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Abdominal X-Ray (AXR) 101

1. Overview & When to Order It

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2. Standard Views

• Supine AXR: The workhorse. Should cover diaphragm to hernial orifices (obturator foramina must be included).
• Erect CXR: The most sensitive plain-film test for pneumoperitoneum — patient should ideally stand for 10 minutes beforehand to allow free gas to rise.
• Left lateral decubitus AXR: Used when the patient cannot stand; free air collects between the liver and right lateral abdominal wall.
• Erect AXR: Historically used for air-fluid levels, but now discouraged — evidence shows it can be misleading.

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3. Systematic Approach — The ABCDE Framework

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4. Normal Bowel Gas Pattern

• Small bowel: Located centrally, calibre ≤3 cm in adults, shows valvulae conniventes (thin, complete folds that span the full bowel width — most prominent in the jejunum)
• Large bowel: Located peripherally, calibre up to 5–6 cm (caecum up to 9 cm), shows haustra (thick, incomplete folds that only partially cross the lumen)
• Rectum: Usually contains some gas/stool
• Gas should be present in stomach, small bowel, colon, and rectum; a gasless abdomen may indicate fluid-filled obstruction or ascites

Key rule (Harriet Lane): Small bowel is central with valvulae; large bowel is peripheral with haustra.

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5. Abnormal Gas Distribution

A. Pneumoperitoneum (Free Intraperitoneal Air)

⚠️ Causes of pneumoperitoneum without peritonitis: recent laparotomy/laparoscopy, pneumatosis intestinalis, vaginal douche — do not always indicate perforation.

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B. Gas in Bowel Wall (Pneumatosis Intestinalis)

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6. Bowel Obstruction

Small Bowel Obstruction (SBO)

• Multiple centrally-placed dilated loops (diameter >3.5 cm, usually <5 cm)
• Valvulae conniventes visible (thin, complete transverse bands — "stacked coins")
• "Stepladder" air-fluid levels on erect view
• "String of beads" sign: small gas pockets trapped between folds in fluid-filled loops
• Paucity of colonic gas (collapsed distal bowel confirms obstruction)

CT is the definitive investigation — identifies the transition point (abrupt calibre change from dilated to collapsed bowel) and defines the cause.

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Large Bowel Obstruction (LBO)

• Peripheral dilatation (haustrated colon around the abdominal frame)
• Diameter typically >5 cm (caecum is most vulnerable — >9 cm risks perforation)
• Site: ~60% due to sigmoid carcinoma; also diverticulitis, volvulus

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Paralytic Ileus vs. Obstruction

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7. Bowel Diameter Limits (Rules of 3, 6, 9)

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8. Calcifications on AXR

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9. Soft Tissue Landmarks

• Psoas shadows: bilateral fusiform density along lumbar spine — loss suggests retroperitoneal pathology (bleeding, infection, AAA)
• Liver/spleen: enlarged if extending below the costal margin
• Kidneys: normally visible in the loin (right lower than left); renal outlines lost in retroperitoneal disease
• Bladder: pelvic soft tissue density; should be empty for a good AXR

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10. AXR in Special Conditions

Acute Pancreatitis (Sleisenger & Fordtran's)

• Sentinel loop: focal ileus of a small bowel loop adjacent to inflamed pancreas
• Colon cut-off sign: spasm of the transverse/descending colon with air cut-off at the splenic flexure
• Pancreatic calcification → chronic pancreatitis

Toxic Megacolon

• Transverse colon dilated >6 cm with loss of haustral pattern + "mucosal islands" (pseudopolyps)
• Complication of UC, Crohn's, infectious colitis — surgical emergency

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11. AXR Limitations — When to go to CT

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• Grainger & Allison's Diagnostic Radiology, Ch. 18
• Harriet Lane Handbook (23rd ed.), Ch. 26
• Sleisenger & Fordtran's GI and Liver Disease, Ch. on Pancreatitis

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CT (Computed Tomography) 101

1. How CT Works

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2. Hounsfield Units (HU) — The Language of CT

Practical rules: Fat is black (negative HU), water is mid-grey (0 HU), bone is bright white (+1000 HU). Anything brighter than expected soft tissue = calcification, contrast, or haemorrhage. Anything darker = fat or gas.

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3. CT Windows — Seeing What You Want

• Window Level (WL) = the centre of the range you want to display (the "brightness")
• Window Width (WW) = the spread of the range (the "contrast")

Key clinical point: Always view a suspected pneumoperitoneum on lung window settings — free gas invisible on soft-tissue windows becomes obvious on lung windows (Grainger & Allison's).

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4. Contrast Media — Types, Phases, and Safety

Types of Contrast

IV Contrast Phases

Wash-in / wash-out: HCC shows arterial hyperenhancement then portal phase washout — hallmark of HCC without biopsy. Haemangiomas show peripheral nodular enhancement with progressive fill-in.

Adrenal Washout Protocol (specific use case)

• Measure HU pre-contrast, at 60 s, and at 15 min
• Absolute contrast washout ≥60% → benign adenoma (sensitivity 86–88%, specificity 92–96%)
• Relative washout (post-contrast only) ≥40% → adenoma (sensitivity 96%, specificity 100%) (Grainger & Allison's)

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5. Contrast Safety

Contrast-Induced Nephropathy (CIN) / Post-Contrast AKI (PC-AKI)

• Absolute creatinine rise >0.3 mg/dL (26.4 µmol/L), OR

• 50% rise in serum creatinine, OR

• Urine output <0.5 mL/kg/h for ≥6 hours

• CKD 3 (moderate impairment): IV hydration with 0.9% saline 100 mL/h starting 6–2 h before CT, continuing 4–12 h after
• CKD 4–5 (severe): avoid iodinated contrast where possible; use alternative modalities
• CIN is real but rare with modern low-osmolar agents in normal renal function

Allergic / Hypersensitivity Reactions

• Mild: nausea, flushing, urticaria — treat with antihistamine
• Moderate: bronchospasm, facial oedema — add corticosteroids
• Severe (anaphylaxis): hypotension, loss of consciousness — epinephrine, resuscitation
• Pre-medication with steroids + antihistamines for patients with prior moderate/severe reactions

Gadolinium Caution

• Nephrogenic Systemic Fibrosis (NSF): fibrocyte proliferation causing skin thickening, contractures, paraesthesia in patients with renal impairment
• Brain deposition: gadolinium accumulates in dentate nuclei, globus pallidus, thalamus with repeated doses — clinical significance still under study
• Linear GBCAs have marketing suspension in Europe; use macrocyclic agents preferentially

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6. Anatomy of an Axial CT Slice — Orientation Rules

• Patient's right = viewer's left (and vice versa)
• Anterior structures are at the top
• Posterior structures (spine) are at the bottom

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7. CT Reconstruction Planes and Techniques

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8. Systematic Approach to Reading an Abdominal/Pelvic CT

1. Check the basics: patient name/DOB, date, scan phase (contrast vs. non-contrast), view plane
2. Lung bases (if included): effusions, consolidation, free air under diaphragm
3. Liver: size, attenuation, focal lesions, biliary dilatation
4. Gallbladder & bile ducts: calculi, wall thickening, pericholecystic fluid
5. Spleen: size (normal <12 cm), focal lesions
6. Pancreas: size, ductal dilatation, peripancreatic fat stranding, calcification
7. Adrenal glands: size (<10 mm limb width), nodules
8. Kidneys & ureters: calculi, hydronephrosis, masses, enhancement pattern
9. Bladder & pelvis: wall thickening, masses, lymphadenopathy
10. Bowel: wall thickening, fat stranding, dilatation, transition points
11. Vasculature: aortic diameter (<3 cm normal), iliac vessels, contrast fill
12. Retroperitoneum: lymph nodes (>1 cm short axis = suspicious), fat stranding
13. Bones: fractures, lytic/sclerotic lesions, vertebral alignment
14. Free fluid / free gas: location, quantity

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9. Common CT Findings and What They Mean

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10. Radiation Dose — Key Concepts

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11. When to Use CT vs. Other Modalities

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• Campbell-Walsh-Wein Urology, Ch. 3 — Hounsfield Units, CT technique
• Grainger & Allison's Diagnostic Radiology, Ch. 1 & 37 — Contrast phases, adrenal washout, CIN
• Brenner & Rector's The Kidney — CIN criteria and management