Radiology

Radiology is one of the highest yield-per-hour subjects in the NEET PG and INI-CET examinations. Although it carries a modest weightage on paper, the questions are overwhelmingly pattern-recognition and association based — meaning a small, finite list of "classic signs", modality-of-choice rules, and numerical criteria can be converted into near-guaranteed marks. Unlike the volume-heavy clinical subjects, Radiology rewards the candidate who has consciously catalogued the recurring image signs, the staging modalities, and the radiation/contrast safety principles. This mother page maps the entire subject system by system, exactly along the official group backbone — Chest, GIT, Genitourinary, Musculoskeletal, Neuroradiology, and Interventional — and layers on the cross-subject integration, recent guideline shifts, study roadmap, and rapid-fire revision material.

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How Radiology Is Tested

Weightage and exam footprint

• In NEET PG, Radiology contributes roughly 8–12 questions (≈4–6% of the 200-question paper). The number fluctuates, but the type is stable.
• In INI-CET (AIIMS/PGI pattern), Radiology is disproportionately loved — image-based and "newer modality / physics" questions appear, and AIIMS examiners enjoy testing radiation dose, MRI physics, and contrast agents in a way conventional NEET PG does not.
• A large fraction of "Radiology" marks are actually embedded inside other subjects — the chest X-ray in a Medicine respiratory stem, the IVP in a Surgery/Urology stem, the CT head in a Neurology stem. The true radiology footprint is therefore larger than the standalone count suggests.

Recurring question styles

Style	What it tests	Example trigger
Classic sign → diagnosis	Eponymous radiographic signs	"Sail sign", "Rigler triad", "double bubble"
Investigation of choice	Best/first modality for a condition	IOC for acute pancreatitis, for renal colic, for cholesteatoma
Image identification	Reading the actual film/CT/MRI	Identify the structure, the level, the lesion
Numerical criteria	Cut-off values & indices	Cardiothoracic ratio, BI-RADS, vertebral angles
Contrast / radiation safety	Pharmacology + physics overlap	Gadolinium & NSF, iodinated contrast & metformin, ALARA
Modality physics	INI-CET favourite	T1 vs T2 weighting, Hounsfield units, half-value layer
Staging / response	RECIST, TNM imaging	How is the M-stage assessed in Ca lung?

Strategic takeaway: memorise the "sign banks" and "IOC tables" first — they are the densest mark-yielders. Physics is a bonus reservoir for INI-CET aspirants.

The recurring traps

• Confusing modality of choice for screening vs staging vs definitive diagnosis (e.g., mammography screens, MRI characterises, biopsy confirms).
• Mixing up T1 vs T2 signal behaviour (fat is bright on T1; water/oedema/CSF bright on T2).
• Forgetting that a "sign" can be shared by multiple diseases (e.g., "tree-in-bud" in TB, but also in any small-airways disease).
• Applying adult criteria to paediatric films (thymic sail sign is normal in infants, not pathology).

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Chest Radiology

The single highest-yield group. Chest imaging overlaps Medicine (Respiratory, Cardiology), Paediatrics, and TB control programmes.

Must-know high-yield topics

• Normal CXR interpretation framework: rotation, inspiration (6 anterior / 10 posterior ribs), penetration; the silhouette sign localises lesions (lost right heart border = right middle lobe; lost left heart border = lingula).
• Cardiothoracic ratio (CTR): normal < 0.5 on a PA film; > 0.5 suggests cardiomegaly (caveat: AP/portable films magnify).
• Air bronchogram = consolidation/alveolar filling (pneumonia, pulmonary oedema, ARDS) — implies patent airways within opacified lung.
• Tuberculosis patterns: primary TB → hilar/paratracheal nodes + Ghon focus; post-primary (reactivation) → apical/posterior segment upper lobe cavitation; miliary TB → diffuse 1–3 mm nodules.
• Pulmonary oedema staging: cephalisation → Kerley B lines (interstitial) → bat-wing/butterfly perihilar alveolar oedema → pleural effusions.
• Pneumothorax & tension: absent lung markings, visceral pleural line; tension = mediastinal shift to contralateral side, depressed hemidiaphragm.
• HRCT for diffuse lung disease: honeycombing + traction bronchiectasis + basal/subpleural reticulation = UIP pattern (IPF). Ground-glass = active alveolitis / infection / COVID.

Classic signs (sign bank)

Sign	Diagnosis
Sail sign	Normal thymus (infant) / left lower lobe collapse
Golden S sign	RUL collapse with central mass (Ca lung)
Westermark sign	Oligaemia in pulmonary embolism
Hampton hump	Wedge infarct in PE
Fleischner sign	Enlarged pulmonary artery in PE
Continuous diaphragm sign	Pneumomediastinum
Deep sulcus sign	Pneumothorax (supine film)
Tree-in-bud	Endobronchial spread (TB, infective bronchiolitis)
Crazy paving	Alveolar proteinosis; also COVID, ARDS
Egg-on-side / egg-on-string	TGA
Boot-shaped heart (coeur en sabot)	Tetralogy of Fallot
Snowman / figure-of-8	TAPVC (supracardiac)
Signet ring sign	Bronchiectasis (dilated bronchus > artery)
Halo sign	Invasive aspergillosis
Air crescent (Monod)	Aspergilloma / recovering angioinvasive aspergillosis

Investigation of choice — chest

• PE: CT pulmonary angiography (CTPA) is IOC; V/Q scan if contrast contraindicated; D-dimer to rule out in low probability.
• Solitary pulmonary nodule: CT thorax to characterise; PET-CT for metabolic activity/staging.
• Mediastinal mass: contrast CT thorax (anterior → 4 Ts: Thymoma, Teratoma, Thyroid, Terrible lymphoma).
• Pleural effusion: USG is best for detecting small effusion + guiding aspiration.

Traps

• "Bat-wing" oedema can be mistaken for bilateral pneumonia — clinical context decides.
• Miliary nodules vs metastases — both diffuse; miliary are tiny and uniform.
• Apical opacity: TB vs Pancoast tumour vs apical cap — Pancoast erodes ribs and causes Horner's.

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Gastrointestinal (GIT) Radiology

A dense source of barium-study signs and acute-abdomen plain-film findings. Overlaps Surgery (acute abdomen, GI malignancy) and Paediatrics (intestinal atresias, intussusception).

Must-know high-yield topics

• Erect/supine abdominal radiograph for acute abdomen: free gas under diaphragm = perforation (best seen erect chest); multiple air-fluid levels = obstruction.
• Pneumoperitoneum signs: Rigler sign (gas on both sides of bowel wall), football sign (large lucency outlining the peritoneal cavity in infants), falciform ligament sign.
• Barium swallow / meal patterns (still examined despite endoscopy era): "rat-tail/bird-beak" achalasia, "apple-core" annular colonic carcinoma, "string sign" of Crohn's terminal ileum, "lead-pipe" colon in chronic ulcerative colitis.
• Intussusception: USG target/doughnut sign (transverse) and pseudokidney sign (longitudinal); air/contrast enema is both diagnostic and therapeutic.
• Paediatric obstruction signs: "double bubble" = duodenal atresia (also malrotation with volvulus); "triple bubble" = jejunal atresia.
• Pancreatitis imaging: CECT is IOC for severity/necrosis (best after 72 h to assess necrosis); USG for gallstones/biliary cause.
• Hepatobiliary IOC: MRCP for biliary tree (non-invasive); ERCP if therapeutic; HIDA scan for cystic duct obstruction (acute cholecystitis) / biliary leak.

Classic signs (sign bank)

Sign	Diagnosis
Bird-beak / rat-tail	Achalasia cardia
Corkscrew oesophagus	Diffuse oesophageal spasm
Apple-core / napkin-ring	Annular colonic carcinoma
String sign (Kantor)	Crohn's terminal ileitis
Lead-pipe colon	Chronic ulcerative colitis
Thumbprinting	Ischaemic / inflammatory colitis
Coffee-bean / inverted-U	Sigmoid volvulus
Bird-of-prey / beak sign	Volvulus on contrast enema
Coiled-spring	Intussusception (barium)
Cobblestone mucosa	Crohn's disease
Stack-of-coins (valvulae)	Small bowel obstruction / intramural haemorrhage
Sentinel loop	Localised ileus over inflammation (pancreatitis, appendicitis)
Colon cut-off sign	Acute pancreatitis
Spoke-wheel pattern	Focal nodular hyperplasia (liver)
Central scar (T2 bright)	FNH
Peripheral nodular discontinuous enhancement → fill-in	Hepatic haemangioma
Capsular retraction / cirrhotic surface	Chronic liver disease

Investigation of choice — GIT

• Acute appendicitis (adult): CT abdomen; USG preferred first in children/pregnancy.
• Acute pancreatitis severity: CECT (Balthazar/CT severity index).
• Liver lesion characterisation: multiphasic (triple-phase) CT or MRI; HCC shows arterial enhancement + portal/delayed washout.
• Suspected perforation: erect CXR (free air) → CT for source.
• GI bleed (obscure): CT angiography; tagged RBC scan / Meckel scan (technetium pertechnetate) for Meckel diverticulum.

Traps

• Free air is best detected on an erect chest X-ray, not the abdominal film.
• "Double bubble" is duodenal atresia and malrotation/volvulus — the latter is the surgical emergency that must be excluded (upper GI contrast study).
• HCC washout vs haemangioma fill-in are opposite enhancement behaviours — a frequent swap.

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Genitourinary Radiology

High yield in both NEET PG and INI-CET because it sits at the Surgery–Urology–Obstetrics crossroads.

Must-know high-yield topics

• Renal colic IOC: Non-contrast CT KUB (NCCT) — most sensitive/specific for calculi, replaced IVP. USG first-line in pregnancy/children.
• IVU/IVP signs (still appearing): "spider-leg / spidery calyces" in polycystic kidney; "drooping lily" in duplex collecting system with upper-moiety obstruction; "soap-bubble" in renal TB (cavities); standing-column / hydronephrosis in PUJ obstruction.
• Renal masses: simple cyst (Bosniak I, no follow-up) vs complex cyst (Bosniak III/IV → surgery). RCC = enhancing solid mass.
• Angiomyolipoma: macroscopic fat (negative HU) on CT — fat in a renal mass = AML (associated tuberous sclerosis).
• Adrenal: adenoma = low HU (< 10 HU) on non-contrast CT + rapid washout; phaeochromocytoma bright on T2 ("light bulb").
• Bladder/urethra: retrograde urethrogram for trauma/stricture; VCUG (MCU) for vesicoureteric reflux in children.
• Obstetric USG dating & anomaly: crown-rump length most accurate in first trimester; nuchal translucency (11–13+6 weeks); anomaly scan 18–20 weeks.
• Doppler in obstetrics: umbilical artery absent/reversed end-diastolic flow = severe IUGR/placental insufficiency.

Classic signs (sign bank)

Sign	Diagnosis
Spider-leg calyces	Adult polycystic kidney disease
Drooping lily	Duplex system, obstructed upper moiety
Soap-bubble / putty kidney	Renal TB (autonephrectomy)
Cobra-head / ureterocele	Ectopic ureterocele
Light-bulb (bright T2)	Phaeochromocytoma; also simple cyst
Fat density in renal mass	Angiomyolipoma
Page kidney	Subcapsular haematoma causing HTN
Snowstorm uterus	Hydatidiform mole
Whirlpool sign (Doppler)	Ovarian/testicular torsion (twisted pedicle)
String-of-beads (renal artery)	Fibromuscular dysplasia

Investigation of choice — GU

• Renal stone: NCCT KUB.
• Prostate cancer local staging: multiparametric MRI (PI-RADS).
• Testicular torsion: colour Doppler USG (absent flow); time-critical.
• VUR in recurrent UTI children: MCU/VCUG.
• Renovascular hypertension: CT/MR angiography; captopril renography functional test.

Traps

• IVP is largely historical; NCCT is now the answer for stones (older books still say IVP — go with CT).
• Fat in a renal lesion = benign AML, but fat in a retroperitoneal mass can mean liposarcoma.
• Whirlpool sign appears in midgut volvulus (GIT) as well as torsion — read the stem.

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Musculoskeletal (MSK) Radiology

Overlaps Orthopaedics, Paediatrics (NAI, skeletal dysplasias), and Medicine (arthropathies, metabolic bone disease).

Must-know high-yield topics

• Bone tumour analysis: age, location (epiphysis/metaphysis/diaphysis), margin (narrow zone of transition = benign), periosteal reaction, matrix.
  • Osteosarcoma: metaphysis around knee, sunburst/spiculated periosteal reaction, Codman triangle.
  • Ewing sarcoma: diaphysis, onion-skin (lamellated) periosteum, permeative.
  • Giant cell tumour: epiphyseal, eccentric, subarticular, "soap-bubble"; after physeal closure.
  • Osteochondroma: most common benign tumour; cortical/medullary continuity with parent bone.
  • Chondrosarcoma: rings-and-arcs / popcorn calcification.
• Arthritis differentiation: RA = symmetrical, periarticular osteopenia, marginal erosions, MCP/PIP; OA = asymmetrical, osteophytes, subchondral sclerosis/cysts, joint-space narrowing; gout = punched-out erosions with overhanging edges, preserved joint space; ankylosing spondylitis = bamboo spine, sacroiliitis.
• Metabolic bone: rickets (cupping/fraying/widened physis), osteomalacia (Looser zones / pseudofractures), hyperparathyroidism (subperiosteal resorption of radial phalanges, salt-and-pepper skull, brown tumours), Paget's (cotton-wool skull, picture-frame vertebra).
• Avascular necrosis: crescent sign (subchondral lucency) on X-ray; MRI is most sensitive (early double-line sign).
• Fracture eponyms: Colles (dorsal angulation), Smith (volar), Monteggia (proximal ulna + radial head dislocation), Galeazzi (radial shaft + DRUJ).

Classic signs (sign bank)

Sign	Diagnosis
Codman triangle / sunburst	Osteosarcoma
Onion-skin (lamellated)	Ewing sarcoma
Soap-bubble (epiphysis)	Giant cell tumour
Looser zones	Osteomalacia
Subperiosteal resorption	Hyperparathyroidism
Rugger-jersey spine	Renal osteodystrophy
Bamboo spine	Ankylosing spondylitis
Pencil-in-cup	Psoriatic arthritis
Sabre tibia	Paget / congenital syphilis
Fallen fragment sign	Simple bone cyst
Ground-glass matrix	Fibrous dysplasia
Cortical break + soft-tissue mass (kids)	Malignant primary bone tumour

Investigation of choice — MSK

• Soft-tissue & marrow / early osteomyelitis & AVN: MRI (most sensitive).
• Occult/stress fracture, metastasis screen (whole body): bone scan (Tc-99m MDP).
• Fracture, alignment, bony detail: plain radiograph first; CT for complex/intra-articular.
• Suspected non-accidental injury (child): skeletal survey + bone scan; metaphyseal corner ("bucket-handle") fractures, posterior rib fractures are specific.

Traps

• Tumour location is the discriminator: epiphysis = GCT/chondroblastoma, metaphysis = osteosarcoma, diaphysis = Ewing.
• "Bone scan negative" does not exclude marrow lesion — MRI may still be positive (e.g., multiple myeloma is often cold on bone scan).

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Neuroradiology

INI-CET examiners' favourite. CT for acute haemorrhage/trauma, MRI for everything subtle. Overlaps Medicine (stroke, infections), Surgery (head injury), Paediatrics (TORCH, neural tube defects).

Must-know high-yield topics

• Stroke imaging: NCCT first to exclude haemorrhage before thrombolysis. Early infarct signs: hyperdense MCA sign, loss of grey-white differentiation, insular ribbon sign. Diffusion-weighted MRI (DWI) is most sensitive for acute infarct (restricted diffusion within minutes).
• Haemorrhage density on CT: acute blood is hyperdense (~60–80 HU); becomes isodense (1–2 weeks), then hypodense.
• Extradural vs subdural: EDH = biconvex/lens-shaped, does not cross sutures (crosses falx), middle meningeal artery; SDH = crescentic, crosses sutures (not falx), bridging veins.
• SAH: blood in sulci/cisterns; CT angiography/DSA for aneurysm (commonest at anterior communicating artery).
• Ring-enhancing lesions (classic differential — "MAGICAL DR"): Metastasis, Abscess, Glioblastoma, Infarct (subacute), Contusion, AIDS (toxoplasmosis), Lymphoma, Demyelination (tumefactive), Radiation necrosis. Abscess shows restricted diffusion centrally; tumour does not.
• MRI sequence logic: T1 — fat bright, CSF dark (anatomy); T2 — CSF/oedema bright (pathology); FLAIR — CSF suppressed, periventricular lesions conspicuous (MS, gliosis); DWI — acute infarct, abscess, epidermoid.
• Demyelination (MS): Dawson fingers (periventricular, perpendicular to ventricles) on FLAIR; open-ring enhancement.
• Neurocysticercosis: scolex within cyst ("dot-in-hole"), staging vesicular → colloidal → granular-nodular → calcified.
• Posterior fossa paediatric tumours: medulloblastoma (midline, restricts diffusion), pilocytic astrocytoma (cystic with enhancing mural nodule), ependymoma ("plastic" extending through foramina).

Classic signs (sign bank)

Sign	Diagnosis
Hyperdense MCA	Acute MCA thrombus
Insular ribbon loss	Early MCA infarct
Lens/biconvex hyperdensity	Extradural haematoma
Crescentic collection	Subdural haematoma
Dawson fingers	Multiple sclerosis
Dot-in-hole / scolex	Neurocysticercosis
Tigroid / leopard-skin	Metachromatic leukodystrophy
Empty delta sign	Dural venous sinus thrombosis (CT venogram)
Eye-of-the-tiger	Pantothenate kinase neurodegeneration (NBIA)
Hummingbird / penguin sign	Progressive supranuclear palsy (midbrain atrophy)
Hot-cross-bun	Multiple system atrophy (pons)
Molar tooth	Joubert syndrome
Double-panda	Wilson disease (midbrain)
Lemon & banana	Chiari II (open spina bifida, antenatal USG)

Investigation of choice — neuro

• Acute head trauma / suspected bleed: NCCT head.
• Acute ischaemic stroke: NCCT to exclude bleed → DWI-MRI / CT perfusion for core vs penumbra.
• Seizure / posterior fossa / soft-tissue / cord lesion: MRI.
• Cholesteatoma, bony temporal lesions: HRCT temporal bone.
• CSF rhinorrhoea / pituitary: dedicated MRI sella ± CT.

Traps

• DWI restriction is seen in acute infarct, abscess, epidermoid, and hypercellular tumours — do not equate it solely with stroke.
• EDH crosses the midline (falx) but not sutures; SDH crosses sutures but not the midline — a perennially reversed pair.
• Hyperacute haemorrhage on MRI can be subtle; CT is superior for acute blood.

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Interventional Radiology (IR)

A growing, examiner-favoured group covering vascular/non-vascular interventions, embolisation, ablation, and the physics/contrast safety attached to image-guided procedures.

Must-know high-yield topics

• Seldinger technique: percutaneous needle → guidewire → catheter exchange — the foundation of all vascular access.
• Image-guided drainage / biopsy: USG- or CT-guided; coaxial technique; the IOC for sampling deep collections and solid masses safely.
• Transarterial chemoembolisation (TACE): intermediate-stage HCC (BCLC-B) — selective hepatic artery delivery exploits the tumour's arterial supply while sparing portal-fed parenchyma. Radiofrequency/microwave ablation for small (< 3 cm) HCC/RCC.
• Uterine artery embolisation (UAE): symptomatic fibroids — uterus-sparing alternative to hysterectomy.
• Embolisation for bleeding: GI bleed, trauma (splenic/pelvic), post-partum haemorrhage, haemoptysis (bronchial artery embolisation).
• TIPS (transjugular intrahepatic portosystemic shunt): refractory variceal bleeding / ascites in portal hypertension; risk = hepatic encephalopathy.
• IVC filter: recurrent PE despite anticoagulation or when anticoagulation contraindicated.
• Central venous access / PICC / port, nephrostomy (obstructed infected system), biliary drainage (PTBD).

Contrast and radiation safety (high-yield, overlaps Pharmacology & Physics)

Issue	Key fact
Iodinated contrast + metformin	Withhold metformin around contrast if eGFR impaired (lactic acidosis risk)
Contrast-induced nephropathy	Hydration is mainstay; identify CKD/diabetes risk
Gadolinium + severe renal failure	Nephrogenic systemic fibrosis — avoid in eGFR < 30
Contrast anaphylactoid reaction	Premedicate (steroids + antihistamine) in prior reactors
ALARA principle	Keep radiation As Low As Reasonably Achievable
Highest radiation modality	CT / interventional fluoroscopy; MRI & USG = no ionising radiation
Pregnancy	USG/MRI preferred; avoid gadolinium; justify any X-ray/CT

Traps

• MRI uses no ionising radiation (a common trick: candidates wrongly attribute dose to MRI). Its hazards are projectiles/implants, not radiation.
• Gadolinium danger is renal (NSF); iodinated contrast danger is renal (CIN) + metformin interaction + anaphylactoid.
• TIPS relieves portal pressure but worsens encephalopathy.

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Cross-Subject Integration

Radiology never lives alone in the exam stem. The recurring overlaps:

Partner subject	Frequent overlap
Medicine	CXR in CCF, TB, PE; CT/MRI in stroke; HRCT in ILD
Surgery	Acute abdomen films, staging CT in malignancy, trauma FAST/CT
Obstetrics	USG dating, anomaly scan, Doppler in IUGR, molar "snowstorm"
Paediatrics	Skeletal dysplasias, NAI survey, intussusception, double bubble
Pathology	Radiologic–pathologic correlation of tumours (HCC, RCC, bone tumours)
Pharmacology	Contrast media, gadolinium, radiopharmaceuticals
Orthopaedics	Fracture eponyms, bone tumour matrix, AVN staging
Anatomy	Cross-sectional CT/MRI levels, mediastinal compartments
PSM/Community Medicine	Radiation protection, screening mammography programmes
Forensic Medicine	Age estimation from epiphyseal fusion radiographs

FAST scan (Focused Assessment with Sonography in Trauma) bridges Surgery + Radiology: detects free fluid in Morrison's pouch, splenorenal space, pelvis, pericardium.

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Recent Update Themes (Current-Exam Relevant)

• Structured reporting systems are increasingly tested: BI-RADS (breast), PI-RADS (prostate MRI), LI-RADS (liver in cirrhosis), TI-RADS (thyroid nodule), Lung-RADS, Bosniak (renal cysts), O-RADS (ovarian). Know what each acronym stands for and the principle (rising number = rising malignancy risk / intervention threshold).
• CT/DWI-MRI thrombectomy window: extended stroke thrombectomy windows (up to 24 h) guided by perfusion imaging (core/penumbra mismatch) — a shift from the old rigid 4.5-h thrombolysis-only paradigm.
• Low-dose CT screening for lung cancer in high-risk smokers (Lung-RADS framework).
• mpMRI before prostate biopsy (PI-RADS) — imaging now precedes/targets biopsy.
• Gadolinium deposition awareness — preference for macrocyclic agents; restraint in renal impairment.
• RECIST 1.1 for solid-tumour response assessment in oncology stems.
• Point-of-care ultrasound (POCUS) and FAST integration in emergency settings.

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Study Roadmap

Phase 1 — Foundation (build the scaffolding)

1. Learn modality physics basics: X-ray attenuation, Hounsfield units (water 0, air −1000, fat negative, bone/calcium high), MRI T1/T2/FLAIR/DWI logic, USG echogenicity, no-radiation modalities.
2. Master the silhouette sign / lobar collapse logic for chest, the most reused concept.

Phase 2 — System sweep (group by group)

• Go through Chest → GIT → GU → MSK → Neuro → IR exactly along this page.
• For each, create a personal two-column sheet: Sign → Diagnosis and Condition → IOC.

Phase 3 — Sign bank consolidation

• Drill the eponymous-sign tables until recall is reflexive. These are the densest mark-yielders and appear verbatim.

Phase 4 — Integration & image reading

• Practise actual images in PYQ banks; INI-CET demands recognising the picture, not just the eponym.
• Layer the structured-reporting acronyms (BI-RADS family) and contrast/radiation safety.

Last-week revision strategy

• Days 7–4: Re-read only your Sign → Diagnosis and IOC sheets + this mother page's tables. Do not start new resources.
• Days 3–2: Rapid image-recognition drills (chest signs, ring-enhancing differential, bone-tumour location triad, MRI signal table).
• Day 1: Memorise the high-frequency one-liners (below), the RADS acronyms, and the contrast-safety table. Sleep adequately — pattern recall degrades with fatigue.
• Exam morning: glance at the T1 vs T2 vs DWI mini-table and the IOC list — these resolve the largest number of questions per fact.

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High-Yield Tables

MRI signal cheat-sheet

Tissue	T1	T2
Fat	Bright	Bright (intermediate on FS)
Water / CSF / oedema	Dark	Bright
Acute haemorrhage (deoxyHb)	Iso/dark	Dark
Subacute haemorrhage (metHb)	Bright	Variable
Air / cortical bone / calcification	Dark	Dark
Flowing blood (flow void)	Dark	Dark

Hounsfield unit landmarks (CT)

Tissue	Approx HU
Air	−1000
Fat	−50 to −100
Water	0
Soft tissue	+30 to +50
Acute blood	+60 to +80
Bone / calcium	+400 to +1000

Investigation-of-choice quick list

Condition	IOC
Renal/ureteric calculus	NCCT KUB
Acute pancreatitis severity	CECT abdomen
Biliary tree (non-invasive)	MRCP
Acute ischaemic stroke	NCCT then DWI-MRI
Acute head trauma	NCCT head
Pulmonary embolism	CTPA
Early osteomyelitis / AVN / marrow	MRI
Bone metastasis whole-body screen	Bone scan (Tc-99m MDP)
Cholesteatoma / temporal bone	HRCT temporal bone
Prostate cancer local staging	mpMRI (PI-RADS)
Testicular / ovarian torsion	Colour Doppler USG
Breast cancer screening	Mammography

Mnemonics

• Ring-enhancing brain lesions — "MAGICAL DR": Metastasis, Abscess, Glioblastoma, Infarct (subacute), Contusion, AIDS-toxoplasma, Lymphoma, Demyelination, Radiation necrosis.
• Anterior mediastinal mass — "4 Ts": Thymoma, Teratoma, Thyroid (retrosternal), Terrible lymphoma.
• EDH vs SDH — "EDH = LEMON (Lens-shaped, crosses midline not sutures); SDH = CRESCENT (crosses sutures not midline)."
• Bone tumour location — "Epiphysis = GCT, Metaphysis = osteosarcoma, Diaphysis = Ewing."
• T2 brightness — "Water Wins on T2" (water/oedema/CSF bright on T2).

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Rapid-Fire One-Liners

1. IOC for renal stone is non-contrast CT KUB, not IVP.
2. DWI-MRI detects acute infarct within minutes; CT is done first only to exclude bleed.
3. Extradural haematoma is biconvex and does not cross sutures; subdural is crescentic and does not cross the midline.
4. Codman triangle + sunburst = osteosarcoma; onion-skin = Ewing sarcoma.
5. Rigler sign (gas on both sides of bowel wall) indicates pneumoperitoneum.
6. Free gas under the diaphragm is best seen on an erect chest X-ray.
7. Fat (negative HU) within a renal mass = angiomyolipoma.
8. MRCP is the non-invasive investigation of choice for the biliary tree.
9. Gadolinium is contraindicated in severe renal failure due to nephrogenic systemic fibrosis.
10. MRI uses no ionising radiation — its hazard is ferromagnetic, not dose.
11. Dawson fingers on FLAIR = multiple sclerosis; empty delta sign = dural venous sinus thrombosis.
12. TACE is used for intermediate-stage (BCLC-B) hepatocellular carcinoma; TIPS relieves portal hypertension but precipitates encephalopathy.