Iran Specialty Board Radiology cheat sheet

Attenuation follows I = I0·e^(−μx); the half-value layer equals ln2/μ = 0.693/μ, and HVL increases with beam hardening (higher kVp, added filtration).

Effective dose is the tissue-weighted sum of equivalent doses: E = Σ wT·HT, where HT = Σ wR·DT,R; wR = 1 for X-rays and gamma rays, 20 for alpha particles.

Typical effective doses: chest radiograph ≈ 0.02 mSv, mammography ≈ 0.4 mSv, head CT ≈ 2 mSv, abdomen-pelvis CT ≈ 10 mSv; natural background ≈ 2.4–3 mSv/year.

ALARA relies on the three cardinal principles: time, distance (inverse-square law), and shielding; lead half-value thickness at 100 kVp is approximately 0.25 mm Pb.

CTDIvol reflects dose per unit volume in a single slice; DLP = CTDIvol × scan length, and effective dose ≈ DLP × k (k ≈ 0.015 mSv/mGy·cm for abdomen).

The photoelectric effect dominates at low photon energies and high atomic number (Z³ dependence), producing high subject contrast; Compton scatter dominates at diagnostic energies in soft tissue and degrades contrast.

MRI safety zones: zone I (public), II (reception), III (screened), IV (magnet room); the static field is always on, and projectile risk persists even when the scanner is powered down.

Gadolinium-based contrast agents carry nephrogenic systemic fibrosis risk (highest with linear agents in severe renal impairment, eGFR < 30) and can cause acute allergic reactions; macrocyclic agents are preferred in renal disease.

Occupational dose limits: 20 mSv/year averaged over 5 years (100 mSv per 5 years) for radiation workers, 150 mSv/year for the lens of the eye, and 500 mSv/year for extremities; public limit is 1 mSv/year.

Silhouette sign: loss of a border between an opacity and an adjacent structure implies they are in contact; right heart border effacement localizes opacity to the right middle lobe, left heart border to the lingula.

Air bronchograms indicate alveolar (airspace) filling; their absence suggests atelectasis or a mass, and CT angiogram showing air bronchograms distally supports a non-obstructive process.

Honeycombing (subpleural cysts 3–10 mm, basal and peripheral) is the hallmark of usual interstitial pneumonia; ground-glass opacity alone reflects active inflammation or early fibrosis.

The tree-in-bud pattern reflects centrilobular bronchiolar impaction (infection, aspiration, tumor spread); the reversed halo sign suggests organizing pneumonia or fungal infection.

Mediastinal compartments: anterior (thymoma, teratoma, thyroid, lymphoma), middle (lymphadenopathy, bronchogenic cyst), posterior (neurogenic tumors, esophageal lesions).

The Fleischner Society criteria for nodule follow-up depend on size and risk: solid nodules ≥8 mm require PET-CT or biopsy; subsolid nodules are followed longer because growth is slow.

Pulmonary embolism on CTPA appears as an intraluminal filling defect; acute emboli distend the vessel and form acute angles with the wall, whereas chronic emboli form obtuse angles with webs and calcification.

Sarcoidosis staging (Scadding): stage I bilateral hilar lymphadenopathy, stage II adenopathy plus parenchymal disease, stage III parenchymal disease alone, stage IV fibrosis.

The crazy-paving pattern (ground-glass with superimposed septal thickening) is seen in pulmonary alveolar proteinosis, ARDS, hemorrhage, and Pneumocystis pneumonia.

Coronary CTA: a calcium score >400 Agatston reduces specificity; fractional flow reserve CT (FFR-CT) estimates hemodynamic significance, and a negative CTA has a very high negative predictive value for significant stenosis.

Cardiac MRI is the reference standard for ventricular volumes and ejection fraction; late gadolinium enhancement shows scar (subendocardial ischemic pattern vs mid-wall in non-ischemic cardiomyopathy).

Aortic dissection: Stanford type A involves the ascending aorta and requires urgent surgery; type B spares the ascending aorta and is usually managed medically unless complicated.

Acute aortic syndromes include dissection, intramural hematoma (crescentic high attenuation without a flap), and penetrating atherosclerotic ulcer; intramural hematoma may lack a visible intimal flap.

Pulmonary hypertension on CT: main pulmonary artery diameter >29 mm, PA-to-aorta ratio >1, and right ventricular enlargement or septal bowing indicate elevated right heart pressure.

Transthyretin cardiac amyloidosis shows diffuse subendocardial late gadolinium enhancement, abnormal native T1, and an elevated extracellular volume; bone scintigraphy with grade 2–3 uptake supports the diagnosis.

Pericardial effusion is first detected on chest radiograph when >200 mL (water-bottle heart); tamponade physiology on echo includes right ventricular diastolic collapse and inferior vena cava plethora.

Myocarditis on MRI follows the Lake Louise criteria: edema (T2), non-ischemic injury (late gadolinium enhancement), and abnormal T1/ECV mapping.

LI-RADS categorizes observations in at-risk livers: LR-5 requires arterial phase hyperenhancement plus washout and/or capsule or threshold growth, with a very high positive predictive value for hepatocellular carcinoma.

Hepatic hemangioma: peripheral nodular enhancement with centripetal fill-in and high T2 signal; flash filling can occur in small lesions (<1.5 cm).

Focal nodular hyperplasia shows a central scar with delayed enhancement and intense arterial enhancement; in contrast, hepatic adenoma lacks a scar and may hemorrhage.

The double duct sign (dilated common bile duct and pancreatic duct) suggests a pancreatic head or ampullary mass; Courvoisier's law states a palpable gallbladder with jaundice is unlikely due to stones.

Acute cholecystitis: gallbladder distension, wall thickening >3 mm, pericholecystic fluid, and a sonographic Murphy sign; a positive HIDA scan shows non-visualization of the gallbladder.

Bowel obstruction: small bowel >3 cm and large bowel >6 cm indicate dilatation; a transition point with collapsed distal bowel on CT confirms mechanical obstruction.

Crohn disease favors the terminal ileum with transmural inflammation, skip lesions, fistulae, and creeping fat; ulcerative colitis is continuous, starts at the rectum, and is mucosal.

Portal hypertension signs: portal vein diameter >13 mm, splenomegaly, varices, ascites, and recanalization of the paraumbilical vein.

Renal cell carcinoma subtypes: clear cell (hypervascular, early enhancement), papillary (hypovascular, homogeneous), chromophobe (spoke-wheel enhancement); clear cell is the most common.

Bosniak classification: category I simple cyst (no follow-up), II benign (no follow-up), IIF (follow-up), III (surgical excision), IV (clearly malignant, nephrectomy).

Angiomyolipoma contains macroscopic fat (CT attenuation < −20 HU) and is associated with tuberous sclerosis; hemorrhage risk increases when >4 cm.

Testicular torsion shows absent or reduced Doppler flow with a twisted spermatic cord; epididymitis shows increased flow with an enlarged epididymis.

Prostate cancer is staged by MRI with PI-RADS: score 1–2 clinically significant cancer unlikely, 3 equivocal, 4–5 highly likely; extraprostatic extension is suggested by capsular bulge or neurovascular bundle involvement.

Ovarian mature cystic teratoma shows fat, calcification, and Rokitansky nodule; endometrioma shows T1-hyperintense shading on T2 (T2 shading sign).

Hydronephrosis grading: mild (pelvis only), moderate (calyceal blunting), severe (cortical thinning); a dilated ureter suggests obstruction below the ureteropelvic junction.

Adrenal adenoma: homogeneous, low attenuation (<10 HU on unenhanced CT), and rapid washout (>60% absolute or >40% relative at 15 minutes); pheochromocytoma is T2-hyperintense (light-bulb sign).

Osteoarthritis: asymmetric joint space narrowing, subchondral sclerosis, osteophytes, and subchondral cysts; rheumatoid arthritis shows symmetric narrowing, marginal erosions, and periarticular osteopenia.

Gout: juxta-articular erosions with overhanging edges, preserved joint space, and tophi; CPPD shows chondrocalcinosis (menisci) and degenerative changes with osteophyte formation.

Bone tumor location aids diagnosis: epiphysis (chondroblastoma, giant cell tumor), metaphysis (osteosarcoma, osteochondroma), diaphysis (Ewing sarcoma, lymphoma).

Osteosarcoma shows aggressive periosteal reaction (Codman triangle, sunburst, hair-on-end) and matrix mineralization; Ewing sarcoma shows a permeative pattern with onion-skin periosteal reaction.

Avascular necrosis of the femoral head: early MRI shows a low-signal band on T1 (double-line sign on T2); later stages show subchondral fracture (crescent sign) and collapse.

Anterior cruciate ligament tear: discontinuity with increased T2 signal, bone contusion at the lateral femoral condyle and posterior tibial plateau, and a positive pivot-shift test.

Rotator cuff tear: full-thickness tear shows a fluid-filled gap on T2/PD fat-saturated images; supraspinatus is the most commonly torn tendon, at the greater tuberosity footprint.

Bone mineral density: T-score ≤ −2.5 defines osteoporosis; a T-score between −1.0 and −2.5 defines osteopenia, and a fragility fracture establishes osteoporosis regardless of T-score.

Stroke imaging: non-contrast CT for hemorrhage exclusion; CT angiography for large-vessel occlusion; CT perfusion for penumbra; MRI with diffusion-weighted imaging is most sensitive for acute infarction within minutes.

The ASPECTS score quantifies early ischemic change on non-contrast CT (10-point scale); a score ≥6 supports mechanical thrombectomy in anterior circulation occlusion.

Intracerebral hemorrhage: hyperdense on non-contrast CT, with a spot sign on CTA predicting expansion; hypertensive hemorrhage favors the putamen, thalamus, pons, and cerebellum.

Multiple sclerosis: periventricular, juxtacortical, infratentorial, and spinal cord lesions; dissemination in space and time (McDonald criteria); Dawson's fingers are typical on sagittal FLAIR.

Glioblastoma: ring-enhancing, necrotic, with surrounding edema and restricted diffusion in solid components; metastasis favors the gray-white junction and is often multiple.

Spinal cord compression: epidural mass, disc herniation, or fracture; cauda equina syndrome requires urgent decompression, with saddle anesthesia and bladder dysfunction.

Head and neck: squamous cell carcinoma is the most common malignancy; nodal necrosis and extracapsular spread worsen prognosis, and HPV-positive oropharyngeal cancer has better outcomes.

Subdural hematoma is crescentic and crosses sutures but not the midline; epidural hematoma is biconvex, limited by sutures, and typically associated with a middle meningeal artery tear.

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

Mammographic signs of malignancy: spiculated mass, clustered pleomorphic microcalcifications, architectural distortion, and asymmetric density; the positive predictive value rises with these features.

Screening benefit: mammography reduces breast cancer mortality, with the greatest benefit in women aged 50–69; screening intervals of 1–2 years are commonly used.

Ultrasound features of malignancy: irregular shape, non-parallel orientation, non-circumscribed margins, hypoechogenicity, and posterior acoustic shadowing.

MRI is the most sensitive modality; indications include high-risk screening (BRCA carriers), problem-solving, and evaluation of implant integrity or extent of disease.

Dense breast tissue reduces mammographic sensitivity and is an independent risk factor; supplemental ultrasound or MRI may be considered.

Stereotactic biopsy is used for microcalcifications; ultrasound-guided biopsy is preferred for sonographically visible masses; a clip is placed for localization.

Male breast cancer: subareolar mass, often invasive ductal carcinoma; gynecomastia is flame-shaped and centered on the nipple, whereas cancer is eccentric and irregular.

Pyloric stenosis: hypertrophied pyloric muscle thickness >3 mm and canal length >15 mm on ultrasound, typically presenting at 3–6 weeks with projectile non-bilious vomiting.

Intussusception: target sign on ultrasound, most commonly ileocolic, in children aged 6 months to 2 years; air or contrast enema is both diagnostic and therapeutic.

Midgut volvulus: bilious vomiting in a neonate with a corkscrew sign on upper GI series; a normal upper GI series does not exclude malrotation, but a low cecal position supports it.

Necrotizing enterocolitis: pneumatosis intestinalis, portal venous gas, and pneumoperitoneum in premature infants; perforation requires surgery.

Developmental dysplasia of the hip: ultrasound is used before ossification (before 4–6 months), with alpha angle <60 degrees and femoral head coverage <50% suggesting dysplasia.

Pediatric bone tumors: osteosarcoma peaks in the second decade at the metaphysis of the distal femur; Ewing sarcoma favors the diaphysis and has a permeative pattern.

Bronchiolitis: hyperinflation, peribronchial thickening, and patchy atelectasis; respiratory syncytial virus is the most common cause in infants.

Radiation protection in children: use lower kVp and mAs, limit the field of view, and prefer ultrasound or MRI; children have higher radiosensitivity and longer life expectancy.

Transjugular intrahepatic portosystemic shunt (TIPS) reduces portal pressure in refractory variceal bleeding and ascites; complications include hepatic encephalopathy and shunt stenosis.

Uterine artery embolization treats symptomatic fibroids; fertility may be preserved, but pregnancy outcomes are less certain than after myomectomy.

Percutaneous drainage: abscesses >3 cm and complex collections benefit from catheter drainage; the Seldinger technique uses a guidewire, whereas the trocar technique is direct.

Radiofrequency ablation is effective for hepatocellular carcinoma <3 cm and for selected metastases; microwave ablation achieves larger zones with less heat-sink effect.

Vascular access: the right internal jugular vein is preferred for central lines; the common femoral artery is the standard access for angiography, with the common femoral vein for IVC filters.

Complications of angiography: contrast nephropathy, cholesterol emboli, pseudoaneurysm, arteriovenous fistula, and retroperitoneal hemorrhage; manual compression or closure devices reduce access-site bleeding.

Varicocele embolization and gonadal vein embolization are performed via the internal jugular or femoral approach; pelvic congestion syndrome is treated by embolizing incompetent ovarian veins.

Bleeding embolization: the middle meningeal artery for chronic subdural hematoma, gastroduodenal artery for duodenal bleeding, and bronchial artery for massive hemoptysis.

FDG-PET: high uptake in malignancy, infection, and inflammation; false negatives in mucinous tumors and small lesions; false positives in sarcoidosis and postoperative inflammation.

Bone scan (Tc-99m MDP): sensitive for osteoblastic metastases (prostate, breast); a superscan shows diffusely increased uptake with absent renal visualization.

Sentinel lymph node mapping uses Tc-99m sulfur colloid or blue dye; the first draining node is identified by lymphoscintigraphy and a gamma probe.

Thyroid imaging: I-123 or Tc-99m pertechnetate for nodules and Graves disease; I-131 is used for ablation and therapy, with pregnancy as a contraindication.

V/Q scan for pulmonary embolism: high-probability patterns include multiple segmental mismatched defects; a normal scan excludes PE.

Myocardial perfusion imaging with Tc-99m sestamibi or tetrofosmin detects ischemia; reversible defects indicate ischemia, and fixed defects indicate scar.

PSMA PET/CT has high sensitivity for prostate cancer recurrence and staging; it is more sensitive than conventional imaging for small nodal and bone metastases.

Radiation safety: I-131 therapy requires patient isolation and instructions; pregnancy testing is mandatory before therapeutic radiopharmaceutical administration.

Doppler principles: the Doppler shift is proportional to blood velocity and transducer frequency and inversely proportional to the cosine of the angle; an angle of 60 degrees or less is required for accurate velocity measurement.

Artifacts: acoustic shadowing (stones, bone), posterior enhancement (cysts), reverberation (gas), comet-tail (adenomyomatosis), and twinkling (calcifications).

Thyroid nodules: suspicious features include hypoechogenicity, irregular margins, microcalcifications, and taller-than-wide shape; ACR TI-RADS assigns points and follow-up or biopsy thresholds.

Liver: increased echogenicity suggests steatosis; a nodular surface and coarse echotexture suggest cirrhosis; portal vein diameter >13 mm suggests portal hypertension.

Obstetric ultrasound: crown-rump length is most accurate for dating in the first trimester; nuchal translucency is measured at 11–14 weeks; the femur length and biparietal diameter are used later.

Deep vein thrombosis: non-compressibility of the vein is the primary criterion; augmentation and Doppler are adjuncts; the common femoral and popliteal veins are most commonly involved.

FAST exam: pericardial, perihepatic (Morison's pouch), perisplenic, and pelvic windows; it detects free fluid but does not identify the source of bleeding.

Elastography measures tissue stiffness; increased stiffness suggests malignancy or fibrosis, but results vary with technique and operator.

FAST exam is the first-line study for hemodynamically unstable trauma patients; CT is preferred for stable patients and detects solid organ injury and retroperitoneal bleeding.

Traumatic aortic injury: intimal flap, pseudoaneurysm, and periaortic hematoma at the aortic isthmus; a normal chest radiograph does not exclude it.

Pneumothorax: absent lung markings and a visible pleural line; tension pneumothorax shows mediastinal shift and requires immediate decompression without imaging delay.

Bowel and mesenteric injury: CT signs include free fluid without solid organ injury, bowel wall thickening, and mesenteric stranding; free air suggests perforation.

Acute appendicitis: dilated appendix >6 mm with wall thickening, periappendiceal fat stranding, and an appendicolith; ultrasound is preferred in children and pregnant women.

Acute mesenteric ischemia: CT angiography shows arterial occlusion or venous thrombosis; pneumatosis and portomesenteric gas indicate late, irreversible ischemia.

Cervical spine clearance: the NEXUS criteria or Canadian C-spine rule guides imaging; CT is the standard for suspected fracture, and MRI is used for ligamentous injury or cord compression.

Ectopic pregnancy: an adnexal mass with free fluid and a positive pregnancy test; rupture with hemoperitoneum is a surgical emergency.

Sarcoidosis: bilateral hilar lymphadenopathy, perilymphatic nodules, and erythema nodosum; the diagnosis requires a compatible clinical picture with non-caseating granulomas.

Tuberculosis: upper lobe cavitary disease, miliary pattern, and lymphadenopathy with central necrosis; the Ghon complex is a peripheral calcified nodule with hilar adenopathy.

Amyloidosis: diffuse organ enlargement and infiltration; cardiac involvement shows late gadolinium enhancement and increased extracellular volume; carpal tunnel syndrome and macroglossia are clues.

Neurofibromatosis type 1: café-au-lait spots, neurofibromas, optic pathway glioma, and sphenoid wing dysplasia; type 2 shows bilateral vestibular schwannomas and meningiomas.

Tuberous sclerosis: cortical tubers, subependymal nodules, renal angiomyolipomas, and lymphangioleiomyomatosis; cardiac rhabdomyomas are common in infants.

Langerhans cell histiocytosis: lytic bone lesions, diabetes insipidus, and pulmonary cysts with nodules; smoking-related pulmonary LCH is upper-lobe predominant.

IgG4-related disease: diffuse organ enlargement with a capsule-like rim, autoimmune pancreatitis (sausage pancreas), and retroperitoneal fibrosis; serum IgG4 is often elevated.

Metastatic disease patterns: lytic vs blastic bone lesions, hepatic metastases (hypovascular or hypervascular), and pulmonary metastases (cannonball, miliary, or lymphangitic).