin should be withheld 48 h if eGFR <30.

Radionuclide dose: effective dose = Σ (organ absorbed dose × tissue weighting factor); typical effective doses: chest radiograph ~0.02 mSv, CT abdomen ~10 mSv, V/Q scan ~2.5 mSv.

Bronchopulmonary segments: right lung has 10 segments (3 upper, 2 middle, 5 lower); left lung has 8–10 (upper and lower lobes separated by oblique fissure).

Liver segments by Couinaud: eight segments based on portal vein branches and hepatic veins; segment I is the caudate lobe, segments II–IV left lobe, V–VIII right lobe.

Kidney: right kidney lies lower than left due to liver; renal hilum transmits renal artery, vein, and pelvis; nephron = glomerulus + tubules; GFR ~125 mL/min.

Circle of Willis: anterior communicating, paired anterior cerebral, internal carotid, posterior communicating, and posterior cerebral arteries; common site of berry aneurysms.

Embryology: neural tube closes by day 28; hindgut (distal transverse colon to anal canal) supplied by inferior mesenteric artery; urogenital sinus forms bladder and urethra.

Thyroid gland: butterfly-shaped, anterior to trachea at C5–T1; parathyroid glands usually four, posterior to thyroid; recurrent laryngeal nerve in tracheoesophageal groove.

Vertebral column: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused), 4 coccygeal; conus medullaris ends at L1–L2 in adults, L3 in neonates.

Physiology relevant to imaging: V/Q ratio ~0.8 overall; pulmonary arterial pressure 25/10 mmHg; portal vein supplies 75% of liver blood flow but 50% of oxygen.

Hounsfield unit ranges: simple cyst 0–20 HU, fat −100 to −50 HU, acute hemorrhage 50–90 HU, calcification >100 HU, enhancing solid lesion >20 HU increase post-contrast.

Tumor markers: AFP elevated in hepatocellular carcinoma and yolk sac tumor; CA-125 in ovarian cancer; PSA in prostate cancer; CEA in colorectal cancer.

Inflammation: acute inflammation shows increased water content (edema) → low T1, high T2 on MRI; fibrosis shows low T1 and low T2 signal.

Cell death: coagulative necrosis (ischemia) preserves tissue architecture; liquefactive necrosis (brain infarct, abscess) shows cystic change with rim enhancement.

Neoplasia: benign tumors are well-circumscribed, slow-growing, no metastasis; malignant tumors show invasion, anaplasia, and metastasis; grading vs staging.

Infection: abscess shows central restricted diffusion (high DWI, low ADC) with rim enhancement; granuloma may calcify (e.g., TB, histoplasmosis).

Genetic syndromes: NF1 (neurofibromas, optic glioma), NF2 (bilateral vestibular schwannomas), tuberous sclerosis (cortical tubers, angiomyolipomas), von Hippel-Lindau (hemangioblastomas, renal cell carcinoma).

Silhouette sign: loss of border between a lesion and adjacent structure of same density indicates they are in contact (e.g., right middle lobe opacity obscures right heart border).

Air bronchogram: air-filled bronchi outlined by consolidated lung; indicates alveolar filling process (pneumonia, edema, hemorrhage) rather than atelectasis.

Pneumothorax: visible visceral pleural line with no lung markings peripheral; tension pneumothorax shows mediastinal shift away from affected side and flattened diaphragm.

Pulmonary embolism: CTPA is first-line; findings include filling defect in pulmonary artery, wedge-shaped peripheral opacity (Hampton hump), and enlarged pulmonary artery.

Cardiomegaly: cardiothoracic ratio >0.5 on PA erect chest radiograph; left atrial enlargement causes double right heart border and splaying of carina.

Aortic dissection: Stanford type A involves ascending aorta (surgical emergency); type B involves descending aorta only; CT shows intimal flap and false lumen.

Pulmonary nodule: ≤3 cm; >3 cm is a mass; Fleischner criteria guide follow-up based on size and risk; ground-glass nodules require longer follow-up.

Sarcoidosis: bilateral hilar lymphadenopathy (potato nodes) and perilymphatic nodules; stage I = lymphadenopathy only, stage II = lymphadenopathy + parenchymal disease.

Bowel obstruction: dilated loops >3 cm (small bowel) or >6 cm (colon); air-fluid levels on erect film; transition point on CT; closed-loop obstruction shows U-shaped loop.

Appendicitis: CT criteria include appendix diameter >6 mm, periappendiceal fat stranding, wall enhancement, and appendicolith; ultrasound shows non-compressible blind-ending tubular structure.

Liver lesions: hemangioma shows peripheral nodular enhancement with centripetal fill-in; hepatocellular carcinoma shows arterial hyperenhancement and portal venous washout.

Pancreatitis: CT findings include pancreatic enlargement, peripancreatic fat stranding, fluid collections; necrosis is non-enhancing; pseudocyst is encapsulated fluid collection after 4 weeks.

Cholelithiasis: gallstones are echogenic with posterior acoustic shadowing on ultrasound; acute cholecystitis shows wall thickening >3 mm, pericholecystic fluid, and sonographic Murphy sign.

Renal colic: non-contrast CT is gold standard; stone appears as hyperdense focus in ureter; hydronephrosis and perinephric stranding may be present.

Diverticulitis: CT shows colonic diverticula, wall thickening, pericolic fat stranding; complications include abscess, fistula, and perforation.

Renal cell carcinoma: clear cell subtype is most common; CT shows enhancing mass with heterogeneous enhancement; staging by TNM; venous invasion common.

Prostate cancer: PI-RADS v2.1 uses multiparametric MRI (T2, DWI, DCE); score 1–5; score ≥3 indicates clinically significant cancer; targeted biopsy for score 4–5.

Ovarian torsion: enlarged ovary with peripheral follicles, decreased Doppler flow; detorsion is treatment; ultrasound is first-line.

Obstetric ultrasound: gestational sac visible at 4–5 weeks (transvaginal); yolk sac at 5–6 weeks; fetal pole with cardiac activity at 6–7 weeks; crown-rump length is most accurate for dating in first trimester.

Ectopic pregnancy: empty uterus with adnexal mass and free fluid; discriminatory hCG level ~1500–2000 mIU/mL for transvaginal ultrasound; methotrexate if unruptured and stable.

Placenta previa: placenta covering internal cervical os; diagnosed by transvaginal ultrasound; major risk is hemorrhage; cesarean delivery required.

Endometrial carcinoma: most common gynecologic malignancy; ultrasound shows thickened endometrium (>4 mm in postmenopausal bleeding); MRI for staging; risk factors include obesity, diabetes, tamoxifen.

Fracture description: location, pattern (transverse, oblique, spiral, comminuted), displacement, angulation, rotation, shortening; open vs closed; neurovascular status.

Osteomyelitis: earliest radiographic sign is soft tissue swelling; bone changes (periosteal reaction, lucency) appear after 10–14 days; MRI is most sensitive (marrow edema, enhancement).

Bone tumors: osteosarcoma (sunburst periosteal reaction, Codman triangle), Ewing sarcoma (onion-skin periosteal reaction, diaphysis), giant cell tumor (epiphyseal, soap-bubble).

Arthritis: osteoarthritis shows joint space narrowing, osteophytes, subchondral sclerosis, cysts; rheumatoid arthritis shows marginal erosions, symmetric involvement, osteoporosis.

Shoulder dislocation: anterior (most common) shows humeral head inferior and medial to glenoid; posterior shows lightbulb sign and trough line; axillary view needed.

MRI of knee: meniscal tear shows linear high signal reaching articular surface; ACL tear shows discontinuity and bone contusion; grading of cartilage lesions by ICRS.

Bone density: DXA T-score ≤ −2.5 defines osteoporosis; −1.0 to −2.5 osteopenia; FRAX estimates 10-year fracture risk.

Stroke: non-contrast CT excludes hemorrhage; early signs include loss of insular ribbon, dense MCA sign, and sulcal effacement; CT perfusion identifies penumbra for thrombectomy.

Brain herniation: subfalcine (cingulate gyrus under falx), transtentorial (uncus), tonsillar (foramen magnum); CT shows midline shift, effaced cisterns.

Multiple sclerosis: periventricular ovoid lesions perpendicular to ventricles (Dawson fingers), juxtacortical, infratentorial, spinal cord; McDonald criteria for diagnosis.

Spinal cord compression: MRI is modality of choice; causes include metastasis, disc herniation, abscess, hematoma; urgent decompression if neurological deficit.

Head and neck: thyroid nodule ultrasound features (hypoechoic, irregular margins, microcalcifications, taller-than-wide) increase malignancy risk; FNA for TI-RADS ≥4.

Brain tumors: glioblastoma (butterfly pattern, necrosis, ring enhancement), meningioma (extra-axial, dural tail, homogeneous enhancement), metastasis (multiple, gray-white junction).

Hydrocephalus: communicating (enlarged ventricles with normal fourth ventricle) vs non-communicating (obstructive); transependymal edema on MRI.

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

Mammography findings: mass, calcification, architectural distortion, asymmetry; suspicious calcifications are fine, linear, branching, or clustered pleomorphic.

Breast density: categories A–D (almost entirely fatty to extremely dense); dense tissue increases cancer risk and reduces mammographic sensitivity.

Ultrasound: useful for differentiating cystic vs solid; simple cyst is anechoic with posterior enhancement; solid mass requires biopsy if suspicious.

MRI breast: high sensitivity, lower specificity; indications include screening high-risk (BRCA), evaluating extent of disease, occult primary with axillary metastasis.

Male breast cancer: presents as subareolar mass; risk factors include BRCA2, Klinefelter syndrome, radiation, estrogen therapy.

Pediatric chest: thymus is prominent in infants (sail sign); respiratory distress syndrome shows ground-glass opacities and air bronchograms; transient tachypnea shows perihilar streaking.

Intussusception: ultrasound shows target sign or pseudokidney; treatment is air or contrast enema; contraindicated if perforation or peritonitis.

Pyloric stenosis: ultrasound shows thickened pyloric muscle (>3 mm) and elongated canal (>15 mm); presentation at 3–6 weeks with projectile vomiting.

Pediatric fractures: Salter-Harris classification (I–V) for physeal fractures; supracondylar fracture is most common elbow fracture; check for fat pad sign.

Neuroblastoma: arises from adrenal medulla or sympathetic chain; calcification common; urinary VMA/HVA elevated; staging by INRG.

Wilms tumor: most common renal tumor in children; presents as abdominal mass; associated with WAGR, Beckwith-Wiedemann; nephrectomy and chemotherapy.

Radiation protection in children: use lower kVp and mAs, limit scan range, use bismuth shielding when appropriate; ALARA especially important due to higher radiosensitivity.

Aortic aneurysm: abdominal aortic diameter >3 cm; repair indicated if >5.5 cm or rapid growth; endovascular aneurysm repair (EVAR) vs open surgery.

Pulmonary embolism: CTPA first-line; massive PE with hemodynamic instability may require thrombolysis or embolectomy; catheter-directed therapy for submassive PE.

Interventional radiology: Seldinger technique for vascular access; embolization for hemorrhage; angioplasty and stenting for stenosis; TIPS for portal hypertension.

Acute mesenteric ischemia: CT angiography shows arterial occlusion, bowel wall thickening, pneumatosis; early diagnosis critical; surgical embolectomy or thrombolysis.

Trauma: FAST exam for free fluid; CT is gold standard for hemodynamically stable patients; contrast extravasation indicates active bleeding; splenic injury grading I–V.

Stroke thrombectomy: indicated for large vessel occlusion within 6–24 hours with salvageable penumbra; CT perfusion or MRI diffusion-perfusion mismatch.

IVC filter: indicated when anticoagulation contraindicated or failed; retrievable filters should be removed when no longer needed.

FDG-PET: glucose analog uptake reflects metabolic activity; normal uptake in brain, myocardium, kidneys, bladder; high uptake in malignancy, infection, inflammation; SUVmax used for quantification.

Bone scan (Tc-99m MDP): sensitive for osteoblastic metastasis, fractures, infection; three-phase for complex regional pain syndrome and osteomyelitis.

Thyroid scan: I-123 or Tc-99m pertechnetate; hot nodule usually benign, cold nodule requires ultrasound/FNA; uptake increased in Graves disease, decreased in thyroiditis.

V/Q scan: high probability when ≥2 segmental mismatched perfusion defects with normal ventilation; used when CTPA contraindicated (renal failure, allergy).

Contrast media: iodinated contrast can cause contrast-induced nephropathy (CIN); risk factors include eGFR <30, diabetes, dehydration; use lowest volume, hydration.

Gadolinium: nephrogenic systemic fibrosis risk with linear agents in severe renal impairment; group II macrocyclic agents preferred; check eGFR before administration.

Anaphylactoid reactions: treat with epinephrine, oxygen, IV fluids, antihistamines; premedication with corticosteroids and antihistamines for prior moderate reaction.