AN54.1-4 | Radiodiagnosis — Gate Quiz

Correct! Fat has a Hounsfield unit value of approximately −100 HU (range −100 to −50). This is why fat appears dark on CT. Bone is +400 to +1000 HU (very bright), soft tissue is +20 to +80 HU (grey), and air is approximately −1000 HU (black).

Hounsfield unit scale: water defined as 0 HU. Air = −1000, fat = −100 to −50, water = 0, soft tissue = 20–80, fresh blood = 50–90, bone = 400–1000, metal > 1000. On CT images, dark structures have low (negative) HU; bright structures have high (positive) HU. Fat density is important for diagnosing lipoma, angiomyolipoma (kidney), and adrenal adenoma.

Incorrect. Hounsfield units: Air ≈ −1000, Fat ≈ −100, Water = 0, Soft tissue = +20 to +80, Bone = +400 to +1000. Fat is at approximately −100 HU.

Correct! The psoas shadow is visible on X-ray because perirenal and retroperitoneal fat outlines the muscle edge. Obliteration of the psoas shadow indicates loss of the fat plane — caused by retroperitoneal pathology: psoas abscess (e.g., from spinal TB, Crohn's disease), retroperitoneal haemorrhage, or retroperitoneal tumour extending into the psoas plane.

Psoas shadow: psoas major runs from T12/L1–L5 transverse processes to lesser trochanter. On X-ray abdomen, the lateral border is visible as a soft tissue stripe due to surrounding retroperitoneal fat. Obliteration = fat replaced by pathology: (1) psoas abscess (TB spondylitis most common cause in India), (2) retroperitoneal haematoma (trauma, AAA rupture), (3) retroperitoneal tumour/lymphadenopathy, (4) perinephric abscess. Bilateral obliteration → severe retroperitoneal disease.

Incorrect. Obliteration of the psoas shadow indicates retroperitoneal pathology affecting the fat plane. Pleural effusion affects the chest, not the psoas shadow. Splenomegaly displaces stomach gas but does not obliterate the psoas shadow. While the psoas is not always equally visible in lean vs obese patients, asymmetric obliteration should prompt investigation.

Correct! The bird-beak (or pencil-point) appearance on barium swallow describes the smooth tapered narrowing of the lower oesophageal sphincter (LOS) with proximal oesophageal dilatation — the classic finding in **achalasia cardia**. The LOS fails to relax (loss of inhibitory myenteric neurons) causing functional obstruction. Rat-tail appearance suggests carcinoma (irregular, asymmetric).

Barium swallow appearances: (1) Bird-beak/pencil-point — achalasia (smooth symmetric taper at LOS, dilated oesophagus above, absence of gastric air bubble due to elevated LOS pressure). (2) Rat-tail — oesophageal carcinoma (irregular, asymmetric, shouldered edges). (3) Filling defect/mucosal irregularity — carcinoma, polyp. (4) Pharyngeal pouch (Zenker's) — posterior pharynx, barium collects in pouch. (5) Hiatus hernia — gastric mucosa above diaphragm on erect film.

Incorrect. Bird-beak appearance = achalasia (smooth tapered narrowing at LOS + dilatation above). Rat-tail appearance = carcinoma (irregular, asymmetric, shouldered). Apple-core = colonic carcinoma on barium enema. Thumbprinting = ischaemic colitis or IBD.

Correct! In IVP, IV contrast is filtered by the glomeruli and concentrated by the tubules. The nephrogram phase (parenchymal opacification, 1–5 min) is followed by the pyelogram phase (progressive filling of the pelvicalyceal system, infundibula, pelvis, ureter). Progressive pyelogram filling indicates normal glomerular filtration and tubular concentration.

IVP phases: (1) Scout film (plain KUB) — baseline, identify calculi. (2) Nephrogram (1–5 min) — glomerular filtration → tubular opacification. (3) Pyelogram (5–15 min) — collecting system fills. (4) Ureterogram — contrast in ureter. (5) Cystogram — bladder. (6) Post-micturition film — residual urine. Delayed/absent nephrogram = poor renal function or obstruction. Persistent nephrogram without pyelogram = acute obstruction (pyelocalyceal back pressure). IVP largely replaced by CT urogram for stone disease.

Incorrect. Normal IVP sequence: injection → nephrogram (parenchyma lights up, 1–5 min) → pyelogram (collecting system fills, 5–15 min) → ureterogram (contrast moves down ureter) → cystogram (bladder fills). A persistent dense nephrogram without pyelogram filling suggests obstruction.

Correct! In a normal HSG, contrast fills the uterine cavity, enters both fallopian tubes, and spills freely into the peritoneal cavity (bilateral peritoneal spillage). Failure of contrast to spill on either side despite adequate filling of the uterine cavity indicates bilateral tubal obstruction — the most common finding requiring further investigation in infertility workup (laparoscopy + chromopertubation).

HSG interpretation: (1) Uterine cavity — shape, filling defects (fibroids/polyps = rounded filling defects; Asherman's = irregular filling defects/synechiae; uterine septum = triangular division). (2) Fallopian tubes — fill, course, bilateral peritoneal spill. Tubal block: cornual (proximal) = spasm vs scar (try antispasmodic, repeat); ampullary (distal) = hydrosalpinx (dilated tube, no spill); fimbrial block. (3) Peritoneal spill — free spill = patent; loculated = adhesions. Complications: infection (PID risk), radiation, contrast reaction.

Incorrect. Normal HSG = contrast fills uterus → tubes → peritoneal spill bilaterally. No spill on either side = bilateral tubal block. Cervical stenosis would prevent even filling of the uterine cavity. Uterine perforation would show contrast outside the uterine cavity in an abnormal location (extraperitoneal or bowel loops).

Correct! MRCP is the preferred first-line non-invasive investigation for suspected CBD stones or biliary obstruction. It clearly visualises the biliary tree and pancreatic duct without radiation or endoscopy. If a CBD stone is confirmed, ERCP is then performed therapeutically (sphincterotomy + stone extraction). This two-step approach avoids the risks of ERCP (pancreatitis, bleeding, perforation) in patients where therapy may not be needed.

Modern biliary imaging algorithm: Ultrasound (first line — screens for dilated CBD, gallstones) → MRCP (non-invasive biliary tree imaging, detects CBD stones, strictures, IPMN) → ERCP (therapeutic — stone extraction, stenting). CT is used for: acute pancreatitis severity, cholangiocarcinoma staging, biliary tumour assessment. Endoscopic ultrasound (EUS) is increasingly used for small CBD stones and ampullary lesions. MRCP has essentially replaced diagnostic ERCP.

Incorrect. ERCP carries significant risks (pancreatitis 3–5%, bleeding, perforation, cholangitis) and should not be used purely for diagnosis when MRCP is available. CT can show biliary dilatation but misses small stones. Barium meal visualises only the stomach and duodenum.

Correct! On CT, acute appendicitis is diagnosed by: (1) dilated appendix >6 mm (or >10 mm some sources), (2) non-compressible with thickened walls, (3) periappendiceal fat stranding (mesenteric inflammation), and (4) possible appendicolith (calcified faecolith). These findings together have >95% sensitivity and specificity for acute appendicitis.

CT appendicitis findings: (1) Appendix >6 mm diameter, non-compressible. (2) Wall thickening and enhancement. (3) Periappendiceal fat stranding. (4) Appendicolith (faecolith — hyperdense calcification). (5) Periappendiceal fluid collection or abscess (complicated). (6) Perforation signs: gas/fluid outside appendix. CT is particularly valuable in: obese patients (ultrasound technically limited), females (to exclude ovarian pathology), and atypical presentations. Alvarado score used clinically before imaging.

Incorrect. Free fluid alone is non-specific (can occur in any peritoneal inflammation). Gas under diaphragm indicates perforation of a hollow viscus (not specific for appendicitis). Caecal wall thickening alone could represent colitis. The combination of dilated appendix >6 mm + fat stranding is the most specific for appendicitis on CT.

Correct! PET-CT with 18F-FDG (fluorodeoxyglucose) is the gold standard for whole-body staging of cancer. Cancer cells have increased glucose metabolism → accumulate more 18F-FDG → appear as "hot spots" on PET. Combined with CT (for anatomical localisation), PET-CT detects metastases in lymph nodes, bones, liver, lungs, and other sites — often before structural changes are visible on conventional imaging.

PET-CT indications: (1) Cancer staging (breast, lung, lymphoma, colorectal, melanoma, etc.). (2) Detecting recurrence after treatment. (3) Treatment response assessment (metabolic response before structural response). (4) Guiding biopsy to most metabolically active site. Limitations: false positives (infections, inflammation also show high FDG uptake — e.g., tuberculosis, sarcoidosis). False negatives: low-grade tumours, mucinous carcinoma, prostate cancer (uses PSMA PET instead). Patient fasting required (4–6h) to reduce background glucose.

Incorrect. Plain X-ray misses small metastases and soft tissue deposits. Bone scan detects osteoblastic activity (useful for bone metastases but misses soft tissue/visceral disease and purely lytic lesions). IVP is for urinary tract evaluation. PET-CT is the most comprehensive staging tool for solid tumours.

Correct! **Rigler's sign** (double-wall or bowel wall sign) is seen on a supine X-ray abdomen when free intraperitoneal gas is present — both the intraluminal gas (inside bowel) and the free gas (outside bowel) make the bowel wall visible as a thin white line on both sides. It indicates pneumoperitoneum on a supine film.

Pneumoperitoneum X-ray signs: (1) Erect film — gas under diaphragm (most sensitive on right). (2) Supine film — Rigler's sign (double-wall), falciform ligament visible (normally invisible), large amount of free gas: football sign (lucency over entire abdomen in neonates/large perforation). (3) Left lateral decubitus — free gas above the liver (right side up). Always request ERECT film if patient can stand; lateral decubitus if cannot. Smallest amounts of free gas detectable: CT > erect X-ray > supine X-ray.

Incorrect. Rigler's sign = double-wall sign on supine film (both sides of bowel wall visible due to free gas on both sides). Psoas sign = pain on psoas stretch (clinical sign of retroperitoneal inflammation). Cullen's sign = periumbilical bruising (haemorrhagic pancreatitis). Murphy's sign = gallbladder tenderness on deep inspiration (clinical sign of acute cholecystitis).

Correct! DSA (selective mesenteric angiography) can detect active GI bleeding at a rate of approximately 0.5 mL/min (compared to nuclear medicine bleeding scan which detects 0.1 mL/min — more sensitive but less anatomically precise). However, DSA has the advantage of being both diagnostic AND therapeutic — the bleeding vessel can be embolised during the same procedure.

GI bleeding investigation: (1) Upper GI: endoscopy first. (2) Lower GI: colonoscopy first; if massive bleed → CT angiography (detects 0.3–0.5 mL/min, non-invasive, fast) → DSA (detects 0.5 mL/min, can embolise). (3) Nuclear scan (Tc-99m RBC) — most sensitive (0.1 mL/min) but poor anatomical detail. DSA therapeutic roles: (a) embolisation of GI bleed, (b) TIPS for portal hypertension, (c) renal artery angioplasty/stenting for renovascular hypertension, (d) hepatic arterial chemoembolisation (TACE) for HCC.

Incorrect. DSA detects active bleeding at ≥0.5 mL/min. Nuclear medicine (Tc-99m RBC scan) is more sensitive at 0.1 mL/min. CT angiography can detect 0.3–0.5 mL/min. DSA's advantage is simultaneous therapeutic embolisation, not sensitivity.