Abdominal x-ray 101
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abdominal x-ray normal bowel gas pattern interpretation
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small bowel obstruction abdominal x-ray dilated loops air fluid levels
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pneumoperitoneum free air under diaphragm erect chest x-ray perforated viscus
Now I have comprehensive material. Let me compile a thorough "Abdominal X-ray 101" guide.
Abdominal X-Ray (AXR) 101
1. Overview & When to Order It
The plain abdominal radiograph (AXR) was the mainstay of acute abdominal investigation for generations. Today, ultrasound and CT have largely replaced it as the first-line modality, but the AXR retains clear value in specific situations.
Valid indications (Grainger & Allison's Diagnostic Radiology):
| Indication | Notes |
|---|---|
| Suspected viscus perforation | Pair with erect CXR |
| Bowel obstruction | Small or large bowel |
| Bowel wall pattern assessment | Ischaemia, colitis |
| Intra-abdominal foreign body | Ingested items, surgical hardware |
| Urinary tract calculi follow-up | Monitoring radiopaque stones |
| Bowel transit time in constipation | Marker studies |
Not helpful for: appendicitis, diverticulitis, cholecystitis, or pancreatitis — CT/US/MRI are preferred for these.
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.
3. Systematic Approach — The ABCDE Framework
Always review an AXR systematically to avoid missing findings:
| Letter | What to Check |
|---|---|
| A — Air | Distribution of bowel gas; free air; pneumobilia |
| B — Bowel | Calibre (dilated?); pattern (SB vs. LB); mucosal folds |
| C — Calcification | Stones, vascular calcification, pancreatic calcification |
| D — Densities/Organs | Liver, spleen, kidneys, psoas shadows, bladder |
| E — Extras | Bones, soft tissues, foreign bodies, tubes/lines |
4. Normal Bowel Gas Pattern
Normal AXR:
- 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.
5. Abnormal Gas Distribution
A. Pneumoperitoneum (Free Intraperitoneal Air)
Almost always indicates perforated viscus (perforated peptic ulcer being the commonest cause; ~70% of perforations produce visible free air on erect CXR).
Signs on AXR/CXR:
| Sign | Description |
|---|---|
| Subdiaphragmatic crescent (erect CXR) | Radiolucent gas between diaphragm and liver — most sensitive plain film sign |
| Rigler's sign (double wall sign) | Gas visible on both sides of the bowel wall on a supine AXR |
| Falciform ligament sign | Free gas outlines the normally invisible falciform ligament |
| Football sign | Massive pneumoperitoneum gives an oval gas-filled appearance of the abdomen |
| Continuous diaphragm sign | Air outlines the inferior diaphragm, including under the heart silhouette |
⚠️ Causes of pneumoperitoneum without peritonitis: recent laparotomy/laparoscopy, pneumatosis intestinalis, vaginal douche — do not always indicate perforation.
B. Gas in Bowel Wall (Pneumatosis Intestinalis)
Linear gas streaks in the bowel wall → suspect intestinal infarction (mesenteric vessel thrombosis/embolism). Also seen in benign pneumatosis cystoides intestinalis.
Advanced ischaemia may show portal venous gas (branching gas in the liver — a very ominous sign).
6. Bowel Obstruction
Small Bowel Obstruction (SBO)
Radiological features:
- 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)
Causes: adhesions (most common in developed world), hernia, tumour, Crohn's stricture, volvulus, gallstone ileus, intussusception
CT is the definitive investigation — identifies the transition point (abrupt calibre change from dilated to collapsed bowel) and defines the cause.
Large Bowel Obstruction (LBO)
Radiological features:
- 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
Special LBO patterns:
| Entity | Appearance |
|---|---|
| Sigmoid volvulus | Inverted U-shaped loop, massively dilated, devoid of haustra; "liver overlap sign," "left flank overlap sign," "pelvic overlap sign" |
| Caecal volvulus | Distended caecum displaced toward left upper quadrant; attached gas-filled appendix sometimes visible |
Paralytic Ileus vs. Obstruction
| Feature | Mechanical Obstruction | Paralytic Ileus |
|---|---|---|
| Gas distribution | Dilated proximal + collapsed distal | Dilated throughout (small AND large bowel) |
| Transition point on CT | Present | Absent |
| Cause | Adhesion, hernia, tumour | Post-op, peritonitis, metabolic, drugs (opioids) |
7. Bowel Diameter Limits (Rules of 3, 6, 9)
| Segment | Upper limit of normal |
|---|---|
| Small bowel | 3 cm |
| Large bowel (general) | 6 cm |
| Caecum | 9 cm (higher → impending perforation risk) |
8. Calcifications on AXR
| Location | Likely Structure |
|---|---|
| Right upper quadrant | Gallstones (~10% are radio-opaque), calcified liver lesions |
| Renal areas / ureters | Renal/ureteric calculi (80–90% radio-opaque) |
| Epigastrium | Pancreatic calcification (chronic pancreatitis) |
| Pelvis | Phleboliths (common/benign), uterine fibroids, bladder stones |
| Vascular (curvilinear) | Aortic / iliac artery calcification, aortic aneurysm |
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
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
11. AXR Limitations — When to go to CT
| Situation | Preferred modality |
|---|---|
| Appendicitis | CT (or US in children/pregnancy) |
| Diverticulitis | CT |
| Cholecystitis | Ultrasound |
| Pancreatitis staging | CT (contrast-enhanced) |
| Confirm perforation when CXR equivocal | CT (lung windows) |
| Confirm SBO + define cause | CT |
CT is particularly powerful for detecting free gas that is invisible on plain films — viewing at lung window settings (W:1500, L:-600) dramatically increases sensitivity.
Key References
- Grainger & Allison's Diagnostic Radiology, Ch. 18
- Harriet Lane Handbook (23rd ed.), Ch. 26
- Sleisenger & Fordtran's GI and Liver Disease, Ch. on Pancreatitis
Ct 101
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CT scan abdomen axial slice anatomy liver spleen kidneys labelled
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CT Hounsfield units window level bone lung soft tissue window settings
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CT contrast phases arterial portal venous delayed enhancement liver timing
I now have all the material needed for a comprehensive CT 101 guide. Let me compile it.
CT (Computed Tomography) 101
1. How CT Works
CT uses a collimated X-ray beam that rotates around the patient while detectors on the opposite side measure transmitted radiation. A computer reconstructs these measurements into cross-sectional images using mathematical interpolation algorithms. The result is a 3D dataset of the body sliced into thin axial planes.
Modern MDCT (Multidetector CT): Current scanners have 64–320+ detector rows, allowing the entire body to be imaged in a single breath-hold with 0.5 mm slices, minimal motion artefact, and far lower radiation than older single-slice machines. Gantry rotation times of ~0.275 seconds are now standard.
2. Hounsfield Units (HU) — The Language of CT
Every pixel on a CT image has an attenuation value expressed in Hounsfield Units (HU), a standardised scale based on how much X-ray the tissue absorbs.
| Tissue / Material | HU Range |
|---|---|
| Air | −1000 |
| Fat | −100 to −50 |
| Water | 0 |
| Soft tissue / muscle | +20 to +80 |
| Blood (acute haemorrhage) | +50 to +90 |
| Calcification / bone | +130 to +1000 |
| Dense cortical bone | ~+1000 |
| Iodinated contrast (vessels) | +150 to +400 |
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.
3. CT Windows — Seeing What You Want
The full HU range (−1000 to +1000) cannot be displayed simultaneously. Windowing selects a subset of that range to optimise contrast for a specific tissue type. Two parameters control this:
- Window Level (WL) = the centre of the range you want to display (the "brightness")
- Window Width (WW) = the spread of the range (the "contrast")
Tissues below WL − WW/2 appear black; tissues above WL + WW/2 appear white.
| Window Preset | Level (WL) | Width (WW) | Used For |
|---|---|---|---|
| Soft tissue / abdomen | +40 | 350–400 | Organs, tumours, fluid |
| Lung | −600 | 1500 | Lung parenchyma, airways, pneumothorax |
| Bone | +700 | 2000–3000 | Fractures, cortical detail, spine |
| Brain | +35 | 80 | Intracranial haemorrhage, oedema |
| Subdural | +75 | 200 | Thin subdural collections |
| Liver | +60 | 160 | Hepatic lesions |
| Free gas | −600 | 1500 | Pneumoperitoneum (lung window on abdominal CT!) |
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).
4. Contrast Media — Types, Phases, and Safety
Types of Contrast
| Type | Agent | Use |
|---|---|---|
| Iodinated IV contrast | Iohexol, iopromide, ioversol | CT vascular/organ enhancement |
| Oral contrast | Dilute iodine or barium | Bowel opacification |
| Rectal contrast | Dilute iodine | Pelvic/colorectal CT |
| Gadolinium | Gadopentetate | MRI (not CT — see NSF risk below) |
IV Contrast Phases
The timing of image acquisition after IV contrast injection determines which phase is captured. Each phase optimises different pathology:
| Phase | Timing After Injection | What It Shows |
|---|---|---|
| Non-contrast (NCCT) | Before injection | Calcification, haemorrhage, fat, baseline attenuation |
| Arterial phase | ~25–35 s | Aorta, hepatic artery, hypervascular tumours (HCC, carcinoid), renal cortex |
| Portal venous phase | ~60–70 s | Portal vein, liver parenchyma (max enhancement), bowel wall, spleen |
| Delayed / nephrographic phase | ~90–120 s | Urothelial tumours, renal collecting system (CTU), abscess characterisation |
| Equilibrium / delayed | 3–15 min | Fibrotic lesions, cholangiocarcinoma, adrenal washout |
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)
5. Contrast Safety
Contrast-Induced Nephropathy (CIN) / Post-Contrast AKI (PC-AKI)
Diagnosis (ACR / AKIN criteria within 48 h of contrast):
- 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
Risk management (Grainger & Allison's, Brenner & Rector's):
- 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
6. Anatomy of an Axial CT Slice — Orientation Rules
Standard axial CT is viewed as if looking up from the patient's feet (radiological convention):
- Patient's right = viewer's left (and vice versa)
- Anterior structures are at the top
- Posterior structures (spine) are at the bottom
Key anatomical landmarks by level:
| Level | Landmarks |
|---|---|
| T10–T12 | Oesophagogastric junction, adrenals, upper kidneys |
| L1 | Coeliac axis, upper pole kidneys, pancreatic body/tail |
| L2 | SMA origin, renal hila, pancreatic head/uncinate |
| L3–L4 | Aortic bifurcation, iliac vessels |
| S1 | Sigmoid colon, distal ureters |
7. CT Reconstruction Planes and Techniques
| Format | Description | Use |
|---|---|---|
| Axial | Standard transverse slices | Default reading view |
| Coronal | Front-to-back reformats | Bowel, kidneys, abdominal overview |
| Sagittal | Side-to-side reformats | Spine, aorta, pelvic organs |
| MPR (Multiplanar Reconstruction) | Any oblique plane | Vessels, ducts, operative planning |
| MIP (Max Intensity Projection) | Highlights brightest voxels | CT angiography, urinary calculi |
| MinIP (Min Intensity Projection) | Highlights darkest voxels | Airways, emphysema |
| VR (Volume Rendering) | 3D surface model | Surgical planning, patient communication |
8. Systematic Approach to Reading an Abdominal/Pelvic CT
Always use a consistent checklist — don't jump straight to the obvious finding:
- Check the basics: patient name/DOB, date, scan phase (contrast vs. non-contrast), view plane
- Lung bases (if included): effusions, consolidation, free air under diaphragm
- Liver: size, attenuation, focal lesions, biliary dilatation
- Gallbladder & bile ducts: calculi, wall thickening, pericholecystic fluid
- Spleen: size (normal <12 cm), focal lesions
- Pancreas: size, ductal dilatation, peripancreatic fat stranding, calcification
- Adrenal glands: size (<10 mm limb width), nodules
- Kidneys & ureters: calculi, hydronephrosis, masses, enhancement pattern
- Bladder & pelvis: wall thickening, masses, lymphadenopathy
- Bowel: wall thickening, fat stranding, dilatation, transition points
- Vasculature: aortic diameter (<3 cm normal), iliac vessels, contrast fill
- Retroperitoneum: lymph nodes (>1 cm short axis = suspicious), fat stranding
- Bones: fractures, lytic/sclerotic lesions, vertebral alignment
- Free fluid / free gas: location, quantity
9. Common CT Findings and What They Mean
| Finding | Significance |
|---|---|
| Free gas | Perforated viscus (surgical emergency) |
| Free fluid (ascites) | Liver disease, malignancy, haemoperitoneum, infection |
| Fat stranding | Inflammation in adjacent tissue (appendicitis, diverticulitis, pancreatitis) |
| Bowel wall thickening | Inflammation, ischaemia, tumour, oedema |
| Target / halo sign | Benign mural oedema (IBD, radiation) |
| Transition point (dilated → collapsed) | Mechanical bowel obstruction |
| Pneumatosis intestinalis | Intramural gas — ischaemia (emergency) or benign |
| Portal venous gas | Ischaemic gut, very ominous |
| Hyperdense aorta | Acute intraluminal thrombus |
| Striated nephrogram | Pyelonephritis |
| Ring-enhancing lesion | Abscess, metastasis, primary brain tumour |
| Double duct sign | Pancreatic head mass (CBD + PD dilatation) |
10. Radiation Dose — Key Concepts
| Parameter | What It Is |
|---|---|
| mSv | Effective dose to patient |
| Typical CXR | ~0.02 mSv |
| Typical CT abdomen/pelvis | ~8–10 mSv |
| Typical CT chest | ~5–7 mSv |
| CTPA | ~5 mSv |
| ALARA principle | As Low As Reasonably Achievable — dose must be justified and optimised |
Dose reduction strategies: lower kVp (tube voltage), automatic tube current modulation, iterative reconstruction algorithms, dual-energy CT to reduce scan phases.
11. When to Use CT vs. Other Modalities
| Clinical Scenario | First Choice |
|---|---|
| Acute abdomen, haemodynamically stable | CT abdomen/pelvis with contrast |
| Suspected perforation | CT (or erect CXR) |
| Bowel obstruction characterisation | CT |
| Right iliac fossa pain (appendicitis) | US first, CT if equivocal |
| Renal colic | CT KUB (non-contrast) |
| Haematuria workup | CT Urogram (3 phases) |
| Pancreatitis severity staging | CECT at 48–72 h |
| Aortic aneurysm | CT angiography |
| Liver lesion characterisation | Multiphase CT ± MRI |
| Pregnancy / paediatrics | US or MRI preferred (no radiation) |
Key References
- 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
Ct Abdomen 101
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